Optical Materials Express

Optical properties of Tm-doped fibers based on different host glass materials, silica and yttrium-aluminum-garnet (YAG), prepared with different technologies, are studied. We show that crystal-derived fibers offer a wider range of core compositions compared to silica fibers prepared by modified chemical vapor deposition method in combination with solution doping and provide the possibility to tune optical properties towards new applications. Absorption and emission behavior, including absorption cross-sections, fluorescence intensities and lifetimes of the 3F4 and 3H4 energy levels, are investigated in the wavelength range from 190 nm to 2150 nm and concentration range from 0.08 mol% up to 1.52 mol% Tm2O3. For a YAG-derived fiber, a new record for silica-based Tm fibers was set with the measured lifetime of the 3H4 transition of 63 µs at a Tm concentration of 0.23 mol% Tm2O3, which provides access to new laser transitions in Tm. As a result of these investigations an energy level scheme with all observed absorption and emission transitions is presented.

Topological effects manifest in a variety of lattice geometries. While square lattices, due to their simplicity, have been used for models supporting nontrivial topology, several exotic topological phenomena such as Dirac points, Weyl points, and Haldane phases are most commonly supported by non-square lattices. Examples of prototypical non-square lattices include the honeycomb lattice of graphene and 2D materials, and the Kagome lattice, both of which break fundamental symmetries and can exhibit quantized transport, especially when long-range hoppings and gauge fields are incorporated. The challenge of controllably realizing such long-range hoppings and gauge fields has motivated a large body of research focused on harnessing lattices encoded in "synthetic" dimensions. Photons in particular have many internal degrees of freedom and hence show promise for implementing these synthetic dimensions; however, most photonic synthetic dimensions have hitherto created 1D or 2D square lattices. Here we show that non-square lattice Hamiltonians such as the Haldane model and its variations can be implemented using Floquet synthetic dimensions. Our construction uses dynamically modulated ring resonators and provides the capacity for direct k-space engineering of lattice Hamiltonians. This k-space construction lifts constraints on the orthogonality of lattice vectors that make square geometries simpler to implement in lattice-space constructions and instead transfers the complexity to the engineering of tailored, complex Floquet drive signals. We simulate topological signatures of the Haldane and the brick-wall Haldane model and observe them to be robust in the presence of external optical drive and photon loss, and discuss unique characteristics of their topological transport when implemented on these Floquet lattices. Our proposal demonstrates the potential of driven-dissipative Floquet synthetic dimensions as a new architecture for k-space Hamiltonian simulation of high-dimensional lattice geometries, supported by scalable photonic integration, that lifts the constraints of several existing platforms for topological photonics and synthetic dimensions.

In this research, the reduced graphene oxide (rGO) was synthesized by Hummer’s method and added to polyurethane (PU/G) with different weight percentages of rGO. Then, the optical phonon modes of the prepared compounds are studied using the Kramers-Kronig (KK) relations. As the results show, the optical phonon modes enhance with increasing the percentage of rGO nanosheets in the polyurethane (PU). Moreover, nonlinear optical (NLO) studies on the compound solutions were performed using the Z-scan method to determine their absorptive and refractive nonlinearities. The samples display saturable absorption (SA) based on Z-scan measurements taken at the open aperture. During the Z-scan measurements, thermal nonlinearity causes the samples to exhibit self-defocusing properties as evidenced by a negative sign of the nonlinear refractive index, n2. Furthermore, the analysis of the figures of merit (FOM) for the prepared sample shows that T < 1 and W > 1. Consequently, there is potential for these samples to be used in lasers and optical switches, based on the results from the experiment.

Fs-laser irradiation is a promising fabrication method for future broadband optoelectronic applications as it creates antireflective micro- and nanoscale structures on semiconductor surfaces and introduces below-bandgap absorption; however, its application has mainly been limited to silicon. This paper demonstrates that fs-laser technology enables high optical absorption both above and below the bandgap in germanium (Ge). With optimized laser parameters, we achieve a maximum above-bandgap absorptance of 95% and over 70% below-bandgap absorptance, due to the creation of surface microstructures and structural defects, respectively. Raman spectroscopy reveals that under intense laser irradiation, Ge may undergo a phase transition to structures with a narrower bandgap extending the absorption to the mid-infrared region. Furthermore, we develop a hyperdoping process using Ti coating pre-laser processing followed by rapid thermal annealing, which results in 90% above-bandgap absorption and a 12% relative increase in below-bandgap absorption along with a high degree of crystallinity. The increased below-bandgap absorption is attributed to Ti doping and is twice as high as reported earlier. Our findings should have significant implications for the future Ge-based infrared applications.

