arXiv subjects

Masayuki Fujita

Publications and source records attributed to Masayuki Fujita.

At least 19 recordsLinked to original sources

arXiv2026

Micrometer-scale displacement and thickness sensing using a single terahertz resonant-tunneling diode

Resonant tunneling diodes (RTDs) support room-temperature terahertz (THz) oscillation and simultaneous THz-band detection, enabling compact monostatic THz sensors for practical and cost-effective sensing applications. In this paper, we present a highly integrated 280 GHz-band radar system based on a single RTD that exploits the self-mixing effect to generate a low-frequency interferometric signal. The resulting self-mixing signal is further analyzed from a radar perspective and processed to extract micrometer-scale displacement and thin-film thickness variations. Experimentally, the proposed system demonstrates a minimum detectable displacement of approximately 5 um and quantitatively resolves polymer film thicknesses of 12.5, 25, and 50 um.

physics.optics↗

arXiv2026

Terahertz Communications Using Effective-Medium-Slot Waveguides

All-dielectric effective-medium-clad waveguides have been widely exploited in terahertz communications owing to their extremely low loss, low dispersion, and broad bandwidth. In this work, we propose a substrateless effective-medium-slot waveguide. Additionally, we introduce a taper-free interface that allows terahertz waves to directly couple from a metallic hollow waveguide without requiring dielectric insertion. By engineering slot couplers with an effectivemedium channel for impedance and modal matching, the waveguide achieves a fractional 3-dB bandwidth of 40% with a maximum coupling efficiency of 90% in the WR-2.2 band (330-500 GHz). By employing a broadband uni-traveling-carrier photodiode transmitter and sub-harmonic mixer receivers, we achieve an aggregated data rate of 0.8 Tbit/s with quadrature amplitude modulation schemes across 14 channels from 330-600 GHz. The effective-medium-slot waveguide platform yields robust broadband coupling with enhanced mechanical protection, offering reliable interconnects for ultra-high-speed terahertz integrated systems.

physics.optics↗

arXiv2026

Resonant tunneling diode-integrated terahertz transceiver module for wireless communications

Terahertz bands enable ultra-broadband wireless communications but require compact, low-cost, and efficient transceiver modules. Conventional implementations based on metallic waveguides or silicon lenses suffer from high loss, bulkiness, and fabrication complexity. Here, we present a compact terahertz transceiver module enabled by a resonant tunneling diode (RTD) integrated with a photonic-electronic antenna chain. The RTD on InP is coupled to a modified Vivaldi antenna and an all-silicon effective-medium-clad waveguide, terminating in a rod antenna interfaced with a 3D-printed cyclic olefin copolymer lens. This architecture enables broadband directive radiation without matching networks or anti-reflection coatings. Packaged in a low-cost 3D-printed PLA enclosure, the module achieves realized gains of 28-33 dBi (E11x) and 30-33 dBi (E11y) across 220-330 GHz. As a receiver, it exhibits a noise voltage density of 5.6 x 10^-9 V/sqrt(Hz), a minimum noise equivalent power of 1.8 pW/sqrt(Hz), and an average responsivity of 6.8 kV/W. It supports error-free transmission up to 30 Gbit/s (OOK) and 80 Gbit/s (16-QAM) over 10 cm, and enables real-time uncompressed high-definition video streaming over 1 m. As a transmitter, it achieves error-free OOK transmission up to 12 Gbit/s at 332 GHz. These results demonstrate a promising terahertz transceiver architecture for 6G systems.

physics.optics↗

arXiv2026

Refractive Index Tuning of Terahertz Photonic Materials Based on a Stretchable Silicon Effective Medium

Dynamically tunable terahertz (THz) photonics requires low-loss dielectric platforms with practical, continuous control of refractive index. Here we present a mechanically reconfigurable THz photonic material platform: a monolithic, all-silicon (Si) stretchable effective medium whose refractive index is tuned by deformation. A 200 micrometer-thick high-resistivity single-crystal Si slab was patterned into a subwavelength spiral-spring through-hole lattice, rendering bulk Si mechanically compliant while preserving its low-loss dielectric response. THz time-domain spectroscopy demonstrates high transmission below 0.6 THz and reveals a monotonic decrease in the effective refractive index under uniaxial stretching. At 12.6% elongation, the effective index decreases by 6% and 8% for polarizations perpendicular and parallel to the stretch direction, respectively, thereby demonstrating deformation-induced, controllable anisotropy without a detectable increase in extinction. This structurally engineered bulk-Si approach offers a process-compatible route to mechanically tunable, low-loss THz components for adaptive wavefront and polarization control.

