arXiv subjects

Virat Tara

Publications and source records attributed to Virat Tara.

14 recordsLinked to original sources

arXiv2026

Inverse designed resistive heaters for uniform switching of Phase Change Materials

Non-volatile phase-change material (PCM)-integrated metasurfaces offer a promising pathway toward next-generation solid-state reconfigurable free-space optics. However, their practical operation is currently bottlenecked by the highly non-uniform thermal profiles generated by the external heaters used to switch the PCM between its amorphous and crystalline states. The non-uniform heat profile in turn severely restricts the active switching area of the PCM integrated devices. In this work, we present an inverse-designed doped silicon resistive heater on a silicon-on-sapphire platform, featuring a spatially varying doping profile explicitly tailored to generate uniform heat. The new heater significantly improves spatial temperature uniformity, reducing the thermal gradient from 110 K in the case of typical conventional heater to merely 25 K in the case of the inverse-designed heater at a target temperature of ~1000 K. By meticulously optimizing the doping and annealing processes to suppress lateral dopant diffusion, we achieve a near-perfect spatial doping resolution capable of patterning two distinct doped Silicon filaments just 100 nm apart. We experimentally fabricate these devices and demonstrate their efficacy by successfully switching a large area (~18 x 14 micrometer square) of the wide-bandgap phase-change material (PCM) Sb2S3 using a compact heater geometry of size 26 x 26 micrometer square. We show that our inverse-designed heater achieves a 10-fold increase in active PCM switching area despite a reduction in the total heater footprint when compared to a conventional heater. Ultimately, this work provides a crucial steppingstone toward the development of non-volatile PCM-based reconfigurable free-space optics.

physics.optics↗

arXiv2026

Complex wavefront engineering via decoupled space-time modulation

Solid-state Spatial Light Modulators (SLMs) are fundamentally limited in their ability to achieve high spatial complexity and high temporal bandwidth simultaneously. High-speed, low-energy modulation requires sub-wavelength active mode volumes, and sophisticated spatial wavefront engineering necessitates an ultra-fine pixel pitch. While small pixels can simultaneously solve both, in conventional architectures, the dense 2D electrical routing required for such pixels creates an insurmountable physical bottleneck. This results in a compromise between the SLM refresh rate, number of pixels and the field of view. Here, we demonstrate a hybrid architecture that overcomes this limit by spatially decoupling the electrical modulation plane from the optical output plane. By integrating a metasurface doublet with a photonic integrated circuit (PIC)-based optical phased array (OPA), we achieve independent 2D electrical control over each phase-element while simultaneously realizing a three-fold reduction in effective pixel pitch. This decoupling allows us to maintain the small active volume required for high-speed operation, while circumventing the routing constraints of dense spatial array of emitters. We utilize this platform to demonstrate tunable varifocal lensing, 2D beam steering, and 2D holography. Our work provides a scalable foundation for next-generation solid-state SLMs that simultaneously offer high speed, low power consumption, and large field of view.

physics.optics↗

arXiv2026

Measurement of complex scattering matrix in a nano-cavity array for boundary scattering tomography

On-chip silicon photonic coupled cavity arrays (CCA) are a promising platform for quantum simulators, with access to high Quality (Q) factor resonators, tunability, and foundry compatibility. Furthermore, scalable two-dimensional (2D) silicon photonic CCAs allow for simulation of rich physical phenomena via Hamiltonian engineering. However, complete reconstruction of the Hamiltonian is limited by access to cavities in the bulk, with current approaches relying on imaging scattered light from bulk resonators. These approaches often require additional scatterers to be built in, limiting scalability, while also being hampered by imaging technology in the near-infrared range. Instead of these approaches, Hamiltonian tomography algorithms that require homodyne boundary measurements have been demonstrated in literature, however measurements of complex scattering measurements along a CCA boundary have not been shown. Here, we experimentally demonstrate an on-chip homodyne measurement setup along a single boundary of a $3\times 3$ silicon photonic racetrack resonator array and reconstruct the system's edge scattering matrix.

physics.optics↗

arXiv2026

Increased endurance of nonvolatile photonics enabled by nanostructured phase-change materials

