Mo Li

Yale University | YU · Department of Electrical Engineering

Rare-earth iron garnets with large magnetic gyrotropy, made with reduced thermal budgets, are ideal magneto-optical materials for integrated isolators. However, reduced thermal budgets impact Faraday rotation by limiting crystallization, and characterization of crystallinity is limited by resolution or scannable area. Here, electron backscatter dif...

Optical computing with integrated photonics brings a pivotal paradigm shift to data-intensive computing technologies. However, the scaling of on-chip photonic architectures using spatially distributed schemes faces the challenge imposed by the fundamental limit of integration density. Synthetic dimensions of light offer the opportunity to extend th...

Engineering the coupling between fundamental quantum excitations is at the heart of quantum science and technologies. A significant case is the creation of quantum light sources in which coupling between single photons and phonons can be controlled and harnessed to enable quantum information transduction. Here, we report the creation of designer qu...

Image sensors with internal computing capability enable in-sensor computing that can significantly reduce the communication latency and power consumption for machine vision in distributed systems and robotics. Two-dimensional semiconductors have many advantages in realizing such intelligent vision sensors because of their tunable electrical and opt...

Excitons are elementary optical excitation in semiconductors. The ability to manipulate and transport these quasiparticles would enable excitonic circuits and devices for quantum photonic technologies. Recently, interlayer excitons in 2D semiconductors have emerged as a promising candidate for engineering excitonic devices due to their long lifetim...

Integrated optoelectronics is emerging as a promising platform of neural network accelerator, which affords efficient in-memory computing and high bandwidth interconnectivity. The inherent optoelectronic noises, however, make the photonic systems error-prone in practice. It is thus imperative to devise strategies to mitigate and, if possible, harne...

Single photon emitters are deterministically created in homobilayer WSe2 by strain engineering. These emitters are highly tunable in energy by the electric field.

Black phosphorus is promising for its electrostatic tunability of optical and electronic properties. We engineered a charge storage layer for non-volatile tuning of the BP-channel and demonstrated programmable electric-conductivities and photo-responsivities for optoelectronic in-memory computing.

The Bayesian neural network (BNN) combines the strengths of neural networks and statistical modeling in that it simultaneously performs posterior predictions and quantifies the uncertainty of the predictions. Integrated photonics has emerged as a promising hardware platform of neural network accelerators capable of energy-efficient, low latency, an...

Image sensors with internal computing capability enable in-sensor computing that can significantly reduce the communication latency and power consumption for machine vision in distributed systems and robotics. Two-dimensional semiconductors are uniquely advantageous in realizing such intelligent visionary sensors because of their tunable electrical...

Among layered and 2D semiconductors, there are many with substantial optical anisotropy within individual layers, including group‐IV monochalcogenides MX (M = Ge or Sn and X = S or Se) and black phosphorous (bP). Recent work has suggested that the in‐plane crystal orientation in such materials can be switched (e.g., rotated through 90°) through an...

Integrated programmable optoelectronics is emerging as a promising platform of neural network accelerator, which affords efficient in-memory computing and high bandwidth interconnectivity. The analog nature of optical computing and the inherent optoelectronic noises, however, make the systems error-prone in practical implementations such as classif...

Excitons are elementary optical excitation in semiconductors. The ability to manipulate and transport these quasiparticles would enable excitonic circuits and devices for quantum photonic technologies. Recently, interlayer excitons in 2D semiconductors have emerged as a promising candidate for engineering excitonic devices due to long lifetime, lar...

A match made for black P: Modifying black P with chemically‐matched Lewis acids imparts remarkable ambient stability, excellent transport properties, and persistent p‐doping. Compatible with device fabrication and relying on commercially available reagents, this facile protocol opens a path for deterministic and persistent tuning of electronic prop...

Herein we introduce a facile, solution‐phase protocol to modify the Lewis basic surface of few‐layer black phosphorus ( b P) and demonstrate its effectiveness at providing ambient stability and tuning of electronic properties. Commercially available group 13 Lewis acids that range in electrophilicity, steric bulk, and Pearson hard/soft‐ness are eva...

