Visible Anisotropic Deformation of Chalcogenide Glass by Optical Force
K. Tanaka, N. Terakado and A. Saitoh
Department of Applied Physics, Graduate School of Engineering, Hokkaido University,
Sapporo, 060-8628, Japan (keiji@eng.hokudai.ac.jp)
Soft materials such as chalcogenide glass and azo-polymer are known to exhibit a variety
of photo-induced mechanical changes [1,2]. The mechanisms are speculative, while in many
cases, photo-electro-atomic processes such as trans-cis conformation changes [2] have been
proposed as triggering processes. We here report that a covalent chalcogenide glass As2S3
exhibits surprisingly prominent anisotropic deformations when exposed to linearly-polarized
bandgap light [3,4]. The deformation seems to be triggered, not by the atomic force, but by
the optical torque.
The photographs shown below give an example. The sample is an annealed As2S3 film (Eg
≈ 2.4 eV) with lateral dimensions of 50-100 μm and a thickness of ~1 μm, which has been put
on a layer of silicone grease (a), and is exposed to an unfocussed linearly-polarized laser
beam with photon energy of 2.3 eV and intensity of 40 mW. After an exposure of 10 min (b),
the film curls as a U shape along the electric field (indicated by the arrow, the length being
scaled to 50 μm) of the light. Then, the film elongates orthogonal to the field, spirals at ~100
min (c), and finally becomes thread-like ((d) 1000 min).
(a)
(b)
(c)
(d)
Mechanism of this anomalous deformation has been investigated through several
experiments. Crystalline As2S3 flakes and amorphous Se show, respectively, orientation
alignment and different deformation. Related microscopic and macroscopic experiments
suggest that the U-shape deformation (b) and elongation (c, d) are induced by photo-induced
birefringence, photoinduced fluidity, and optical torque. However, there remain quantitative
problems, and also the spiral (d) is puzzling.
[1] V.M. Kryshenik, M.L. Trunov, and V.P. Ivanitsky, J. Optoelectron. Adv. Mater. 9, 1949
(2007).
[2] C.J. Barett, J. Mamiya, K.G. Yager, and T. Ikeda, Soft Matter 3, 1249 (2007).
[3] K. Tanaka, Appl. Phys. Express 1, 12006 (2008).
[4] K. Tanaka, N. Terakado, and A. Saitoh, J. Optoelectron. Adv. Mater. 10, 124 (2008).