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Integrated all-photonic non-volatile multi-level memory
Abstract
Implementing on-chip non-volatile photonic memories has been a long-term, yet elusive goal. Photonic data storage would dramatically improve performance in existing computing architectures by reducing the latencies associated with electrical memories and potentially eliminating optoelectronic conversions. Furthermore, multi-level photonic memories with random access would allow for leveraging even greater computational capability. However, photonic memories have thus far been volatile. Here, we demonstrate a robust, non-volatile, all-photonic memory based on phase-change materials. By using optical near-field effects, we realize bit storage of up to eight levels in a single device that readily switches between intermediate states. Our on-chip memory cells feature single-shot readout and switching energies as low as 13.4 pJ at speeds approaching 1 GHz. We show that individual memory elements can be addressed using a wavelength multiplexing scheme. Our multi-level, multi-bit devices provide a pathway towards eliminating the von Neumann bottleneck and portend a new paradigm in all-photonic memory and non-conventional computing.
... Phase change materials (PCMs) have received attention for their ability to reversibly transition between stable solid phases with different electrical conductivities and optical reflectivities. [2][3][4][5][6] These property differences allow PCMs to encode information in the solid phases-crystalline and amorphous-which correspond to bits of binary data, 3,5,6 as illustrated in Fig. 1. This phase transition can occur rapidly and locally due to temperature changes, which may be induced by a laser for optical storage or electrically for nonvolatile random access memory. ...
... Phase change materials (PCMs) have received attention for their ability to reversibly transition between stable solid phases with different electrical conductivities and optical reflectivities. [2][3][4][5][6] These property differences allow PCMs to encode information in the solid phases-crystalline and amorphous-which correspond to bits of binary data, 3,5,6 as illustrated in Fig. 1. This phase transition can occur rapidly and locally due to temperature changes, which may be induced by a laser for optical storage or electrically for nonvolatile random access memory. ...
... This phase transition can occur rapidly and locally due to temperature changes, which may be induced by a laser for optical storage or electrically for nonvolatile random access memory. 2,3,5,6 Currently, the most commonly used PCM for memory applications is Ge 2 Sb 2 Te5 (GST), 2,6 and hence there is wide interest in better characterizing this material both experimentally and through atomistic modeling. Our focus is on developing an accurate and efficient computational model for atomistic simulations of GST for use by the broader community. ...
Phase change materials such as Ge2Sb2Te5 (GST) are ideal candidates for next-generation, non-volatile, solid-state memory due to the ability to retain binary data in the amorphous and crystal phases and rapidly transition between these phases to write/erase information. Thus, there is wide interest in using molecular modeling to study GST. Recently, a Gaussian Approximation Potential (GAP) was trained for GST to reproduce Density Functional Theory (DFT) energies and forces at a fraction of the computational cost [Zhou et al., Nat. Electron. 6, 746 (2023)]; however, simulations of large length and time scales are still challenging using this GAP model. Here, we present a machine-learned (ML) potential for GST implemented using the Atomic Cluster Expansion (ACE) framework. This ACE potential shows comparable accuracy to the GAP potential but performs orders of magnitude faster. We train the ACE potentials both directly from DFT and also using a recently introduced indirect learning approach where the potential is trained instead from an intermediate ML potential, in this case, GAP. Indirect learning allows us to consider a significantly larger training set than could be generated using DFT alone. We compare the directly and indirectly learned potentials and find that both reproduce the structure and thermodynamics predicted by the GAP and also match experimental measures of GST structure. The speed of the ACE model, particularly when using graphics processing unit acceleration, allows us to examine repeated transitions between crystal and amorphous phases in device-scale systems with only modest computational resources.
... Leveraging these advantages, PICs hold significant development potential in high-speed data computing and processing, as well as in ultra-large capacity communication network applications. Photonic integrated devices are continuously being developed, including optical switches [1,2], microring resonators [3][4][5], photonic memory [6,7], power dividers [8], mode converters [9][10][11], and more. And the mode division multiplexing (MDM) system on the SOI platform has attracted significant attention due to its propagation speed and transmission capacity. ...
... The crystalline state of PCMs can be continuously adjusted by applying electrical pulses, enabling the continuous programming of multiple intermediate levels of mode purity values. Multilevel phase transitions have been successfully implemented in various applications, such as multistage memory [6] and photonic convolutional neural networks [11], underscoring the importance of studying intermediate states of PCMs. Mode purity is quantified by the equation β TE0(TM0) = P TE0 (P TM0 )/(P TE0 + P TM0 ). ...
Subwavelength gratings serve as a pivotal tool in optical devices, enabling the flexible modulation of the effective refractive index of waveguide modes and modulating the guided mode through intense optical scattering at subwavelength intervals. Nevertheless, the modulation space remains limited. Incorporating phase change materials (PCMs) can achieve significantly higher modulation efficiency. This paper proposes a compact reconfigurable polarization converter based on PCMs, which is shaped into a subwavelength tilted grating. Effects of the tilted angle on effective refractive indices of TE mode and TM mode are systematically investigated for the subwavelength tilted grating. Through precise control of the transition between the crystalline and amorphous states of the phase-change material, the reconfiguration of the polarization converter is achieved with high efficiency and low insertion loss. In the crystalline state of the PCM, the slight difference in effective refractive index, along with the perturbation caused by the tilted grating, promotes mode coupling, allowing the conversion of the input TE0 mode into the TM0 mode. After the crystalline-to-amorphous transition, the periodic perturbation has almost no effect on the guided mode in the waveguide, and the device is in the normal on-state. The device realizes the free conversion of TE0 mode and TM0 mode with a small coupling length (17.89μm) and low extra loss (<1.5 dB). It has high conversion efficiency and mode purity in the broad range of 1500nm-1600 nm. Through dynamically controlling electrical pulses, we achieve 21-level programming operations, demonstrating multiple levels of tunability. Our work provides a feasible method to solve the polarization sensitivity of silicon-based photonic devices and shows a prospect of application in neuromorphic computing networks due to its multistage tunability.
... This makes them a very promising platform for developing integrated optical devices, methods that combine PCMs with silicon-on-insulator (SOI) waveguides as tuning media [6] to overcome the bottleneck of traditional optical switches. Reconfigurable silicon and PCMs hybrid integrated waveguides may be employed in optical switches [3,7], photonic memory [8], optical computing [9] and optical neural networks [10] due to the nonvolatility and strong refractive index modulation (usually ∆n > 1) [11] of PCMs. As the most widely used PCMs, GST has high relative loss due to its small band gap. ...
... It has been a focal point of research in the field of silicon-based photonics. Nevertheless, most of the MRR optical switches directly cover the PCMs on top of the waveguide [8,[17][18][19][20]. Although this method is convenient to operate, the weak interaction between the waveguide mode and PCMs results in limited optical modulation efficiency. ...
Integrated silicon photonics based on phase-change materials have the advantages of energy efficiency, ultra-compactness, and nonvolatility, with a promising prospect in reconfigurable photonic systems. A wavelength-selective 1 × 2 nonvolatile optical switch based on a periodic Ge2Sb2Se4Te1 (GSST)-assisted racetrack silicon micro-ring is proposed, where the GSST is embedded in the micro-ring as a one-dimensional photonic crystal. The switch state changes from ‘OFF’ to ‘ON’ by changing the GSST state from amorphous to crystal. The results show that the extinction ratio of the device is 27.99 dB and 19.57 dB at the drop and through ports, respectively. The insertion loss is as low as 1.21 dB at the through port in the crystal state and 0.61 dB at the drop port in the amorphous state. The proposed on-chip optical switch significantly enhances device integration, offers a potential path toward the development of nonvolatile Si-GSST hybrid optical switches, and is crucial to realizing reconfigurable photonic systems. Furthermore, the resonant wavelength of the switch changes very little, by only 0.66 nm between the ‘ON’ and ‘OFF’ states, which is suitable for multichannel wavelength-division-multiplexing systems.
... Moreover, from the viewpoint of future large-scale chip integration, its process flow should be simple and compatible with the standardized (or front-end/back-end) CMOS process [3,4], along with high scalability and reliability. For instance, phase-change materials, featuring substantial changes of real and imaginary refractive indices in the transition between amorphous and crystalline states, are a promising material platform for on-chip optical memristors, which can be physically deposited on large-scale photonic chips directly and compatible with the back-end CMOS process [5][6][7][8][9][10][11]. However, in the operation of phase-change transition, the local ramping up to several hundred degrees Celsius undoubtedly results in issues of thermal crosstalk, material fatigue, and equilibration period [2,10]. ...
