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Takuo Tanemura

Publications and source records attributed to Takuo Tanemura.

16 recordsLinked to original sources
arXiv2026

High-Resolution Speckle-based Single-Pixel Imaging using Silicon Photonic Non-Redundant Optical Phased Array

Optical phased arrays (OPAs) are promising wavefront controlling devices for imaging applications due to their compact and high-speed nature. However, conventional periodic OPAs have a limited spatial resolution, which scales only linearly with the number of optical antennas $N$. Here, we experimentally demonstrate high-resolution imaging using a non-redundant OPA (NR-OPA). Due to the non-redundant antenna layout of NR-OPA based on the Costas array, the resolution scales quadratically with $N$. Combined with the speckle-based single-pixel imaging (SSPI) scheme, which avoids the need for precise phase calibration, we experimentally achieve a large number of resolvable imaging points exceeding 10,000 using a silicon photonic NR-OPA chip with only $N=127$ without sweeping the wavelength. The demonstrated scheme provides a promising route toward mega-pixel imaging by a compact OPA chip with reduced number of phase shifters.

physics.optics↗
arXiv2026

Scalable optical neural network with nonlocally coupled coherent photonic processor

Optical neural networks (ONNs) based on programmable photonic integrated circuits (PICs) offer a promising route toward low-latency and energy-efficient deep learning. However, conventional photonic implementations of matrix-vector multiplication (MVM) rely on locally connected architectures, such as Mach-Zehnder interferometer (MZI) meshes, whose number of active components scales quadratically with matrix size, severely limiting scalability. Here, we present a scalable ONN that overcomes this limitation by exploiting the intrinsically diffractive and nonlocal nature of coherent light inside a silicon photonic chip. Our approach employs cascaded stages of multiport directional couplers (MDCs) interleaved with compact phase-shifter arrays, enabling strong nonlocal coupling among multiple optical modes. We show that an MDC-based optical unitary converter (OUC) requires only $3N$ phase shifters to achieve uniform coverage over the $N$-dimensional complex unitary group, in stark contrast to the $O(N^2)$ scaling of conventional MZI meshes. Based on the singular value decomposition, we demonstrate that an $N\times N$ MVM can be realized using only $7N$ phase shifters, breaking the traditional $O(N^2)$ scaling barrier. We experimentally implement a 32-input silicon photonic MVM chip with a tenfold reduction in active components and validate its performance on various classification tasks. Our results establish a practical pathway toward large-scale, energy-efficient, and reconfigurable photonic neural networks.

physics.optics↗
arXiv2025

17 GHz Lossless InP-Membrane Active Metasurface

High-speed active metasurfaces enable spatiotemporal control of incident light within an ultra-thin layer, offering new possibilities for optical communication, computing, and sensing. However, a fundamental tradeoff between electrical conductivity and optical absorption of the material has hindered the realization of active metasurfaces that simultaneously achieve broad modulation bandwidth and low optical loss. Here, we experimentally demonstrate a high-speed active metasurface operating in the 1.5-μm wavelength range that realizes a record-high modulation bandwidth of 17.5 GHz, while maintaining a high quality (Q) factor of 102 and an ultra-low optical loss of 0.56 dB. The key enabling technology is the indium-phosphide (InP) membrane platform; an n-type InP offers both high electron mobility and low free-carrier optical absorption, making it an ideal material for active metasurface devices. The high-Q Friedrich-Wintgen quasi-bound-states-in-the-continuum mode inside the InP-membrane high-contrast grating (HCG) is utilized to trap the normally incident light within an organic electro-optic (OEO) material, enabling efficient modulation. InP HCG also serves as an ultralow-resistance interdigitated electrodes for applying high-speed electrical signals to the OEO material, thereby offering 50-fold improvement in modulation bandwidth compared to conventional silicon-based counterparts. Our work paves the way towards high-speed, low-loss active metasurfaces for spatiotemporal control of light beyond the gigahertz regime.