physics.optics↗

arXiv2026

Advances in All-Silicon Waveguides for Terahertz Integration

In chip-to-chip communication, terahertz waves provide a promising approach to achieve high data capacity with improved energy efficiency, effectively bridging the gap between electrical and optical domains. By realizing this potential, all-dielectric waveguides have emerged as high-speed interconnects, offering broad bandwidth and low-loss characteristics while being compatible with planar integration. This paper reviews different types of terahertz all-dielectric waveguides, including photonic-crystal and effective-medium-clad structures, and highlights recent progress in implementing these platforms to develop various passive functional devices, paving the way for high-speed operation and large-scale integration in terahertz systems.

physics.app-ph↗

arXiv2024

Distributed Coverage Hole Prevention for Visual Environmental Monitoring with Quadcopters via Nonsmooth Control Barrier Functions

This paper proposes a distributed coverage control strategy for quadcopters equipped with downward-facing cameras that prevents the appearance of unmonitored areas in between the quadcopters' fields of view (FOVs). We derive a necessary and sufficient condition for eliminating any unsurveilled area that may arise in between the FOVs among a trio of quadcopters by utilizing a power diagram, i.e. a weighted Voronoi diagram defined by radii of FOVs. Because this condition can be described as logically combined constraints, we leverage nonsmooth control barrier functions (NCBFs) to prevent the appearance of unmonitored areas among a team's FOV. We then investigate the symmetric properties of the proposed NCBFs to develop a distributed algorithm. The proposed algorithm can support the switching of the NCBFs caused by changes of the quadcopters composing trios. The existence of the control input satisfying NCBF conditions is analyzed by employing the characteristics of the power diagram. The proposed framework is synthesized with a coverage control law that maximizes the monitoring quality while reducing overlaps of FOVs. The proposed method is demonstrated in simulation and experiment.

eess.SY↗

arXiv2022

Phase-resolved measurement and control of ultrafast dynamics in terahertz electronic oscillators

As a key component for next-generation wireless communications (6G and beyond), terahertz (THz) electronic oscillators are being actively developed. Precise and dynamic phase control of ultrafast THz waveforms is essential for high-speed beam steering and high-capacity data transmission. However, measurement of such ultrafast dynamic process is beyond the scope of electronics due to the limited bandwidth of the electrical measuring instruments. Here we surpass this limit by applying photonic technology. Using a femtosecond laser, we generate offset-free THz pulses to phase-lock the electronic oscillators based on a resonant tunneling diode. This enables us to perform phase-resolved measurement of the emitted THz electric field waveform in time-domain with sub-cycle time resolution. Ultrafast dynamic response such as anti-phase locking behaviour is observed, which is distinct from in-phase stimulated emission observed in laser oscillators. We also show that the dynamics follows the universal synchronization theory for limit cycle oscillators. This provides a basic guideline for dynamic phase control of THz electronic oscillators, enabling many key performance indicators to be achieved in the new era of 6G and beyond.

physics.optics↗

arXiv2022

Visual Pursuit Control based on Gaussian Processes with Switched Motion Trajectories

This paper considers a scenario of pursuing a moving target that may switch behaviors due to external factors in a dynamic environment by motion estimation using visual sensors. First, we present an improved Visual Motion Observer with switched Gaussian Process models for an extended class of target motion profiles. We then propose a pursuit control law with an online method to estimate the switching behavior of the target by the GP model uncertainty. Next, we prove ultimate boundedness of the control and estimation errors for the switch in target behavior with high probability. Finally, a Digital Twin simulation demonstrates the effectiveness of the proposed switching estimation and control law to prove applicability to real world scenarios.

