arXiv subjects

Shinichi Takagi

Publications and source records attributed to Shinichi Takagi.

17 recordsLinked to original sources

arXiv2026

Substrate-Voltage-Controlled Temporal Nonlinearity in Ferroelectric FET-based Reservoir Computing

Physical reservoir computing exploits inherent nonlinearity and short-term memory of physical dynamics to achieve efficient processing of time-series data with extremely-low training cost. In this study, we demonstrate a ferroelectric field-effect transistor (FeFET)-based reservoir computing system with augmented temporal and spatial nonlinearity by utilizing both gate and substrate terminals as inputs. The ferroelectric polarization state in the next time step can additionally be controlled by modifying the electric field distribution in the gate stack of FeFET through a substrate input, enabling more diverse internal states compared with the case where inputs are applied only to the gate. To introduce a nonlinearity in the time domain, we introduce a delay between a gate input and a substrate input, which facilitates efficient nonlinear mixing between the current and past inputs. As a result, both the short-term memory and nonlinearity of the FeFET reservoir computing system are enhanced with an improved capability of feature extraction of complex input time-series. These findings demonstrate that introducing substrate input provides an additional degree of freedom for controlling ferroelectric polarization dynamics, enabling a flexible, energy-efficient, and highly integrable FeFET-based reservoir computing platform suitable for diverse time-series processing applications.

physics.app-ph↗

arXiv2025

Low-loss Sb$_2$S$_3$ Optical Phase Shifter Enabled by Optimizing Sputtering Conditions

By quantitatively evaluating the atomic concentrations of sputtered Sb$_2$S$_3$ films with different sputtering powers and Ar flows, we reveal that a sputtered Sb$_2$S$_3$ film becomes close to the stoichiometric composition as the sputtering power and Ar flow decrease. We characterize the optical properties of Sb$_2$S$_3$ and show that the lower sputtering power leads to a better figure of merit of Sb$_2$S$_3$ as an optical phase shifter in the near infrared (NIR) range. Based on these results, we achieve a loss per phase shift of 0.33 dB/$π$ at a wavelength of 1.55 $μ$m, one of the lowest losses among Sb$_2$S$_3$ phase shifters in the NIR range.

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

Scalable intensity-based photonic matrix-vector multiplication processor using single-wavelength time-division-multiplexed signals

Photonic integrated circuits provide a compact platform for ultrafast and energy-efficient matrix-vector multiplications (MVMs) in the optical domain. Recently, schemes based on time-division multiplexing (TDM) have been proposed as scalable approaches for realizing large-scale photonic MVM processors. However, existing demonstrations rely on coherent detection or multiple wavelengths, both of which complicate their operations. In this work, we demonstrate a scalable TDM-based photonic MVM processor that uses only single-wavelength intensity-modulated optical signals, thereby avoiding coherent detection and enabling simplified operations. A 32-channel processor is fabricated on a Si-on-insulator (SOI) platform and used to experimentally perform convolution operations in a convolutional neural network (CNN) for handwritten digit recognition, achieving a classification accuracy of 93.47% for 1500 images.

physics.optics↗

arXiv2024

Waveguide-multiplexed photonic matrix-vector multiplication processor using multiport photodetectors

The slowing down of Moore's law has driven the development of application-specific processors for deep learning. Analog photonic processors offer a promising solution for accelerating matrix-vector multiplications (MVMs) in deep learning by leveraging parallel computations in the optical domain. Intensity-based photonic MVM processors, which do not utilize the phase information of light, are appealing due to their simplified operations. However, existing intensity-based schemes for such processors often employ wavelength multiplexing or mode multiplexing, both of which have limited scalability due to high insertion loss or wavelength crosstalk. In this work, we present a scalable intensity-based photonic MVM processor based on the concept of waveguide multiplexing. This scheme employs multiport photodetectors (PDs) to sum the intensities of multiple optical signals, eliminating the need for multiple wavelengths or modes. A 16-port Ge PD with a 3 dB bandwidth of 11.8 GHz at a bias voltage of -3 V is demonstrated, and it can be further scaled up to handle 250 ports while maintaining a 6.1 GHz operation bandwidth. A 4 $\times$ 4 circuit fabricated on a Si-on-insulator (SOI) platform is used to perform MVMs in a 3-layer neural network designed for classifying Iris flowers, achieving a classification accuracy of 93.3%. Furthermore, the performance of large-scale circuits in a convolutional neural network (CNN) for Fashion-MNIST is simulated, resulting in a classification accuracy of 90.53%. This work provides a simplified and scalable approach to photonic MVM, laying a foundation for large-scale and multi-dimensional photonic matrix-matrix multiplication in optical neural networks.