physics.optics↗
arXiv2025

Subvolt high-speed free-space modulator with electro-optic metasurface

Active metasurfaces incorporating electro-optic (EO) materials enable high-speed free-space optical modulators that show great promise for a wide range of emerging applications, including free-space optical communication, light detection and ranging, and optical computing. However, the limited light-matter interaction lengths in ultrathin metasurfaces typically require high driving voltages exceeding tens of volts. Here we present ultralow-voltage, high-speed free-space optical modulators based on silicon-organic-hybrid metasurfaces with dimerized-grating-based nanostructures. By exploiting a high-Q quasi-bound state in the continuum, normally incident light is effectively trapped within a submicrometer-scale silicon slot region embedded with organic EO material. Consequently, highly efficient modulation is obtained, enabling data transmission at 50 Mbps and 1.6 Gbps with driving voltages of only 0.2 V and 1 V, respectively. These unprecedented metasurface modulators operating at complementary metal-oxide-semiconductor (CMOS)-compatible voltage levels provide the pathway toward energy-efficient high-speed active metasurface devices.

physics.optics↗
arXiv2025

Ultrafast one-chip optical receiver with functional metasurface

High-speed optical receivers are crucial in modern optical communication systems. While complex photonic integrated circuits (PICs) are widely employed to harness the full degrees of freedom (DOFs) of light for efficient data transmission, their waveguide nature inherently constrains two-dimensional spatial scaling to accommodate a large number of optical signals in parallel. Here, we present a novel optical receiver platform that fully exploits the high spatial parallelism and ultrabroad bandwidth of light, while leveraging all DOFs - intensity, phase, and polarization. Our solution integrates a thin metasurface, composed of silicon nanoposts, with ultrafast membrane photodetectors on a compact chip. The metasurface provides all the functionalities of conventional PICs for normal-incident spatially parallelized light, enabling high-speed detection of optical signals in various modulation formats, including simultaneous detection of 320-gigabits-per-second four-channel signals and coherent detection of a 240-gigabits-per-second signal.

physics.optics↗
arXiv2024

Complete vectorial optical mode converter using multi-layer metasurface

A vectorial optical mode converter that can transform an orthogonal set of multiple input vector beams into another orthogonal set of vector beams is attractive for a wide range of applications in optics and photonics. While multi-plane light conversion (MPLC) and metasurface (MS) technologies have been explored to individually address multiple spatial mode conversion and polarization mode manipulation, there has been no universal methodology to simultaneously convert a set of multiple vectorial modes, having non-uniform spatial distributions in both their complex amplitude and polarization, to another set of multiple vectorial modes. In this paper, we demonstrate versatile devices based on the MPLC concept incorporating multi-layer locally birefringent MSs and present a general design formalism for complete vectorial mode conversion in arbitrary cases. The effectiveness of our proposed method is confirmed experimentally by demonstrating a 6-mode (3 spatial modes $\times$ 2 polarization modes) multiplexer, fabricated on a compact chip with a ~0.65 mm$^2$ lateral size in a folded MS configuration. Additionally, we verify its applicability to more advanced functional devices through numerical demonstration of a mode-division-multiplexed dual-polarization coherent receiver and spatial-mode-multiplexed vectorial holography. The versatility of our protocol makes it suitable for designing a myriad of multi-input-multi-output devices, providing a powerful tool for realizing universal optical mode converters for a wide range of applications.

physics.optics↗
arXiv2024

Metasurface-enabled non-orthogonal four-output polarization splitter for non-redundant full-Stokes imaging

Imaging polarimetry plays an essential role in various fields since it imparts rich information that cannot be obtained through mere intensity and spectral measurements. To retrieve full Stokes parameters, at least four sensor pixels are required, each of which projects incident light to a different polarization state in the Stokes space. Conventional full-Stokes division-of-focal-plane (DoFP) cameras realize this function by integrating angled polarizers and retarders on top of image sensors. Due to the inevitable absorption at the polarizers, however, the maximum efficiency of these schemes is limited to 50% in theory. Instead of polarizers, three sets of lossless polarization beam splitters can be used to achieve higher-efficiency polarimetry, however, at the cost of reduced spatial resolution due to the need for six redundant sensor pixels. In this paper, we reveal, for the first time to our knowledge, that low-loss four-output polarization splitting (without filtering) is possible using a single-layer dielectric metasurface. Although these four states are not orthogonal to each other, our metasurface enables simultaneous sorting and focusing onto four sensor pixels with an efficiency exceeding 50\%, which is not feasible by a simple combination of space-optic components. The designed metasurface composed of silicon nanoposts is fabricated to experimentally demonstrate complete retrieval of full Stokes parameters at the near-infrared wavelength range from 1500 to 1600 nm with $-$2.28-dB efficiency. Finally, simple imaging polarimetry is demonstrated using a 3$\times$4 superpixel array.

