「様式３」

プロジェクト番号 P-20

Ultrafast imaging of plasmonic-phononic structures[1] Leader: Name Oliver B. WrightAffiliation Hokkaido University, Professor
Participants:

nm on glass substrates were used (see Adv. Mater.
16, 90, 2004). The samples have a graded heightand so the void depth can be varied by choosing
the excitation point on the sample

1) Ronald Ulbricht, Hokkaido University, JSPS
Fellow
2) Hidenori Mimura, Research Institute of Electronics,
Professor[2]
Purpose of research
We propose to optically image plasmonicnanostructures with sub-micron resolution usingtightly focused ultrashort optical pulses thatgenerate coherent phonons with acoustic
frequencies up to 100 GHz and wavelengths
down to 100 nm. We shall thereby determine theeffect of the surface plasmon polariton (SPP)
modes on the phonon modes.
The samples consist of gold nano-voids withdiameters in the several-100 nm range.
Experimental results that we recently published(see T. Kelf et al., N. J. Phys. 15, 023013, 2013)
have allowed us to demonstrate that the proposed
imaging is possible, but the role of the plasmonicmodes has not been properly elucidated.
By collaboration with Shizuoka University wesought to better characterize the sample opticalmodes by white-light spectroscopy, and thereby
use the results to carry out tuned optical pumpand probe experiments on ultrashort time scales.
This should reveal the role of the plasmonicmodes on the optical modulation produced by thephononic modes of the samples.
Research content

Samples:Nano-void samples of Au void diameter 1600 nm

Measurements
We carried out characterization with white-light
spectroscopy using a white-light laser
spectrometer from the near-IR to visible
wavelengths. The spectra were taken for voids
with a height-to-diameter ratio ~0.9. This allowed
us to indentify several Mie modes representativeof the near-hollow spherical voids.
GHz vibrations for spectrally characterized voidsof varying diameter heights were measured withcoincident 10-micron diameter pump and probe
spots (200 fs pump and 1 ps probe pulsedurations) at normal incidence with sub-nJpulses at 80 MHz repetition rate. By temporal
Fourier analysis we identified the excited GHz
vibrational modes
The dependence of the vibrational mode
frequencies and phase on the optical probewavelength
wasinvestigatedby
optical-wavelength scanning of the probe in therange 720-920 nm with a fixed pump wavelengthof 415 nm.

[3]Results
Figure 1 shows the white-light spectroscopy
results, where the Mie mode resonances areclearly visible.

model for acousto-optic coupling.
Future prospects and research

Fig. 1 broadband reflection spectrum of nanovoids(grey-shaded); the ultrafast measurements shownin Fig. 2 were performed on the blue side (800nm)
and the red side (860nm) of the plasmon peak at
830nm.
In the pump-probe measurements, whichmeasure the excitation-induced change in the
optical reflectivity at the probe wavelength, we
found several oscillations that correspond to
acoustic resonances in the GHz range, as shownin Fig. 2.

These results show great promise formanipulating the coupling between plasmonsand phonons in complex periodic structures. In
the future there remains much research to bedone to determine the effect of the void size,
truncation and to explain the results
quantitatively. It is particularly important to try
to distinguish localized SPP contributions(probably dominant in the present experiments)
and non-localized SPP contributions. The resultswith different void truncations, availablel withour current sample, should help elucidate this.
Also, what determines the maximum
opto-acoustic coupling should be determined.

[4] Documentation
A conference paper was submitted to Meta14 in
Singapore on the subject of this work entitled“Gigahertzvibrations
inagradedplasmonic–phononic crystal.”(Also see T. Kelf et al., N. J. Phys. 15, 023013,2013)

Fig. 2 Pump-probe measurements, yielding thepump-induced differential change in reflection∆R/R, at probe wavelengths of 800nm (blue) and860nm (red); several oscillations are visible,originating from the excitation of phonon modes
that couple to the plasmon mode at 830nm (seeFig. 1); the modulation of the plasmon peak
position due to the induced vibrations is evident
from the change of the phase of the oscillations.

We also found that the phase of the acoustic
vibration inverted across the localized-plasmonicMie resonances. In the example shown in thefigues the peak wavelength of the resonance is at830nm, and the phase of the acoustics oscillations
flips between probing on the blue side (800nm)
and the red side (860nm) of the plasmon
resonance. This is as expected from a simple

出張報告
氏 名：Oliver B. Wright
所 属：Hokkaido University
期 間：Sep.9-11, 2013
用務先：Research Institute of Electronics
用務内容：Research Meeting
主たる対応者：Hidenori Mimura

氏 名：Ronald Ulbricht
所 属：Hokkaido University
期 間：Feb.23-25, 2014
用務先：Research Institute of Electronics
用務内容：Research Meeting
主たる対応者：Hidenori Mimura