On the electromagnetic spectrum, the terahertz (THz) frequency range lies between radio waves and infrared waves and shares the characteristics of both. Like radio waves, THz-waves can pass through a wide variety of materials, including plastics, ceramics, and papers. THz-waves can also be guided using mirrors and lenses, like infrared waves. Additionally, a large number of materials have unique absorption characteristics, e.g., a fingerprint spectrum, at THz frequencies.
We are working on the development of high power THz wave sources to realize many applications using such unique characteristics as shown below.
Development of high-power THz wave sources
injection-seeded THz wave parametric generator
The injection-seeded terahertz (THz) parametric generator (is-TPG) is one of the most high-power single-longitudinal mode THz-wave source. Our system is less influenced by scattering, refraction, and multiple reflections by samples because it is a narrow-linewidth source, and the detection area of the THz parametric detector is large. Thus, it is suitable for nondestructive inspection of practical samples in the real world. And recently, we have focused our efforts on a real-time measurement system using a multiwavelength is-TPG, which gives rise to numerous potential applications, given the significantly shorter measurement times.
- K. Murate and K. Kawase, “Perspective: Terahertz wave parametric generator and its applications,” Journal of Applied Physics, vol. 124, no. 16, p. 160901, Oct. 2018.
- K. Murate, S. Hayashi, and K. Kawase, “Multiwavelength terahertz-wave parametric generator for one-pulse spectroscopy,” Appl. Phys. Express, vol. 10, no. 3, p. 032401, Feb. 2017.
THz wave generation from LiNbO3 wave guide
When combined with a nonlinear waveguide crystal, Cherenkov phase matching allows for highly effective generation of high power and broadband terahertz (THz) waves. Using a ridged Lithium Niobate (LiNbO3) waveguide coupled with a specially designed silicon lens, we successfully generated THz waves with intensity of approximately three orders of magnitude stronger than those from conventional photoconductive antenna. The broadband spectrum was from 0.1 THz to 7 THz with a maximum dynamic range of 80 dB. The temporal shape of time domain pulse is a regular single cycle which could be used for high depth resolution time of flight tomography.
Applications using our system
Non-destructive drug inspection in thick materials
In 2003, we reported the first-ever development of a spectral imaging system for illicit drugs detection using a terahertz (THz) wave parametric oscillator (TPO) [K. Kawase et al., Opt. Exp. 11(20), 2549 2003]. The system has a dynamic range below four orders of magnitude, which enables it to identify reagents only through thin envelopes using spectral imaging. Recently, we succeeded in developing a high power and high sensitivity THz wave spectral imaging system using injection-seeded THz parametric generation and detection. A dynamic range in excess of 80 dB has been obtained, which is much higher than that of the 2003 system. In this study, the new spectral imaging system successfully identified reagents through thicker material than the thin envelopes used previously.
- M. Kato, S. R. Tripathi, K. Murate, K. Imayama, and K. Kawase, “Non-destructive drug inspection in covering materials using a terahertz spectral imaging system with injection-seeded terahertz parametric generation and detection,” Optics Express, vol. 24, no. 6, p. 6425, Mar. 2016.
Terahertz wave three-dimensional computed tomography
We demonstrated a high dynamic range, three-dimensional (3-D) terahertz (THz) wave computed tomography system by an is-TPG and ultrasensitive THz parametric detector. As an illustration, we obtained 3-D computed tomographic images of strong absorbed samples, that demonstrates the potential applications of our imaging system in non-destructive testing and evaluation of industrial products.
- S. R. Tripathi, Y. Sugiyama, K. Murate, K. Imayama, and K. Kawase, “Terahertz wave three-dimensional computed tomography based on injection-seeded terahertz wave parametric emitter and detector,” Optics Express, vol. 24, no. 6, p. 6433, Mar. 2016.