Time-Resolved X-ray Absorption Measurement
of Optically Excited Silicon by Pump-Probe
Spectroscopy using Femtosecond Laser-Plasma
X-rays
Hidetoshi Nakano, Peixiang Lu, Tadashi Nishikawa,
and Naoshi Uesugi
Physical Science Laboratory
The recent development of high-power ultrafast
laser technologies has made laser-produced
plasmas attractive as potential bright X-ray
sources. High-density plasmas created near
a solid surface by femtosecond laser pulses
emit short X-rays pulses in the energy range
from sub-keV to MeV energies. Moreover, these
X-rays are synchronized to the incident laser
pulse. Such X-rays are extremely important
as diagnostic probes in the pump-probe-type
experiments for observing the dynamic responses
of optically excited materials.
We demonstrated time-resolved measurement
of soft X-ray absorption in optically pumped
silicon membrane by means of pump-probe spectroscopy
and found that soft X-ray absorption in Si
near its LII,III edge could be rapidly modulated by an intense
laser pulse irradiation [1]. Measurements
were carried out using a soft X-ray pulse
emitted from a femtosecond laser plasma created
on a Ta film as a probe and a 100-fs laser
pulse with a wavelength of 790 nm as a pump.
The duration of the probe pulse near a photon
energy of 100 eV was measured to be 40 ps
by using an X-ray streak camera. The sample
was 100-nm-thick silicon without supporting
structures. When the intensity of the pumping
laser pulse on Si membrane was 1010 W/cm2, which is well below the damage threshold,
a 5% increase in soft X-ray absorption near
LII,III absorption edge was observed as shown in
Fig. 1. Figure 1 shows the result when the
pump and probe pulses arrived on a sample
simultaneously. The most significant dip
in the differential transmission spectrum
(C) appeared near 99.5 eV. The sharp change
in soft X-ray transmission only appeared
near 99.5 eV, which is slightly lower than
the LII,III edge. This change was observed when the
relative time delay was within 40 ps. Therefore,
in this case, the dependence of the transmission
change at 99.5 eV on the time delay of the
probe soft X-ray pulse well fitted the probe
pulse shape, as shown in Fig. 2. The origin
of this absorption change is most likely
an additional absorption line and/or band
gap shift due to high-density photo-induced
electron-hole plasma creation.
[1] H. Nakano et al., Appl. Phys. Lett. 75 (1999) 2350.
Fig. 1. Transmission spectra near the LII,III edge of Si. Dotted (A) and thick solid (B)
curves represent transmission spectra observed
with and without laser irradiation. The thin
dotted curve (C) shows the differential transmission.
Fig. 2. Depth of the dip in differential
transmission at 99.5 eV as a function of
the probe soft X-ray pulse delay. The thin
dotted curve shows the probe pulse shape
measured by an X-ray streak camera.
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