Neuromorphic photonics has recently emerged as a promising hardware accelerator, with significant potential speed and energy advantages over digital electronics for machine learning algorithms, such as neural networks of various types. Integrated photonic networks are particularly powerful in performing analog computing of matrix-vector multiplicat...

Advances in mid-IR lasers, detectors, and nanofabrication technology have enabled new device architectures to implement on-chip sensing applications. In particular, direct integration of plasmonic resonators with a dielectric waveguide can generate an ultra-compact device architecture for biochemical sensing via surface-enhanced infrared absorption...

The electro-optical modulation of black phosphorus is promising in broad mid-IR range. We demonstrated 10% of modulation at 3-µm wavelength by a multi-passing waveguide device and proposed a plasmonic waveguide to achieve higher modulation depth and smaller footprint.

The integration of nanoplasmonic devices with a silicon photonic platform affords a new approach for efficient light delivery by combining the high field enhancement of plasmonics and the ultralow propagation loss of dielectric waveguides. Such a hybrid integration obviates the need for a bulky free-space optics setup and can lead to fully integrat...

Waveguide-integrated plasmonics is a growing field with many innovative concepts and demonstrated devices in the visible and near-infrared. Here, we extend this body of work to the mid-infrared for the application of surface-enhanced infrared absorption (SEIRA), a spectroscopic method to probe molecular vibrations in small volumes and thin films. B...

The spin-orbit torque (SOT) that arises from materials with large spin-orbit coupling promises a path for ultralow power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BixSe(1-x) thin films in BixSe(1-x)/Co20Fe60B20 heterostructures by using d.c. planar Hall and spin-torque ferromagnetic...

Waveguide-integrated plasmonics is a growing field with many innovative concepts and demonstrated devices in the visible and near-infrared. Here, we extend this body of work to the mid-infrared for the application of surface-enhanced infrared absorption (SEIRA), a spectroscopic method to probe molecular vibrations in small volumes and thin films. B...

The optical absorption in black phosphorus could be modulated by electric field due to the QCFK effect. We extracted the modulation of extinction coefficient from free space device and showed 5dB modulation would be promising by integrated black phosphorus on Si waveguide.

Black phosphorus stands out from the family of two-dimensional materials as a semiconductor with a direct, layer-dependent bandgap in energy corresponding to the spectral range from the visible to the mid-infrared (mid-IR), as well as many other attractive optoelectronic attributes. It is, therefore, a very promising material for various optoelectr...

Few-layer black phosphorus (BP) has emerged as a promising 2D material for photodetection in the mid-infrared spectral range given its narrow bandgap. However, a comprehensive understanding of BP photodetector’s response in the mid-infrared is still lacking. In this paper, we study the photoresponse of few-layer BP photodetector in the mid-infrared...

With its high mobility, narrow bandgap, and unique anisotropy, black phosphorus (BP) is a promising material for optoelectronic applications. Waveguide-integrated photodetectors with RC-limited speeds up to 3 GHz have been recently demonstrated at telecom wavelengths. To truly be competitive, however, BP photodetectors must reach speeds of tens of...

We demonstrate the integration of black phosphorus photodetector in a hybrid, three-dimensional architecture of silicon photonics and metallic nanoplasmonics structures. This integration approach combines the advantages of low propagation loss of silicon waveguides, high field confinement of a plasmonic nanogap, and the narrow bandgap of black phos...

Black phosphorus has been the subject of growing interest due to its unique band structure that is both layer dependent and anisotropic. While many have studied the linear optical response of black phosphorus, the nonlinear response has remained relatively unexplored. Here we report on the observation of third-harmonic generation in black phosphoru...

Owing to enormous growth in both data storage and the demand for high-performance computing, there has been a major effort to integrate telecom networks on-chip. Silicon photonics is an ideal candidate, thanks to the maturity and economics of current CMOS processes in addition to the desirable optical properties of silicon in the near IR. The basic...

We observe third-harmonic generation in black phosphorus and find that χ(3) is strongly dependent on both the number of layers and incident polarization. We also use the third-harmonic signal to measure the pulse width of our ultrafast laser.