The rapid rise of artificial intelligence, and in-memory computing has reinvigorated research on scalable, energy-efficient, and reconfigurable photonic hardware. Non-volatile phase-change materials (PCMs) are attractive, as they offer large refractive index contrast, wavelength-scale footprints, and zero static power consumption. However, current PCM-based electrically controlled photonic devices are plagued by high insertion loss and low endurance. One prevalent hypothesis for these material limitations come from electromagnetic scattering in the interface and large programming volumes, respectively. Here, we validate this hypothesis by showing that nano-structuring of PCM minimizes optical loss and enhances the endurance. By tapering both ends of a wide bandgap PCM Sb2Se3 segment on a silicon waveguide, we suppressed the insertion loss by ~94% (resulting in a loss of ~0.1 dB per π phase shift). Through combining tapering and segmentation, we achieved high optical modulation amplitude (~70%), low loss (~0.5 dB per π phase shift), low-voltage (< 5V) actuation, and record high endurance greater than 100 million cycles. This work showcases the substantial advantage of nanopatterning PCMs to attain low loss and high cyclability.

physics.optics↗

arXiv2026

All-Dielectric Metasurface with a Two-Dimensional Locally Flat Photonic Band

Photonic flatbands offer promising light-matter interaction due to their unique slow-light nature. In recent years, flatbands have also attracted significant interest in optical engineering because of their angle-insensitive resonant characteristics. However, to date, no studies have reported the dispersionless behavior of flatbands under arbitrary two-dimensional incident angles. Here, we present a two-dimensional photonic flatband created using a silicon metasurface with a Lieb lattice-inspired structure which demonstrates a locally flat photonic band for both transverse electric (TE) and transverse magnetic (TM) polarized light. Employing Fourier imaging, we analyze the energy-momentum dispersion of the flatband metasurface under arbitrary two-dimensional incident angles, demonstrating dispersionless flatbands with a change in resonance within $\pm2 nm$ up to $\pm24ô$ or $\pm5ô$, depending on the polarization state and incident angle. This geometry can be adapted for various applications in local field enhancement, enhanced photodetection, and augmented reality displays.

physics.optics↗

arXiv2025

Electrically reconfigurable nonvolatile transmissive metasurface in visible

The synergy between metasurfaces and non-volatile phase change materials (PCMs) has created many reconfigurable photonic devices for applications in optical memory, optical computing and optical communications. But these advances have been limited to the infrared wavelengths due to the high loss of PCMs in the visible regime. Here we demonstrate a nonvolatile visible metasurface that is electrically reconfigurable using wide bandgap PCM Sb2S3. Our device supports a resonant mode at 610 nm, a wavelength largely under-explored for PCM-based metasurfaces. By incorporating only a 20 nm thick layer of Sb2S3, we experimentally demonstrate a resonance tuning range of 16 nm. Reversible switching of the metasurface is accomplished in situ using a carefully engineered, ultrathin doped silicon micro-heater. Our work paves the way for integrating PCMs into visible-frequency systems, particularly for human-centric applications such as augmented and virtual reality displays.

physics.optics↗

arXiv2025

Compact broadband thermal absorbers based on plasmonic fractal metasurfaces

The ability to efficiently absorb thermal radiation within a small material volume is crucial for the realization of compact and high spatial resolution thermal imagers. Here we propose and experimentally demonstrate a compact plasmonic metasurface for broadband absorption in the 6 to 14 microns wavelength range. As opposed to previous works, our metasurface leverages strongly localized electromagnetic modes to achieve high absorption within a compact form factor. We numerically investigate the spectral response of finite arrays of fractals and show that the absorption enhancement provided by arrays with greater than 6x6 fractals covering a total area of only 30x30 microns squared is similar to that of an infinitely periodic array. Furthermore, we experimentally validate our metasurface's absorption enhancement and demonstrate a good qualitative agreement between the measured and simulated spectral responses. Owing to its ability to achieve broadband absorption enhancement in a compact footprint, our metasurface provides new avenues for the realization of next generation infrared sensors and bolometers.