High repetition rate, high peak, and average power laser systems are crucial for next-generation particle accelerators, inertial confinement fusion, and secondary particle sources. These applications demand durable laser optics, particularly interference coatings on optics lasting millions of shots at high fluence. This study focuses on designing, testing, and simulating multi-layer dielectric (MLD) mirrors for pulse durations of 260 fs, 77 fs, and 25 fs at 1030 nm wavelength and 45-degree incidence angle with p-polarization. S-on-1 laser-induced damage thresholds (LIDT) for varying pulse numbers were determined, with single-shot LIDT values of 0.98 Jcm⁻², 1.63 Jcm⁻², and 2.3 Jcm⁻² for 25 fs, 77 fs, and 260 fs respectively. A strong correlation between blister shape and local fluence was observed, implying that the layer expansion in a blister depends on local fluence. We have also examined mechanisms responsible for laser-induced stress generation and energy release rates in blister formation. Damage mechanisms are further explored by finite-difference time-domain (FDTD) simulations, incorporating Keldysh strong field ionization, whose predictions were in excellent agreement with the onset of damage determined experimentally. These findings offer insights for enhancing MLD coating technology, promising more efficient and resilient laser systems for diverse scientific and industrial applications.

The process of creating a photosensitive structure with a double-distributed heterojunction based on a combination of fullerene and non-fullerene acceptors (PC71BM and ITIC-F) is described in this work. P3HT was used as the donor in both layers. The spectral photosensitivity response of the resulting FTO/P3HT:ITIC-F/P3HT:PC71BM/InGaSn structure covers a wide range of 470–980 nm. The peak photosensitivity of the structure reaches 40.98 mA/W, which is many times higher than the values for structures based on individual layers and a blend of all materials in one bulk heterojunction. The studies and measurements show that such a structure can be used in optics and electronics to detect radiation in the visible and near-IR ranges. In addition, the principle of creating a double-distributed heterojunction structure used in this study can be applied to other organic materials to obtain previously unattainable performance levels.

We report on the near-infrared intersubband (ISB) absorption properties of strain-free Sc0.14Al0.86N/GaN multiple quantum wells (MQWs) grown on c-plane GaN substrates by molecular beam epitaxy. These MQWs exhibit strong, sharp, and tunable absorption energies between 515 meV and 709 meV, for well widths ranging from 7 nm to 1.5 nm, respectively. Observation of ISB absorption in ultra-thin Sc0.14Al0.86N/GaN MQWs not only extends the near-infrared range accessible with Sc-containing nitrides but also highlights the challenges of growing nanometer-thick GaN quantum wells. We explore the effects of growth temperature on absorption characteristics and find that substrate temperatures above 600°C significantly enhance ISB absorption intensity but also introduce an energy redshift for the narrowest wells. The redshift is attributed to increased interface roughness due to ScAlN surface morphology degradation at higher temperatures. Additionally, a comparison of experimental results with simulated band-structures indicates that the magnitude of net polarization rises faster with Sc-composition than previously suggested by theoretical calculations. This study advances the prospects of ScAlN/GaN heterostructures for novel photonic devices in the technologically important near-infrared range.

The gold nanoparticle (AuNP) dimer represents an interesting structure that combines the chemical properties of AuNPs and its ability to generate a powerful plasmonic hotspot in the interparticle gap, which can enable the detection of single molecules via surface-enhanced Raman spectroscopy (SERS). This attribute can be furthermore used to study the orientation and binding mechanism of molecules on surfaces due to the selective enhancement properties of SERS. This work investigates the characteristics of AuNP dimers formed after adsorption of DNA bases on the surface which modify the Zeta-potential of the particles and enable controlled aggregation. We observe variations in the gap separation for different nucleobase concentrations and pH values that correspond well with changes in the relative SERS spectra and can be quantified by UV-vis spectroscopy of AuNP dimer solutions. The observed dependency of the gap separation on concentration and pH is speculated to originate from reorientations of the adsorbed molecular species. This method offers a way to estimate the vertical extension of surface monolayers on colloids, representing an additional tool for elucidating the orientation of various molecules on AuNPs. By applying this approach to the case of adenine, we suggest a new binding model for this nucleobase, thus making a unique contribution to the vast amount of literature on this complex interaction.

Exciton polaritons are bosonic quasi-particles that arise from the quantum superposition of photons and excitons. It can be utilized in the design of polaritonic devices such as low-threshold lasers and optical switches. However, exciton-polaritons are typically observed at low temperatures due to the small binding energy of excitons in traditional inorganic semiconductors. Here, using Tamm microcavities, demonstrate a sandwich-structured WS2-based device that realizes exciton polaritons at room-temperature. A clear anti-crossing is observed in both angular-resolved reflectivity and photoluminescence (PL) spectra with a pronounced Rabi splitting of 39.2 meV. The realization of room temperature exciton polaritons is conducive to the practical device applications of exciton-polaritons.

