「様式３」P-23

Ultrafast imaging of the phononic responseof split-ring-resonator［３］Results(３－１) Research results We have fabricatedand carried out experiments on a gold split-ringresonator (SRR) array of thickness 60 nm asshown by the design and SEM (scanning electronmicrograph) of Fig. 2.

［１］Organization
Project Leader：Prof. Oliver B. Wright（Affiliation）Hokkaido University
Representative at RIE：Prof. Hidenori Mimura（Affiliation）Shizuoka University
Participants：Paul Otsuka, Yuta Imade（Affiliation）Hokkaido University
Prof. Vygantas Mizeikis（Affiliation）Shizuoka University［２］Research Progress
High acoustic-frequency experiments werecarried out with tunable excitation (pump) opticalpulses to locally excite the picosecond acoustic
waves, and delayed independently-tunable probe
optical pulses for excitation and detection of
surface motion or refractive index changes. Thissystem is shown in Fig. 1

Fig. 2: Schematic diagram of the fabricated SRR unit cell structure
together with a SEM micrograph.

This structure is designed to show an opticalresonance in the near infrared, at a wavelength~800 nm accessible with our femtosecond lasersystem. Simulations with COMSOL for theelectromagnetic response gives reasonableagreement with the experimentally measured
transmission spectrum of the sample in thenear-infrared to visible region .
Results for the relative reflectivity change in thetime domain for normal incidence with pumping
at wavelength 400 nm and probing at wavelength
800 nm are Fourier transformed to reveal thevibrational resonant response of the SRRs.
Results for different pump and probe
polarizations are shown in Fig. 3.

Fig. 1: Schematic diagram of the laser system used forexperiments on GHz excitation and detection of vibrationsof acoustic metamaterials.

This system was put to use to excite GHz
vibrations in arrays of split ring resonators, anelectromagnetic metamaterial.
We have also made use of acoustic and opticalsimulation packages for surface acoustic waves:
PzFlex and COMSOL Multiphysics.

1

We observe a much stronger response when theprobe is polarized in the X direction (see Fig. 8), as
expected because this probe polarization couples
strongly to the electromagnetic modes of the SRR.

also published (see achievements list).

In order to interpret the observed vibrational
resonances below ~10 GHz, we carried out
simulations with both COMSOL and PZFlex.

In future we wish to extend the results to avariety of probe optical wavelengths to investigate
the effect of the plasmonic resonance on theacoustic response. Moreover, we wish to extend
this research to plasmonic nanolenses incollaboration with Shizuoka University.
［４］Achievements (List of Publications)（１）'Optical tracking of picosecond coherent
phonon pulse focusing inside a sub-micron object'
T. Dehoux, K. Ishikawa, P. H. Otsuka, M. Tomoda,O. Matsuda, M. Fujiwara, S. Takeuchi, I. A. Veres,
V. E. Gusev and O. B. WrightLight Sci. Appl. 5, 16082 (2016)（２）'On the origin of negative density andmodulus in acoustic metamaterials'
S. H. Lee and O. B. WrightPhys. Rev. B 93, 024302 (2016)

Fig. 3: Modulus of the temporal Fourier transform of thereflectance change variation for pumping and probing ananoscale gold SRR array at different linear polarizations.
Structural vibrations of the SRR are evident up to ~10GHz.

We find reasonable agreement with experimentfor the position of the vibrational modes, asshown in Fig. 4, but with different amplitudes, asexpected because the details of the plasmonicinteractions with the sample were not taken
account of in the simulations. The frequency
resolution of the COMSOL simulations is better
because of the longer time taken for thesimulation.

Fig. 4: Comparison of the frequency spectrum inexperiment (Pump Y and Probe X) and in simulations with
COMSOL and PzFlex, showing broad agreement of theacoustic resonance positions but with different amplitudes.
The simulation shows the longitudinal strain along the zaxis on the SRR surface.

(３－２) Ripple effects and further developments
Applications of this work include high frequency
acousto-optic modulators. This work is being
prepared for submission to the journal Applied
Physics Letters. Related papers in this field were2

Travelling Report（Mention each travel by CRP budget.）Name：Oliver B. WrightAffiliation：Hokkaido University
Period of time：August 3-August 6, 2015
Period of time：March 2-March 5, 2016
Destination：Shizuoka University
Purpose：Research discussions and experiments
Name of receiver：Oliver B. Wright3