Silicon photonics, plasmonic structures and two dimensional material are integrated vertically on SOI (Silicon on Insulator) substrate to produce a short channel photodetector. Its estimated average intrinsic responsivity is 220 mA/W.

Ultrafast photocurrent spectroscopy is used to investigate the carrier lifetime of a black phosphorus heterostructure. Tuning the polarization reveals anisotropic response of the excitons with the optical field. Individual ultrafast pulses with a spacing of ~1ns are resolved with our device.

Layered two-dimensional materials have shown novel optoelectronic properties and are well suited to be integrated in planar photonic circuits. For example, graphene has been utilized as a wideband photodetector. However, because it lacks a band gap, graphene photodetectors suffer from extremely high dark current8. In contrast, few-layer black phosp...

A dual-mode, graphene optical modulator and detector for the near-IR is demonstrated in a single device. Gate dependent photocurrent and optical transmission allow the device to operate in a highly novel mode of simultaneous optical modulation and detection.

Graphene's unique optoelectronic properties have been exploited for many photonic applications. Here, we demonstrate a single graphene-based device that simultaneously provides both efficient optical modulation and photodetection. The graphene device is integrated on a silicon waveguide and is tunable with a graphene gate to achieve near-infrared p...

The present invention relates to devices which operate on gradient optical forces, in particular, nanoscale mechanical devices which are actuable by gradient optical forces. Such a device comprises a waveguide and a dielectric body, with at least a portion of the waveguide separated from the dielectric body at a distance which permits evanescent co...

A piezoelectric aluminum nitride film on oxidized silicon wafer is used to realize high frequency surface acoustic wave devices. Optical ring resonator is integrated with the surface acoustic wave device to demonstrate a high speed acousto-optic modulation.

We demonstrate gradient optical forces in metal-dielectric hybrid plasmonic waveguides (HPWG) for the first time. The magnitude of optical force is quantified through excitation of the nanomechanical vibration of the suspended waveguides. Integrated Mach-Zehnder interferometry is utilized to transduce the mechanical motion and characterize the prop...

We demonstrate a large grid of individually addressable superconducting single photon detectors on a single chip. Each detector element is fully integrated into an independent waveguide circuit with custom functionality at telecom wavelengths. High device density is achieved by fabricating the nanowire detectors in traveling wave geometry directly...

Optomechanical phenomena in photonic devices provide a new means of light-light interaction mediated by optical force actuated mechanical motion. In cavity optomechanics, this interaction can be enhanced significantly to achieve strong interaction between optical signals in chip-scale systems, enabling all-optical signal processing without resortin...

Flexible microelectronics has shown tremendous promise in a broad spectrum of applications, especially those that cannot be addressed by conventional microelectronics in rigid materials and constructions. These unconventional yet important applications range from flexible consumer electronics to conformal sensor arrays and biomedical devices. A rec...

The fundamental switching energy limitations for waveguide coupled graphene-on-graphene optical modulators are described. The minimum energy is calculated under the constraints of fixed insertion loss and extinction ratio. Analytical relations for the switching energy both for realistic structures and in the quantum capacitance limit are derived an...

To fully utilize graphene's remarkable optical properties for optoelectronic applications, it needs to be integrated in planar photonic systems. Here, we demonstrate integration of graphene on silicon photonic circuits and precise measurement of the optical absorption coefficient in a graphene/waveguide hybrid structure. A method based on Mach-Zehn...

An electro-absorption optical modulator concept based upon a dual-graphene layer is presented. The device consists of a silicon-on-insulator waveguide upon which two graphene layers reside, separated by a thin insulating region. The lower graphene acts as a tunable absorber, while the upper layer functions as a transparent gate electrode. Calculati...

A new class of nano-optomechanical systems based on a hybrid plasmonic waveguide device built on a silicon photonic platform is proposed and experimentally investigated. Theoretical analysis and numerical modeling reveals that, due to the ability of sub-wavelength field localization, these hybrid plasmonic devices could enhance the optical forces b...

We demonstrate flexible silicon photonic devices, including ring resonators and Mach-Zehnder interferometers, by transferring from SOI wafer to PDMS. Optical characteristics of these devices could be tuned by deforming the flexible substrate.