... The optical memristor based on the graphene-MRR memristor is encoded by its resonant wavelength. It promises the construction of integrated optical memristors with WDM capabilities, which is expected to supplement or even surpass the performance of electrical memristors [1,6]. As shown in Fig. 4(a), two graphene-MRR memristor cells are evanescently coupled with the same bus-waveguide. ...
Chip-integrated optical memristors, modulating light in a nonvolatile and semicontinuous manner, are attractive to revolutionize on-chip optical signal processing via the constructions of nonvolatile reconfigurable photonic circuits, in-memory computing, brain-inspired architectures, etc. Mechanisms, including phase-change, filamentation, and ferroelectricity, have been attempted to implement on-chip optical memristors, though their intricate tradeoffs between fabrication compatibility, modulation depth, power consumption, retention time, and cyclability make it desired to pursue new architectures. Here, we demonstrate graphene-based on-chip optical amplitude and phase memristors by electrostatically doping the graphene integrated on a silicon nitride waveguide with a ferroelectric film. Benefiting from graphene’s significant dependence of complex refractive index on its carrier density and the ferroelectric remnant doping, semicontinuous nonvolatile modulation with a maximum depth of is realized with a low programming energy of {\sim}{1.86}\;{\rm pJ/}\unicode{x00B5}{\rm m}^2 , exhibiting good cyclability (fluctuation ratio ) and long retention time (over 10 years). By integrating the graphene-based optical memristor with cascaded microring resonators, in-memory computings with multiple wavelength channels are demonstrated by analogue matrix-vector multiplication and digital logic gate operations. Combining these merits with CMOS-compatible on-chip graphene integration, the demonstrated graphene-based optical memristor has proven to be a competitive candidate for high-bandwidth neuromorphic computing, convolutional processing, and artificial intelligence on photonic integrated circuits.
... Using PCMs, the data can be written and erased using short pulses that switch the phase of the 54 material between crystalline and amorphous [22][23][24]. Despite being very compact and the fact 55 that they enable in-memory computing, they require post-fabrication processing, have hundreds 56 of nanoseconds write/erase time, introduce non-negligible optical loss (few dB/um [25]), need 57 accurate optical pulse generation, and typically have limited number of programming cycles. ...
... Although the focus of this work is on high-speed volatile optical memory for temporary data storage, it is worth mentioning that phase change materials (PCMs) enable non-volatile optical memory. Using PCMs, the data can be written and erased using short pulses that switch the phase of the material between crystalline and amorphous [22][23][24]. Despite being very compact and the fact that they enable in-memory computing, they require post-fabrication processing, have hundreds of nanoseconds write/erase time, introduce non-negligible optical loss (few dB/um [25]), need accurate optical pulse generation, and typically have limited number of programming cycles. ...
Significant advancements in integrated photonics have enabled high-speed and energy efficient systems for various applications, from data communications and high-performance computing to medical diagnosis, sensing, and ranging. However, data storage in these systems has been dominated by electronic memories that in addition to signal conversion between optical and electrical domains, necessitates conversion between analog to digital domains and electrical data movement between processor and memory that reduce the speed and energy efficiency. To date, scalable optical memory with optical control has remained an open problem. Here, we report an integrated photonic set-reset latch as a fundamental optical static memory unit based on universal optical logic gates. As a proof of concept, experimental implementation of the universal logic gates and realistic simulation of the latch are demonstrated on a programmable silicon photonic platform. Optical set, reset, and complementary outputs, scalability to a large number of memory units via the independent latch supply light, and compatibility with wavelength division multiplexing scheme and different photonic platforms enable more efficient and lower latency optical processing systems.
... A key application of the memristor is non-volatile memory for data storage. The number of states corresponds to Fig. 4 Power consumption of different low-power memristors when performing synaptic plasticity [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58], where biological synaptic power consumption is ~ 10 fJ. The reported power consumption of novel memristors range from 5 nJ to 4.28 aJ, exhibiting great potential in neuromorphic computing discrete resistors that can be read, reflecting the storing ability of memory. ...
As an emerging memory device, memristor shows great potential in neuromorphic computing applications due to its advantage of low power consumption. This review paper focuses on the application of low-power-based memristors in various aspects. The concept and structure of memristor devices are introduced. The selection of functional materials for low-power memristors is discussed, including ion transport materials, phase change materials, magnetoresistive materials, and ferroelectric materials. Two common types of memristor arrays, 1T1R and 1S1R crossbar arrays are introduced, and physical diagrams of edge computing memristor chips are discussed in detail. Potential applications of low-power memristors in advanced multi-value storage, digital logic gates, and analogue neuromorphic computing are summarized. Furthermore, the future challenges and outlook of neuromorphic computing based on memristor are deeply discussed.
... The most important family of PCM consists of the Ge-Sb-Te alloys along the GeTe-Sb2Te3 pseudo-binary line [11][12][13][14][15][16][17][18][19], e.g. Ge2Sb2Te5 (GST), which has been extensively used in electronic, photonic as well as hybrid optoelectronic devices [20][21][22][23][24][25][26][27][28]. The basic working principle is the large contrast in electrical resistance or optical transmission between the crystalline and amorphous phases of PCM for memory encoding [1]. ...
Chalcogenide phase-change materials (PCMs) are a leading candidate for advanced memory and computing applications. Epitaxial-like growth of chalcogenide thin films at the wafer scale is important to guarantee the homogeneity of the thin film but is challenging with magnetron sputtering, particularly for the growth of phase-change heterostructure (PCH), such as TiTe2/Sb2Te3. In this work, we report how to obtain highly textured TiTe2/Sb2Te3 heterostructure thin films with atomically sharp interfaces on standard silicon substrates. By combining atomic-scale characterization and ab initio simulations, we reveal the critical role of the Sb2Te3 seed layer in forming a continuous Si-Sb-Te mixed transition layer, which provides a wafer-scale flat surface for the subsequent epitaxial-like growth of TiTe2/Sb2Te3 thin film. By gradually reducing the thickness of the seed layer, we determine its critical limit to be ~2 nm. Non-negligible in-plane tensile strain was observed in the TiTe2 slabs due to the lattice mismatch with the adjacent Sb2Te3 ones, suggesting that the chemical interaction across the structural gaps in the heterostructure is stronger than a pure van der Waals interaction. Finally, we outline the potential choices of chalcogenides for atomically flat seed layers on standard silicon substrates, which can be used for wafer-scale synthesis of other high-quality PCM or PCH thin films.
... The large refractive index change (Δn) and non-volatility allow for the creation of compact, reconfigurable devices (∼100 μm) 52,53 with zero static energy consumption. 50,[54][55][56][57][58] Furthermore, PCMs are suitable for large-scale integration as they can be easily deposited by sputtering 50,54,56,[58][59][60][61][62] or thermal evaporation 53 onto various integrated photonics material platforms, including silicon and silicon nitride. While traditional PCMs such as Ge 2 Sb 2 Te5 (GST) and GeTe show strong optical absorption, emerging wide-bandgap PCMs, such as Ge 2 Sb 2 SeTe 4 (GSST), 63 Sb 2 S 3 , 64,65 and Sb 2 Se 3 , 66-69 offer new opportunities to reduce this absorption loss. ...
Programmable photonic integrated circuits are expected to play an increasingly important role in enabling high-bandwidth optical interconnects and large-scale in-memory computing as needed to support the rise of artificial intelligence and machine learning technology. To that end, chalcogenide-based non-volatile phase-change materials (PCMs) present a promising solution due to zero static power. However, high switching voltage and a small number of operating levels present serious roadblocks to the widespread adoption of PCM-programmable units. Here, we demonstrate an electrically programmable wide bandgap Sb2S3-clad silicon ring resonator using a silicon microheater at a complementary-metal–oxide–semiconductor compatible voltage of <3 V. Our device shows a low switching energy of 35.33 nJ (0.48 mJ) for amorphization (crystallization) and reversible phase transitions with high endurance (>2000 switching events) near 1550 nm. Combining a volatile thermo-optic effect with non-volatile PCMs, we demonstrate 7-bit (127 levels) operation with excellent repeatability and reduced power consumption. Our demonstration of low-voltage and low-energy operation, combined with the hybrid volatile–nonvolatile approach, marks a significant step toward integrating PCM-based programmable units in large-scale optical interconnects.
