Researchers at RIKEN have made significant progress in harnessing the terahertz band of the electromagnetic spectrum, thanks to the development of palm-sized devices. These devices have the potential to “X-ray” objects without the use of harmful ionizing radiation, offering a safer alternative to traditional X-ray technology.
While various technologies already exploit different sections of the electromagnetic spectrum, the terahertz band has remained relatively overlooked. Terahertz waves have exciting applications, particularly in their ability to penetrate and see through materials, similar to X-rays. Unlike X-rays, however, terahertz waves do not pose the same risks associated with ionizing radiation.
The challenge has been adapting microwave or visible-light technologies to operate within the terahertz range at practical sizes and power outputs. One approach has been to develop electrical devices that generate higher frequency, ultrashort-wavelength microwaves to produce terahertz waves. However, achieving optimal parameters for these devices and ensuring sufficient electrical performance has proven to be a difficult task.
Alternatively, researchers have explored using nonlinear crystals to convert shorter, higher-frequency waves of infrared light into terahertz waves. At the RIKEN Center for Advanced Photonics, a team led by Hiroaki Minamide has focused on this strategy, aiming to produce terahertz waves by converting the output from an infrared laser. However, the challenge has been generating sufficiently powerful terahertz waves for practical applications without relying on large lasers.
The Tera-Photonics Research Team, headed by Minamide, aims to develop compact yet powerful terahertz wave sources for various industrial and fundamental research applications. They have made significant progress in this area and have established multiple collaborations with industrial partners.
The team’s focus has been on using lithium niobate, a nonlinear crystal known for generating terahertz waves when exposed to near-infrared laser light. In the early stages, despite years of effort, it was challenging to produce terahertz waves with the desired power using this method.
An unexpected breakthrough came after a major earthquake struck Sendai, Japan, where the RIKEN campus is located, leading to a temporary halt in lab research. During that period, Minamide recalled the results of a previous experiment and noticed a promising hint for a potential solution.
In the earlier experiment, a near-infrared laser with nanosecond pulse durations was used. However, when shorter, sub-nanosecond laser pulses were employed, the generation of terahertz waves varied in response to the input laser pulse. This observation piqued Minamide’s curiosity, prompting further investigation.
Minamide came across a 1993 paper that explored the effects of laser pulse duration on nonlinear crystals using visible light. The study suggested that using shorter pulses reduced a phenomenon known as Brillouin scattering, which causes light scattering. Minamide wondered if by reducing the laser pulse duration, Brillouin scattering could be minimized in lithium niobate crystals, potentially enhancing the conversion of laser light into terahertz waves and increasing the power output.
This discovery opened up new possibilities for the team’s research, and they have since made significant strides towards their goal of developing compact and powerful terahertz wave sources. The collaboration with industrial partners is expected to further advance the application of terahertz technology in various fields.