physics.optics↗

arXiv2024

Si-waveguide-based optical power monitoring of 2$\times$2 Mach$\unicode{x2013}$Zehnder interferometer based on InGaAsP/Si hybrid MOS optical phase shifter

Transparent in-line optical power monitoring in Si programmable photonic integrated circuits (PICs) is indispensable for calibrating integrated optical devices such as optical switches and resonators. A Si waveguide (WG) photodetector (PD) based on defect-mediated photodetection is a promising candidate for a transparent in-line optical power monitor owing to its simplicity and ease of integration with a fully complementary metal-oxide-semiconductor (CMOS)-compatible process. Here, we propose a simple optical power monitoring scheme for a 2$\times$2 Mach$\unicode{x2013}$Zehnder interferometer (MZI) optical switch based on InGaAsP/Si hybrid MOS optical phase shifters. In the proposed scheme, a low-doped p-type Si WG PD with a response time of microseconds is utilized as a transparent in-line optical power monitor and the ground terminal of the MOS optical phase shifter is shared with that of the Si WG PD to enable the simple monitoring of the output optical power of the MZI. Based on this scheme, we experimentally demonstrate that the output optical power of a 2$\times$2 MZI can be simply monitored by applying a bias voltage to the Si slabs formed at the output WGs of the MZI without excess optical insertion loss.

physics.optics↗

arXiv2024

Symmetric silicon microring resonator optical crossbar array for accelerated inference and training in deep learning

Photonic integrated circuits are emerging as a promising platform for accelerating matrix multiplications in deep learning, leveraging the inherent parallel nature of light. Although various schemes have been proposed and demonstrated to realize such photonic matrix accelerators, the in-situ training of artificial neural networks using photonic accelerators remains challenging due to the difficulty of direct on-chip backpropagation on a photonic chip. In this work, we propose a silicon microring resonator (MRR) optical crossbar array with a symmetric structure that allows for simple on-chip backpropagation, potentially enabling the acceleration of both the inference and training phases of deep learning. We demonstrate a $4 \times 4$ circuit on a Si-on-insulator (SOI) platform and use it to perform inference tasks of a simple neural network for classifying Iris flowers, achieving a classification accuracy of 93.3%. Subsequently, we train the neural network using simulated on-chip backpropagation and achieve an accuracy of 91.1% in the same inference task after training. Furthermore, we simulate a convolutional neural network (CNN) for handwritten digit recognition, using a $9 \times 9$ MRR crossbar array to perform the convolution operations. This work contributes to the realization of compact and energy-efficient photonic accelerators for deep learning.

cs.ET↗

arXiv2023

Non-volatile hybrid optical phase shifter driven by a ferroelectric transistor

Optical phase shifters are essential elements in photonic integrated circuits (PICs) and function as a direct interface to program the PIC. Non-volatile phase shifters, which can retain information without a power supply, are highly desirable for low-power static operations. Here a non-volatile optical phase shifter is demonstrated by driving a III-V/Si hybrid metal-oxide-semiconductor (MOS) phase shifter with a ferroelectric field-effect transistor (FeFET) operating in the source follower mode. Owing to the various polarization states in the FeFET, multistate non-volatile phase shifts up to 1.25π are obtained with CMOS-compatible operation voltages and low switching energy up to 3.3 nJ. Furthermore, a crossbar array architecture is proposed to simplify the control of non-volatile phase shifters in large-scale PICs and its feasibility is verified by confirming the selective write-in operation of a targeted FeFET with a negligible disturbance to the others. This work paves the way for realizing large-scale non-volatile programmable PICs for emerging computing applications such as deep learning and quantum computing.

physics.app-ph↗

arXiv2023

Non-volatile optical phase shift in ferroelectric hafnium zirconium oxide

A non-volatile optical phase shifter is a critical component for enabling large-scale, energy-efficient programmable photonic integrated circuits (PICs) on a silicon (Si) photonics platform. While ferroelectric materials like BaTiO3 offer non-volatile optical phase shift capabilities, their compatibility with complementary metal-oxide-semiconductor (CMOS) fabs is limited. Hence, the search for a novel CMOS-compatible ferroelectric material for non-volatile optical phase shifting in Si photonics is of utmost importance. Hafnium zirconium oxide (HZO) is an emerging ferroelectric material discovered in 2011, which exhibits CMOS compatibility due to the utilization of high-k dielectric HfO2 in CMOS transistors. Although extensively studied for ferroelectric transistors and memories, its application in photonics remains relatively unexplored. Here, we show the optical phase shift induced by ferroelectric HZO deposited on a SiN optical waveguide. We observed a negative change in refractive index at a 1.55 um wavelength in the pristine device regardless of the direction of an applied electric filed. We achieved approximately pi phase shift in a 4.5-mm-long device with negligible optical loss. The non-volatile multi-level optical phase shift was confirmed with a persistence of > 10000 s. This phase shift can be attributed to the spontaneous polarization within the HZO film along the external electric field. We anticipate that our results will stimulate further research on optical nonlinear effects, such as the Pockels effect, in ferroelectric HZO. This advancement will enable the development of various devices, including high-speed optical modulators. Consequently, HZO-based programmable PICs are poised to become indispensable in diverse applications, ranging from optical fiber communication and artificial intelligence to quantum computing and sensing.