physics.optics↗
arXiv2024

Lower-depth programmable linear optical processors

Programmable linear optical processors (LOPs) can have widespread applications in computing and information processing due to their capabilities to implement reconfigurable on-chip linear transformations. A conventional LOP that uses a mesh of Mach-Zehnder interferometers (MZIs) requires $2N+3$ stages of phase shifters for $N \times N$ matrices. However, it is beneficial to reduce the number of phase shifter stages to realize a more compact and lower-loss LOP, especially when long and lossy electro-optic phase shifters are used. In this work, we propose a novel structure for LOPs that can implement arbitrary matrices as long as they can be realized by previous MZI-based schemes. Through numerical analysis, we further show that the number of phase shifter stages in the proposed structure can be reduced to $N+2$ and $N+3$ for a large number of random dense matrices and sparse matrices, respectively. This work contributes to the realization of compact, low-loss, and energy-efficient programmable LOPs.

physics.optics↗
arXiv2023

High-speed metasurface modulator using critically coupled bimodal plasmonic resonance

Free-space electro-optic (EO) modulators operating at gigahertz and beyond are attractive for a wide range of emerging applications, including high-speed imaging, free-space optical communication, microwave photonics, and diffractive computing. Here we experimentally demonstrate a high-speed plasmonic metasurface EO modulator operating at a near-infrared wavelength range with a gigahertz modulation bandwidth. To achieve efficient intensity modulation of reflected light from an ultrathin metasurface layer, we utilize the bimodal plasmonic resonance inside a subwavelength metal-insulator-metal grating, which is precisely tuned to satisfy the critical coupling condition. As a result, perfect absorption of -27 dB (99.8%) and a high quality (Q) factor of 113 are obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer inside the grating, we achieve a modulation depth of up to 9.5 dB under an applied voltage of $\pm$30 V. The 3-dB modulation bandwidth is confirmed to be 1.25 GHz, which is primarily limited by the undesired contact resistance. Owing to the high electrical conductivity of metallic gratings and a compact device structure with a minimal parasitic capacitance, the demonstrated device can potentially operate at several tens of gigahertz, which opens up exciting opportunities for ultrahigh-speed active metasurface devices in various applications.

physics.optics↗
arXiv2023

Iterative configuration of programmable unitary converter based on few layer redundant multi-plane light conversion

Programmable unitary photonic devices are emerging as promising tools to implement unitary transformation for quantum information processing, machine learning, and optical communication. These devices typically use a rectangular mesh of Mach-Zehnder interferometers (MZIs), which has a clear mathematical structure and can be configured deterministically. However, this mesh architecture is sensitive to fabrication errors, and the correction techniques are still under investigation. In contrast, the multi-plane light conversion (MPLC) architecture is more robust against fabrication errors, but a deterministic method for configuring the converter has not yet been developed due to its complex mathematical structure. In this work, we propose a fast iterative configuration method for MPLC, following the mathematical review of the matrix distance and proposal of a new norm. We show through numerical simulations that adding a few redundant layers significantly improves the convergence of the MPLC architecture, making it a practical and attractive option. We also consider the effects of finite resolution and crosstalk in phase shifters in our simulations. In addition, we propose a phase-insensitive distance suited for applications using only intensity detections. Our method demonstrates orders of magnitude better accuracy and a 20-fold speed-up compared to previous approaches.

cs.ET↗
arXiv2023

Compact and scalable polarimetric self-coherent receiver using dielectric metasurface

The polarimetric self-coherent system using a direct-detection-based Stokes-vector receiver (SVR) is a promising technology to meet both the cost and capacity requirements of the short-reach optical interconnects. However, conventional SVRs require a number of optical components to detect the state of polarization at high speed, resulting in substantially more complicated receiver configurations compared with the current intensity-modulation-direct-detection (IMDD) counterparts. Here, we demonstrate a simple and compact polarimetric self-coherent receiver based on a thin dielectric metasurface and a photodetector array (PDA). With a single 1.05-$μ$m-thick metasurface device fabricated on a compact silicon-on-quartz chip, we implement functionalities of all the necessary passive components: a 1$\times$3 splitter, three polarization beam splitters with different polarization bases, and six focusing lenses. Combined with a high-speed PDA, we demonstrate self-coherent transmission of 20-GBd 16-ary quadrature amplitude modulation (16QAM) and 50-GBd quadrature phase-shift keying (QPSK) signals over a 25-km single-mode fiber. Owing to the surface-normal configuration, it can easily be scaled to receive spatially multiplexed channels from a multicore fiber or a fiber bundle, enabling compact and low-cost receiver modules for the future highly parallelized self-coherent systems.