In this study, we demonstrate the integration of novel two-dimensional (2D) vanadium carbide (V2CTx) MXene nanosheets as saturable-absorber (SA) into an erbium-doped fiber laser (EDFL) cavity, enabling both polarization-independent passive Q-switched (PI-PQS) and polarization-dependent passive Q-switched (PD-PQS) pulse operations. The V2CTx nanosheets were synthesized then the morphogy as well structure of the prepared nanosheets were characterized through scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. In PI-PQS operation, pulse generation was initiated at a pump power of 17.5 mW, while in PD-PQS operation, the pulse operation was initiated at 92.9 mW. With a maximum pump power of 312.5 mW, the pulse duration and average output power were 4.92 µs and 1.19 mW for the PI-PQS EDFL and 4.25 µs and 2.09 mW for the PD-PQS EDFL, respectively. Furthermore, employing density functional theory (DFT) calculations, we obtained the band structure and optical properties of V2CTx, confirming its importance as an SA in EDFL. These results validate the optimized performance of PD-PQS EDFL relative to PI-PQS EDFL. This study further suggests that other MXenes may have potential as SAs in EDFLs for producing Q-switched pulses, warranting further investigation.

This paper presents an ultrathin and flexible metamaterial absorber (MA) for Ka-band applications. The MA with a five-layer structure is composed of three copper layers separated by two FR4 dielectric layers. Due to the use of ultrathin but robust FR4, the thickness of the proposed MA is only 0.35 mm corresponding to 0.035λ at the operating frequency range. The MA can achieve absorptivity of over 95% in the frequency band of 30-36 GHz. The investigation of electric field distribution is used to interpret the absorption mechanism. Additionally, the MA exhibits excellent polarization and angular stability. A prototype of the proposed MA was fabricated using PCB technology. The measured results of absorption performance agree well with the simulated ones, thereby demonstrating the validity of the design. More importantly, the absorption performance of the flexible MA in bent states is examined, proving the feasibility of the MA for conformal applications. This work provides a good candidate for electromagnetic (EM) shielding at Ka-band.

Design of a long-range hybrid plasmonic waveguide with graphene-based electrical tuning of propagation length Long-range hybrid plasmonic waveguides (HPWs) have attracted significant research attention as they help to achieve ultra-low nanoscale mode confinement with low propagation losses. Here, we present numerical studies of long-range HPWs consisting of a combination of plasmonic metal thin films and nanoscale structures of a high refractive index material (such as silicon) with a low refractive index material (such as silica) surrounding the nanoscale structures and the plasmonic metal thin film. The HPWs show hybrid mode characteristics due to the interaction between dielectric modes and SPP modes. Moreover, electrical tuning of optical losses is formed by introducing partially overlapping graphene monolayers between silicon nanowires and plasmonic metal film. The tunability in the HPW is implemented by using graphene as an electro-absorption material. The effective refractive index and the corresponding propagation length obtained for these plasmonic waveguides using an Eigenmode solver demonstrate the viability of these hybrid plasmonic waveguides in applications that demand a long propagation range with reasonable field confinement. Moreover, we study the effect of the variation of different waveguide parameters on the propagation length and mode area. There is a significant electrical tuning of propagation length in graphene-based HPWs by applying an external voltage at the telecom wavelength of 1550 nm, owing to the strong field confinements between metal and silicon nanowires. The waveguides we proposed can be fabricated with relative ease using standard processes, and protocols for the fabrication of these hybrid plasmonic waveguides are provided in this paper. The proposed hybridization in nanoscale waveguides can be used in nanolasing, nanofocusing, sensing, switching, and subwavelength optical guiding applications in future nanoscale photonic/plasmonic integrated circuits.

Achieving optimal band alignment and efficient p-type conductivity is a critical challenge for the heterogeneous integration of wide bandgap materials onto silicon (Si), a key step in revolutionizing next-generation integrated circuits (ICs). In this work, we report what we believe to be the first investigation of the heterojunction formed by pulsed laser deposition (PLD) growth of lithium-doped iridium oxide (IrO2:Li) on (100)-oriented Si. The IrO2:Li films exhibit a polycrystalline structure with a preferred (200) out-of-plane orientation, as confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Monochromated electron energy loss spectroscopy (EELS) measurements revealed an electronic bandgap of 2.90 eV for the IrO2:Li film, which is corroborated by photoluminescence (PL) measurements and consistent with prior work on undoped IrO2. Electrical characterization demonstrated p-type conductivity with a high carrier concentration, comparable to that of epitaxial IrO2 films. The valence and conduction band offsets at the IrO2:Li/Si heterointerface were determined to be 0.76 ± 0.10 and 2.54 ± 0.10 eV, respectively, using high-resolution X-ray photoelectron spectroscopy (HRXPS), indicating a type-II (staggered) band alignment. The combination of wide bandgap, p-type conductivity, and favorable band alignment with Si makes PLD-grown IrO2:Li a promising candidate for future optoelectronic and power devices integrated with Si technology.