... 20 Here, the influence of the above discussed stimuli can trigger a crystallographic transition in VO 2 (M1), shifting from a high-temperature metallic phase to a low-temperature insulating phase, transitioning from a tetragonal (R) (space group P4 2 /mnm) to a monoclinic (M) (space group P2 1 /c) crystallographic structure. Its remarkable ability to undergo a reversible phase transition at room temperature makes VO 2 an intriguing material for a range of applications, including thermal radiators, 21 optical modulators, 22 resonators, 23,24 energy-efficient switching devices, 25,26 terahertz devices, 27 microbolometers, 28 photonic memory devices, 29,30 etc. ...
Vanadium dioxide (VO2) (M1) exhibits a unique metal–insulator transition (MIT) near room temperature, garnering considerable attention for its applications in bolometer, terahertz/infrared detectors, and microelectronic devices. Here, we explore the potential of epitaxially grown VO2 (M1) thin films for near-infrared (IR) detection by optimizing the growth conditions, followed by structural characterization and device fabrication. Alongside the VO2 (M1) phase, two other oxides from the vanadium oxide family, VO2 (A) and V2O5, were also grown on a c-cut sapphire substrate using a pulsed laser deposition (PLD) system. In-depth analysis using temperature-dependent XRD and Raman spectroscopy confirmed the crystalline structure and the quality of epitaxial thin film formation of VO2 (M1), while also unveiling structural phase transition (SPT) behavior and the critical temperature of transition. At elevated temperatures during electrical measurement, the VO2 (M1) epilayer exhibits a first-order phase transition from the metallic to the insulating state, accompanied by a significant change in resistance exceeding three orders of magnitude unveiling its potential in thermal switches, memory-based devices etc. In depth, electrical analysis on all the grown oxides shows that VO2 (M1) and V2O5 exhibit a higher temperature coefficient of resistance (TCR) (3%/K and 2%/K) and a lower 1/f noise (in the order pA / Hz at 0.1 Hz) as compared to VO2 (A), paving scope for further analysis of these two oxides toward important applications in the domain of thermal sensors. Additionally, VO2 (M1) exhibited good bolometric response (in the order of ms) to IR radiation, proving its candidature for the application in IR detectors as well.
... Non-volatile optical memories, on the other hand, can maintain the stored data without power. They can be achieved by altering the material properties interacting with light, including phase-change memories (PCMs) [9][10][11][12] , ferroelectric memories [13][14][15] , micro-electro-mechanical systems (MEMS) [16][17][18] , floating-gate memories [19][20][21][22][23] , and optical memristors [24][25][26][27] . PCMs are characterized by thermal energy-dependent properties, transitioning between amorphous and crystalline states. ...
Implementing on-chip non-volatile optical memories has long been an actively pursued goal, promising significant enhancements in the capability and energy efficiency of photonic integrated circuits. Here, we demonstrate an non-volatile optical memory exclusively using the most common semiconductor material, silicon. By manipulating the photon avalanche effect, we introduce a trapping effect at the silicon-silicon oxide interface, which in turn demonstrates a non-volatile reprogrammable optical memory cell with a record-high 4-bit encoding, robust retention and endurance. This silicon avalanche-induced trapping memory provides a distinctively cost-efficient and high-reliability route to realize optical data storage in standard silicon foundry processes. We demonstrate its applications in trimming in optical interconnects and in-memory computing. Our in-memory computing test case reduces energy consumption by approximately 83% compared to conventional optical approaches.
... Chalcogenide phase-change materials (PCMs) present a promising solution to realize programmable photonics due to their nonvolatile, reversible microstructural phase transition, the ability for multilevel operation, and significant refractive index contrast between crystalline and amorphous phases (Δn > 0.5). [11][12][13][14] PCMs, including Ge 2 Sb 2 Te5, GSST, Sb 2 Se 3 , or Sb 2 S 3 , have been incorporated in Mach-Zehnder interferometers (MZIs) 15,16 and directional couplers 17,18 for phase modulation and in ring resonators for resonance tuning. 19,20 Typically, the functionality of these devices is determined by the phase state of the entire PCM film, whether in binary or multilevel configurations. ...
Chalcogenide phase-change materials (PCMs) offer a promising approach to programmable photonics thanks to their nonvolatile, reversible phase transitions and high refractive index contrast. However, conventional designs are limited by global phase control over entire PCM thin films between fully amorphous and fully crystalline states, which restricts device functionality and confines design flexibility and programmability. In this work, we present a novel approach that leverages pixel-level control of PCM in inverse-designed photonic devices, enabling highly reconfigurable, multi-functional operations. We integrate low-loss Sb2Se3 onto a multi-mode interferometer and achieve precise, localized phase manipulation through direct laser writing. This technique allows for flexible programming of the photonic device by adjusting the PCM phase pattern rather than relying on global phase states, thereby enhancing device adaptability. As a proof of concept, we programmed the device as a wavelength-division multiplexer and subsequently reconfigured it into a mode-division multiplexer. Our results underscore the potential of combining inverse design with pixel-wise tuning for next-generation programmable phase-change photonic systems.
... Currently, the mainstream methods for modulating GST-based microring synaptic devices are to induce a phase transition in GST by applying the optical pulse from above [14] or inputting it through the waveguide [15,16]. When modulating synaptic devices with the latter method, the resonance wavelength of the microring is mostly chosen as the wavelength of the pump pulse [17]. ...
A synaptic device is fundamental for memory and learning functions of neural networks. In this work, we demonstrated a GST (Ge2Sb2Te5)-based microring synapse with nonvolatile reconfigurable characteristics. The device shows bidirectional weight modulation during unidirectional crystallization or amorphization of GST, simulating long-term depression (LTD) and long-term potentiation (LTP). In addition, we adopted an anti-resonance pump scheme to reduce the fluctuations in pump energy coupled into the device to less than 12%. This scheme significantly enhances the programming precision of the microring synapse, achieving 24 resolvable states over 15 cycles. This work holds promise for laying the foundation for novel, to the best of our knowledge, photonic computing architectures and provides possibilities for the implementation of vision and adaptive optical neural networks that rely on bidirectional plasticity.
... GST can be quickly excited by output laser pulses and temperature variations, demonstrating faster times compared to CMOS devices 32,40 . Its distinct electrical and optical properties in both amorphous and crystalline states make it useful in optical switches and resonators 28,39 . ...
In the drive toward efficient neuromorphic computing, photonic technologies offer promising solutions for implementing neural functionalities. Here we demonstrate the first all-optical double micro-ring resonator incorporating Ge2Sb2Te5 (GST) as a phase-change material to realize precise nonlinear activation functions (NLAF). Our device architecture achieves switching speeds of 0.5 ns through a novel approach to GST integration, where angular positioning of GST segments within the rings enables unprecedented control over optical transmission characteristics. Through systematic investigation of sixteen distinct phase configurations, we identify optimal GST positioning (180 degrees in the first ring, 90 degrees in the second) that achieves ultra-narrow band transmission with 0.47 nm full width at half maximum. Operating at significantly lower temperatures (100 degrees centigrade) than conventional GST implementations, our device maintains high contrast ratios with transmission coefficient modulation from near-zero to 0.85 across a 4 nm spectral window. The dual-ring architecture enables independent optimization of spectral selectivity and switching contrast a capability previously unattainable in single ring designs. These results demonstrate a viable pathway toward efficient neuromorphic photonic systems that can operate at speeds relevant for practical computing applications while maintaining the precision required for neural processing.
... One of the successful materials in photonic computing is the Ge−Sb−T e alloy, which recently demonstrated nanosecond recording speed using optical pulses [137]. This development has led to the creation of photonic memory devices [138], switches [139] and non-volatile computers [140]. In addition, PCM has established itself as a platform for neuromorphic bio-inspired on-chip computing and has already demonstrated spike-timing dependent plasticity [141] and control of spiking neurons [142] in such systems. ...
The extensive development of the field of spiking neural networks has led to many areas of research that have a direct impact on people's lives. As the most bio-similar of all neural networks, spiking neural networks not only allow the solution of recognition and clustering problems (including dynamics), but also contribute to the growing knowledge of the human nervous system. Our analysis has shown that the hardware implementation is of great importance, since the specifics of the physical processes in the network cells affect their ability to simulate the neural activity of living neural tissue, the efficiency of certain stages of information processing, storage and transmission. This survey reviews existing hardware neuromorphic implementations of bio-inspired spiking networks in the "semiconductor", "superconductor" and "optical" domains. Special attention is given to the possibility of effective "hybrids" of different approaches
... This design leads to a minimum 0.19 dB and maximum 0.238 dB optical insertion loss of depending on the ferroelectric phase state of SnSe on top of the silicon nitride waveguide. Note that the achieved optical insertion loss values for SnSe-based photonic memory cells when silicon and silicon nitride waveguide is used is significantly lower than the conventional GST-based photonic memory cells with minimum 1 dB loss reported by [6,9]. ...