physics.optics↗

arXiv2023

Breakdown-limited endurance in HZO FeFETs: mechanism and improvement under bipolar stress

Breakdown is one of main failure mechanisms that limit write endurance of ferroelectric devices using hafnium oxide-based ferroelectric materials. In this study, we investigate the gate current and breakdown characteristics of Hf0.5Zr0.5O2/Si ferroelectric field-effect transistors (FeFETs) by using carrier separation measurements to analyze electron and hole leakage currents during time-dependent dielectric breakdown (TDDB) tests. Rapidly increasing substrate hole currents and stress-induced leakage current (SILC)-like electron currents can be observed before the breakdown of the ferroelectric gate insulator of FeFETs. This apparent degradation under voltage stress is recovered and the time-to-breakdown is significantly improved by interrupting the TDDB test with gate voltage pulses with the opposite polarity, suggesting that defect redistribution, rather than defect generation, is responsible for the trigger of hard breakdown.

cond-mat.mtrl-sci↗

arXiv2022

Memory Window in Ferroelectric Field-Effect Transistors: Analytical Approach

A memory window of ferroelectric field-effect transistors (FeFETs), defined as a separation of the HIGH-state and the LOW-state threshold voltages, is an important measure of the FeFET memory characteristics. In this study, we theoretically investigate the relation between the FeFET memory window and the P-E hysteresis loop of the ferroelectric gate insulator, and derive a compact model explicitly described by material parameters. It is found that the memory window is linearly proportional to the ferroelectric polarization for the small polarization regime, and converges to the limit value of 2 x coercive field x thickness when the remanent polarization is much larger than permittivity x coercive field. We discuss additional factors that possibly influence the memory window in actual devices such as the existence of interlayer (no direct impact), interface charges (invalidity of linear superposition between the ferroelectric and charge-trapping hysteresis), and minor-loop operation (behavior equivalent to the generation of interface charges).

physics.app-ph↗

arXiv2021

Ultrahigh-sensitivity optical power monitor for Si photonic circuits

A phototransistor is a promising candidate as an optical power monitor in Si photonic circuits since the internal gain of photocurrent enables high sensitivity. However, state-of-the-art waveguide-coupled phototransistors suffer from a responsivity of lower than $10^3$ A/W, which is insufficient for detecting very low power light. Here, we present a waveguide-coupled phototransistor consisting of an InGaAs ultrathin channel on a Si waveguide working as a gate electrode to increase the responsivity. The Si waveguide gate underneath the InGaAs ultrathin channel enables the effective control of transistor current without optical absorption by the gate metal. As a result, our phototransistor achieved the highest responsivity of approximately $10^6$ A/W among the waveguide-coupled phototransistors, allowing us to detect light of 621 fW propagating in the Si waveguide. The high responsivity and the reasonable response time of approximately 100 $μ$s make our phototransistor promising as an effective optical power monitor in Si photonics circuits.

physics.app-ph↗

arXiv2021

Si microring resonator crossbar array for on-chip inference and training of optical neural network

Deep learning is one of the most advancing technologies in various fields. Facing the limits of the current electronics platform, optical neural networks (ONNs) based on Si programmable photonic integrated circuits (PICs) have attracted considerable attention as a novel deep learning scheme with optical-domain matrix-vector multiplication (MVM). However, most of the proposed Si programmable PICs for ONNs have several drawbacks such as low scalability, high power consumption, and lack of frameworks for training. To address these issues, we have proposed a microring resonator (MRR) crossbar array as a Si programmable PIC for an ONN. In this article, we present a prototype of a fully integrated 4 ${\rm \times}$ 4 MRR crossbar array and demonstrated a simple MVM and classification task. Moreover, we propose on-chip backpropagation using the transpose matrix operation of the MRR crossbar array, enabling the on-chip training of the ONN. The proposed ONN scheme can establish a scalable, power-efficient deep learning accelerator for applications in both inference and training tasks.

