188
N. Tamai and S. Masuo
by the electric field of the LSP under 465 nm excitation because of the overlap
between the LSPR band and the absorption spectrum of the QD. On the other hand,
the relaxation rate of the QD could also be enhanced by the AgTip independent of
the excitation wavelength because of the spectral overlap between the PL spectrum
and LSPR band. To generate the electric field of z-direction on the AgTip, a radially
polarized laser beam was introduced to the microscope.
The PL and AFM images of the single QDs dispersed on a glass coverslip are
summarized in Fig. 10.15. In the PL images, individual QDs exhibit a double-lobed
PL intensity pattern, which is characteristics of radially polarized beam excitation.
The full-width at half-maximum (fwhm) of the center lobe (Fig. 10.15a) was estimated to be 220 nm (Fig. 10.15b). The PL and AFM images of the same area as
image (a) were then measured again under 405 nm excitation with approaching the
AgTip (Fig. 10.15c, d). The center lobe of each PL spot was vanished by approaching
the AgTip (Fig. 10.15c), which was also confirmed by the cross section of the PL
spot shown in Fig. 10.15g. This result indicates that the PL from the individual
QDs was quenched, namely the nonradiative decay rate of the QD was enhanced by
approaching the AgTip. On the other hand, in the PL image under 465 nm excitation
with approaching the AgTip (Fig. 10.15e), the center lobe of the PL spots was not
vanished. The fwhm of the center lobe was smaller (97 nm, Fig. 10.15h) than the
cross section (Fig. 10.15b) obtained under 405 nm excitation without approaching
the AgTip. These results indicated that the PL was not quenched in the case of 465 nm
excitation, and the spatial resolution of the PL image was increased by the approach
of the AgTip.
The representative emission behavior of a single QD depending on the distance
between a single QD and the AgTip (z-distance) is summarized in Fig. 10.16. In
the time traces of the PL intensity (Fig. 10.16a–e), the PL intensity decreased with
decreasing the z-distance. In the PL decay curves (Fig. 10.16k–o), the curves was
shortened with decreasing z-distance. The decrease in the PL intensity with shortening lifetime depending on the z-distance clearly indicated that the PL of the QD was
quenched, namely the nonradiative decay rate of the QD was enhanced by the AgTip
due to the resonance energy transfer from the QD to the AgTip. In the photon correlation histogram (Fig. 10.16f–j), the center peak gradually increased with decreasing
the z-distance. The g
(2) (0) before the approach of the AgTip was estimated to be 0.09
(Fig. 10.16f). This value increased to 0.17 for z = 10 nm (Fig. 10.16g) and 0.85 for
z = 6 nm (Fig. 10.16h). In the case of z = 2 nm (Fig. 10.16i), the photon correlation
histogram could not be constructed because of the low PL intensity. After retracting
the AgTip (Fig. 10.16j), the g
(2) (0) value returned to the original value of 0.08. These
results indicated that single-photon emission from the single QD (before the approach
of the AgTip) changed to multiphoton emission with the approach of the AgTip, i.e.,
the emission photon statistics of the QD can be controlled by the resonance energy
transfer. This control of the emission photon statistics can be interpreted as follows.
In the case of 405 nm excitation, no excitation enhancement occurs because the LSP
is not generated on the AgTip. On the other hand, enhancement of the nonradiative
rate (quenching) by the energy transfer occurs through the spectral overlap between
the PL and LSPR band. Thus, we conclude that, in the case of the QD-AgTip system,
N. Tamai and S. Masuo
by the electric field of the LSP under 465 nm excitation because of the overlap
between the LSPR band and the absorption spectrum of the QD. On the other hand,
the relaxation rate of the QD could also be enhanced by the AgTip independent of
the excitation wavelength because of the spectral overlap between the PL spectrum
and LSPR band. To generate the electric field of z-direction on the AgTip, a radially
polarized laser beam was introduced to the microscope.
The PL and AFM images of the single QDs dispersed on a glass coverslip are
summarized in Fig. 10.15. In the PL images, individual QDs exhibit a double-lobed
PL intensity pattern, which is characteristics of radially polarized beam excitation.
The full-width at half-maximum (fwhm) of the center lobe (Fig. 10.15a) was estimated to be 220 nm (Fig. 10.15b). The PL and AFM images of the same area as
image (a) were then measured again under 405 nm excitation with approaching the
AgTip (Fig. 10.15c, d). The center lobe of each PL spot was vanished by approaching
the AgTip (Fig. 10.15c), which was also confirmed by the cross section of the PL
spot shown in Fig. 10.15g. This result indicates that the PL from the individual
QDs was quenched, namely the nonradiative decay rate of the QD was enhanced by
approaching the AgTip. On the other hand, in the PL image under 465 nm excitation
with approaching the AgTip (Fig. 10.15e), the center lobe of the PL spots was not
vanished. The fwhm of the center lobe was smaller (97 nm, Fig. 10.15h) than the
cross section (Fig. 10.15b) obtained under 405 nm excitation without approaching
the AgTip. These results indicated that the PL was not quenched in the case of 465 nm
excitation, and the spatial resolution of the PL image was increased by the approach
of the AgTip.
The representative emission behavior of a single QD depending on the distance
between a single QD and the AgTip (z-distance) is summarized in Fig. 10.16. In
the time traces of the PL intensity (Fig. 10.16a–e), the PL intensity decreased with
decreasing the z-distance. In the PL decay curves (Fig. 10.16k–o), the curves was
shortened with decreasing z-distance. The decrease in the PL intensity with shortening lifetime depending on the z-distance clearly indicated that the PL of the QD was
quenched, namely the nonradiative decay rate of the QD was enhanced by the AgTip
due to the resonance energy transfer from the QD to the AgTip. In the photon correlation histogram (Fig. 10.16f–j), the center peak gradually increased with decreasing
the z-distance. The g
(2) (0) before the approach of the AgTip was estimated to be 0.09
(Fig. 10.16f). This value increased to 0.17 for z = 10 nm (Fig. 10.16g) and 0.85 for
z = 6 nm (Fig. 10.16h). In the case of z = 2 nm (Fig. 10.16i), the photon correlation
histogram could not be constructed because of the low PL intensity. After retracting
the AgTip (Fig. 10.16j), the g
(2) (0) value returned to the original value of 0.08. These
results indicated that single-photon emission from the single QD (before the approach
of the AgTip) changed to multiphoton emission with the approach of the AgTip, i.e.,
the emission photon statistics of the QD can be controlled by the resonance energy
transfer. This control of the emission photon statistics can be interpreted as follows.
In the case of 405 nm excitation, no excitation enhancement occurs because the LSP
is not generated on the AgTip. On the other hand, enhancement of the nonradiative
rate (quenching) by the energy transfer occurs through the spectral overlap between
the PL and LSPR band. Thus, we conclude that, in the case of the QD-AgTip system,