Amorphization and crystallization of GeSbTe thin films induced by ananoscale femtosecond light source
Takashi Hira, Yuki Hongo, Kazuyuki Tajima, Takashi Honma, Nariaki Kitamura and Toshiharu SaikiGraduate School of Science and Technology, Keio UniversityABSTRACT
Writing of amorphous marks in a GeSbTe thin film with a femtosecond localized electric field generated
on an Au nanoparticle was demonstrated. A drastic modification (depletion) of plasmon scattering
accompanied the amorphization of the underlying GeSbTe film. The amorphous mark was erased and theplasmon scattering recovered by irradiation of annealing laser. The repeatability of the writing and erasing
processes was confirmed. A polarization-based readout method of nanoscale recording mark is also
discussed.
Key words: femtosecond laser, localized surface plasmon, gold nanoparticle
INTRODUCTION
The demand for high density optical data storage with fast recording and retrieval rates is still growing
ever faster. Also, to meet the explosive growth in data volume to be stored, drastic improvement in storage
capacity is strongly required. Toward ultra high density storage over hard disk magnetic recording,
Hamann et al. demonstrated phase-change (crystallization) recording at a storage density of 3.3 Tb/inch2 byusing a heated atomic force microscope tip1. In actual device operation, however, heat transfer through atip-surface point contact is not appropriate in the sense of mechanical damage to the tip. More importantly,
lateral heat spreading in the thermal recording process inherently limits the recording density. The problem
is more serious in the amorphization process, where a higher temperature gradient is required compared tothe crystallization process.
Recently we have demonstrated sub-picosecond nonthermal amorphization of a GeSbTe thin film withfemtosecond laser pulse excitation2. In the process high density carrier excitation on a femtosecond timescale makes a significant contribution to the amorphization that is completed without the formation of aliquid phase. This nonthermal amorphization could solve the essential problem of the heat flow in thewriting and erasing process.
For recording and retrieving small marks much beyond the diffraction limit of light, localized surface
plasmon on a metal nanoparticle or nanostructure provides an ideal nanoscale light source3. In addition tothis, electric field enhancement due to the plasmon resonance enables a reduction of threshold fluence forboth amorphization and crystallization.
In this presentation, amorphization of a GeSbTe thin film with a femtosecond localized electric field
generated on an Au nanoparticle is demonstrated. As a mechanism for reading of nanoscale recording
marks, modification of plasmon resonance associated with the phase change of the underlying substrate isalso discussed.

EXPERIMENTS
The sample investigated was a Ge10Sb2Te13 (GST) film with a thickness of 20 nm sputtered on a glass
substrate. The GST film is covered with 10-nm thick ZnS-SiO2. The thin film was annealed at 280 °C toobtain the crystalline phase. Au nanoparticles with a diameter of 40 nm were uniformly dispersed on thefilm.
A schematic of the experimental setup is illustrated in Fig. 1. Surface plasmon resonance scattering fromindividual Au nanoparticles was observed with an optical microscope under white light illumination using acolor CCD camera. For amorphization of the GST film a single femtosecond pulse (=800 nm) wasdelivered through a microscope objective and was focused on an Au nanopaticle. A continuous wave (cw)laser diode (=830 nm) was used to erase the amorphous region by thermal annealing crystallization.
RESULTS and DISCUSSIONS
Figure 2 shows sequential optical micrographs of a single Au nanoparticle taken during repeatedamorphization and crystallization of the underlying GST film by alternative irradiations of femtosecond
laser and cw annealing laser. We found that the amorphization of GST substrate leads to a drastic reductionof scattered light of the nanoparticle. Also by the irradiation of annealing laser the scattered light recovers
the initial intensity. A good repeatability in phase transformation was confirmed.
In order to understand the mechanism of the significant modification of plasmon resonance of the Au
nanoparticle, we performed FDTD simulation and calculated the difference in plasmon resonance for theamorphous and crystal states of GST substrate. Although a small resonance shift and broadening isexpected, the drastic change obtained experimentally could not be reproduced. One possible explanation isthat a strong electric field originating from localized spatial charges generated by the photoexcitation may
attribute to the modification of plasmon oscillation.
CONCLUSION
Femtosecond local surface plasmon assisted recording and erasing of amorphous marks and theirrepeatability was demonstrated. A significant switching contrast of plasmon resonant scattering is
promising as a mechanism of reading out of nanoscale recording marks.
We are grateful to K. Yusu (Toshiba Corporation) for providing us with GST thin film samples. This
work was supported by the Strategic Information and Communications R&D Promotion Program from theMinistry of Internal Affairs and Communications.
REFERENCES
1. H. F. Hamann et al.: “Ultra-high-density phase-change storage and memory”, Nature Materials 5 (2006)
383.
2. M. Konishi et al.: “Ultrafast amorphization in Ge10Sb2Te13 thin film induced by single femtosecondlaser pulse”, Applied Optics 49 (2010) 3470.
3. T. Matsumoto et al.: “Writing 40 nm marks by using a beaked metallic plate near-field optical probe”,Optics Letters 31 (2006) 259.

cw-LD(crystallization)
fs-pulse(amorphization)

Au =40 nm
ZnS-SiO2
GeSbTeglass

Figure 1: Schematic illustration of experimental setup for amorphization and crystallization of aGeSbTe thin film with a gold nanoparticle under femtosecond pulse irradiation.

fs-pulse

fs-pulse

cw-LD

cw-LD

fs-pulse

Figure 2: Sequential optical micrographs of a single Au nanoparticle on a GeSbTe film taken byalternative irradiations of femtosecond laser and cw annealing laser. When a small portion of GeSbTe
film directly below the Au particle changed to amorphous, the scattered light completely vanished.