... The large refractive index change (∆n) and non-volatility allow for the creation of compact reconfigurable devices (∼ 100 µm) 52,53 with zero static energy consumption 50,[54][55][56][57][58] . Furthermore, PCMs are suitable for large-scale integration as they can be easily deposited by sputtering 50,54,56,[58][59][60][61][62] or thermal evaporation 53 onto various integrated photonics material platforms, including silicon and silicon nitride. While traditional PCMs like Ge 2 Sb 2 Te 5 (GST) and GeTe show strong optical absorption, emerging wide-bandgap PCMs, such as Ge 2 Sb 2 SeTe 4 (GSST) 63 , Sb 2 S 3 64,65 , and Sb 2 Se 3 66-69 , offer new opportunities to reduce this absorption loss. ...
Programmable photonic integrated circuits are expected to play an increasingly important role to enable high-bandwidth optical interconnects, and large-scale in-memory computing as needed to support the rise of artificial intelligence and machine learning technology. To that end, chalcogenide-based non-volatile phase-change materials (PCMs) present a promising solution due to zero static power. However, high switching voltage and small number of operating levels present serious roadblocks to widespread adoption of PCM-programmble units. Here, we demonstrate electrically programmable wide bandgap Sb2S3-clad silicon ring resonator using silicon microheater at CMOS compatible voltage of < 3V. Our device shows low switching energy of 35.33 nJ (0.48 mJ) for amorphization (crystallization) and reversible phase transitions with high endurance (> 2000 switching events) near 1550 nm. Combining volatile thermo-optic effect with non-volatile PCMs, we demonstrate 7-bit (127 levels) operation with excellent repeatability and reduced power consumption. Our demonstration of low-voltage and low-energy operation, combined with the hybrid volatilenonvolatile approach, marks a significant step towards integrating PCM-based programmable units in large-scale optical interconnects.
The democratization of AI encourages multi-task learning (MTL), demanding more parameters and processing time. To achieve highly energy-efficient MTL, Diffractive Optical Neural Networks (DONNs) have garnered attention due to extremely low energy and high computation speed. However, implementing MTL on DONNs requires manually reconfiguring & replacing layers, and rebuilding & duplicating the physical optical systems. To overcome the challenges, we propose LUMEN-PRO, an automated MTL framework using DONNs. We first propose to automate MTL utilizing an arbitrary backbone DONN and a set of tasks, resulting in a high-accuracy multi-task DONN model with small memory footprint that surpasses existing MTL. Second, we leverage the rotability of the physical optical system and replace task-specific layers with rotation of the corresponding shared layers. This replacement eliminates the storage requirement of task-specific layers, further optimizing the memory footprint. LUMEN-PRO provides flexibility in identifying optimal sharing patterns across diverse datasets, facilitating the search for highly energy-efficient DONNs. Experiments show that LUMEN-PRO provides up to 49.58% higher accuracy and 4× better cost efficiency than single-task and existing DONN approaches. It achieves memory lower bound of MTL, with memory efficiency matching single-task models. Compared to IBM-TrueNorth, LUMEN-PRO achieves an energy efficiency gain, while it matches Nanophotonic in efficiency but surpasses it in per-operator efficiency due to its larger system.
Ge-Sb-Sn/Se with a superlattice-like structure (SLL) is a promising material candidate for multi-level phase change photonics memory technology. However, its multi-stage phase transition process has not been elucidated so far due to the limitations of traditional research approaches. The most critical issue is to efficiently construct its composition-process-structure-property multi-parameter coupled constitutive relationship. In this work, we develop a high-throughput approach to systematically study the multi-level phase transition mechanisms of Ge-Sb-Sn/Se SLL combinatorial thin films. For the Ge-Sb-Sn system, phase evolution is observed from trigonal to hexagonal/tetrahedral structures. In contrast, the Ge-Sb-Se system behaves differently. We further examine the optical properties of the Ge-Sb-Sn/Se SLL combinatorial thin films. The results identify the GeSbSn3 SLL thin film as a standout from the Ge-Sb-Sn ternary system under Sb→Sn→Ge deposition sequence, with a figure of merit (FOM) greater than 0.4 and high thermal stability. The present study serves as a foundation for further exploration of the Ge-Sb-based quaternary system and accelerates the application of advanced phase change materials (PCMs) in the big data era.
Reconfigurable photodetectors are crucial for applications such as adaptive sensing and dynamic imaging. However, conventional devices based on materials such as silicon typically require external electric fields and additional memory units, resulting in increased system complexity and energy consumption. Here, a self‐driven, reconfigurable, and controllable photoresponse with a large responsivity contrast of up to 100 is achieved using a Ge2Sb2Te5 (GST)/MoS2 heterojunction, leveraging the integration of the reversible phase‐transition capability of phase‐change materials (PCMs) with the nonvolatile reconfigurable optoelectronic merits of two‐dimensional (2D) materials. The reconfigurable GST/MoS2 heterojunction photodetector also demonstrates fast response times, excellent cycling stability, and a linear photocurrent–power relationship, supporting its use in real‐time imaging systems. Furthermore, a 3 × 3 reconfigurable photodetector array is implemented as an optical convolution kernel for in‐sensor image processing, achieving high‐quality image recognition, contrast enhancement, and edge detection. This work positions phase‐change‐2D heterojunctions as a promising platform for next‐generation reconfigurable photodetection and intelligent sensing technologies.
Nanofabrication, a pivotal technology at the intersection of nanoscale engineering and high-resolution patterning, has substantially advanced over recent decades. This technology enables the creation of nanopatterns on substrates crucial for developing nanophotonic devices and other applications in diverse fields including electronics and biosciences. Here, this mega-review comprehensively explores various facets of nanofabrication focusing on its application in nanophotonics. It delves into high-resolution techniques like focused ion beam and electron beam lithography, methods for 3D complex structure fabrication, scalable manufacturing approaches, and material compatibility considerations. Special attention is given to emerging trends such as the utilization of two-photon lithography for 3D structures and advanced materials like phase change substances and 2D materials with excitonic properties. By highlighting these advancements, the review aims to provide insights into the ongoing evolution of nanofabrication, encouraging further research and application in creating functional nanostructures. This work encapsulates critical developments and future perspectives, offering a detailed narrative on the state-of-the-art in nanofabrication tailored for both new researchers and seasoned experts in the field.
In this work, we propose a novel differential photonic static random access memory (pSRAM) bitcell design using fabrication-friendly photonic components. The proposed pSRAM overcomes the key limitations of traditional electrical SRAMs, which struggle with speed and power efficiency due to increasing bitline/wordline capacitance and interconnect resistance associated with long electrical wires as technology scales. By utilizing cross-coupled micro-ring resonators and differential photodiode structures, along with optical waveguides instead of traditional wordlines and bitlines, our pSRAM exhibits high-speed, and energy-efficient performance. The pSRAM bitcell demonstrates a read/write speed of 40 GHz, with a switching (static) energy consumption of approximately 0.6 pJ (0.03 pJ) per bit and a footprint of 330x290 um^2 using the GlobalFoundries 45SPCLO process node. These bitcells can be arranged into a 2D memory array, enabling large-scale, on-chip photonic memory subsystems ideal for high-speed memory, data processing and computing applications.
Optoelectronics are crucial for developing energy‐efficient chip technology, with phase‐change materials (PCMs) emerging as promising candidates for reconfigurable components in photonic integrated circuits, such as nonvolatile phase shifters. Antimony sulfide (Sb2S3) stands out due to its low optical loss and considerable phase‐shifting properties, along with the non‐volatility of both phases. This study demonstrates that the crystallization kinetics of Sb2S3 can be switched from growth‐driven to nucleation‐driven by altering the sample dimension from bulk to film. This tuning of the crystallization process is critical for optical switching applications requiring control over partial crystallization. Calorimetric measurements with heating rates spanning over six orders of magnitude, reveal that, unlike conventional PCMs that crystallize below the glass transition, Sb2S3 exhibits a measurable glass transition prior to crystallization from the undercooled liquid (UCL) phase. The investigation of isothermal crystallization kinetics provides insights into nucleation rates and crystal growth velocities while confirming the shift to nucleation‐driven behavior at reduced film thicknesses—an essential aspect for effective device engineering. A fundamental difference in chemical bonding mechanisms was identified between Sb2S3, which exhibits covalent bonding in both material phases, and other PCMs, such as GeTe and Ge2Sb2Te5, which demonstrate pronounced bonding alterations upon crystallization.
