Kazuki Horita

Efficient electro-optic sampling detection of terahertz radiation via Cherenkov phase matching

We experimentally demonstrate an efficient electro-optic sampling scheme based on Cherenkov phase matching of broadband terahertz radiation with 800-nm femtosecond probe beam in a 0.5 mm-thick LiNbO3 crystal coupled to a Si prism. The electro-optic signal from a Cherenkov-phase-matched LiNbO3 crystal is found to be comparable to that with a 4 mm-thick ZnTe crystal using a collinear phase matching. The Cherenkov phase matching technique can be achieved with any probe wavelength and hence has an advantage over the collinear phase matching method.

Fast-Scan Terahertz Time Domain Spectrometer Based on Laser Repetition Frequency Modulation

We propose a high-speed scanning terahertz (THz) time-domain spectrometer (TDS) based on optical sampling by repetition frequency modulation (OSREFM) using a repetition-frequency-tunable femtosecond fiber laser. Repetition frequency modulation is controlled by a highly stabilized external sweep generator. The fast scan of the time-delay between the pump and probe pulses is similar to an optical sampling with cavity tuning (OSCAT) THz-TDS system. However, the advantage of this technique is that it does not require physically moving components and uses only one laser system having an electronically modulated repetition rate. The time window and scanning time of this system depend on the modulation frequency range and the sweeping speed of the laser. In this study, we demonstrate the acquisition of a ~300 ps-window THz time waveform within 3 ms at a scan rate of 333 Hz.

Efficient electro-optic sampling detection and generation of intense THz radiation via Cherenkov-type phase matching in a LiNbO 3 crystal coupled to a Si prism

Efficient terahertz (THz) wave generation via Cherenkov-type phase matching were demonstrated with a thin lithium niobate (LiNbO3, LN) crystal coupled to a Si prism structure (Si-LN). This design relies on wave-guiding effects for the optical pump and decreased THz absorption in the LN crystal to achieve intense THz wave emission. Results showed that the peak value of the THz emission from a 40-μm-thick LN crystal is 7 times stronger than that of a 0.5 mm-thick crystal. In addition, an effective THz electro-optic (EO) sampling scheme was experimentally demonstrated using the same Si-LN structure and Cherenkov-phase matching scheme. The detected broadband THz peak signal via Cherenkov phase-matched EO-sampling using an 800-nm femtosecond probe beam and a Si prism-coupled 0.5 mm-thick LN crystal was measured to be 2.5 times higher than by standard EO sampling using collinear phase matching in a 4 mm-thick ZnTe crystal. The Cherenkov phase matching technique can be achieved with any probe wavelength and hence has an advantage over the collinear phase matching method.

Development of fast scan THz-TDS system by using repetition rate tunable femtosecond laser

In order to completely prevent the mixture of different medicines and also their contamination by foreign objects it is necessary to inspect all medicines in the manufacturing process. In line with this, THz time-domain spectroscopy (THz-TDS) is considered to be a promising technique applicable to the quality-control inspection system along the production line of chemicals and medicinal tablets. However, previous THz-TDS utilizes mechanical type time-delay scanning stage that requires several minutes in order to acquire a THz time waveform. Therefore, it is difficult to introduce this THz-TDS into the system. In the present study, we aim to develop a method for high-speed sampling of the THz waveform to significantly shorten the measurement time in the THz-TDS by changing the repetition rate of the femtosecond laser. The high-speed sampling method proposed was applied to a THz-TDS and the proof of principal experiment was done.