Photoinduced Dichroism in Amorphous As2Se3 Thin Film

by DeForrest, Dan

The dichroism in amorphous As2Se3 induced by a polarized beam of near band-gap light (? = 632.8 nm) was measured in films that ranged in thickness from 0.25 µm to 1.93 µm. Most noncrystalline materials are initially isotropic and homogeneous. When amorphous As2Se3 (a chalcogenide glass) absorbs an intense pump-beam of polarized light, the absorption coefficient for light polarized in the same direction as the pump-beam is less than for the perpendicular polarization, i.e. the sample becomes anisotropic (dichroic). The induced dichroism is reversible by rotating the polarization of the pump light by 90°. Induced dichroism is potentially useful in various photonic devices including optical switches, optical memory, and photowritable polarizers.

Several aspects of photoinduced anisotropy in a-As2Se3 have been analyzed:

(i) rate of photoinduced anisotropy buildup as a function of inducing beam intensity and sample thickness,

(ii) the kinetics of the photoinduced anisotropy buildup in terms of a stretched exponential curve,

(iii) the stretched exponent, ?, as a function of inducing beam intensity and sample thickness,

(iv) reversibility of the photoinduced anisotropy, (v) saturation level of photoinduced anisotropy as a function of inducing beam intensity and sample thickness.

The anisotropy buildup kinetics has been found to follow a stretched exponential behavior and that there exists an inverse relationship between the pump intensity and the time constant, ?. The ?(I) vs intensity (I) relationship more closely follows a logistic dose response curve than a simple straight line or power law relationship. There exists a direct relationship between the time contact ? and the sample thickness, where a longer anisotropy buildup time is required as the sample thickness increases. The stretched exponent, ?, was found to be approximately 0.6 and has a slight dependence on the inducing light intensity. The correlation of the stretching exponent, ?, to sample thickness, L, was found to have a weak inverse relationship, that is ? tends to decrease as the sample thickness increases.

The findings in this work demonstrate that the anisotropy orientation could be changed indefinitely since it was found that even after 100 orientation changes the anisotropy saturation had no measurable fatiguing. The anisotropy saturation level was found to be independent of the inducing beam intensity and linearly proportional to the sample thickness.