physics.optics↗
arXiv2022

Ultra-broadband surface-normal coherent optical receiver with nanometallic polarizers

A coherent receiver that can demodulate high-speed in-phase and quadrature signals of light is an essential component for optical communication, interconnects, imaging, and computing. Conventional waveguide-based coherent receivers, however, exhibit large footprints, difficulty in coupling a large number of spatial channels efficiently, and limited operating bandwidth imposed by the waveguide-based optical hybrid. Here, we present a surface-normal coherent receiver with nanometallic-grating-based polarizers integrated directly on top of photodetectors without the need for an optical hybrid circuit. Using a fabricated device with the active section occupying a 70-μm-square footprint, we demonstrate demodulation of high-speed (up to 64 Gbaud) coherent signals in various formats. Moreover, ultra-broadband operation from 1260 nm to 1630 nm is demonstrated, thanks to the wavelength-insensitive nanometallic polarizers. To our knowledge, this is the first demonstration of a surface-normal homodyne optical receiver, which can easily be scaled to a compact two-dimensional arrayed device to receive highly parallelized coherent signals.

physics.optics↗
arXiv2021

Non-redundant optical phased array

Optical phased array (OPA) is a promising beam-steering device for various applications such as light detection and ranging (LiDAR), optical projection, free-space optical communication and switching. However, the previously reported OPAs suffer from either an insufficiently small number of resolvable points, or a complicated control requirement due to an extremely large number of phase shifters. This work introduces a novel array configuration for OPA devices based on the non-redundant array (NRA) concept. Based on this design, we can realize high-resolution OPA whose number of resolvable points scales with N2. In contrast, that of traditional OPAs scales only with N. Thus, a significant reduction in the number of required phase shifters can be attained without sacrificing the number of resolvable points. We first investigate the impact of employing the NRA theoretically by considering the autocorrelation function of the array layout. We then develop a Costas-array-based silicon OPA and experimentally demonstrate 2D beam steering with ~19,000 resolvable points using only 127 phase shifters. This corresponds to the largest number of resolvable points achieved by an OPA without sweeping the wavelength to the best of our knowledge.

physics.optics↗
arXiv2021

Scalable and Robust Photonic Integrated Unitary Converter

Optical unitary converter (OUC) that can convert a set of N mutually orthogonal optical modes into another set of arbitrary N orthogonal modes is expected to be the key device in diverse applications, including the optical communication, deep learning, and quantum computing. While various types of OUC have been demonstrated on photonic integration platforms, its sensitivity against a slight deviation in the waveguide dimension has been the crucial issue in scaling N. Here, we demonstrate that an OUC based on the concept of multi-plane light conversion (MPLC) shows outstanding robustness against waveguide deviations. Moreover, it becomes more and more insensitive to fabrication errors as we increase N, which is in clear contrast to the conventional OUC architecture, composed of 2 $\times$ 2 Mach-Zehnder interferometers. The physical origin behind this unique robustness and scalability is studied by considering a generalized OUC configuration. As a result, we reveal that the number of coupled modes in each stage plays an essential role in determining the sensitivity of the entire OUC. The maximal robustness is attained when all-to-all-coupled interferometers are employed, which are naturally implemented in MPLC-OUC.

physics.optics↗
arXiv2017

Electrically Pumped Orbital Angular Momentum (OAM) Laser at Telecom Wavelengths

Semiconductor lasers capable of generating a vortex beam with a specific orbital angular momentum (OAM) order are highly attractive for applications ranging from nanoparticle manipulation, imaging and microscopy to fibre and quantum communications. In this work, an electrically pumped OAM laser operating at telecom wavelengths is fabricated by monolithically integrating an optical vortex emitter with a distributed feedback (DFB) laser on the same InGaAsP/InP epitaxial wafer. A single-step dry etching process is adopted to complete the OAM emitter, equipped with specially designed top gratings. The vortex beam emitted by the integrated laser is captured, and its OAM mode purity characterized. The electrically pumped OAM laser eliminates the external laser required by silicon- or silicon-on-insulator (SOI)-based OAM emitters, thus demonstrating great potential for applications in communication systems and quantum domain.

physics.optics↗