Integrated silicon photonic devices, especially microring resonators, are susceptible to fabrication variations, with even nanoscale errors potentially leading to suboptimal performance, thereby hindering their large-scale production and commercialization. This study demonstrates femtosecond-laser-based post-fabrication trimming of microring resonators, achieving controllable and permanent adjustments to the resonance wavelengths and coupling regimes. Micro-Raman spectroscopy revealed the trimming mechanism, whereas systematic analysis highlighted performance variations among the on-chip devices. The experimental results showed directional resonance wavelength shifts with long-term stability and minimal reduction in the quality factor. The overcoupling devices were tuned to near-critical coupling to improve the extinction ratio exceeding 22 dB. This approach promises to accelerate the industrialization of high-performance, cost-effective photonic devices.
Programmable on‐chip terahertz (THz) topological photonic devices are poised to address the rising need for high‐capacity data systems, offering broad bandwidth, minimal loss, and reconfigurability. However, current THz topological chips rely on volatile tuning mechanisms that require continuous power to function. Here, a nonvolatile, programmable THz topological silicon chip is demonstrated that integrates a waveguide‐cavity coupled system with phase‐change material, Ge2Sb2Te5 (GST), enabling persistent and efficient functionality without constant power input. Through precise tuning of the intermediate phase states of GST between amorphous and crystalline forms, a stable, non‐volatile reconfiguration of the topological cavity is achieved, enabling transitions across over‐coupling, critical coupling, and under‐coupling states. Multi‐level modulation of resonance transmission with a modulation depth of 70 dB is demonstrated, enabling precise control over the onset and disappearance of resonance modes and dynamic tuning of critical coupling states. The THz topological chip facilitates phototunable, volatile modulation across nonvolatile configurations, allowing controlled resetting of the coupling states of the cavity. Here, the first nonvolatile, programmable terahertz topological integrated chip is demonstrated, offering flexible control over resonance modes. This advancement significantly paves the way for integrating phase change materials into silicon topological chips for programmable photonic devices, including interconnects, modulators, and logic circuits.
High‐performance signal processing and telecommunication systems absolutely necessitate analog‐to‐digital converters (ADCs) that offer extensive bandwidth, exceptional precision, and minimal power consumption, in order to efficiently convert real‐world analog signals into digital signals. While current electronic ADCs are constrained by limitations such as low bandwidth, high jitter noise, susceptibility to electromagnetic interference, and excessive energy consumption, photonic ADCs present promising solutions to overcome these challenges. Here, a programmable photonic ADC is developed by integrating phase‐change materials (PCMs) with silicon photonics fabricated using foundry processes. Thanks to the programmability and non‐volatile nature of PCMs, 2‐ and 4‐bit photonic ADCs are demonstrated on a single chip, achieving zero energy consumption during the quantization. Through the experimental demonstration of 65‐state PCMs, photonic ADCs can attain a resolution of 8‐bit, marking a significant milestone as the highest resolution reported to date for ADCs leveraging optical technologies. As a proof of concept, an all‐optical analog‐to‐digital conversion system is demonstrated by integrating 2‐bit photonic ADCs with optical sampling using a mode‐locked laser (MLL). This system achieves the conversion of a 321 MHz radio frequency (RF) signal at a sampling rate of 40 MS s⁻¹. The programmable, energy‐efficient, and high‐speed photonic ADCs represent a significant advancement in the evolution of signal processing systems.
The increasing demand for energy supply in sensing units and the computational efficiency of computation units has prompted researchers to explore novel, integrated technology that offers high efficiency and low energy consumption. Self‐powered sensing technology enables environmental perception without external energy sources, while neuromorphic computation provides energy‐efficient and high‐performance computing capabilities. The integration of self‐powered sensing technology and neuromorphic computation presents a promising solution for an all‐in‐one system. This review examines recent developments and advancements in self‐powered artificial neuron devices based on triboelectric, piezoelectric, and photoelectric effects, focusing on their structures, mechanisms, and functions. Furthermore, it compares the electrical characteristics of various types of self‐powered artificial neuron devices and discusses effective methods for enhancing their performance. Additionally, this review provides a comprehensive summary of self‐powered perception systems, encompassing tactile, visual, and auditory perception systems. Moreover, it elucidates recently integrated systems that combine perception, computing, and actuation units into all‐in‐one configurations, aspiring to realize closed‐loop control. The seamless integration of self‐powered sensing and neuromorphic computation holds significant potential for shaping a more intelligent future for humanity.
In this work, we demonstrate stable multi-bit (discrete) modulations of the optical properties of thin-film GST225 (Ge2Sb2Te5 or GST) samples as a result of phase transitions induced by femtosecond (≈ 50 fs) laser pulses. An ultra-high crystallization rate of the SET material is shown, ~13 m/s. One- and many-step modulations of optical properties are obtained during crystallization of the sample material. Reamorphization or RESET of the samples occurs at a rate of 5–6 m/s; it is realized in several impacting pulses. This fact indicates the thermal nature of the phase transitions.
Advancements in nanofabrication processes have propelled nonvolatile phase change materials (PCMs) beyond storage‐class applications. They are now making headway in fields such as photonic integrated circuits (PIC), free‐space optics, and plasmonics. This shift is owed to their distinct electrical, optical, and thermal properties between their different atomic structures, which can be reversibly switched through thermal stimuli. However, the reliability of PCM‐based optical components is not yet on par with that of storage‐class devices. This is in part due to the challenges in maintaining a uniform temperature distribution across the PCM volume during phase transformation, which is essential to mitigate stress and element segregation as the device size exceeds a few micrometers. Understanding thermal transport in PCM‐based devices is thus crucial as it dictates not only the durability but also the performance and power consumption of these devices. This article reviews recent advances in the development of PCM‐based photonic devices from a thermal transport perspective and explores potential avenues to enhance device reliability. The aim is to provide insights into how PCM‐based technologies can evolve beyond storage‐class applications, maintain their functionality, and achieve longer lifetimes.
As a phase change material (PCM), antimony exhibits a set of desirable properties that make it an interesting candidate for photonic memory applications. These include a large optical contrast between crystalline and amorphous solid states over a wide wavelength range. Switching between the states is possible on nanosecond timescales by applying short heating pulses. The glass state is reached through melting and rapid quenching through a supercooled liquid regime. While initial and final states are easily characterized, little is known about the optical properties on the path to forming a glass. Here we resolve the entire switching cycle of antimony with femtosecond resolution in stroboscopic optical pump‐probe measurements and combine the experimental results with ab‐initio molecular dynamics simulations. The glass formation process of antimony is revealed to be a complex multi‐step process, where the intermediate transient states exhibit distinct optical properties with even larger contrasts than those observed between crystal and glass. The provided quantitative understanding forms the basis for exploitation in high bandwidth photonic applications.
Traditional von Neumann computing, with physical separation of memory and processing units, cannot satisfy the development of artificial intelligence and cloud computing. Neuromorphic computing, inspired by the human brain, has drawn much attention. All-optical neuromorphic (AON) devices employ optical signals as information carriers and leverage the neuromorphic functions to implement fast operation speed, low energy consumption, and high bandwidth of neuromorphic computing. Here, we discuss the recent progress in AON devices, including materials, device performance, working mechanisms, and applications. Moreover, the advantages and limitations of AON are presented and discussed. Finally, the perspective of AON devices points out the future research direction of neuromorphic computing.
Memristors enable non‐volatile memory and neuromorphic computing. Optical memristors are the fundamental element for programmable photonic integrated circuits due to their high‐bandwidth computing, low crosstalk, and minimal power consumption. Here, an optical memristor enabled by a non‐volatile electro‐optic (EO) effect, where refractive index modulation under zero field is realized by deliberate control of domain alignment in the ferroelectric material Pb(Mg1/3Nb2/3)O3‐PbTiO3(PMN‐PT) is proposed. The non‐volatile EO memristor is designed exclusively for the modulation of the optical phase without degrading the optical transparency, and it allows the support for deterministic and repeated non‐volatile multilevel EO states. A non‐volatile tunable waveplate composed of the optical memrisor for free‐space optics, which allows for deterministic multilevel, and non‐volatile phase shifts from 0 to π/2 is presented. The state switching rate of the memristor is less than 100 ms, with a switching energy consumption of 234 nJ, and the states can be retained for up to 12 h without requiring static power consumption. These results demonstrate a novel approach to fully realizing non‐volatile optical memristors, where only optical phase modulation is involved, providing unprecedented opportunities for the development of new ferroelectric memristors.