physics.optics↗

arXiv2025

NEO-PGA: Nonvolatile electro-optically programmable gate array

Programmable photonic integrated circuits (PICs) offer a unique opportunity to create a flexible platform, akin to electronic field programmable gate array (FPGA). These photonic PGAs can implement versatile functionalities for applications ranging from optical interconnects to microwave photonics. However, state-of-the-art programmable photonics relies predominantly on volatile thermo-optic tuning, which suffers from high static power consumption, large footprints, and thermal crosstalk. All these dramatically limit the gate density and pose a fundamental limit to the scalability. Chalcogenide-based phase-change materials (PCMs) offer a superior alternative due to their nonvolatility and substantial optical contrast, though challenges such as optical loss, and bit precision severely limited their application in large-scale PICs. Here, we demonstrate precise, multi-bit, low-loss tuning of the emerging PCM Sb2Se3 using a closed-loop, "program-and-verify" method. Electrically reconfigurable PCM-integrated silicon photonic gates are implemented on a 300mm silicon photonic platform, using circulating and forward Mach-Zehnder interferometer (MZI) meshes. In the circulating mesh, we realize broadband optical switching fabrics and high-Q coupled resonators with unprecedented local control of coupling rates, which further enable exploration of coupled-cavity systems. The forward mesh supports self-configurable MZIs that sort two orthogonal beams to different ports. These results showcase a new type of scalable photonic PGA enabled by PCMs, offering a pathway toward general-purpose, on-chip programmable photonic systems.

physics.optics↗

arXiv2025

Electrically reconfigurable nonvolatile flatband absorbers in the mid-infrared with wide spectral tuning range

While recent advances in reconfigurable photonics have provided new avenues for manipulating light on the subwavelength scale, on-demand control of infrared absorption remains elusive. Here, we experimentally demonstrate a plasmonic metasurface based on the phase change material Ge2Sb2Te5 with in-situ electrically-switchable infrared absorption in the 3-5 microns wavelength range. Unlike traditional infrared microstructures based on volatile phase change materials, our device does not require the external stimuli to be continuously applied in order to maintain a given optical state, thus enabling zero static power operation. Furthermore, the 400x deep-subwavelength field localization supported by our device not only allows robust tuning of its spectral response but also makes its absorptivity independent of the angle of incidence, thus enabling a flatband behavior. We conduct switching of our device using rapid thermal annealing and reversible switching using electrical pulses over 26 cycles. Our device provides new avenues for infrared absorption control and serves as a steppingstone for the next generation of mid-wave infrared photonics.

physics.optics↗

arXiv2025

Free Space Few-Photon Nonlinearity in Critically Coupled Polaritonic Metasurfaces

Few-photon optical nonlinearity in planar solid-state systems is challenging yet crucial for quantum and classical optical information processing. Polaritonic nonlinear metasurfaces have emerged as a promising candidate to push the photon number down -- but have often been hindered by challenges like the poor photon-trapping efficiency and lack of modal overlap. Here, we address these issues in a self-hybridized perovskite metasurface through critical coupling engineering, and report strong polaritonic nonlinear absorption at an ultra-low incident power density of only 519 W/cm2 (2 orders of magnitude lower than the state of art in free-space planar devices), with an estimated photon number of 6.12 per cavity lifetime. Taking advantage of a quasi-bound-state-in-the-continuum design with asymmetry-controlled quality-(Q)-factor, we systematically examine the Q-dependent device nonlinearity and determine the optimal cavity critical coupling condition. With the optimized device, we demonstrate at 6 Kelvin a tunable nonlinear response from reverse saturable absorption to saturable absorption at varying pump powers, with a maximal effective nonlinear absorption coefficient up to 29.4+-5.8 cm/W (6 orders of magnitude larger than unpatterned perovskites) at 560 nm wavelength. In addition, the cavity-exciton detuning dependent device response is analyzed and well explained by a phase-space-filling model, elucidating the underlying physics and the origin of giant nonlinearity. Our study paves the way towards practical flat nonlinear optical devices with large functional areas and massive parallel operation capabilities.

