A simple method to find temporal overlap between THz and X-ray pulses using X-ray-induced carrier dynamics in semiconductors

Abstract

X-ray-induced carrier dynamics in silicon and gallium arsenide were investigated through intensity variations of transmitted terahertz (THz) pulses in the pico to microsecond time scale with X-ray free-electron laser and synchrotron radiation. We observed a steep reduction in THz transmission with a picosecond scale due to the X-ray-induced carrier generation, followed by a recovery on a nano to microsecond scale caused by the recombination of carriers. The rapid response in the former process is applicable to a direct determination of temporal overlap between THz and X-ray pulses for THz pump-X-ray probe experiments with an accuracy of a few picoseconds.

Figures (4)

• Figure 1: (a) Schematic of the X-ray pump-THz probe experiment for semiconductors. (b) Schematic of the X-ray and THz pulses reaching the sample. At positive (negative) delay times, the X-ray (THz) pulse reaches the sample first.
• Figure 2: (a) Schematic of the THz pulse generation setup at SPring-8 BL19LXU. Red solid line: optical path for $800$ nm beam, red dashed line: optical path for THz diagnostic, blue bold line: optical path for THz beam. Grating: groove density of $1200$ mm$^{-1}$, L1, L2: lenses, LN: LiNbO$_3$ crystal, M1: dielectric coated plane mirror, M2: gold-coated plane mirror, OAP: Off-axis parabolic mirror. (b, c) The waveform of the THz pulse evaluated with the EO sampling method (b) and its Fourier spectrum (c). (d, e) Time evolution of the differential intensity of transmitted THz pulses between the pumped and unpumped conditions for Si with the delay range of (d) [$-2$$\mu$s, $10$$\mu$s] and (e) [$-20$$\mu$s, $100$$\mu$s]. The red, green, and blue circles represent data for Si with the thickness of $200$, $500$, and $1000$$\mu$m, respectively. The black solid lines represent fitting curves with an exponential decay function convoluted with a Gaussian function. The curves are offset for clarity.
• Figure 3: Time evolution of the differential intensity of transmitted THz pulses between the pumped and unpumped conditions for GaAs with the thickness of $700$$\mu$m (purple circles). For comparison, the data for Si with the thickness of $500$$\mu$m are also plotted (green circles). The black solid line represents a fitting curve with an exponential decay function convoluted with a Gaussian function.
• Figure 4: (a) Schematic of the THz pulse generation setup at SACLA BL3. Red bold line: optical path for $800$ nm beam, red dashed line: optical path for THz diagnostic, cyan bold line: optical path for $400$ nm beam, blue bold line: optical path for THz beam. BBO: Beta Barium Borate crystal, ITO: indium tin oxide coated mirror, OAP: Off-axis parabolic mirror. (b, c) The waveform of the THz pulse evaluated with the EO sampling method (b) and its Fourier spectrum (c). (d, e) Time evolution of the differential intensity of transmitted THz pulses between the pumped and unpumped conditions for Si with the thickness of $500$$\mu$m (d) and GaAs with the thickness of $700$$\mu$m (e). Delay time zero is defined as the center of the falling. Black solid and dashed lines represent the average and $3 \sigma$ values in the $20$ to $170$ ps range, respectively.