This paper proposes an alive monitoring system for small self-powered sensors that uses 2.45-GHz microwave power transfer, where a second harmonic generated by a receiver is directly radiated through an antenna and monitored by the transmitter for location detection at ultra-low energy. The location is detected by intermittently transmitting and receiving the harmonic in the transmitter and then executing alive monitoring with continuous transmission to reduce the transmitting energy. The key technology of our alive monitoring system is the slim dual-band antenna configuration, which includes a 2.45-GHz sleeve receiving and 4.9-GHz dipole transmitting single-feed antennae for the receiver, enabling omnidirectional power reception and transmission. The configuration also includes 2.45-GHz transmitting and 4.9-GHz receiving Yagi-Uda antennae for the transmitter, which makes it feasible to improve the harmonic-reception sensitivity by arranging the polarization vertically and shortening the distance between the antennae. To demonstrate the effectiveness of this system, we prototyped our system for 2.45-GHz microwave power transfer targeting self-powered water-leakage sensors. The system was evaluated under simulated test conditions in which the transmitter performed intermittent operation with a duty ratio of 10%. The test results confirmed that the energy consumption could be reduced by 62.5% compared to that of continuous transmission alone.

Terahertz (THz) spectroscopy offers a non-contact analysis of biological samples; however, it faces challenges due to water absorption within the THz range. To address this issue, a THz metamaterial sensor was developed for high-sensitivity moisture measurements. The sensor is composed of a subwavelength metal layer, porous insulator, and metal layer, with its principle relying on resonant frequency shifts derived from changes in capacitance with and without the target gas within the porosity. The sensor experimentally demonstrated a significant frequency shift in response to humidity compared to the simulation results, indicating enhanced sensitivity because of the porous structure. This novel approach has the potential for practical analysis using THz waves.

A bipolar driving technique is presented to improve the displacement accuracy of an electrostatic microactuator at the submicron scale. A lateral comb-type electrostatic actuator with a full stroke of 34 µm is fabricated. Owing to the charging inside the device, the displacement error reaches a maximum of 1 µm when a DC driving voltage is applied. The displacement error reduces when a 1 Hz bipolar square wave driving voltage with the same magnitude is applied instead of a monopolar DC signal. The maximum fluctuation is reduced to 0.25 µm. The proposed driving technique, which is simple and does not require additional components or control systems, is effective for controlling the gap in nanophotonics.