physics.optics↗

arXiv2025

Heteroepitaxial growth of highly anisotropic $Sb_{2}Se_{3}$ films on GaAs

The epitaxial integration of anisotropic materials with mainstream cubic semiconductors opens new routes to advanced electronic and photonic devices with directional properties. In this work, we synthesize heteroepitaxial thin films of orthorhombic "quasi-1D" $Sb_{2}Se_{3}$ on cubic GaAs(001) using molecular beam epitaxy. Traditionally, the synthesis of anisotropic films with low symmetry materials is challenging due to multiple grain orientations that form. On a macroscopic scale, such a film tends towards isotropic properties, even if individual grains possess anisotropic responses. We achieve epitaxial $Sb_{2}Se_{3}$ grains on pristine homoepitaxial GaAs templates at low temperatures of 180-200 °C. With the $Sb_{2}Se_{3}$ 1D axis aligned in-plane to GaAs [110] and the primary van der Waals direction lying out-of-plane, we find a birefringence of 0.2 between in-plane orthogonal directions and a giant out-of-plane birefringence greater than 1 at telecom wavelengths. Growth at higher temperatures up to 265 °C yields $Sb_{2}Se_{3}$ of an unusual in-plane rotated texture that further enhances the in-plane optical index anisotropy to 0.3.

cond-mat.mtrl-sci↗

arXiv2024

Low-power 7-bit hybrid volatile/ nonvolatile tuning of ring resonators

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.

physics.optics↗

arXiv2024

Low-loss multilevel operation using lossy PCM-integrated silicon photonics

Chalcogenide phase-change materials (PCMs) offer new paradigms for programmable photonic integrated circuits (PICs) thanks to their zero static energy and significant refractive index contrast. However, prototypical PCMs, such as GeSbTe (GST), are lossy in their crystalline phase, albeit transparent in the amorphous state. Moreover, electrically switching PCMs to intermediate states is a stochastic process, limiting programming accuracy. As a result, achieving both low-loss and deterministic multi-level operation with GST remains challenging. Although low-loss PCMs, such as Sb2S3 and Sb2Se3, have been discovered in recent years, they are much less technologically mature. In this work, we propose a design with multiple GST segments to overcome the challenge of deterministic multilevel operation. GST segments are individually controlled by interleaved silicon PIN diode heaters in a binary but reliable fashion, and multiple levels are encoded in their phase sequence. A 1 x 1 programmable unit with two unequal GST segments is experimentally demonstrated, showcasing four distinct operation levels and negligible thermal crosstalk with only one pair of metal contacts. We then extend the design to 1 x 2 and 2 x 2 programmable units. For the 2 x 2 programmable unit design, we propose a phase-detuned three-waveguide directional coupler structure to mitigate the absorption and radiation loss, showing < -1.2 dB loss and three splitting ratios. Our work provides a new path toward low-loss and multi-level optical switches using lossy PCMs.

physics.optics↗

arXiv2023

Non-volatile Phase-only Transmissive Spatial Light Modulators

Free-space modulation of light is crucial for many applications, from light detection and ranging to virtual or augmented reality. Traditional means of modulating free-space light involves spatial light modulators based on liquid crystals and microelectromechanical systems, which are bulky, have large pixel areas (~10 micron x 10 micron), and require high driving voltage. Recent progress in meta-optics has shown promise to circumvent some of the limitations. By integrating active materials with sub-wavelength pixels in a meta-optic, the power consumption can be dramatically reduced while achieving a faster speed. However, these reconfiguration methods are volatile and hence require constant application of control signals, leading to phase jitter and crosstalk. Additionally, to control a large number of pixels, it is essential to implement a memory within each pixel to have a tractable number of control signals. Here, we develop a device with nonvolatile, electrically programmable, phase-only modulation of free-space infrared radiation in transmission using the low-loss phase-change material (PCM) Sb2Se3. By coupling an ultra-thin PCM layer to a high quality (Q)-factor (Q~406) diatomic metasurface, we demonstrate a phase-only modulation of ~0.25pi (~0.2pi) in simulation (experiment), ten times larger than a bare PCM layer of the same thickness. The device shows excellent endurance over 1,000 switching cycles. We then advance the device geometry, to enable independent control of 17 meta-molecules, achieving ten deterministic resonance levels with a 2pi phase shift. By independently controlling the phase delay of pixels, we further show tunable far-field beam shaping. Our work paves the way to realizing non-volatile transmissive phase-only spatial light modulators.

physics.optics↗