Vanadium dioxide (VO2) has received significant interest in the context of nanophotonic metamaterials and memories owing to its reversible insulator−metal transition associated with significant changes in its optical and electronic properties. The phase transition of VO2 has been extensively studied for several decades, and the ways how to control its hysteresis characteristics relevant for memory applications have significantly improved. However, the hysteresis dynamics and stability of coexisting phases during the transition have not been studied on the level of individual single-crystal VO2 nanoparticles (NPs), although they represent the fundamental component of ordinary polycrystalline films and can also act like nanoscale memory units on their own. Here, employing transmission electron microscopy techniques, we investigate phase transitions of single VO2 NPs in real time. Our analysis reveals the statistical distribution of the transition temperature and steepness and how they differ during forward (heating) and backward (cooling) transitions. We evaluate the stability of coexisting phases in individual NPs and prove the persistent multilevel memory at near-room temperatures using only a few VO2 NPs. Our findings unveil the physical mechanisms that govern the hysteresis of VO2 at the nanoscale and establish VO2 NPs as a promising component of optoelectronic and memory devices with enhanced functionalities.
Modulating memristors optically paves the way for new optoelectronic devices with applications in computer vision, neuromorphic computing, and artificial intelligence. Here, we report on memristors based on a hybrid material of vertically aligned zinc oxide nanorods (ZnO NRs) and poly(methyl methacrylate) (PMMA). The memristors require no forming step and exhibit the typical electronic switching properties of a bipolar memristor. The devices can also be switched optically and demonstrate an optically tunable multilevel switching behavior upon illumination with UV light. Additionally, the devices demonstrate high-performance photonic synaptic functionalities, including excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), and enhanced potentiation/depression and learning-forgetting characteristics. Notably, after the removal of the UV light, the optoelectronic memristor exhibits a short-term memory due to a persistent photoconductance (PPC) effect. Such a behavior has application in the fabrication of cloned neural networks with pretrained information. The work provides a promising pathway for the fabrication of simple, easy-to-make, and low-cost optoelectronic devices for memory and optically tuned neuromorphic computing applications.
All‐optical and fully reconfigurable transmissive diffractive optical neural network (DONN) architectures emerge as high‐throughput and energy‐efficient machine learning (ML) hardware accelerators in broad applications. However, current device and system implementations have limited performance. In this work, a novel transmissive diffractive device architecture, a digitized phase‐change material (PCM) heterostack, which consists of multiple nonvolatile PCM layers with different thicknesses, is demonstrated. Through this architecture, the advantages of PCM electrical and optical properties can be leveraged and challenges associated with multilevel operations in a single PCM layer can be mitigated. Through proof‐of‐concept experiments, the electrical tuning of one PCM layer is demonstrated in a transmissive spatial light modulation device, and thermal analysis guides the design of multilayer devices and DONN systems to avoid thermal cross talk if individual heterostacks are assembled into an array. Further, a heterostack containing three PCM layers is designed based on experimental results to produce a large‐phase modulation range and uniform coverage, and the ML performance of DONN systems with the designed heterostack is evaluated. The developed device architecture is practically feasible and scalable for future energy‐efficient, fast‐reconfigured, and compact transmissive DONN systems.
Phase change memory (PCM) is an emerging technology that combines the unique properties of phase change materials with the potential for novel memory devices, which can help lead to new computer architectures. Phase change materials store information in their amorphous and crystalline phases, which can be reversibly switched by the application of an external voltage. This article describes the advantages and challenges of PCM. The physical properties of phase change materials that enable data storage are described, and our current knowledge of the phase change processes is summarized. Various designs of PCM devices with their respective advantages and integration challenges are presented. The scaling limits of PCM are addressed, and its performance is compared to competing existing and emerging memory technologies. Finally, potential new applications of phase change devices such as neuromorphic computing and phase change logic are outlined.
Conventional flash memory devices are voltage driven and found to be unsafe for confidential data storage. To ensure the security of the stored data, there is a strong demand for developing novel nonvolatile memory technology for data encryption. Here we show a photonic flash memory device, based on upconversion nanocrystals, which is light driven with a particular narrow width of wavelength in addition to voltage bias. With the help of near-infrared light, we successfully manipulate the multilevel data storage of the flash memory device. These upconverted photonic flash memory devices exhibit high ON/OFF ratio, long retention time and excellent rewritable characteristics.
An optical switch operating at a wavelength of 1.55 μm and showing a 12 dB modulation depth is introduced. The device is implemented in a silicon racetrack resonator using an overcladding layer of the phase change data storage material Ge2Sb2Te5, which exhibits high contrast in its optical properties upon transitions between its crystalline and amorphous structural phases. These transitions are triggered using a pulsed laser diode at λ = 975 nm and used to tune the resonant frequency of the resonator and the resultant modulation depth of the 1.55 μm transmitted light.
The phase transformation dynamics induced in Ge2Sb2Te5 films by picosecond laser pulses were studied using real-time reflectivity measurements. With subnanosecond resolution. Evidence was found that the thermal diffusivity of the substrate plays a crucial role in, determining the ability of. the films to crystallize and amorphize. A film/substrate configuration with optimized heat flow. conditions for ultrafast phase cycling with picosecond laser pulses was designed and produced. In this system; we achieved reversible phase transformations with large optical contrast (>20%) using single laser pulses with a duration of 30 ps within well-defined fluence windows. The amorphization (writing) process is completed within less than 1 ns, whereas, crystallization (erasing) needs approximately 13 ns to be completed. (C) 2004 American Institute of Physics.
Three-dimensional integration may allow for continued improvements in the speed, density and cost of non-volatile memory.
Light is intrinsically very difficult to store in a small space. The ability to trap photons for a long time (photon lifetime, τph) and to slow the propagation of light plays a significant role in quantum information1, 2, 3 and optical processing4, 5, 6. Photonic-crystal cavities with an ultrahigh quality factor (Q) are attracting attention7, 8 because of their extremely small volume; however, high-Q demonstrations have been accomplished only with spectral measurements9, 10, 11. Here we describe time-domain measurements on photonic-crystal cavities with the highest Q among wavelength-scale cavities, and show directly that photons are trapped for one nanosecond. These techniques constitute clear and accurate ways of investigating ultrasmall and long τph systems. We also show that optical pulses are delayed for ~1.45 ns, corresponding to light propagation at ~2×10−5 c the speed of light in a vacuum, which is the slowest for any dielectric slow-light medium. Furthermore, we succeeded in dynamically changing the Q within the τph, which is key to realizing the dynamic control of light12, 13 and photon-trapping memory14.
We propose an all-photonic, non-volatile memory and processing element based
on phase-change thin-films deposited onto nanophotonic waveguides. Using
photonic microring resonators partially covered with Ge2Sb2Te5 (GST)
multi-level memory operation in integrated photonic circuits can be achieved.
GST provides a dramatic change in refractive index upon transition from the
amorphous to crystalline state, which is exploited to reversibly control both
the extinction ratio and resonance wavelength of the microcavity with an
additional gating port in analogy to optical transistors. Our analysis shows
excellent sensitivity to the degree of crystallization inside the GST, thus
providing the basis for non-von Neuman neuromorphic computing.
A comprehensive and thorough review of PCM technologies, including a discussion of material and device issues, is provided in this paper. ABSTRACT | In this paper, recent progress of phase change memory (PCM) is reviewed. The electrical and thermal proper-ties of phase change materials are surveyed with a focus on the scalability of the materials and their impact on device design. Innovations in the device structure, memory cell selector, and strategies for achieving multibit operation and 3-D, multilayer high-density memory arrays are described. The scaling prop-erties of PCM are illustrated with recent experimental results using special device test structures and novel material synthe-sis. Factors affecting the reliability of PCM are discussed.
Phase-change memory (PCM) has emerged as one among the most promising technologies for next-generation non-volatile solid-state memory. Multilevel storage, namely storage of non-binary information in a memory cell, is a key factor for reducing the total cost-per-bit and thus increasing the competiveness of PCM technology in the nonvolatile memory market. In this paper, we present a family of advanced programming schemes for multilevel storage in PCM. The proposed schemes are based on iterative write-and-verify algorithms that exploit the unique programming characteristics of PCM in order to achieve significant improvements in resistance-level packing density, robustness to cell variability, programming latency, energy- per-bit and cell storage capacity. Experimental results from PCM test-arrays are presented to validate the proposed programming schemes. In addition, the reliability issues of multilevel PCM in terms of resistance drift and read noise are discussed.
