P11

Improvement of Spatial Resolution of Nano Spectroscopy Imaging Using
Phase-Change Mask
Yu Sato, Nobuhiro Tsumori, Nurrul Syafawati Humam, Shohei Kanazawa, and Toshiharu Saiki
Graduate School of Science and Technology, Keio University
3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa 223-8522, Japanyuu.satou@saiki.elec.keio.ac.jp
ABSTRACTAn optical mask layer using phase-change material (PCM) has been proposed for photoluminescence spectroscopy
and emission energy control of semiconductor single quantum dot with band gap in the near-infrared wavelength
region. In this approach, a thin film of crystalline PCM is deposited on a quantum dot sample and amorphous markwhich can act as rewritable nanoaperture is formed. In this study, we reduced the size of amorphous aperture andimproved the spatial resolution of this method by recrystallizing four points at the outer region of an amorphous
aperture and by recrystallizing using an azimuthally polarized doughnut spot.
Key words: GeSbTe, Quantum dot, photoluminescence spectroscopy
1. INTRODUCTION
Semiconductor quantum dots (QDs) emitting in the wavelength of 1.55µm (optical communication wavelength) areremarkable as single photon sources [1]. To clarify and control the optical properties of QD, photoluminescence (PL)spectroscopy of a single QD is essential. The useful method available for single QD PL spectroscopy is near-fieldscanning optical microscopy (NSOM), which has resolution comparable to the aperture. However, since the apertureis much smaller than emission wavelength of QDs, light collection efficiency is attenuated [2].
As a solution to the low PL collection efficiency, an optical mask layer using phase-change material (PCM)
has been shown to be promising for high spatial resolution photoluminescence spectroscopy at near-infrared
wavelengths [3]. In this approach, a thin film of crystalline PCM is deposited on a quantum dot sample andamorphous mark is formed. This amorphous mark can act as a rewritable nanoaperture. An amorphous nanoaperture
allows high spatial resolution of photoluminescence spectroscopy and high collection efficiency. This is due to thelarge optical contrast between the crystalline and amorphous phases of the PCM at visible wavelengths and its hightransparency at near-infrared wavelengths. Moreover, PCM mask enables emission energy control of QDs by using
volume expansion of amorphization [4].
In the previous study [3], an amorphous mark with diameter of approximately 300 nm was made with amicroscope objective (NA = 0.80). For measurements at low temperature using a cryostat, however, it is very difficult
to use an objective with high NA (>0.8) due to the short working distance. In this study, under restriction to use anobjective with NA = 0.65, we formed smaller amorphous apertures by recrystallization at four points around theamorphous aperture and by recrystallization using azimuthally polarized beam.
2. EXPERIMENTS
As a sample investigated, a 40-nm-thick Ge2Sb2Te5 film was deposited by sputtering method on InAs/InP selfassembled double-capped QDs. The surface of Ge2Sb2Te5 film was covered with a 50-nm-thick SiO2 layer to preventfrom oxidation. The sample was subsequently annealed to form a crystalline phase. An amorphous aperture wasformed using picosecond laser pulses (λ=532nm) through microscope objective (NA=0.65). Confocal laser scanningmicroscopy measurements revealed that the amorphous aperture had a diameter of approximately 500nm. In PL
measurements, the sample was cooled down to 10K in a cold-finger cryostat. For PL excitation, a He-Ne laser(λ=632.8nm) was focused onto the amorphous aperture through the same objective. We employed two different
methods for recrystallization to reduce the area of amorphous aperture: (i) recrystallization at four points around theamorphous aperture (1μm away from the center) (Fig.1 (a)), (ii) recrystallization using azimuthally polarized beamirradiation (Fig.1 (b)). After these processes, we performed PL measurements.

Fig.1 (a) recrystallization at four points (b) recrystallization using azimuthally polarized beam.
3. RESULTS AND DISCUSSION
Figure 2(a) shows PL spectra of the InAs QDs obtained through the amorphous aperture before (blue line) and after(red line) recrystallization at four points. After recrystallization the number of PL peaks decreased to one third,
implying that the area of amorphous aperture was reduced to the same degree. Figure 2 (b) compares PL spectra
obtained before (blue line) and after (red line) recrystallization with azimuthally polarized beam irradiation. We
confirmed a reduction of the number of PL peaks and background emission from a large number of QDs.

Fig.2 (a) PL spectra of the InAs QDs obtained through the amorphous aperture before (blue line) and after (red line)
recrystallization at 4 points. (b) same as (a) but recrystallization using azimuthally polarized beam.
4. CONCLUSION
In conclusion, the present study has demonstrated that we improved the spatial resolution of phase-change mask by
reducing the amorphous aperture size using two different methods of recrystallization. By recrystallization using anazimuthally polarized beam, we expect to realize much higher spatial resolution.
ACKNOWLEDGMENT
This research was supported in part by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education,Culture, Sport, Science, and Technology in Japan. We would like to thank Y. Sakuma for providing a QD sample, andT. Shintani for forming GeSbTe films on the QDs.
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