Stress-induced and photo-induced birefringences in As2S3 glass

The birefringence in As ₂S ₃ glass that is induced by either uniaxial stress or illumination with linearly-polarized light ( ħω = 2.0 and 2.3 eV) is comparatively studied using a specially-designed phase meter operating at a wavelength of 633 nm (∼2.0 eV). The photoelastic constant of As ₂S ₃ is negative (−6 × 10 ⁻¹² Pa ⁻¹ ): the refractive index parallel to an applied uniaxial stress becomes smaller, which is contrastive to that (+3.5 × 10 ⁻¹² Pa ⁻¹) in SiO ₂ glass. On the other hand, the photoinduced birefringence in As ₂S ₃ is also negative; i.e., the refractive index in the direction parallel to the electric field of excitation light becomes smaller than the perpendicular. These resembling features with comparable magnitudes of the stress- and photo-induced birefringences imply common structural origin. We propose that the negative birefringences can be understood by a unified model that assumes anisotropically-aligned segmental layer structures which are produced by mechanical and photo-structural atomic motions.