Here we report on the optical engineering of Si nanopillar arrays (NPAs) through the integration of Mie-type resonant nanostructures. We first use finite difference time domain (FDTD) simulations to find optimal nanopillar geometry and array layout as a function of incident photon wavelength. The simulations reveal up to 50% optical absorption within 750 nm tall nanopillars which reduces the effective absorption depth by a factor of nearly 2.7 which can lead to a significant enhancement in quantum efficiency (QE) and reduction in response time. Simultaneously, the reflection of target wavelengths can be effectively eliminated. Next, our simulations guide the fabrication of devices, and we experimentally validate the optical performance predicted by these results. Our next aim is to simulate and experimentally demonstrate quantum efficiency and low response time. A photocathode with high quantum efficiency and low response time can enable charge density photo modulation and the operation of vacuum power amplifiers in the X-band and beyond.

The rapid growth in machine learning and artificial intelligence workloads has increased the demand for computing power. Although digital computing accelerators dominate today's market, analog computing architectures are emerging as promising energy-efficient alternatives. In this work, we propose an approach to address these challenges by replacing the first layer of a convolutional neural network (CNN) with a network of oscillatory retinal neurons (ORNs) composed of weakly coupled, optically activated negative differential resistance (NDR) devices. Each ORN consists of a photodetector exhibiting NDR behavior under illumination, coupled with an inductor forming a self-oscillating circuit without needing external voltage sources. We model the nonlinear oscillator dynamics using experimentally measured device characteristics and simulate their behavior under varying optical inputs. Simulations performed on the Fashion-MNIST (Modified National Institute of Standards and Technology) dataset demonstrate that the hybrid ORN-CNN architecture maintains high recognition accuracy and achieves 95% training accuracy and 92% testing accuracy comparable to the fully software-defined CNN with 93% test accuracy while dramatically reducing energy consumption to 24 aJ/operation. Different ORN network topologies were investigated, with asymmetric inductive coupling achieving consistently high performance. These results highlight the potential of oscillator-based networks for low-power analog computing and demonstrate the possibility of using nonlinear physical systems for energy-efficient machine learning applications. This work demonstrates a possible path for integrating device-level oscillator dynamics into future neuromorphic and analog computing platforms.

This work demonstrates a Hot Electron Light Assisted Cathode (HELAC) inspired vacuum-channel silicon edge emitter that enables emission directly from a Si edge exposed to vacuum, avoiding vertical transport through an insulating layer. Under high vacuum and edge-focused optical excitation, the device exhibits an electronically tunable photoemission process that is measurable across the vacuum gap. Optical modulation experiments demonstrate that the emission current tracks optical switching, indicating potential for high-speed modulation and further optimization.

Achieving an electron source that simultaneously offers high efficiency, high brightness, and long operational lifetime remains a fundamental challenge due to intrinsic trade-offs in existing cathode technologies. Here, we demonstrate a graphene-encapsulated, hydrogen-terminated diamond heterostructure that acts as an atomically engineered gain medium while maintaining a stable negative electron affinity (NEA) emission interface. This architecture allows efficient carrier amplification with continuous charge neutralization and thereby enables operation beyond pulsed mode for the first time.

Electrostatically tunable negative electron affinity (NEA) semiconductor photocathode can be a stable alternative to chemically activated NEA surfaces, which are highly sensitive to atmospheric exposure. Here we present a planar silicon-insulator-graphene heterostructure-based Hot-Electron Light-Assisted Cathode (HELAC) [1, 2], that enables reversible, air-stable NEA operation. Unlike conventional cathodes, HELAC offers independent electronic control over emission parameters, eliminating its dependence on surface chemistry. Together with its structural simplicity and scalability, these features establish HELAC as a transformative platform for the development of nextgeneration compact, optically modulated, and highfrequency electron source.

Work function plays a pivotal role in technologies ranging from energy conversion and electronics to catalysis. In this work, we integrated machine learning (ML) with multi-fidelity screening to develop a data-driven framework for accelerating the discovery of materials with extreme work functions. We augmented a previously published Random Forest (RF) model for work function to include prediction uncertainty calibration and domain of applicability assessment to enhance prediction robustness. By combining the augmented RF model with universal ML interatomic potential simulations and targeted ab initio calculations, we screened 5.5 million compounds from the GNoME and Alexandria databases. This workflow identified 209 surfaces with extreme low work functions below 2.0 eV and 227 surfaces with extreme high work functions above 6.0 eV, corresponding to 136 and 172 unique materials, respectively. The resulting candidates revealed trends consistent with established chemical principles, including the tendency of alkali- and alkaline-earth-terminated surfaces to exhibit low work functions. While it also uncovered less conventional motifs: lanthanide-rich surface terminations were strongly associated with extremely low work functions, whereas surfaces containing metalloids or phosphorus at the top layer were correlated with exceptionally high work functions. This work demonstrates a scalable strategy that leverages ML models and multi-fidelity computational efforts to accelerate the discovery of materials with extreme work functions for advanced electronic, energy-conversion, and catalytic applications.