Ultra-small, low-power, all-optical switching and memory elements, such as all-optical flip-flops, as well as photonic integrated circuits of many such elements, are in great demand for all-optical signal buffering, switching and processing. Silicon-on-insulator is considered to be a promising platform to accommodate such photonic circuits in large-scale configurations. Through heterogeneous integration of InP membranes onto silicon-on-insulator, a single microdisk laser with a diameter of 7.5 µm, coupled to a silicon-on-insulator wire waveguide, is demonstrated here as an all-optical flip-flop working in a continuous-wave regime with an electrical power consumption of a few milliwatts, allowing switching in 60 ps with 1.8 fJ optical energy. The total power consumption and the device size are, to the best of our knowledge, the smallest reported to date at telecom wavelengths. This is also the only electrically pumped, all-optical flip-flop on silicon built upon complementary metal-oxide semiconductor technology.
Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.
The search for a universal memory storage device that combines rapid read and write speeds, high storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phase-change materials, which can be reversibly switched between amorphous and crystalline states, are promising in this respect, but top-down processing of these materials into nanostructures often damages their useful properties. Self-assembled nanowire-based phase-change material memory devices offer an attractive solution owing to their sub-lithographic sizes and unique geometry, coupled with the facile etch-free processes with which they can be fabricated. Here, we explore the effects of nanoscaling on the memory-storage capability of self-assembled Ge2Sb2Te5 nanowires, an important phase-change material. Our measurements of write-current amplitude, switching speed, endurance and data retention time in these devices show that such nanowires are promising building blocks for non-volatile scalable memory and may represent the ultimate size limit in exploring current-induced phase transition in nanoscale systems.
Phase-change materials offer a promising route for the practical realisation of new forms of general-purpose and 'brain-like' computers. An experimental proof-of-principle of such remakable capabilities is presented that includes (i) the reliable execution by a phase-change 'processor' of the four basic arithmetic functions of addition, subtraction, multiplication and division, (ii) the demonstration of an 'integrate and fire' hardware neuron using a single phase-change cell and (iii) the expostion of synaptic-like functionality via the 'memflector', an optical analogue of the memristor.
We survey the current state of phase change memory (PCM), a non-volatile solid-state memory technology built around the large electrical contrast between the highly-resistive amorphous and highly-conductive crystalline states in so-called phase change materials. PCM technology has made rapid progress in a short time, having passed older technologies in terms of both sophisticated demonstrations of scaling to small device dimensions, as well as integrated large-array demonstrators with impressive retention, endurance, performance and yield characteristics. We introduce the physics behind PCM technology, assess how its characteristics match up with various potential applications across the memory-storage hierarchy, and discuss its strengths including scalability and rapid switching speed. We then address challenges for the technology, including the design of PCM cells for low RESET current, the need to control device-to-device variability, and undesirable changes in the phase change material that can be induced by the fabrication procedure. We then turn to issues related to operation of PCM devices, including retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. Several factors that can be expected to enhance PCM in the future are addressed, including Multi-Level Cell technology for PCM (which offers higher density through the use of intermediate resistance states), the role of coding, and possible routes to an ultra-high density PCM technology. Comment: Review article
Could optical technology offer a solution to the heat generation and bandwidth limitations that the computing industry is starting to face? The benefits of energy-efficient passive components, low crosstalk and parallel processing suggest that the answer may be yes.
Phase-change materials are some of the most promising materials for data-storage applications. They are already used in rewriteable optical data storage and offer great potential as an emerging non-volatile electronic memory. This review looks at the unique property combination that characterizes phase-change materials. The crystalline state often shows an octahedral-like atomic arrangement, frequently accompanied by pronounced lattice distortions and huge vacancy concentrations. This can be attributed to the chemical bonding in phase-change alloys, which is promoted by p-orbitals. From this insight, phase-change alloys with desired properties can be designed. This is demonstrated for the optical properties of phase-change alloys, in particular the contrast between the amorphous and crystalline states. The origin of the fast crystallization kinetics is also discussed.
Photonic signals were efficiently stored in a semiconductor-based memory cell. The incident photons were converted to electron-hole pairs that were locally stored in a quantum well that was laterally modulated by a field-effect tunable electrostatic superlattice. At large superlattice potential amplitudes, these pairs were stored for a time that was at least five orders of magnitude longer than their natural lifetime. At an arbitrarily chosen time, they were released in a short and intense flash of incoherent light, which was triggered by flattening the superlattice amplitude.
Photonic integration has long been pursued, but remains immature compared with electronics. Nanophotonics is expected to change this situation. However, despite the recent success of nanophotonic devices, there has been no demonstration of large-scale integration. Here, we describe the large-scale and dense integration of optical memories in a photonic crystal chip. To achieve this, we introduce a wavelength-addressable serial integration scheme using a simple cavity-optimization rule. We fully exploit the wavelength-division-multiplexing capability, which is the most important advantage of photonics over electronics, and achieve an extremely large wavelength-channel density. This is the first demonstration of the large-scale photonic integration of nanophotonic devices coupled to waveguides in a single chip, and also the first dense wavelength-division-multiplexing nanophotonic devices other than filters. This work paves the way for optical random-access memories and for a large-scale wavelength-division-multiplexing photonic network-on-chip.
An effective solution to enhance the capacity of an optical-interconnect link is utilizing advanced multiplexing technologies, like wavelength-division-multiplexing (WDM), polarization-division multiplexing (PDM), spatial-division multiplexing (SDM), bi-directional multiplexing, etc. On-chip (de)multiplexers are necessary as key components for realizing these multiplexing systems and they are desired to have small footprints due to the limited physical space for on-chip optical interconnects. As silicon photonics has provided a very attractive platform to build ultrasmall photonic integrated devices with CMOS-compatible processes, in this paper we focus on the discussion of silicon-based (de)multiplexers, including WDM filters, PDM devices, and SDM devices. The demand of devices to realize a hybrid multiplexing technology (combining WDM, PDM and SDM) as well as a bidirectional multiplexing technologies are also discussed to achieve Peta-bit optical interconnects.
Inspired by the brain’s structure, we have developed an efficient, scalable, and flexible non–von Neumann architecture that
leverages contemporary silicon technology. To demonstrate, we built a 5.4-billion-transistor chip with 4096 neurosynaptic
cores interconnected via an intrachip network that integrates 1 million programmable spiking neurons and 256 million configurable
synapses. Chips can be tiled in two dimensions via an interchip communication interface, seamlessly scaling the architecture
to a cortexlike sheet of arbitrary size. The architecture is well suited to many applications that use complex neural networks
in real time, for example, multiobject detection and classification. With 400-pixel-by-240-pixel video input at 30 frames
per second, the chip consumes 63 milliwatts.
The development of materials whose refractive index can be optically transformed as desired, such as chalcogenide-based phase-change materials, has revolutionized the media and data storage industries by providing inexpensive, high-speed, portable and reliable platforms able to store vast quantities of data. Phase-change materials switch between two solid states--amorphous and crystalline--in response to a stimulus, such as heat, with an associated change in the physical properties of the material, including optical absorption, electrical conductance and Young's modulus. The initial applications of these materials (particularly the germanium antimony tellurium alloy Ge2Sb2Te5) exploited the reversible change in their optical properties in rewritable optical data storage technologies. More recently, the change in their electrical conductivity has also been extensively studied in the development of non-volatile phase-change memories. Here we show that by combining the optical and electronic property modulation of such materials, display and data visualization applications that go beyond data storage can be created. Using extremely thin phase-change materials and transparent conductors, we demonstrate electrically induced stable colour changes in both reflective and semi-transparent modes. Further, we show how a pixelated approach can be used in displays on both rigid and flexible films. This optoelectronic framework using low-dimensional phase-change materials has many likely applications, such as ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses and artificial retina devices.
Phase change materials are widely considered for application in non-volatile memories due to their ability to achieve phase transformation in nano-second time scale. However the knowledge of fast crystallization dynamics in these materials are limited due to the lack of fast and accurate temperature control methods. In this work we have developed an experimental methodology that enables ultra-fast characterization of phase-change dynamics on a more technologically relevant melt-quenched amorphous phase using practical PCM device structures. We have extracted the crystallization growth velocity (U) in a functional capped PCM device over eight orders of magnitude (10(-10) m/s < U < 10(-1) m/s) spanning a wide temperature range (415 K < T < 580 K). We also observed direct evidence of non-Arrhenius crystallization behavior in programmed PCM devices at very high heating rates (>10(8) K/s), which reveals the extreme fragility of Ge2Sb2Te5 in its super-cooled liquid phase. Furthermore, these crystallization properties were studied as a function of device programming cycles and the results show degradation in the cell retention properties due to elemental segregation. The above experiments are enabled by the use of an on-chip fast heater and thermometer called as Micro Thermal Stage (MTS) integrated with a vertical phase change memory (PCM) cell. The temperature at the PCM layer can be controlled up to 600 K using MTS and with a thermal time constant of 800 ns leading to heating rates ~ 10(8) K/s that are close to the typical device operating conditions during PCM programming. The MTS allows us to independently control the electrical and thermal aspects of phase transformation (inseparable in a conventional PCM cell) and extract the temperature dependence of key material properties in real PCM devices.
