Browsing by Author "Shirai, Hideto"

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Femtosecond time-evolution of mid-infrared spectral line shapes of Dirac fermions in topological insulators
(2020-06-17)
Yeh, Tien-Tien
;
Tu, Chien-Ming
;
Lin, Wen-Hao
;
Cheng, Cheng-Maw
;
Tzeng, Wen-Yen
Mid-infrared (MIR) light sources have much potential in the study of Dirac-fermions (DFs) in graphene and topological insulators (TIs) because they have a low photon energy. However, the topological surface state transitions (SSTs) in Dirac cones are veiled by the free carrier absorption (FCA) with same spectral line shape that is always seen in static MIR spectra. Therefore, it is difficult to distinguish the SST from the FCA, especially in TIs. Here, we disclose the abnormal MIR spectrum feature of transient reflectivity changes (Delta R/R) for the non-equilibrium states in TIs, and further distinguish FCA and spin-momentum locked SST using time-resolved and linearly polarized ultra-broadband MIR spectroscopy with no environmental perturbation. Although both effects produce similar features in the reflection spectra, they produce completely different variations in the Delta R/R to show their intrinsic ultrafast dynamics.
Ultrabroadband Infrared Pump-Probe Spectroscopy Using Chirped-Pulse Upconversion
(2015-01-01)
Shirai, Hideto
;
Yeh, Tien-Tien
;
Nomura, Yutaka
;
Luo, Chih-Wei
;
Fuji, Takao
We have demonstrated infrared pump-probe spectroscopy using chirped-pulse upconversion with a nonlinear mixing in a gas. The ultrabroadband mid-infrared spectrum spanning from 200 to 5,000 cm(-1) was upconverted by using a 0.4 ps chirped pulse to visible wavelength generation in gas. This technique allow for the high speed and high sensitivity detection in ultrabroadband mid-infrared region. Ultrafast dynamics of free-carrier in Ge was clearly observed in the range 200-5,000 cm(-1).
Ultrabroadband Mid-Infrared Pump-Probe Spectroscopy using Chirped-Pulse Upconversion
(2015)
Shirai, Hideto
;
Yeh, Tien-Tien
;
Nomura, Yutaka
;
Luo, Chih-Wei
;
Fuji, Takao
We demonstrated ultrabroadband mid-infrared pump-probe spectroscopy combined with chirped-pulse upconversion in a gas. Ultrafast reflectivity-change signal of an intrinsic germanium was observed in the region from 200 to 5000 cm(-1). (C) 2014 Optical Society of America
Ultrabroadband Midinfrared Pump-Probe Spectroscopy Using Chirped-Pulse Up-conversion in Gases
(2015-05-28)
Shirai, Hideto
;
Yeh, Tien-Tien
;
Nomura, Yutaka
;
Luo, Chih-Wei
;
Fuji, Takao
We demonstrate ultrabroadband and ultrafast midinfrared (MIR) pump-probe spectroscopy by employing the chirped-pulse up-conversion technique with four-wave difference-frequency generation in a gas. The full spectra of the MIR probe pulses, which spread from 200 to 5000 cm(-1), are recorded on a single-shot basis. We apply this MIR pump-probe spectroscopy to measure the ultrafast carrier dynamics of an intrinsic germanium crystal wafer. Ultrafast reflectivity change induced by the excitation of carriers is observed in the region from far to near infrared. The behavior of the reflectivity-change signals at each delay time is explained by the Drude model.
Ultrafast carrier dynamics in Ge by ultra-broadband mid-infrared probe spectroscopy
(2017-01-11)
Yeh, Tien-Tien
;
Shirai, Hideto
;
Tu, Chien-Ming
;
Fuji, Takao
;
Kobayashi, Takayoshi
In this study, we carried out 800-nm pump and ultra-broadband mid-infrared (MIR) probe spectroscopy with high time-resolution (70 fs) in bulk Ge. By fitting the time-resolved difference reflection spectra [Delta R(omega)/R(omega)] with the Drude model in the 200-5000 cm(-1) region, the time-dependent plasma frequency and scattering rate have been obtained. Through the calculation, we can further get the time-dependent photoexcited carrier concentration and carrier mobility. The Auger recombination essentially dominates the fast relaxation of photoexcited carriers within 100 ps followed by slow relaxation due to diffusion. Additionally, a novel oscillation feature is clearly found in time-resolved difference reflection spectra around 2000 cm(-1) especially for high pump fluence, which is the Lorentz oscillation lasting for about 20 ps due to the Coulomb force exerted just after the excitation.