We present a comprehensive device physics model of the Hot Electron Light Assisted Cold-cathode (HELAC) with $\mathrm{Si} / \mathrm{SiO}_{2} /$ graphene photocathode architecture. HELAC enables air-stable electron emission and electronically tunable negative electron affinity without surface activation. The model couples optical carrier generation, self-consistent MOS electrostatics, and modified Fowler-Nordheim tunneling framework appropriate for photoexcited carriers at the tunneling interface. Validated against experimental measurements, the model reveals that bandwidth limitations arise from inversion charge dynamics rather than carrier transit or circuit parasitics. Consequently, under reliabilitysafe field conditions ($E_{o x}{\lt}1 \mathrm{~V} / \mathrm{nm}$), the 3-dB bandwidth remains in the sub-MHz range. Despite this, at 1 MHz modulation emission current, $J_{\text {emission }}\gt1 \mathrm{mAcm}^{-2}$ with Quantum Efficiency $\gt10^{-3} \%$ is achievable. By increasing $E_{o x}$ to $1.5 \mathrm{~V} / \mathrm{nm}, J_{\text {emission }}$ in $\mu \mathrm{Acm}^{-2}$ range becomes possible for GHz operation with high risk of rapid breakdown. Therefore, for bright beam emission and GHz modulation, the insulator layer needs to be resilient to high fields, potentially replaced with vacuum for maximum performance at high frequencies.

Semiconductor thin films are foundational to a broad range of optoelectronic technologies. Solution deposition offers a low‐cost, energy‐efficient alternative to vapor‐based methods, but its practical scalability is hindered by poor reproducibility and high defect densities arising from complex interdependencies among processing variables. Statistical design of experiments (DoE) enables critical insight into the non‐intuitive interdependencies that are not accessible by conventional one‐variable‐at‐a‐time (OVAT) approaches. Using Sb 2 Se 3 as an example semiconductor, DoE is employed with a central composite design (CCD) to systematically vary six processing parameters encompassing ink formulation and deposition parameters. From only 77 experiments, predictive models are constructed for four macro‐ and microscopic defect types known to interrupt charge transport. All six variables are found to significantly and synergistically influence film quality and are optimized to minimize defect density. Importantly, the optimal film exhibited a threefold enhancement in photoresponse in single‐junction devices with no detectable change in composition, nanostructure, or film stability, implicating defect suppression as the critical driver of improved performance. This work highlights DoE as a powerful methodology for uncovering latent structure–processing–property relationships in thin films and provides a general framework for accelerating the development of optoelectronic‐grade materials via solution processing.

Scaling of transistors has enabled continuous improvements in logic device performance, especially through materials engineering. However, surpassing horizontal limitations in chip manufacturing requires a vertical, third dimension. Three-dimensional integration of high-performance logic demands solving the challenge of low-temperature (less than 450C) synthesis of high-mobility n-type and p-type semiconductor thin films for back-end-of-line (BEOL) compatible transistors. Metal oxides, particularly indium oxides alloyed with gallium and tungsten, are promising n-type channel materials, but suitable p-type materials for BEOL remain scarce. Zinc sulfide (ZnS), a wide band-gap semiconductor, shows room-temperature p-type conductivity when doped with copper and crystallizes below 400C. Here, we report growth of crystalline ZnS thin films by pulsed laser deposition on amorphous and polycrystalline surfaces including silicon nitride, thermal silicon dioxide, yttrium oxide, hafnium dioxide, sapphire, platinum, and titanium nitride. X-ray diffraction reveals out-of-plane texturing across all surfaces, while grazing incidence wide-angle X-ray scattering probes in-plane crystalline quality. Surface and interface properties are assessed using X-ray reflectivity and atomic force microscopy. Electrical characterization via J-V measurements (ZnS on Pt) and metal-oxide-semiconductor capacitor (ZnS on silicon dioxide) measurements show low leakage current (10^-5 A/cm^2 at 0.40 MV/cm) and bilayer capacitor behavior, suggesting ZnS is highly intrinsic with minimal electrically active defects. Further work on doping ZnS with copper or other p-type elements is needed to realize ZnS as a dopable wide band-gap semiconductor for BEOL integration. This work demonstrates a novel thin-film growth method for sulfide semiconductors under BEOL-compatible conditions.