We demonstrate the electrical tuning of InAs/GaAs quantum dots (QDs) embedded in a photonic-crystal (PhC) cavity with transparent indium tin oxide (ITO) electrodes hybrid-integrated by transfer printing. The design includes spacer layers between the cavity and the electrodes to reduce absorption loss while still maintaining a significant bias field. The bottom electrode, PhC cavity, and top electrode were fabricated independently and integrated by transfer printing, and an electrical connection was made by inkjet printing, both implemented locally without affecting the entire chip. We observed the electrical tuning of QD emissions into the cavity mode and Purcell enhancement. This work paves the way for utilizing multiple hybrid-integrated QDs on the same chip.

This study presents a comprehensive and systematic investigation of the deposition and patterning of hydrogenated amorphous silicon carbide (a-SiC:H) using advanced plasma-based techniques to precisely tailor its optical characteristics for micro-optical applications. We demonstrate the ability to adjust the refractive index, deposition rate, and bandgap of a-SiC:H thin films utilizing chemical vapor deposition. Optimizations are accomplished based on the response surface methodology from the statistical design of experiment. Furthermore, we provide a detailed investigation of the reactive ion etching of a-SiC:H, also guided by response surface methodology. This approach enables fine-tuned patterning of a-SiC:H, resulting in tunable sidewall angles, defect-free etch profiles, and high etch rates. Finally, we conduct FEM and RCWA simulations using the measured dispersion properties for the design of diffraction gratings. Comparisons between the simulations and the measured diffraction efficiencies confirm the performance and reliability of the fabricated a-SiC:H-based optical elements. This study highlights the potential of a-SiC:H for advanced micro-optical applications, particularly in scenarios where high refractive index materials are useful.

In laser-induced graphitization, decreasing the linewidth of patterned structures is key to enhancing the potential in microdevice fabrication. In this study, we demonstrated that graphitic carbon structures with narrow linewidth can be directly patterned in laser-induced graphitization of cellulose nanofiber (CNF) film by immersing the film in CaI2 solution. The linewidth of the structures increased with increasing laser power, indicating that the linewidth can be readily controlled. Based on the above findings, we patterned multiple concentric structures with precisely controlled linewidth to fabricate a Fresnel zone plate, which can focus laser light in the visible wavelength range.

Due to the current limitations of angle-selective surface structures that cannot simultaneously meet the ideal angular selection characteristics, adjustable passbands, and dual polarization, an angle-selective surface structure with a wide-angle selection characteristic and adjustable passbands based on a three-layer frequency-selective surface is presented. Through theoretical analysis, calculation, and simulation derivation, a three-layer cross-slotted structure was ultimately selected as the unit structure for this design. The proposed three-layer angle-selective surface structure features identical upper and lower structures, while the middle layer structure is different from both. The simulation results indicate that the proposed design displays a passband (|S 21 |>-1 dB) within the incident angle range of 0° to 23.5° at a frequency of 3 GHz for the TE and TM polarization modes, and shows a stopband (|S 21 |<-20 dB) when the incident angle is greater than 36°. Although various factors influenced the simulation results, we believe that the simulation results will provide valuable reference for researchers in the relevant field.

A quasi-bound state in the continuum (QBIC) is a hot topic in optics, as it brings extremely high Q factors by slightly breaking the structural symmetry or tuning geometric parameters. However, QBIC devices are usually not dynamically tunable; thus, achieving this ultra-high Q factor demands ultra-high fabrication accuracy. Here, by first proposing an elastomer-based double resonant gratings (DRG) in the THz band, a strategy to realize dynamically tunable QBIC devices is presented. By simple stretching, structural parameters can be tuned, leading to dynamic QBIC-BIC switching, dynamic Q factors tuning to the order of 10⁶, and enhancement of sensing performance. Moreover, by varying the incident angle, this dynamic tuning is achievable at a constant frequency. The strategy is expected to neutralize the demanding requirements for fabrication accuracy in the implementation of QBIC devices. Hence, it is a promising solution to realize tunable QBIC devices for applications such as THz biosensing.