eess.SY↗

arXiv2022

Terahertz Oscillator Chips Backside-coupled to Unclad Microphotonics

Terahertz technology is largely dependent upon planar on-chip antennas that radiate downwards through the substrate, and so an effective means to interface these antennas with integrated waveguides is an attractive prospect. We present a viable methodology for backside coupling between a terahertz oscillator chip and a broadband all-intrinsic-silicon dielectric waveguide. Our investigation employs resonant tunneling diodes as compact two-terminal electronic terahertz oscillators, and terahertz waves are observed from 270 GHz to 409 GHz. The fact that this power is accessed via a curved length of silicon waveguide validates successful backside coupling.

physics.optics↗

arXiv2022

Distributed Gaussian Process Based Cooperative Visual Pursuit Control for Drone Networks

In this paper, we propose a control law for camera-equipped drone networks to pursue a target rigid body with unknown motion based on distributed Gaussian process. First, we consider the situation where each drone has its own dataset, and learned the unknown target motion in a distributed manner. Second, we propose a control law using the distributed Gaussian processes, and show that the estimation and control errors are ultimately bounded. Furthermore, the effectiveness of the proposed method is verified by using simulations and experiments with actual drones.

eess.SY↗

arXiv2021

Steering of beam trajectory by distorted photonic crystals

Electromagnetic waves follow linear paths in homogenous index media, with the exception of band edges. In this study, we introduced spatially distorted photonic crystals (D-PCs) that are capable of beam-steering light waves, even when a homogeneous refractive index is maintained. We analyzed their equifrequency contours to investigate the correspondence between the direction of distortion and the direction of the group velocity vector in the D-PC. Thereafter, we experimentally verified the beam-steering phenomenon in the terahertz range using an all-silicon D-PC. Our structures serve as on-chip beam trajectory control without the need for any specially-engineered materials, using only lattice distortion.

physics.app-ph↗

arXiv2021

Dielectric slot-coupled half-Maxwell fisheye lens as octave-bandwidth beam expander for terahertz-range applications

We present a paradigm for integrated photonic devices based upon broadband slab-confined collimated beams that are launched with half-Maxwell fisheye lenses. Although it is challenging to match to the low-index focus of the lens whilst maintaining adequate field confinement for a well-defined point source, integrated dielectric slot waveguides prove highly suitable, yielding collimators of 90% efficiency and over one octave bandwidth. Terahertz technology will benefit from such broadband slab-confined beams to replace free-space optics, toward compact, mass-producible systems that do not require manual optical alignment. We present two prototype systems to demonstrate the versatility of this concept, namely a diagonally-set distributed Bragg reflector as frequency-division diplexer for terahertz communications, and an attenuated total reflection-based liquid sensor. Both are enabled by oblique in-slab reflections that are collected at a location other than the originating lens, which is not attainable using ordinary single-mode channel waveguides.

physics.optics↗

arXiv2019

Effective-medium-cladded dielectric waveguides for terahertz communications

Terahertz communications is a promising modality for future short-range point-to point wireless data transmission at rates up to terabit per second. A milestone towards this goal is the development of an integrated transmitter and receiver platforms with high efficiency. One key enabling component is a planar waveguiding structure with wide bandwidth and low dispersion. This work proposes substrate-less all-dielectric waveguides cladded by an effective medium for low-loss and low dispersion terahertz transmission in broadband. This self-supporting structure is built solely into a single silicon wafer with air perforations to mitigate significant absorptions in metals and dielectrics at terahertz frequencies. The realized waveguides can cover the entire 260 to 400 GHz with single dominant modes in both orthogonal polarizations. The simulation shows that for the E_11^x mode the attenuation ranges from 0.003 to 0.024 dB/cm over the entire band, while it varies from 0.008 to 0.023 dB/cm for the E_11^y mode. Limited by the measurement setup, the maximum error-free data rate of 28 Gbit/s is experimentally achieved at 335 GHz on a 3-cm waveguide. We further demonstrate the transmission of uncompressed 4K-resolution video across this waveguide. This waveguide platform promises integration of diverse active and passive components. Thus, we can foresee it as a potential candidate for the future terahertz integrated circuits, in analogy to photonic integrated circuits at optical frequencies.

