192
N. Tamai and S. Masuo
decreased (Fig. 10.17d, e), the intensity decreased. After the AgTip was retracted, the
PL intensity returned to the same value as before the approach of the AgTip. In the
photon correlation histograms (Fig. 10.17g–l), the center peak gradually increased
with decreasing the z-distance, which was same as in the case of the 405 nm excitation.
Thus, the probability of multiphoton emission increased with decreasing z-distance.
The decay curves (Fig. 10.14m–r) were also shortened with decreasing the z-distance.
Interestingly, when the z-distance was 10 nm, the PL intensity increased with a slight
increase in the center peak. As the z-distance further decreased, the center peak
increased, i.e., the probability of multiphoton emission increased, with increasing
the PL intensity (z = 8 and 6 nm). These results demonstrated that the control of
single-photon and multiphoton emission associated with the increased PL intensity
could be achieved through nanometer-scale control of the z-distance. Above emission
behavior observed under 465 nm excitation can be interpreted as follows. The PL
intensity increased by the enhancement of the excitation rate because the LSP could
be generated on the AgTip under 465 nm excitation. Furthermore, the quenching
of SX also occurred as in the case of the 405 nm excitation, which resulted in an
increase in multiphoton emission with a shortening of PL lifetime. The mechanism
of the emission behavior is further discussed below.
The observed enhancements of the PL intensity and g
(2) (0) value as a function of
the z-distance built from average values of ten single QD under 465 nm excitation are
summarized in Fig. 10.18. In Fig. 10.18a, the PL intensity increased with decreasing
z-distance, and the intensity reached the maximum value, i.e., enhancement of 2.3
times on average was obtained near z ≈ 10 nm. The intensity then decreased with
decreasing the z-distance. In Fig. 10.18b, the g
(2) (0) value increased with decreasing zdistance below 10 nm. The enhancement of 10.5 times was obtained as the maximum
value at z ≈ 2 nm. To reveal the mechanism of the modification of the emission
behavior induced by the approach of the AgTip, theoretical analysis was applied to
the enhancement of the PL intensity (Fig. 10.18a) [54]. In this theoretical analysis,
the enhancement of the excitation rate by the LSP of the AgTip and the quenching of
the PL by resonance energy transfer from the single QD to the AgTip were considered
to fit the PL enhancement shown in Fig. 10.18a. The red and blue lines in Fig. 10.18a
exhibit the theoretical curves obtained by assuming Φ
0
PL = 30 and 40%, respectively,
with tip radius a = 20 nm. The theoretical curves well reproduced the experimental
results using reasonable Φ
0
PL and a values. This result supports that the observed PL
enhancement can be explained by the combination of the excitation rate enhancement
by LSP of the AgTip and the quenching of SX by the resonance energy transfer from
the QD to the AgTip. Therefore, the observed increase in multiphoton emission can
be explained by the quenching of the SX due to the energy transfer. The g
(2) (0) value
increased with decreasing the z-distance (Fig. 10.18b), because the rate of energy
transfer increases with decreasing z-distance. This quenching occurred independent
of the excitation wavelength because the spectral overlap between PL of the QD and
LSPR of the AgTip is important for the quenching. Thus, the increase in multiphoton
emission was observed under both 405 and 465 nm excitation. In the case of 465 nm
excitation, enhancement of the excitation rate also occurred. Hence, the PL intensity increased with decreasing z-distance, and then, the PL intensity decreased with
N. Tamai and S. Masuo
decreased (Fig. 10.17d, e), the intensity decreased. After the AgTip was retracted, the
PL intensity returned to the same value as before the approach of the AgTip. In the
photon correlation histograms (Fig. 10.17g–l), the center peak gradually increased
with decreasing the z-distance, which was same as in the case of the 405 nm excitation.
Thus, the probability of multiphoton emission increased with decreasing z-distance.
The decay curves (Fig. 10.14m–r) were also shortened with decreasing the z-distance.
Interestingly, when the z-distance was 10 nm, the PL intensity increased with a slight
increase in the center peak. As the z-distance further decreased, the center peak
increased, i.e., the probability of multiphoton emission increased, with increasing
the PL intensity (z = 8 and 6 nm). These results demonstrated that the control of
single-photon and multiphoton emission associated with the increased PL intensity
could be achieved through nanometer-scale control of the z-distance. Above emission
behavior observed under 465 nm excitation can be interpreted as follows. The PL
intensity increased by the enhancement of the excitation rate because the LSP could
be generated on the AgTip under 465 nm excitation. Furthermore, the quenching
of SX also occurred as in the case of the 405 nm excitation, which resulted in an
increase in multiphoton emission with a shortening of PL lifetime. The mechanism
of the emission behavior is further discussed below.
The observed enhancements of the PL intensity and g
(2) (0) value as a function of
the z-distance built from average values of ten single QD under 465 nm excitation are
summarized in Fig. 10.18. In Fig. 10.18a, the PL intensity increased with decreasing
z-distance, and the intensity reached the maximum value, i.e., enhancement of 2.3
times on average was obtained near z ≈ 10 nm. The intensity then decreased with
decreasing the z-distance. In Fig. 10.18b, the g
(2) (0) value increased with decreasing zdistance below 10 nm. The enhancement of 10.5 times was obtained as the maximum
value at z ≈ 2 nm. To reveal the mechanism of the modification of the emission
behavior induced by the approach of the AgTip, theoretical analysis was applied to
the enhancement of the PL intensity (Fig. 10.18a) [54]. In this theoretical analysis,
the enhancement of the excitation rate by the LSP of the AgTip and the quenching of
the PL by resonance energy transfer from the single QD to the AgTip were considered
to fit the PL enhancement shown in Fig. 10.18a. The red and blue lines in Fig. 10.18a
exhibit the theoretical curves obtained by assuming Φ
0
PL = 30 and 40%, respectively,
with tip radius a = 20 nm. The theoretical curves well reproduced the experimental
results using reasonable Φ
0
PL and a values. This result supports that the observed PL
enhancement can be explained by the combination of the excitation rate enhancement
by LSP of the AgTip and the quenching of SX by the resonance energy transfer from
the QD to the AgTip. Therefore, the observed increase in multiphoton emission can
be explained by the quenching of the SX due to the energy transfer. The g
(2) (0) value
increased with decreasing the z-distance (Fig. 10.18b), because the rate of energy
transfer increases with decreasing z-distance. This quenching occurred independent
of the excitation wavelength because the spectral overlap between PL of the QD and
LSPR of the AgTip is important for the quenching. Thus, the increase in multiphoton
emission was observed under both 405 and 465 nm excitation. In the case of 465 nm
excitation, enhancement of the excitation rate also occurred. Hence, the PL intensity increased with decreasing z-distance, and then, the PL intensity decreased with