The generation, detection, and applications of terahertz waves (electromagnetic waves whose frequency ranges from about 0.1 THz ~ 10 THz) have been attracting great interest for about 20 years. Unfortunately, the generation and detection of terahertz waves are complicated because this band is located on the border between electronics and photonics. However, recent advancements, particularly in devices that emit continuous or quasi-continuous terahertz waves and in unique applications of time domain spectroscopy are opening up real-world applications. In this preface, we briefly summarize the related history and give an overview of this special issue.

View full abstract

Since terahertz (THz) spectra are sensitive to crystal structures, they are available for crystal polymorph identification and quantification. They can also be utilized to detect defects in crystal, such as impurity molecules. Thus, THz spectroscopy provide high sensitivity and a low detection limit for pharmaceutical inspection. In this paper, we first present our originally developed high-frequency accurate CW Gallium Phosphide (GaP) THz spectrometer and then demonstrate trace impurity detection in pharmaceuticals. Such high-frequency accurate spectroscopy has potential for the inspection of middle molecular weight (MMW) pharmaceuticals, which are expected to become mainstream in the near future.

View full abstract

We demonstrated backward terahertz-wave parametric oscillation and used optical injection for the forward- propagating idler to increase the output power, reduce the threshold, and stabilize the oscillation. We generated backward-propagating terahertz-wave pulses with a high peak intensity of about 200 W at 0.3 THz. Terahertz-wave imaging was performed using our developed backward terahertz-wave parametric oscillator to confirm its suitability for nondestructive testing applications. Due to the long-time stability of terahertz-wave oscillation, transmission images with good visibility and inside information were obtained by point scanning imaging. As a result of measuring various materials, we found that the system is applicable to versatile nondestructive inspection applications.

View full abstract

A terahertz difference-frequency generation quantum cascade laser is a compact and ultra-broadband semiconductor source. An electrically pumped monolithic source is based on intra-cavity, difference-frequency generation and can be designed to achieve an ultra-broadband, multi-mode terahertz emission spectrum that extends from 1 to 4 THz without any external optical setup. We demonstrated spectroscopic imaging using a THz source. We performed spectroscopic imaging with frequency bandpass filters, and with this imaging technique, we clearly identified three different tablet components: polyethylene, D-histidine, and DL-histidine. This spectroscopic imaging technique may be suitable for the quality monitoring of pharmaceutical materials.

View full abstract

We report on a recent progress on resonant-tunneling-diode (RTD) terahertz oscillators and its radar applications including three-dimensional imaging. A fundamental oscillation up to 1.98 THz was obtained by a slot antenna with small conduction loss and a high-power operation of 0.7 mW at 1 THz was achieved by a large-scale oscillator array. For higher frequency and higher output power, structures with cavity resonators were proposed. For good uniformity and reduction in fabrication process, structure- simplified devices were developed. Using the high frequency modulation characteristics of RTD oscillators, we proposed and developed ranging systems using the RTD oscillators with the amplitude- modulated continuous-wave (AMCW)，the frequency-modulated continuous-wave (FMCW)，and the discrete Fourier transform (DFT) radar principles. These radar system shows fine ranging errors of below 1 mm by optimization in modulation setup and signal processing.

View full abstract

Terahertz waves are expected to be used in future 6G/Beyond 5G communication technology. A terahertz wave source is a key component for achieving a practical communication system. Photomixing is one promising method for terahertz-wave generation. Utilizing photonics technology and photomixing, we made an arrayed-terahertz source and successfully combined their power. We also devised an optical- phased array and demonstrated terahertz beam steering in combination with the arrayed-terahertz source.

View full abstract

When an optical frequency comb is incident on a photoconductive antenna (PCA) for terahertz (THz) detection, a THz comb of a photocarrier (PC-THz comb) is generated inside the PCA. This is equivalent to incorporating the function of a THz frequency ruler into the PCA, and can be used for various applications. In this review, we introduce the application of a PC-THz comb for the absolute frequency measurement of CW-THz waves and distance measurement with its phase slope.

View full abstract

We demonstrate that terahertz spectroscopy, based on ultrashort optical pulse technology, is a powerful method for evaluating the solid electrolytes of fuel cells and all-solid-state batteries. The results of terahertz time-domain spectroscopy include information on fast ion motion in solids, which cannot be accessed with conventional experimental techniques.

View full abstract