496
Y. Ishibashi and T. Asahi
divided into two pulses with the same energy. One of the two pulses was frequencydoubled (397 nm) with a 2-mm thickness BBO crystal, and it was used as a pump
pulse, which was chopped at a 20-kHz repetition rate by a high-speed optical chopper
to generate transient modulations of target nanocrystal. After passing through an
optical delay stage (STM-150, SIGMA Koki) which can control delay time between
−50 and 700 ps, the pump pulse was guided into an inverted optical microscope
(IX71, Olympus) and focused into the sample with an objective lens (60×, N.A.
0.7) (LUCPLFLN60X, Olympus). The other fundamental pulse was focused into an
80-cm photonic crystal fiber (PCF) (NL-PM-750, Crystal Fibre) to generate a supercontinuum in the wavelength range of 500–900 nm as shown in Fig. 29.1b, which
was used as a probe pulse. The tuned probe pulse was focused on the sample with the
same objective lens co-linearly with the pump pulse. This system has two detection
manners; transmitted (absorption-based) light and back-scattering light detections.
The back-scattering light of the probe pulse was collected with a ×60 objective
lens and passed through a confocal pinhole, and then the intensity was detected
with a fiber-coupled avalanche photodiode module (APD1). In the absorption-based
mode experiment, the transmitted light through the sample was collected with a ×
20 objective lens, and its intensity was measured with APD1. To extract modulation
of weak transient signal by pump light in Fig. 29.1a, the APD1 output was directed
into the signal input of a lock-in amplifier (SR830, Stanford Research) referenced to
the optical chopper. A part (10%) of the probe light was split before the microscope,
and the light intensity detected with the avalanche photodiode (APD2) was used as
the reference in a differential amplification of the signals. The output signals of the
lock-in amplifier were sent to the analog-to-digital converter (NI USB-6356, National
Instruments) and a personal computer to calculate the transient signals induced by the
pump pulses. The transient signals were recorded as a function of the position of the
translational stage (i.e., delay time between pump and probe pulses). The obtained
kinetic data were analyzed by using commercial software and homemade programs.
The fundamental specifications of the developed system are summarized as
follows. The diameter of the focusing spot (Fig. 29. 1c) was estimated from images of
a reflection at the glass coverslip with a Gaussian function. The diameter of the probe
pulse was 590 nm in FWHM, and that of the pump pulse was 730 nm in FWHM. The
fluence of the excitation pulse on the microscope stage was varied with a variable
neutral density filter (VND) in the range of 0.01–30 mJ cm
−2 pulse
−1 . To select the
appropriate probe wavelength, we inserted an optical bandpass filter (BPF) having
10-nm bandwidth in FWHM (Asahi Spectra) after generating the supercontinuum,
Fig. 29.1d shows the frequency chirp of the PCF-generated supercontinuum, which
was measured from the time profiles of transient absorbance of an organic laser
dye (DCM) solution in the probe wavelength range of 500–800 nm. By taking into
accounts the frequency chirp curve, we reconstructed transient absorption spectra
from the time profile of the transient signals at each probe wavelength. The FWHM
of the cross-correlation between the pump (397 nm) and probe (680 nm) pulses was
350 fs at the sample position (Fig. 29.1c). The polarization of the pump and probe
lights were linear and circular, respectively (Fig. 29.1e).
Y. Ishibashi and T. Asahi
divided into two pulses with the same energy. One of the two pulses was frequencydoubled (397 nm) with a 2-mm thickness BBO crystal, and it was used as a pump
pulse, which was chopped at a 20-kHz repetition rate by a high-speed optical chopper
to generate transient modulations of target nanocrystal. After passing through an
optical delay stage (STM-150, SIGMA Koki) which can control delay time between
−50 and 700 ps, the pump pulse was guided into an inverted optical microscope
(IX71, Olympus) and focused into the sample with an objective lens (60×, N.A.
0.7) (LUCPLFLN60X, Olympus). The other fundamental pulse was focused into an
80-cm photonic crystal fiber (PCF) (NL-PM-750, Crystal Fibre) to generate a supercontinuum in the wavelength range of 500–900 nm as shown in Fig. 29.1b, which
was used as a probe pulse. The tuned probe pulse was focused on the sample with the
same objective lens co-linearly with the pump pulse. This system has two detection
manners; transmitted (absorption-based) light and back-scattering light detections.
The back-scattering light of the probe pulse was collected with a ×60 objective
lens and passed through a confocal pinhole, and then the intensity was detected
with a fiber-coupled avalanche photodiode module (APD1). In the absorption-based
mode experiment, the transmitted light through the sample was collected with a ×
20 objective lens, and its intensity was measured with APD1. To extract modulation
of weak transient signal by pump light in Fig. 29.1a, the APD1 output was directed
into the signal input of a lock-in amplifier (SR830, Stanford Research) referenced to
the optical chopper. A part (10%) of the probe light was split before the microscope,
and the light intensity detected with the avalanche photodiode (APD2) was used as
the reference in a differential amplification of the signals. The output signals of the
lock-in amplifier were sent to the analog-to-digital converter (NI USB-6356, National
Instruments) and a personal computer to calculate the transient signals induced by the
pump pulses. The transient signals were recorded as a function of the position of the
translational stage (i.e., delay time between pump and probe pulses). The obtained
kinetic data were analyzed by using commercial software and homemade programs.
The fundamental specifications of the developed system are summarized as
follows. The diameter of the focusing spot (Fig. 29. 1c) was estimated from images of
a reflection at the glass coverslip with a Gaussian function. The diameter of the probe
pulse was 590 nm in FWHM, and that of the pump pulse was 730 nm in FWHM. The
fluence of the excitation pulse on the microscope stage was varied with a variable
neutral density filter (VND) in the range of 0.01–30 mJ cm
−2 pulse
−1 . To select the
appropriate probe wavelength, we inserted an optical bandpass filter (BPF) having
10-nm bandwidth in FWHM (Asahi Spectra) after generating the supercontinuum,
Fig. 29.1d shows the frequency chirp of the PCF-generated supercontinuum, which
was measured from the time profiles of transient absorbance of an organic laser
dye (DCM) solution in the probe wavelength range of 500–800 nm. By taking into
accounts the frequency chirp curve, we reconstructed transient absorption spectra
from the time profile of the transient signals at each probe wavelength. The FWHM
of the cross-correlation between the pump (397 nm) and probe (680 nm) pulses was
350 fs at the sample position (Fig. 29.1c). The polarization of the pump and probe
lights were linear and circular, respectively (Fig. 29.1e).