Light in flight was captured by a single shot of a newly developed backside-illuminated multi-collection-gate image sensor at a frame interval of 10 ns without high-speed gating devices such as a streak camera or post data processes. This paper reports the achievement and further evolution of the image sensor toward the theoretical temporal resolution limit of 11.1 ps derived by the authors. The theoretical analysis revealed the conditions to minimize the temporal resolution. Simulations show that the image sensor designed following the specified conditions and fabricated by existing technology will achieve a frame interval of 50 ps. The sensor, 200 times faster than our latest sensor will innovate advanced analytical apparatuses using time-of-flight or lifetime measurements, such as imaging TOF-MS, FLIM, pulse neutron tomography, PET, LIDAR, and more, beyond these known 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.

The theoretical temporal resolution limit $t_T$ of a silicon photodiode (Si PD) is 11.1 ps. We call “super temporal resolution” the temporal resolution that is shorter than that limit. To achieve this resolution, Germanium is selected as a candidate material for the photodiode (Ge PD) for visible light since the absorption coefficient of Ge for the wavelength is several tens of times higher than that of Si, allowing a very thin PD. On the other hand, the saturation drift velocity of electrons in Ge is about 2/3 of that in Si. The ratio suggests an ultra-short propagation time of electrons in the Ge PD. However, the diffusion coefficient of electrons in Ge is four times higher than that of Si. Therefore, Monte Carlo simulations were applied to analyze the temporal resolution of the Ge PD. The estimated theoretical temporal resolution limit is 0.26 ps, while the practical limit is 1.41 ps. To achieve a super temporal resolution better than 11.1 ps, the driver circuit must operate at least 100 GHz. It is thus proposed to develop, at first, a short-wavelength infrared (SWIR) ultra-high-speed image sensor with a thicker and wider Ge PD, and then gradually decrease the size along with the progress of the driver circuits.

A burst image sensor named Hanabi, meaning fireworks in Japanese, includes a branching CCD and multiple CMOS readout circuits. The sensor is backside-illuminated with a light/charge guide pipe to minimize the temporal resolution by suppressing the horizontal motion of signal carriers. On the front side, the pixel has a guide gate at the center, branching to six first-branching gates, each bifurcating to second-branching gates, and finally connected to 12 ($=6\times 2$) floating diffusions. The signals are either read out after an image capture operation to replay 12 to 48 consecutive images, or continuously transferred to a memory chip stacked on the front side of the sensor chip and converted to digital signals. A CCD burst image sensor enables a noiseless signal transfer from a photodiode to the in-situ storage even at very high frame rates. However, the pixel count conflicts with the frame count due to the large pixel size for the relatively large in-pixel CCD memory elements. A CMOS burst image sensor can use small trench-type capacitors for memory elements, instead of CCD channels. However, the transfer noise from a floating diffusion to the memory element increases in proportion to the square root of the frame rate. The Hanabi chip overcomes the compromise between these pros and cons.

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.

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.