Phase-change materials integrated into nanophotonic circuits provide a flexible way to realize tunable optical components. Relying on the enormous refractive-index contrast between the amorphous and crystalline states, such materials are promising candidates for on-chip photonic memories. Non-volatile memory operation employing arrays of microring resonators is demonstrated as a route toward all-photonic chipscale information processing.
A class of two-terminal passive circuit elements that can also act as
memories could be the building blocks of a form of massively parallel
computation known as memcomputing.
Optical computers will be more interesting if they take advantage of
phenomena that are unique to optics. In this respect, telecommunications
hardware might have something to offer.
Historically, the application of phase-change materials and devices has been limited to the provision of non-volatile memories. Recently, however, the potential has been demonstrated for using phase-change devices as the basis for new forms of brain-like computing, by exploiting their multilevel resistance capability to provide electronic mimics of biological synapses. Here, a different and previously under-explored property that is also intrinsic to phase-change materials and devices, namely accumulation, is exploited to demonstrate that nanometer-scale electronic phase-change devices can also provide a powerful form of arithmetic computing. Complicated arithmetic operations are carried out, including parallel factorization and fractional division, using simple nanoscale phase-change cells that process and store data simultaneously and at the same physical location, promising a most efficient and effective means for implementing beyond von-Neumann computing. This same accumulation property can be used to provide a particularly simple form phase-change integrate-and-fire “neuron”, which, by combining both phase-change synapse and neuron electronic mimics, potentially opens up a route to the realization of all-phase-change neuromorphic processing.
Ultra-compact four-channel wavelength division multiplexing (WDM) devices in add/drop filter and multi-mode interferometer configurations were demonstrated on silicon-on-insulator for future on-chip optical interconnects. Both devices show a crosstalk level ≤-13dB and a footprint ≤0.006mm2.
A rapid and reversible transition between a highly resistive and conductive state effected by an electric field, which we have observed in various types of disordered semiconducting material, is described in detail. The switching parameters and chemical composition of a typical material are presented, and microscopic mechanisms for the conduction phenomena are suggested.
Silicon photonics is currently a very active and progressive area of research, as silicon optical circuits have emerged as the replacement technology for copper-based circuits in communication and broadband networks. The demand for ever improving communications and computing performance continues, and this in turn means that photonic circuits are finding ever increasing application areas. This text provides an important and timely overview of the hot topics in the field, covering the various aspects of the technology that form the research area of silicon photonics. With contributions from some of the worlds leading researchers in silicon photonics, this book collates the latest advances in the technology. Silicon Photonics: the State of the Art opens with a highly informative foreword, and continues to feature: the integrated photonic circuit; silicon photonic waveguides; photonic bandgap waveguides; mechanisms for optical modulation in silicon; silicon based light sources; optical detection technologies for silicon photonics; passive silicon photonic devices; photonic and electronic integration approaches; applications in communications and sensors. Silicon Photonics: the State of the Art covers the essential elements of the entire field that is silicon photonics and is therefore an invaluable text for photonics engineers and professionals working in the fields of optical networks, optical communications, and semiconductor electronics. It is also an informative reference for graduate students studying for PhD in fibre optics, integrated optics, optical networking, microelectronics, or telecommunications.
Phase-change random-access memory (PCRAM) is one of the leading candidates for next-generation data-storage devices, but the
trade-off between crystallization (writing) speed and amorphous-phase stability (data retention) presents a key challenge.
We control the crystallization kinetics of a phase-change material by applying a constant low voltage via prestructural ordering
(incubation) effects. A crystallization speed of 500 picoseconds was achieved, as well as high-speed reversible switching
using 500-picosecond pulses. Ab initio molecular dynamics simulations reveal the phase-change kinetics in PCRAM devices and
the structural origin of the incubation-assisted increase in crystallization speed. This paves the way for achieving a broadly
applicable memory device, capable of nonvolatile operations beyond gigahertz data-transfer rates.
The structure of laser-crystallized thin films of Ge 2 Sb 2+x Te 5 (0.0≪x≤1.0) formed by the sputtering method were identified by x-ray diffraction studies to be composed of two phases: one phase is chiefly NaCl type crystal with a lattice constant of about 6 Å and a composition corresponding to Ge 2 Sb 2 Te 5 ; the other phase comprises a small amount of an amorphous component such as Sb metal. Results of the Rietveld and the whole-powder-pattern fitting analyses show good agreement when assuming that (i) the 4(a) site is wholly occupied by only Te, (ii) the 4(b) site is randomly occupied by Ge or Sb atoms, and (iii) a little less than 20% of the 4(b) site is always vacant independent of the x value. The above results and the fact that halo noise rises with x increasing from 0.0 to 1.0 indicate a more precise model of crystal structure as follows. That is, Sb atoms added beyond the stoichiometric ratio, Ge 2 Sb 2 Te 5 , never fill up the vacancies of the 4(b) site in the NaCl type structure; the excess Sb atoms will remain in the amorphous state and concentrate, for example, at the grain boundary. The authors conclude that the amounts of the amorphous component produced through the crystallization process predominantly determine the crystallization rates and the critical temperatures of Ge–Sb–Te amorphous films, reportedly that they show a gentle and continuous dependence on the compositional deviation from the GeTe–Sb 2 Te 3 pse-
udobinary line. © 2000 American Institute of Physics.
An optical gate switch using Ge2Sb2Te5 phase-change material integrated with a silicon waveguide is reported. The switch is very small (~2 ¿m) owing to the large difference in absorption coefficient between the crystalline state and the amorphous state. The prototype switch has been fabricated and successfully switched from the transparent on-state to the opaque off-state by laser pulse irradiation. An extinction ratio of more than 12.5 dB was achieved over a wavelength range of 75 nm.
The increasing speed of fibre-optic-based telecommunications has focused attention on high-speed optical processing of digital information. Complex optical processing requires a high-density, high-speed, low-power optical memory that can be integrated with planar semiconductor technology for buffering of decisions and telecommunication data. Recently, ring lasers with extremely small size and low operating power have been made, and we demonstrate here a memory element constructed by interconnecting these microscopic lasers. Our device occupies an area of 18 x 40 microm2 on an InP/InGaAsP photonic integrated circuit, and switches within 20 ps with 5.5 fJ optical switching energy. Simulations show that the element has the potential for much smaller dimensions and switching times. Large numbers of such memory elements can be densely integrated and interconnected on a photonic integrated circuit: fast digital optical information processing systems employing large-scale integration should now be viable.
Non-volatile 'flash' memories are key components of integrated circuits because they retain their data when power is interrupted. Despite their great commercial success, the semiconductor industry is searching for alternative non-volatile memories with improved performance and better opportunities for scaling down the size of memory cells. Here we demonstrate the feasibility of a new semiconductor memory concept. The individual memory cell is based on a narrow line of phase-change material. By sending low-power current pulses through the line, the phase-change material can be programmed reversibly between two distinguishable resistive states on a timescale of nanoseconds. Reducing the dimensions of the phase-change line to the nanometre scale improves the performance in terms of speed and power consumption. These advantages are achieved by the use of a doped-SbTe phase-change material. The simplicity of the concept promises that integration into a logic complementary metal oxide semiconductor (CMOS) process flow might be possible with only a few additional lithographic steps.
The authors also acknowledge support from the DFG and the State of Baden-Württemberg through the DFG-Center for Functional Nanostructures (CFN) within subproject A6.4
- Wirtschaft
Wirtschaft (sdw). H.B. acknowledges support from the John Fell Fund and the EPSRC
(EP/J00541X/2 and EP/J018694/1).The authors also acknowledge support from the DFG
and the State of Baden-Württemberg through the DFG-Center for Functional
Nanostructures (CFN) within subproject A6.4. This work was partly carried out with the
support of the Karlsruhe Nano Micro Facility (KNMF, http://www.knmf.kit.edu), a
Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, http://www.
kit.edu). The authors thank S. Diewald for assistance with device fabrication and M.
Blaicher for technical assistance with device design.