Semiconductor thin films are foundational to a broad range of optoelectronic technologies. Solution deposition offers a low-cost, energy-efficient alternative to vapor-based methods, but its practical scalability is hindered by poor reproducibility and high defect densities arising from complex interdependencies among processing variables. Statistical design of experiments (DoE) enables critical insight into the non-intuitive interdependencies that are not accessible by conventional one-variable-at-a-time (OVAT) approaches. Using Sb2Se3 as an example semiconductor, DoE is employed with a central composite design (CCD) to systematically vary six processing parameters encompassing ink formulation and deposition parameters. From only 77 experiments, predictive models are constructed for four macro- and microscopic defect types known to interrupt charge transport. All six variables are found to significantly and synergistically influence film quality and are optimized to minimize defect density. Importantly, the optimal film exhibited a threefold enhancement in photoresponse in single-junction devices with no detectable change in composition, nanostructure, or film stability, implicating defect suppression as the critical driver of improved performance. This work highlights DoE as a powerful methodology for uncovering latent structure-processing-property relationships in thin films and provides a general framework for accelerating the development of optoelectronic-grade materials via solution processing.

We experimentally investigated the impact on electron emission of adding thermal energy to a Hot Electron Laser Assisted Cathode (HELAC). We observed an increase in photo-gated emission with temperature up to a certain point where the photo-gated emission mechanism was destroyed due to insulator breakdown in the device. We observed that this breakdown is imminent after a sudden increase in device current. We hypothesize this results from ohmic heating thermal runaway in the insulator, permanently damaging the device's response to absorbed photons. We aim to more precisely predict and avoid this breakdown, while operating the HELAC in a temperature range where sustainable improvement in quantum efficiency is feasible.

Photoemission of electrons from solid surfaces into vacuum is routinely used in two configurations: reflection mode, in which light is incident on the emitting surface, and transmission mode, in which light illuminates the photoemissive material from behind. Here, using silicon nitride waveguides integrated beneath a high quantum efficiency cesium antimonide thin film, we demonstrate a new photoemission regime─the evanescent mode photoemission─in which light travels parallel to the surface in the waveguide while evanescently coupling into the thin film to excite and emit photoelectrons. Our experiments show that this configuration enables direct visualization of guided optical modes over an unprecedented range (∼100 nm to 1 mm) and allows electron beam shaping at the source with transverse features well below ∼600 nm, establishing evanescent mode photoemission as a new platform for nanoscale optical field mapping and precise generation of electron beams.

Scaling of transistors has enabled continuous improvement in the performance of logic devices, especially with contributions from materials engineering. However, there is a need to surpass the horizontal limitations in chip manufacturing and incorporate the vertical or third dimension. To enable monolithic three-dimensional integration of high-performance logic, one needs to solve the fundamental challenge of low temperature (<450 degrees C) synthesis of high mobility n-type and p-type semiconductor thin films that can be utilized for the fabrication of back-end-of-line (BEOL) compatible transistors. Metal oxides, especially indium oxides alloyed with gallium and tungsten, are promising n-type semiconductor channel materials; however there is a lack of p-type channel materials that can meet the stringent synthesis conditions of BEOL manufacturing. Zinc sulfide (ZnS), a transparent wide bandgap semiconductor, has shown room temperature p-type conductivity when doped with copper and nitrogen, and crystallizes below 400 degrees C. Here, we report growth of crystalline thin films of ZnS by pulsed laser deposition on a variety of amorphous and polycrystalline surfaces, including silicon nitride (SiNx), thermal silicon dioxide (SiO2), yttrium oxide (Y2O3), hafnium dioxide (HfO2), sapphire (Al2O3), platinum (Pt), and titanium nitride (TiN). X-ray diffraction scans show out-of-the-plane texturing of ZnS on all surfaces. In-plane crystalline quality is investigated using grazing incidence wide-angle x-ray scattering measurements. Surface and interface quality is measured using x-ray reflectivity and atomic force microscopy measurements. Electrical characterization of the ZnS films is done by J-V measurements of ZnS on platinum and metal-oxide-semiconductor capacitor (MOSCAP) measurements of ZnS on SiO2 on heavily doped silicon. The J-V measurements indicate low leakage current on the order of 10(-5) A/cm(-2) with an electric field of 0.40 MV cm(-1), and the MOSCAP characteristics show bilayer capacitor behavior, which points to ZnS being highly intrinsic with very low unintentional, electrically active point defects. Further work on doping ZnS with copper or other p-type candidate dopants is needed to demonstrate ZnS as a dopable wide bandgap semiconductor for channels compatible with BEOL manufacturing. This work showcases the capability of the novel thin film growth technique of a wide bandgap sulfide semiconductor under BEOL compatible conditions with potential for technological applications in transistor manufacturing.

