Research areas

Ultrafast Lasers & Compact Waveguide Lasers 

Coherent light sources emitting ultrashort pulse and high peak power are essential for investigation on diverse nonlinear and ultrafast phenomena. We are developing ultrafast solid-state lasers operating in various spectral ranges using Ti:sapphire, Yb:KYW, Yb:KLuW, Cr:forsterite, Cr:YAG, Cr:ZnS, etc. Recently, waveguide lasers which simplify a solid-state laser cavity into on-chip scale are in great interests. In particular, waveguide lasers can generate high-repetition-rate (GHz-level) mode-locked pulses to be utilized in fast sampling and high precision metrology. In addition, novel types of saturable absorbers necessary to achieve stable and self-starting pulse formation are developed based on low-dimensional carbon nanostructures.

Research highlights

Nanocarbon Photonics 

Nanocarbons such as carbon nanotubes (CNTs) and graphene are one of the most attractive materials for applications in photonics and optoelectronics. We are working on developing novel ultrafast switching devices based on nanocarbons for laser mode-locking and Q-switching. The broadband absorption and nonlinear optical properties of CNTs can be optimized through precise control of synthesis and fabrication processes. We have demonstrated CNT-based saturable absorbers applicable for broadband femtosecond laser mode-locking in a spectral range between 0.8 and 2.4 µm. We are also developing graphene saturable absorbers with almost 'unlimited' operation wavelength range due to unique point bandgap structure.

Research highlights

THz wave generation and spectroscopy

Terahertz (THz) wave has recently attracted attention as a future communication platform, non-destructive imaging, and spectroscopy methods. THz pulses can be typically generated by transient current and optical rectification in photo-conductive antennas and nonlinear crystals, respectively. Based on Ti:sapphire oscillator and regenerative amplifier system, we establish various THz setups for different measurements, such as linear/nonlinear THz time-domain spectroscopy, optical-pump/THz-probe, and THz emission spectroscopy, etc. We study characteristics of various materials, including 2D materials, magnetic materials, and nonlinear materials, through the above THz setups. 

Research highlights

Terahertz nonlinear photonics

We focus on THz nonlinear & time-resolved spectroscopy and THz nonlinear phenomena in various materials including highly nonlinear organic crystals, low-dimensional nanomaterials, metamaterials, hybrid nanostructures and their applications. In addition, we develop ultrafast high-power (> 10 mW), high-field(~1 MV/cm) broadband THz source to investigate THz nonlinearities and their enhancement. Different THz setups based on Ti:sapphire oscillator and regenerative amplifier are established for different applications. 

Research highlights

UV-to-THz Time-resolved & Nonlinear Spectroscopy

In order to understand ultrafast carrier dynamics and nonlinear responses, we investigate degenerate and non-degenerate pump-probe & nonlinear spectroscopy, whereas different femtosecond excitation sources (Ti:sapphire laser, Yb-doped, Cr-doped and Er-doped lasers, OPO, OPA and THz sources) are used. We are able to measure both transmission- and reflection-type samples in a broad spectral range from UV to THz.


Research highlights

Nonlinear optical characterization of novel materials

One of our research interests is the nonlinear optical characterization of various materials, including 2D materials, with high resolution. We are able to investigate second/third-order nonlinearity such as second harmonic generation and nonlinear absorption, which provide quantitative information for ultrafast photonics. The transmission change can be resolved with a resolution of less than 0.1%.


Research highlights

Ultrafast spintronics

Future electronic devices require extended functionality beyond electronic charge and operating speeds beyond gigahertz frequencies. Optical manipulation of spin angular momentum has attracted attention due to its potential to accelerate the speed of information engineering to femtosecond time scales, and  thus, terahertz (THz) frequencies. Strong THz field pulse and optical pulse can manipulate magnetic excitations with THz frequencies (Fig. a-c), and conversely, spin transfer and relaxation can emit THz pulse, making it efficient spintronic THz emitter (Fig. d-e).

Research highlights