29 Femtosecond Pump-Probe Microspectroscopy …
499
10
4 nm
2 ), (2σ ext /πω
2
0 ) and (2σ scat /πω
2
0 ) were calculated to be 0.02. The value of r is
0.035 for a normal incidence of light at the interface between the glass (n 1 = 1.46)
and air (n 0 = 1.00). According to Eqs. (29.1) and (29.2), the ratio of I S /I S to I T /I T
is approximate 27 by assuming that the value of σ ext is equal to that of σ scat .
Consequently, the back-scattering mode will improve signal gain for nanoparticles
having a smaller size than the beam west.
To demonstrate a high sensitivity of the back-scattering light detection experimentally, we measured the transient signals of single Au nanoparticle on a cover glass
slip. Figure 29.3a shows the comparison of the time profiles of a 100-nm diameter
particle between in transmitted light and back-scattering light detections at the probe
wavelength of 540 nm. The values of σ ext and σ scat are by chance the same to each
other value [14]. In the experiment, the scattering light intensity (I S ) detected with
the APD1 was set to be the same to the transmission one (I T ) by tuning the probe
pulse intensities before the microscope using the variable neutral density filters. The
setups of the lock-in amplifier and the accumulation time (3 min/1 trace) were set to
be the same in both experiments. The signal at negative delay times in both experiments showed almost the same noise levels to each other. However, it is obvious
that the back-scattering light detection experiment gave a much larger magnitude
in the transient signals than that of the transmittance mode. The signal gain in the
back-scattering mode was higher by about 20 times than that in the transmittance
mode. This experimental value was close to the calculated one ((I S /I S )/(I T /I T )
= 27) described above. It was demonstrated that the back-scattering light detection
could improve the sensitivity of the pump-probe signal. This high-sensitivity is a
great advantage in the ultrafast spectroscopic studies of single nanoparticles. We can
suppress the intensity of the pump pulse in order to avoid the photodecomposition
of the sample in the pump-probe experiments.
According to Eq. (29.2), decreasing the value of r will improve the sensitivity
of the back-scattering mode measurement. For example, when the glass coverslip
is covered with water, the value of r is calculated to be 0.0022 which is smaller by
15 times than that of the coverslip in air. That is, the transient signal intensity of a
nanoparticle is expected to increase 15 times. We have confirmed the idea experimentally as shown in Fig. 29.3b, where the transient signal intensities of the identical
single gold nanoparticle (100 nm in diameter) on a glass cover slip covered with and
without water. The signal intensity of the Au nanoparticle in water was 10 times
larger than that in air.
29.3 Application to Single Organic Nanocrystals
In this section, we describe the applications of the above setup to the investigations of
the excited-state dynamics of single organic nanocrystals of perylene and CuPc. First,
we explain the results of perylene nanocrystals. Figure 29.4a is the dark-field image
of perylene crystals on a glass coverslip prepared by recrystallization from an acetone
solution. Target nanocrystal is in the circle and showed a broad fluorescence peak at
499
10
4 nm
2 ), (2σ ext /πω
2
0 ) and (2σ scat /πω
2
0 ) were calculated to be 0.02. The value of r is
0.035 for a normal incidence of light at the interface between the glass (n 1 = 1.46)
and air (n 0 = 1.00). According to Eqs. (29.1) and (29.2), the ratio of I S /I S to I T /I T
is approximate 27 by assuming that the value of σ ext is equal to that of σ scat .
Consequently, the back-scattering mode will improve signal gain for nanoparticles
having a smaller size than the beam west.
To demonstrate a high sensitivity of the back-scattering light detection experimentally, we measured the transient signals of single Au nanoparticle on a cover glass
slip. Figure 29.3a shows the comparison of the time profiles of a 100-nm diameter
particle between in transmitted light and back-scattering light detections at the probe
wavelength of 540 nm. The values of σ ext and σ scat are by chance the same to each
other value [14]. In the experiment, the scattering light intensity (I S ) detected with
the APD1 was set to be the same to the transmission one (I T ) by tuning the probe
pulse intensities before the microscope using the variable neutral density filters. The
setups of the lock-in amplifier and the accumulation time (3 min/1 trace) were set to
be the same in both experiments. The signal at negative delay times in both experiments showed almost the same noise levels to each other. However, it is obvious
that the back-scattering light detection experiment gave a much larger magnitude
in the transient signals than that of the transmittance mode. The signal gain in the
back-scattering mode was higher by about 20 times than that in the transmittance
mode. This experimental value was close to the calculated one ((I S /I S )/(I T /I T )
= 27) described above. It was demonstrated that the back-scattering light detection
could improve the sensitivity of the pump-probe signal. This high-sensitivity is a
great advantage in the ultrafast spectroscopic studies of single nanoparticles. We can
suppress the intensity of the pump pulse in order to avoid the photodecomposition
of the sample in the pump-probe experiments.
According to Eq. (29.2), decreasing the value of r will improve the sensitivity
of the back-scattering mode measurement. For example, when the glass coverslip
is covered with water, the value of r is calculated to be 0.0022 which is smaller by
15 times than that of the coverslip in air. That is, the transient signal intensity of a
nanoparticle is expected to increase 15 times. We have confirmed the idea experimentally as shown in Fig. 29.3b, where the transient signal intensities of the identical
single gold nanoparticle (100 nm in diameter) on a glass cover slip covered with and
without water. The signal intensity of the Au nanoparticle in water was 10 times
larger than that in air.
29.3 Application to Single Organic Nanocrystals
In this section, we describe the applications of the above setup to the investigations of
the excited-state dynamics of single organic nanocrystals of perylene and CuPc. First,
we explain the results of perylene nanocrystals. Figure 29.4a is the dark-field image
of perylene crystals on a glass coverslip prepared by recrystallization from an acetone
solution. Target nanocrystal is in the circle and showed a broad fluorescence peak at