A voltage-tunable negative electron affinity (NEA) semiconductor photocathode offers one key advantage over current materials such as cesiated NEA photocathodes: stability under ambient conditions. A semiconductor/insulator/graphene heterostructure can inject electrons into the conduction band of the insulator, where an electric field "heats" them up so that the effective emission barrier seen by the "hot" electrons is negative, enabling a voltage-tunable NEA surface. Here, we have experimentally demonstrated a peak emission current density of 2.253 × 10-3 A/cm2 and a peak external quantum efficiency (EQE) of ∼1.53% from a p-Si/amorphous-Al2O3/graphene-based hot electron laser-assisted cathode (HELAC). We have developed a full-band Monte Carlo Boltzmann Transport Equation (MCBTE) solver to study the hot electron transport behavior in three different crystalline insulators: SiO2, Al2O3, and MgO. Through MCBTE and semiconductor device simulations, we have predicted a peak emission current density of ∼103 A/cm2, far above our experimental value, indicating that the optimal performance of state-of-the-art HELACs has not yet been realized. This theoretical framework provides an understanding of the key performance limitations of the device and can be used to guide the optimal design (e.g., through the selection of new materials) of voltage-tunable NEA semiconductor photocathodes.

Here, we introduce an electronic circuit that mimics the functionality of a biological spiking neuron following the Fitzhugh-Nagumo (FN) model. The circuit consists of a tunnel diode that exhibits negative differential resistance (NDR) and an active inductive element implemented by a single MOSFET. The FN neuron converts a DC voltage excitation into voltage spikes analogous to biological action potentials. We predict an energy cost of 2 aJ/cycle through detailed simulation and modeling for these FN neurons. Such an FN neuron is CMOS compatible and enables ultralow power oscillatory and spiking neural network hardware. We demonstrate that FN neurons can be used for oscillator-based computing in a coupled oscillator network to form an oscillator Ising machine (OIM) that can solve computationally hard NP-complete max-cut problems while showing robustness toward process variations.

In this work, we explore III-V based metal-semiconductor-metal structures for tunable metasurfaces. We use an epitaxial transfer technique to transfer a III-V thin film directly on metallic surfaces, realizing III-V metal-semiconductor-metal (MSM) structures without heavily doped semiconductors as substitutes for metal layers. The device platform consists of gold metal layers with a p-i-n GaAs junction. The target resonance wavelength can be tuned by modifying the geometry of the top metal grating on the GaAs, while systematic resonance tunability has been shown through the modulation of various carrier concentration injections in the mid-IR range. Electrically tunable metasurfaces with multilevel biasing can serve as a fundamental building block for electrically tunable metasurfaces. We believe that our demonstration can contribute to understanding the optical tuning of III-V under various biased conditions, inducing changes in metasurfaces.

Negative electron affinity (NEA) photocathodes have long been pursued as an efficient electron source due to their enhanced quantum efficiency, narrow energy spread and high brightness or current density. Conventionally, NEA is achieved by applying a thin layer of cesium-rich oxide onto a semiconductor surface, rendering it highly reactive, unstable, and susceptible to environmental conditions [1]. In a recent investigation focused on electrostatically generated tunable NEA surface [2], an NEA photocathode is explored that exhibits remarkable stability in ambient conditions and allows for electronic tuning of its electron affinity. The present work discusses an enhanced iteration of this device, characterized by improved quantum efficiency and reliability.

Developing cost-effective III-V integration has posed a critical challenge in integrated photonics. To date, the integration of III-V with LiNbO3 has been demonstrated through epitaxial transfer or wafer bonding; however, challenges persist in terms of both cost and alignment. In this paper, we demonstrated various patterned single-crystalline III-V mesas achieved through low-temperature templated liquid-phase (LT-TLP) growth on LiNbO3. Both electrical and optical characterizations reveal the distinctive quality of III-V even without using lattice-matched substrates. InAs on LiNbO3 shows high electron mobility (2500 cm(2)/Vs) at room temperature, while InP on LiNbO3 displays uniform photoluminescence and the ability to tune the optical properties of the grown material through in situ doping. We project that the reported LT-TLP monolithic III-V growth on LiNbO3 reported here will prove beneficial in designing photonic devices such as photon generation and detection or gain materials.