Supplementary Material
Synthesis of GaAs nanowires with very small diameters and their
optical properties with the radial quantum-confinement effect
Guoqiang Zhang,a) Kouta Tateno, Haruki Sanada, Takehiko Tawara, Hideki Gotoh, and
Hidetoshi Nakano
NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya,
Atsugi, Kanagawa 243-0198, Japan
a)
Electronic mail: zhang@will.brl.ntt.co.jp.
The material includes the following three parts:
1. Details of structural and PL characterizations
2. Calculation of the theoretical polarization anisotropy
1
1. Details of structural and PL characterizations
For structural characterization, NWs were dispersed on a copper grid and then
analyzed by TEM. For optical measurement, freestanding GaAs NWs were dispersed on
Au-coated (200 nm in thickness) SiO2/Si substrates (SiO2 thickness: 500 nm). The
substrates were then loaded into a cryostat with an optical window and cooled to 3.6 ±
0.5 K in a liquid helium flow. We used a CW Ti:sapphire laser (710-nm wavelength;
power of 0.1-1 kW/cm2) as the excitation source. By using an objective lens, a spot size
diameter as small as 2 μm can be obtained, which enables us to perform micro-PL
measurement for individual NWs. The PL from single NWs was dispersed through a
spectrometer and detected with a liquid-nitrogen-cooled charge coupled device camera.
During the measurement, we defined an arbitrary reference axis on the substrate for NW
dispersion and used the relative angle θ between the reference axis and the optical
polarization axis as a parameter. The angle, θ, in Fig. 3 was calibrated and 0º
corresponds to the NW long axis.
Notably, only when using Au-coated SiO2/Si substrates for NW dispersion, we can
observe the luminescence from these NWs. It is very difficult to obtain NWs with
detectable luminescence intensity when using bare SiO2/Si substrates without Au
coating film. We consider that the absorption and emission of the NWs may be
enhanced considerably by the Au film with a mirror face. On the other hand, the NW PL
efficiency may also be significantly enhanced by surface plasmon polaritons on the
metal-dielectric interface.1 Similar phenomenon has been observed in InGaN quantum
wells2 and semiconductor nanocrystals with metallic layer.3
2
2. Calculation of the theoretical polarization anisotropy of freestanding GaAs NWs
The polarization anisotropy is caused by the large dielectric contrast between
freestanding NWs and air. When the incident field is polarized parallel to the NW long
axis, the electric field inside the NW is not reduced. But when polarized perpendicular
to the NW, the electrical field amplitude is attenuated according to the following
equation:
Ei=2Ecε0/(ε+ε0)
where Ei is the electric field inside the NW, Ec is the excitation field, ε (ε0) is the
dielectric constant of the NW (vacuum).4 Using the dielectric constant for bulk GaAs of
13.1, we calculated the theoretical polarization ratio ρ to be 96% by the following
equation:
ρ = (Ec2 - Ei2)/( Ec2 + Ei2)
References:
1
J. B. Khurgin, G. Sun, and R. A. Soref, J. Opt. Soc. Am. B 24, 1968 (2007).
2
K. Okamoto, I. Niki, A. Shvortser, Y. Narukawa, T. Mukai, and A. Scherer, Nature
Mater. 3, 601 (2004).
3
K. Okamoto, A. Scherer, and Y. Kawakami, phys. stat. sol. (c) 5, 2822 (2008).
4
Electrodynamics of Continuous Media, edited by L. D. Landau, E. M. Lifshitz, and L. P.
Pitaevskii, (Pergamon, Oxford, 1984), pp. 34-42.
3