A novel multichannel cavity optomechanical system consisting of a micro-disk resonator and a nano-cantilever has been proposed and implemented, which subsequently led to the first demonstration of optically induced tunable mechanical nonlinearity.

The ability to control mechanical motion with optical forces has made it possible to cool mechanical resonators to their quantum ground states. The same techniques can also be used to amplify rather than reduce the mechanical motion of such systems. Here, we study nanomechanical resonators that are slightly buckled and therefore have two stable con...

We propose a scheme for generating pairs of de‐correlated photons in silicon nanowire waveguides. By properly engineering the geometrical dispersion of nanoscale waveguides we achieve both four‐wave‐mixing and group velocity matching. Factorable pure photon states are found that do not require spectral filtering. Highly anti‐correlated two‐photon...

Cavity optomechanics enables active manipulation of mechanical resonators through backaction cooling and amplification. This ability to control mechanical motion with retarded optical forces has recently spurred a race towards realizing a mechanical resonator in its quantum ground state. Here, instead of quenching optomechanical motion, we demonstr...

We demonstrate optical gradient force-tunable directional couplers in free-standing silicon nitride slot waveguides. Utilizing device geometries optimized for strong optomechanical interactions allows us to control the optical transmission without the aid of a cavity. Static, wideband tuning as well as low-power optical modulation is achieved.

We describe measurement results of silicon photonic circuits at cryogenic temperatures. The interplay between optically induced heating and free carrier dynamics in nano-photonic ring resonators is investigated at temperatures down to 1.8 K. We find that the life-time of free carriers generated by two-photon absorption in silicon waveguides is redu...

Photonic crystal slot cavities are designed with Q up to 2×106. Strong field confinement allows for significant gradient optical force actuation. Measured devices show high extinction ratio and mechanical resonance at low optical driving power.

We report the piezoresisitivity in magnetic thin films of FeGa and their use for fabricating self transducing microcantilevers. The actuation occurs as a consequence of both the ferromagnetic and magnetostrictive property of FeGa thin films, while the deflection readout is achieved by exploiting the piezoresisitivity of these films. This self-sensi...

We demonstrate integrated nano-optomechanical systems with driven flexural resonance up to 760 MHz in the ultrahigh frequency band. The mechanical element of the device is embedded in a slot waveguide racetrack optical resonator with an optical quality factor of 60 000. Displacement sensitivity of 0.45×10−15 m/ at 127 MHz is achieved in this circui...

A brand new photonic platform for nanomechanical systems integration is proposed . We are able to overcome all these difficulties of current NEMS by harnessing optical force for device transduction and using integrated photonic circuit for interconnects.

Miniaturized gas chromatography (GC) systems can provide fast, quantitative analysis of chemical vapors in an ultrasmall package. We describe a chemical sensor technology based on resonant nanoelectromechanical systems (NEMS) mass detectors that provides the speed, sensitivity, specificity, and size required by the microscale GC paradigm. Such NEMS...

We present a novel platform to construct high-performance nanophotonic devices in low refractive index dielectric films at telecoms wavelengths. The formation of horizontal slots by PECVD deposition of high index amorphous silicon provides a convenient and low-cost way to tailor nanophotonic devices to application needs. Low propagation loss of les...

We demonstrate integrated photonic circuits made from stoichiometric silicon nitride for effective integration of high Q micromechanical resonators and nano-optical components. Using silicon bulk micromachining techniques we fabricate free-standing highly tensile nanostrings exceeding 400 μm in length. The nanostrings are actuated using gradient op...

We perform time-domain measurements of optical transport dynamics in silicon nano-photonic devices. Using pulsed optical excitation the thermal and carrier induced optical nonlinearities of micro-ring resonators are investigated, allowing for identification of their individual contributions. Under pulsed excitation build-up of free carriers and hea...

We report a circuit cavity optomechanical system in which a nanomechanical resonator is adiabatically embedded inside an optical ring resonator with ultralow transition loss. The nanomechanical device forms part of the top layer of a horizontal silicon slot ring resonator, which enables dispersive coupling to the dielectric substrate via a tapered...