cs.ET↗

arXiv2021

Symmetry-preserving enforcement of low-dissipation method based on boundary variation diminishing principle

A class of high-order shock-capturing schemes, P$_n$T$_m$-BVD (Deng et al., J. Comp. Phys., 386:323-349, 2019; Comput. & Fluids, 200:104433, 2020.) schemes, have been devised to solve the Euler equations with substantially reduced numerical dissipation, which enable high-resolution simulations to resolve flow structures of wider range scales. In such simulations with low dissipation, errors of round-off level might grow and contaminate the numerical solutions. A typical example of such problems is the loss of symmetry in the numerical solutions for physical problems of symmetric configurations even if the schemes are mathematically in line with the symmetry rules. In this study, the mechanisms of symmetry-breaking in a finite volume framework with the P$_4$T$_2$-BVD reconstruction scheme are thoroughly examined. Particular attention has been paid to remove the possible causes due to the lack of associativity in floating-point arithmetic which is associated with round-off errors. Modifications and new techniques are proposed to completely remove the possible causes for symmetry breaking in different components of the P$_4$T$_2$-BVD finite volume solver. Benchmark tests that have symmetric solution structures are used to verify the proposed methods. The numerical results demonstrate the perfect symmetric solution structures.

math.NA↗

arXiv2020

Evaluation of polarization characteristics in metal/ferroelectric/semiconductor capacitors and ferroelectric field-effect transistors

In this study, we propose a measurement technique for evaluating ferroelectric polarization characteristics in ferroelectric field-effect transistors (FeFETs). Different from standard metal/ferroelectric/metal capacitors, the depletion and inversion phenomena in semiconductor substrates have to be carefully taken into account when evaluating the ferroelectric properties using fast voltage sweep as input. The non-equilibrium deep depletion is found to be the limiting factor for the accurate evaluation of ferroelectric properties in metal/ferroelectric/semiconductor capacitors. By connecting the source, drain, and substrate of the FeFET together during the polarization measurement, the deep depletion can be suppressed and the ferroelectricity of the ferroelectric gate can be accurately evaluated. The present technique is a powerful method for capturing the polarization states in FeFETs, enabling new approaches for device characterization and fundamental study, and overcomes the limitation found in the conventional polarization measurement on 2-terminal metal/ferroelectric/semiconductor capacitors.

physics.app-ph↗

arXiv2017

High-modulation-efficiency InGaAsP/Si hybrid MOS optical modulator with Mach-Zehnder interferometer

A high-modulation-efficiency optical modulator integrated on silicon (Si) is a key enabler for low-power and high-capacity optical interconnects. However, Si-based optical modulators suffer from low phase modulation efficiency owing to the weak plasma dispersion effect in Si. Therefore, it is essential to find a novel modulation scheme that is compatible with a Si photonics platform. Here, we demonstrate an InGaAsP/Si hybrid metal-oxide-semiconductor (MOS) optical modulator with a Mach-Zehnder interferometer (MZI) formed by direct wafer bonding with an Al2O3 bonding interface. Electron accumulation at the InGaAsP MOS interface enables the extraction of the electron-induced refractive index change in InGaAsP, which is significantly greater than that in Si. The presented modulator exhibits a phase modulation efficiency of 0.047 Vcm, which is approximately 5 times higher than that of Si MOS optical modulators. This approach provides a new efficient scheme of phase modulation on a Si photonics platform for low-power, high-speed, and high-density optical links.

physics.ins-det↗

arXiv2013

Strain-induced enhancement of plasma dispersion effect and free-carrier absorption in SiGe optical modulators

The plasma dispersion effect and free-carrier absorption are widely used for changing refractive index and absorption coefficient in Si-based optical modulators. However, these free-carrier effects in Si are not large enough for making the footprint of the Si modulators small. Here, we have theoretically and experimentally investigated the enhancement of the plasma dispersion effect and free-carrier absorption by strain-induced mass modulation in silicon-germanium (SiGe). The application of compressive strain to SiGe reduces the conductivity hole mass, resulting in the enhanced free-carrier effects. Thus, the strained SiGe-based optical modulator exhibits more than twice modulation efficiency as large as that of the Si modulator. To the best of our knowledge, it is the first demonstration of the enhanced free-carrier effects in strained SiGe at the near-infrared telecommunication wavelength. The strain-induced enhancement technology for the free-carrier effects is expected to boost the modulation efficiency of the most Si-based optical modulators thanks to high complementary metal-oxide-semiconductor (CMOS) compatibility.

physics.optics↗