Research

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.

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. 

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. 

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.

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%.

Optical manipulation of the spin orders has been studied recently as an important topic in science and information engineering due to its high speed. Spin-orbit interaction allows light to control and measure the magnetic state of materials, and femtosecond laser enables time-resolved measurement of ultrafast spin dynamics. Many parameters such as magnetic field, strain, photon energy level, and polarization can be applied to the optical studies of magnetism, which will benefit future magnetic information technology.