6 Second-Order Nonlinear Optical Properties of Plasmonic Nanostructures
219
6.6 Improved Samples
6.6.1 Key Improvement in Fabrication
It is evident from the results discussed so far that SHG is extremely sensitive to
the nanoscale morphology and defects of metal nanostructures even when the linear
response behaves as expected. In order to design nanostructures with the desired nonlinear response, samples with higher quality are needed. It is therefore quite fortunate
to note that during the last few years, a huge improvement in fabrication methods has
been achieved [86], resulting in high-quality nanostructures with parameters pushed
up to the fabrication limit [87]. Such an improvement is also evident for our samples.
Figure 6.7 shows two samples from different years. Both low- (Fig. 6.7a) and highquality (Fig. 6.7b) samples were fabricated with the same method. The improvement
in the fabrication process is mainly due to a new electron-beam lithography system
with better accuracy and stability than the old one. The line quality is improved by
smaller beam step size and the improvement of the shape control is due to higher
acceleration voltage (100 kV) of the new system.
The effect of the sample quality is seen not only by comparing the scanning
electron microscope images (Fig. 6.7), but also in the linear extinction spectra of the
plasmon resonances. For the new samples, the extinction peaks are enhanced and
their linewidths are narrowed compared to the old samples (Table 6.1). Such results
are due to smaller variation in the sample dimensions, leading to less inhomogeneous
broadening of the resonance for the new samples.
Fig. 6.7 Scanning electron microscope images of arrays of L-shaped nanoparticles of a low-quality
(from 2004) and b high-quality samples (from 2009)
Table 6.1 The details of resonances for low and high-quality samples
Sample
Resonance wavelength (nm)
FWHM (nm)
Low-quality
1060
260
(length 200 nm, width 100 nm, period 400 nm)
1490
420
High-quality
1031
88
(length 250 nm, width 100 nm, period 500 nm)
1555
203
219
6.6 Improved Samples
6.6.1 Key Improvement in Fabrication
It is evident from the results discussed so far that SHG is extremely sensitive to
the nanoscale morphology and defects of metal nanostructures even when the linear
response behaves as expected. In order to design nanostructures with the desired nonlinear response, samples with higher quality are needed. It is therefore quite fortunate
to note that during the last few years, a huge improvement in fabrication methods has
been achieved [86], resulting in high-quality nanostructures with parameters pushed
up to the fabrication limit [87]. Such an improvement is also evident for our samples.
Figure 6.7 shows two samples from different years. Both low- (Fig. 6.7a) and highquality (Fig. 6.7b) samples were fabricated with the same method. The improvement
in the fabrication process is mainly due to a new electron-beam lithography system
with better accuracy and stability than the old one. The line quality is improved by
smaller beam step size and the improvement of the shape control is due to higher
acceleration voltage (100 kV) of the new system.
The effect of the sample quality is seen not only by comparing the scanning
electron microscope images (Fig. 6.7), but also in the linear extinction spectra of the
plasmon resonances. For the new samples, the extinction peaks are enhanced and
their linewidths are narrowed compared to the old samples (Table 6.1). Such results
are due to smaller variation in the sample dimensions, leading to less inhomogeneous
broadening of the resonance for the new samples.
Fig. 6.7 Scanning electron microscope images of arrays of L-shaped nanoparticles of a low-quality
(from 2004) and b high-quality samples (from 2009)
Table 6.1 The details of resonances for low and high-quality samples
Sample
Resonance wavelength (nm)
FWHM (nm)
Low-quality
1060
260
(length 200 nm, width 100 nm, period 400 nm)
1490
420
High-quality
1031
88
(length 250 nm, width 100 nm, period 500 nm)
1555
203