physics.optics↗

arXiv2019

Terahertz topological photonics for on-chip communication

The computing speeds in modern multi-core processors and big data servers are no longer limited by the on-chip transistor density that doubles every two years following the Moores law, but are limited by the on-chip data communication between memories and microprocessor cores. Realization of integrated, low-cost, and efficient solutions for high speed, on-chip data communications require terahertz (THz) interconnect waveguides with tremendous significance in future THz technology including THz-wave integrated circuits and THz communication. However, conventional approaches to THz waveguiding suffer from sensitivity to defects and considerable bending losses at sharp bends. Here, building on the recently-discovered topological phase of light, we experimentally demonstrate robust THz topological valley transport on low-loss, all-silicon chips. We show that the valley polarized topological kink states exhibit unity transmission over a bulk band gap even after propagating through ten sharp corners. Such states are excellent information carriers due to their robustness, single-mode propagation, and linear dispersion-key properties for next generation THz communications. By leveraging the unique properties of kink states, we demonstrate error-free communication through a highly-twisted domain wall at an unprecedented data rate (10 Gbit/s) and uncompressed 4K high-definition video transmission. Our work provides the first experimental demonstration of the topological phases of THz wave, which could certainly inspire a plethora of research on different types of topological phases in two and three dimensions.

physics.app-ph↗

arXiv2018

Passivity-Based Generalization of Primal-Dual Dynamics for Non-Strictly Convex Cost Functions

In this paper, we revisit primal-dual dynamics for convex optimization and present a generalization of the dynamics based on the concept of passivity. It is then proved that supplying a stable zero to one of the integrators in the dynamics allows one to eliminate the assumption of strict convexity on the cost function based on the passivity paradigm together with the invariance principle for Caratheodory systems. We then show that the present algorithm is also a generalization of existing augmented Lagrangian-based primal-dual dynamics, and discuss the benefit of the present generalization in terms of noise reduction and convergence speed.

eess.SY↗

arXiv2017

A Passivity-Based Distributed Reference Governor for Constrained Robotic Networks

This paper focuses on a passivity-based distributed reference governor (RG) applied to a pre-stabilized mobile robotic network. The novelty of this paper lies in the method used to solve the RG problem, where a passivity-based distributed optimization scheme is proposed. In particular, the gradient descent method minimizes the global objective function while the dual ascent method maximizes the Hamiltonian. To make the agents converge to the agreed optimal solution, a proportional-integral consensus estimator is used. This paper proves the convergence of the state estimates of the RG to the optimal solution through passivity arguments, considering the physical system static. Then, the effectiveness of the scheme considering the dynamics of the physical system is demonstrated through simulations and experiments.

cs.MA↗

arXiv2013

3-D Visual Coverage Based on Gradient Descent Techniques on Matrix Manifold and Its Application to Moving Objects Monitoring

This paper investigates coverage control for visual sensor networks based on gradient descent techniques on matrix manifolds. We consider the scenario that networked vision sensors with controllable orientations are distributed over 3-D space to monitor 2-D environment. Then, the decision variable must be constrained on the Lie group SO(3). The contribution of this paper is two folds. The first one is technical, namely we formulate the coverage problem as an optimization problem on SO(3) without introducing local parameterization like Eular angles and directly apply the gradient descent algorithm on the manifold. The second technological contribution is to present not only the coverage control scheme but also the density estimation process including image processing and curve fitting while exemplifying its effectiveness through simulation of moving objects monitoring.

eess.SY↗

arXiv2013

Cooperative Environmental Monitoring for PTZ Visual Sensor Networks: A Payoff-based Learning Approach

This paper investigates cooperative environmental monitoring for Pan-Tilt-Zoom (PTZ) visual sensor networks. We first present a novel formulation of the optimal environmental monitoring problem, whose objective function is intertwined with the uncertain state of the environment. In addition, due to the large volume of vision data, it is desired for each sensor to execute processing through local computation and communication. To address the issues, we present a distributed solution to the problem based on game theoretic cooperative control and payoff-based learning. At the first stage, a utility function is designed so that the resulting game constitutes a potential game with potential function equal to the group objective function, where the designed utility is shown to be computable through local image processing and communication. Then, we present a payoff-based learning algorithm so that the sensors are led to the global objective function maximizers without using any prior information on the environmental state. Finally, we run experiments to demonstrate the effectiveness of the present approach.

eess.SY↗