Research

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.

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.

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.

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.

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.


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%.


Ultrafast magneto-optics

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.

Equipments

⊙ 1-kHz ultrafast laser (Amplifier system)

▶ Center wavelength: 800 nm

- Pulse duration: < ~100 fs

- Average power: < 6 W

- Repetition rate: 1 kHz

▶ Center wavelength: 1140~2000 nm

- Pulse duration: < 130~150 fs

- Average power: < 100 mW

- Repetition rate: 1 kHz

⊙ 80-MHz ultrafast laser (Oscillator)

▶ Center wavelength: 690~1040 nm

- Pulse duration: < 90 fs~100 fs

- Average power: < 2 W

- Repetition rate: 80 MHz

▶ Center wavelength: 1100~1500 nm

- Pulse duration: < 100~120 fs

- Average power: < 100 mW

- Repetition rate: 80 MHz

⊙ Cryostat system with magnetic module

- Temperature range: 3.5 K – 350 K

- Temperature stability: <10 mK

- Magnetic field : 0.7 Tesla

- Magnetic field resolution: <5 μTesla


⊙ Spectrometer

- Scan range: 800-2600 nm (resolution : 0.5 nm)

- Wavelength accuracy:± 0.1 nm

- Scan rate: approx. 6 Hz

Lock-in amplifier, Oscilloscope, Boxcar ...