Ultrabroadband impulsive stimulated Raman scatter

In a scientific mission collaborated with CEA (Paris, France) and CFEL (Hamburg, Germany), I designed a high-speed deep-UV spectrometer necessary for high-sensitivity measurements. The high-speed referenced spectrometer is equipped with two identical 2048-pixel high-sensitivity (1300 V/(lx･s)) CMOS linear detectors with a spectral response down to 200 nm and a maximum line rate of 4,500 lines/s (10MHz video rate digitizer with 16-bit resolution inside). The noise level of for a single scan is below 280 μOD, which is essential to this mission. As discussed above, the dynamics are probed in the liquid phase with a path length of only a few microns limiting the amount of signal from the experiment. To see the consecutive internal conversion steps, it is key to obtain a high signal-to-noise ratio with the high-sensitivity deep-UV time spectrometer.

We established state-of-the-art time-resolved transient absorption spectroscopy for liquid nanosheets. The OH stretching mode of liquid water in real-time was revealed for the first time thanks to the sub-5 fs resolution. The mission was achieved by joint expertise from groups in CFEL(Hamburg), EPFL (Lausanne), and CEA(France). Previous literature only reports this mode via vibrational spectroscopy; however, it’s never been temporally resolved due to the lack of short pulses in the deep ultraviolet (DUV) region to probe the OH bond. Moreover, the characteristic lifetime of the OH-stretching mode, around 50 fs, is interestingly on the same scale as the formation lifetime of the OH radical in ionized water. 

Ultrafast dynamics in honeycomb Mott insulator

We combine optical transient spectroscopy and time-resolved RIXS (tr-RIXS) spectroscopy to study 5-d orbital dynamics in the honeycomb Mott insulator α-Li2IrO3. Commissioned ultrafast X-ray spectroscopy apparatus at the Swiss free-electron laser facility (SwissFEL) and executed the pilot experiment to uncover perturbated inter-site transition in Kitaev honeycomb α-Li2IrO3. (Struct. Dyn.)

Ultrafast Carrier Injection Dynamics in Perovskite

Wide bandgap oxides (WBGOs) have widely emerged in modern photonic devices, but early dynamics right after carrier injection into the conduction band has not been fully understood. In this study, we mimic transient carrier injection into BaSnO3 via ultrafast above-gap photoexcitation with sub-picosecond UV transient absorption (TA) spectroscopy probing across the bandgap. A fast hot-carrier cooling lifetime of 1ps due to electron-phonon scattering along with clear exciton bleach and excitonic enhancement resulting from photo-excited free carriers are unveiled. The s methodology exemplifies a novel approach to investigating carrier injection dynamics in perovskite WBGOs relevant to applications in power electronics and photovoltaics.