Masayuki Fujita

High-resistivity single-crystal silicon (Si) is a low-loss and microfabrication-compatible platform for terahertz (THz) photonics, but its brittleness has largely limited Si THz components to mechanically rigid structures. Here, we demonstrate a structurally stretchable Si effective medium in which the optical properties of a Si/air dielectric composite are mechanically tuned by deforming the Si framework itself. A 200µm-thick high-resistivity Si slab was patterned by MEMS micromachining into a subwavelength spiral-spring through-hole lattice, converting bulk Si into a mechanically compliant, substrate-free photonic medium. In this architecture, the same monolithic Si microstructure provides both mechanical compliance and electromagnetic functionality: Uniaxial elongation changes the Si/air filling fraction and structural anisotropy, thereby tuning the effective permittivity and birefringence of the medium. THz time-domain spectroscopy verifies this concept by showing high transmission below 0.6 THz and a monotonic, polarization-dependent decrease in the retrieved effective refractive index; at 12.6% elongation, the effective index decreases by approximately 6%–8% without a detectable increase in extinction. These results establish structurally flexibilized bulk Si as a MEMS-compatible platform for mechanically tunable and structurally flexible all-dielectric THz effective media, providing a basis for future deformable THz photonic components after device-specific optical and mechanical optimization.

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 μm and quantitatively resolves polymer film thicknesses of 12.5, 25, and 50 μm.

All-dielectric effective-medium-clad waveguides have been widely exploited in terahertz communications owing to their extremely low loss, low dispersion, and broad bandwidth. Here, we propose a substrateless effective-medium-slot waveguide. In addition, we introduce a taper-free interface that allows terahertz waves to directly couple from a metallic hollow waveguide without requiring dielectric insertion. We achieve a 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 demonstrate an aggregated data rate of 0.8 Tbit/s with quadrature amplitude modulation schemes across 14 channels from 330 to 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.

The integration of terahertz (THz) sensing technology into compact, on-chip platforms is essential to the advancement of high-precision chemical and biomedical analysis, promising to bring analytics closer to the point of care and to enable in situ analysis of industrial processes. This study presents an integrated quasi-optical THz liquid sensor that features a longitudinal cavity in a silicon slab waveguide, in which a capillary-confined analyte interacts with guided slab modes on resonance. The sensor design leverages mode-parity-dependent field distributions: even-parity resonances exhibit strong analyte-field interaction, whilst odd-parity modes remain largely unaffected by the presence of the analyte, enabling intrinsic self-calibration. The device is fabricated using deep reactive ion etching of high-resistivity silicon and monolithically integrates all required components. Experimental measurements with water and isopropanol demonstrate alternating resonance peaks with distinct sensitivity to refractive index and absorption, validated by linear shifts in frequency and transmission loss. The self-calibrating feature allows for real-time compensation of system fluctuations towards automated continuous monitoring applications. These findings establish the sensor’s capability for simultaneous, precise material characterization and calibration, highlighting its potential for in-line process monitoring and other high-bandwidth sensing applications.

A silicon waveguide-based interconnection is proposed for terahertz on-chip communications in the 300-GHz band, featuring the integration of a uni-traveling-carrier photodiode (UTC-PD) as a transmitter and a resonant tunneling diode (RTD) as a receiver. The interconnection achieves low transmission loss over a broad bandwidth spanning the WR-2.8 band (260–390 GHz). Experimental results successfully demonstrate intermediate frequency (IF) transmission at a data rate of up to 100 Gbit/s using 32-QAM modulation, with the bit error rate remaining within the hard-decision forward-error correction (HD-FEC) limit. These achievements highlight the potential of the proposed interconnection scheme to advance high-performance, compact, and scalable terahertz integrated systems for nextgeneration communications applications.

To meet the growing need for higher data rates with improved energy efficiencies, this article demonstrates the application of resonant tunneling diodes (RTDs) for wireless communications at a frequency of approximately 860 GHz. By incorporating a low-loss air-bridge transmission line to form a cavity resonator and a ring-slot antenna, a two-RTD oscillator realized coherent terahertz radiation with a power output of 0.23 mW and a dc-to-RF efficiency of 0.2%. Experimental validation confirmed wireless data transmission for data rates up to 1.2 Gbit/s using on-off keying modulation with bit error rates below 10⁻⁹. To the best of our knowledge, this is the first reported demonstration of all-electronic oscillators for wireless communication beyond 850 GHz, paving the way for next-generation networks and advanced integrated circuits in the artificial intelligence-driven era.

Polarization‐division multiplexing is crucial for increasing channel capacity and spectral efficiency in communications. A key component is the polarization multiplexer, which combines or separates signals with orthogonal polarizations. Existing planar multiplexers lack an ultrawide band for terahertz communications. While microwave‐inspired orthomode transducers (OMTs) provide broadband operation, they suffer from high ohmic loss and bulkiness at terahertz frequencies. Optical integrated polarization beam splitters (PBSs) offer low loss and good integrability but have narrow bandwidths. This work presents a substrateless all‐silicon polarization multiplexer based on tapered directional couplers and air‐silicon effective mediums, integrated monolithically on a compact footprint. The device demonstrates a 37.8% fractional bandwidth, an average insertion loss of 1 dB, and a polarization extinction ratio above 20 dB over 225–330 GHz. This superior performance is enabled by the anisotropy of the effective medium claddings, affecting the two orthogonal guided modes differently. The multiplexer enables simultaneous two‐channel terahertz fiber communications with polarization diversity. It supports aggregated data rates up to 155 and 190 Gbit with bit error rates below hard‐ and soft‐decision forward‐error‐correction limits, respectively. This advancement holds promise for 6G terahertz integrated systems with enhanced channel capacities.

As a key component for next-generation wireless communications (6 G 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 and control of such ultrafast dynamic process is beyond the scope of electronics due to the limited bandwidth of the electronic equipment. 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 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 6 G and beyond.

Terahertz radars based on leaky-wave antennas are promising for the realization of radar systems with high resolution over short ranges. This type of radar relies on spatial frequency mapping to realize a wide field of view without mechanical actuation or electrical beam steering. Previously, integrated leaky-wave antennas based on metallic wave confinement have been implemented, but the high ohmic losses limit the realized antenna gain, which is essential for extending the range of such a system when limited power is available. Here, we demonstrate an all-dielectric leaky-wave antenna fabricated monolithically from silicon and then apply leaky-wave radar techniques to realize a terahertz radar system capable of real-time data acquisition. Through this all-dielectric approach, we can avoid metallic losses, achieving an experimentally measured maximum realized gain of 25 dBi and a 34.3° 3-dB field-of-view while utilizing established and scalable fabrication techniques essential for the wide-spread adoption of terahertz technologies. We foresee this technique being applied to a variety of real-time radar applications, and here we demonstrate two potential use-cases: multi-object tracking and differentiation of liquids.