Surface Plasmon Resonance Switching of Gold Nanoparticles
Based on Ultrafast Phase Change of GeSbTe
T. Hira, T. Homma, and T. Saiki
Graduate School of Science and Technology, Keio University
3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa 223-8522, Japansaiki@elec.keio.ac.jp
Abstract
We demonstrated localized surface plasmon resonance (LSPR) switching of Au nanoparticles (AuNPs)
with a phase change of a GeSbTe (GST) material. We observed difference in the magnitude of LSPR shift
and damping depending on the degree of interaction between AuNPs and the GST film. In a staticmeasurement, LSPR spectra of AuNPs on a crystalline GST were significantly different from those on
amorphous GST, which result is almost consistent with numerical simulation. In a dynamic measurement,
for appropriate degree of interaction, we obtained giant switching of LSPR, spectral shift and reduction ofscattering intensity, upon repeated amorphization and crystallization by alternate irradiations offemtosecond laser and cw annealing laser.

1. Introduction
Phase change materials such as GeSbTe (GST) compounds are commonly used for rewritablememory devices due to high contrast between the amorphous and the crystalline phase in refractive index
and resistance, fast write and erase speeds as well as non-volatility, and low energy consumption.
Regarding the write speed, recently we demonstrated sub-picosecond nonthermal amorphization of aGeSbTe thin film with femtosecond laser pulse excitation [1]. Also in the process of erasing, we havefound that for nonthermally amorphized phase, crystallization proceeds more efficiently than for the meltquenched amorphous phase.
Localized surface plasmon resonance (LSPR) of noble metal nanoparticle such as Au and Ag
provides us with a strong and highly confined electromagnetic field. The resonance can be modulated by
the change of refractive index of the surrounding medium. By combining this unique property of LSPR
with the femtosecond phase change, we can achieve ultrahigh density storage, ultrafast switching devices
and, more interestingly, neuron-inspired devices by using plasticity and threshold responses of phasechange materials.
In this presentation, we demonstrated LSPR switching of Au nanoparticles (AuNPs) on a GST thinfilm upon phase change between crystalline and amorphous phase.
2. Experiments
Figure 1 shows the sample structure used in this experiment. AuNPs with a diameter of 85 nm wasembedded in a polymer (polyvinyl alcohol; PVA) thin film and was partially covered with a Ge2Sb2Te5(GST) film. The thickness of GST film was 30 nm. By changing the thickness d of the polymer film, we
can control the degree of interaction (contact area) between an AuNP and the GST film. For d > 85 nm,AuNPs were not in contact with the GST film. We also investigated LSPR behavior of AuNPs directly
dispersed on a GST (20 nm)/SiO2 (10 nm) thin film (Fig. 1(b)).
LSPR scattering from individual AuNPs was observed with a dark-field optical microscope setup
under white light illumination. Single-particle scattering spectra were measured by using a liquid-nitrogencooled multi-channel CCD spectrometer. For amorphization of the GST film a single femtosecond pulse(=800 nm) was delivered through a microscope objective and was focused on an AuNP and theunderlying GST film. A continuous wave laser diode (=830 nm) was used to erase the amorphous region(recrystallization) by thermal annealing.

GST
PVA

SiO2
GST
GlassAuNP

Glass(a)(b)
FIG. 1: Schematic of sample structure

3. Results and discussions
Figures 2(a) and (b) are dark-field optical micrographs for d =100 nm and 30 nm, respectively. In
both images left half part is as-deposited amorphous phase and right half part is crystalline phase preparedby laser annealing. In the case of polymer thickness d =100 nm, where there was a 15 nm average gap
between AuNPs and the GST film, AuNPs exhibit various colors depending on their LSPR frequency. We
do not see visible difference in color distribution between the amorphous and crystalline area. In the case of
d =30 nm, on the other hand, AuNPs do not exhibit specific colors and look almost white (broad spectrum)
as shown in Fig. 2(b). Large contact area with the GST film probably causes strong damping of LSPR due
to a significant large imaginary part Im of dielectric constant of GST. Brighter scattering in the amorphous
area implies less LSPR damping, which corresponds to smaller Im (for Ge2Sb2Te5 Im (cry)> Im (amo)in visible wavelength).

As-deposited(amorphous)annealed(crystalline)

As-deposited(amorphous)(a)annealed(crystalline)(b)FIG. 2: Dark-field optical micrographs of AuNPs partially covered with a GST
film for PVA thickness of (a) 100 nm and (b) 30 nm.
For appropriate d, at which AuNPs undergoes less LSPR damping compared to the case in Fig. 2(b),we expect a peak shift of LSPR (change in color of scattered light) upon phase change of the GST film.
Figure 3 shows sequential dark-field optical micrographs of a single AuNP (encircled) taken during
repeated amorphization and crystallization of the underlying GST film by alternate irradiations offemtosecond laser and cw annealing laser. A drastic and dynamic change in color of AuNP was observed.
In particular a large blue shift cannot be accounted for even qualitatively in the context of opticalfs pulse

annealingfs pulse

annealing

FIG. 3: LSPR switching of a single AuNP partially covered with a GST film upon
phase change. The AuNP was irradiated with a femtosecond laser pulse andannealing laser alternately.

interaction between AuNP and GST. Transformation of the polymer film, such as melting andresolidification, might be responsible for the LSPR switching.
We also investigated LSPR switching of AuNPs directly dispersed on a GST/SiO2 film (Fig. 1(b)).
Figures 4(a) and 4(b) are micrographs and scattering spectra of two different single AuNPs. In some cases,
as shown in Fig. 4(a), AuNPs exhibit a LSPR peak shift as large as 25 nm. In other cases we obtained
LSPR switching just with a change of its scattering cross section and not accompanied by a change inspectral shape. In all measurements we confirmed repeatability of the switching process.

FIG. 4: LSPR switching of single AuNP dispersed on a GST thin film upon phasechange. The AuNP was irradiated with a femtosecond laser pulse andannealing laser alternately.
4. Conclusion
We demonstrated LSPR switching of Au nanoparticles through the interaction with a phase-change
GeSbTe thin film. In dynamic measurements, where alternate irradiations of femtosecond laser and cw
annealing laser was given, giant LSPR shift and damping was observed with good repeatability. The
ultrafast and high contrast switching mechanism will be implemented in future active nanophotonicdevices.
We are grateful to K. Yusu (Toshiba) and T. Shintani (Hitachi) for providing us with GST thin film
samples. This work was supported by the Strategic Information and Communications R&D Promotion
Program from the Ministry of Internal Affairs and Communications.

References
1.

M. Konishi, H. Santo, Y. Hongo, K. Tajima, M. Hosoi, and T. Saiki: “Ultrafast amorphization in
Ge10Sb2Te13 thin film induced by single femtosecond laser pulse”, Appl. Opt. 49 (2010) 3470