We employ the finite-difference time-domain method to calculate the dominant short range forces in optomechanical devices, Casimir and gradient optical forces. Numerical results are obtained for typical silicon optomechanical devices and are compared to metallic reference structures, taking into account geometric and frequency dispersion of silicon...

We analyze the effect of spatial mode beating on optical forces in coupled waveguide resonators. Continuous sign and amplitude change is achieved through optical phase tuning. Competing force components are decomposed via optical mode expansion.

We demonstrate enhanced gradient optical force between a high-Q micro-disk resonator and a waveguide. We find that the total optical force is composed of contributions from the cavity backaction, the reactive and evanescent coupling.

We demonstrate spectrally tuned dispersive and reactive optical force in a cavity optomechanics system that comprises a microdisk and a vibrating nanomechanical beam waveguide. The waveguide coupled to the microdisk acts as a bosonic dissipation channel and its motion modulates the cavity's damping rate. As a result a reactive optical force arises...

We theoretically introduce a mechanism for achieving the mechanical analog of optical Kerr nonlinearity. By exploiting optomechanical interactions in deformable waveguide structures, a nonlinear phase shift proportional to the optical field intensity is induced. Resulting Kerr coefficients several orders of magnitude larger than the intrinsic silic...

We present a rigorous analysis of the optical gradient force between coupled single-mode waveguides in three dimensions. Using eigenmode expansion we determine the optical mode patterns in the coupled system. In contrast to previous work, the sign and amplitude of the optical force is found to vary along the waveguide with a characteristic beating...

We report enhanced optomechanical coupling by embedding a nano-mechanical beam resonator within an optical race-track resonator. Precise control of the mechanical resonator is achieved by clamping the beam between two low-loss photonic crystal waveguide couplers. The low insertion loss and the rigid mechanical support provided by the couplers yield...

Nanoelectromechanical systems based on cantilevers have consistently set records for sensitivity in measurements of displacement, force and mass over the past decade. Continued progress will require the integration of efficient transduction on a chip so that nanoelectromechanical systems may be operated at higher speeds and sensitivities. Conventio...

The optical binding forces between guided lightwaves in dielectric waveguides can be either repulsive or attractive. So far only attractive force has been observed. Here we experimentally demonstrate a bipolar optical force between coupled nanomechanical waveguides. Both attractive and repulsive optical forces are obtained. The sign of the force ca...

We present a study of transverse optical forces arising in a free-standing silicon nanowire waveguide. A theoretical framework is provided for the calculation of the optical forces existing between a waveguide and a dielectric substrate. The force is evaluated using a numerical procedure based on finite-element simulations. In addition, an analytic...

We analyze the photothermal response of nanoelectromechanical systems (NEMS) integrated in a silicon photonic circuit over a wide frequency range. The dynamic response of NEMS devices is studied using a two-color pump-probe scheme in an on-chip photonic Mach–Zehnder interferometer. The measured response is composed of three contributions: (i) the m...

We present a theoretical and experimental study of the frequency response of the photothermal effect in silicon waveguides. The effect is studied for modulation frequencies up to 3 GHz using integrated photonic circuits in Mach–Zehnder and ring oscillator configurations. The thermal behavior of silicon waveguides is described by a diffusive substra...

The force exerted by photons is of fundamental importance in light-matter interactions. For example, in free space, optical tweezers have been widely used to manipulate atoms and microscale dielectric particles. This optical force is expected to be greatly enhanced in integrated photonic circuits in which light is highly concentrated at the nanosca...

We report the development a new class of self-sensing, nanometer-scale cantilevers with fundamental-mode mechanical resonances up to very high frequencies (VHF). The sensors employ integrated piezoresistive displacement transducers; we show that, at the nanoscale, these are optimally realized using thin, metallic-density films. Our approach enables...

Scanning probe microscopies (SPM) and cantilever-based sensors generally use low-frequency mechanical devices of microscale dimensions or larger. Almost universally, off-chip methods are used to sense displacement in these devices, but this approach is not suitable for nanoscale devices. Nanoscale mechanical sensors offer a greatly enhanced perform...