Novel photoinduced anisotropy in amorphous As50Se50 films at near the glass transition temperature

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Abstract

The results of an experimental study of high-temperature measurements of photoinduced anisotropy and the first Raman spectra of the photoinduced anisotropic state of amorphous As50Se50 films are given. Two novel photo-anisotropic phenomena were observed near the glass-transition temperature, Tg, and a possible explanation of these phenomena in terms of light-induced preferential bond breaking within crystalline clusters or rotation of these clusters is suggested.

Introduction

The absorption of linearly polarized light in vitreous chalcogenide semiconductors excites predominantly centres (i.e., molecular units, lone-pair electrons or defect micro-domains) having the transition dipole moment parallel to the electric vector of the light [1]. As a result of non-radiative recombination at these excited centres, a macro-anisotropic structure is formed in a previously isotropic material having a preferential orientation of transition dipole moments. A subsequent change of the direction of the electric vector of the inducing light to the orthogonal orientation leads again to a redistribution of the dipole moments and, hence reversible photoinduced anisotropy (PA) can be produced in a relatively flexible glassy network, manifesting on a macroscopic scale as reversible dichroism (αα) or birefringence (nn), where α is the optical absorption coefficient and n is the refractive index for a probe beam polarized parallel (‖) or perpendicular (⊥) to the electric vector of the inducing light, respectively. In certain semiconducting vitreous chalcogenides, it has been observed that such cycles of directional `photodarkening and photobleaching' can be repeated virtually indefinitely with no significant reduction of magnitude.

In this paper, we present the results of an experimental study of the PA around the glass-transition temperature Tg of amorphous As50Se50 films.

Section snippets

Experimental procedures

Thin-film samples were prepared by thermal evaporation of As50Se50 bulk glass onto silica substrates. The film thickness was ∼2 μm. Raman spectra were taken in back-scattering geometry with an excitation laser wavelength λ=1064 nm. The random error in measuring band maxima was ±1 cm−1. The set-up allowed the polarization of the inducing Raman beam to be changed with a λ/2 waveplate so that the orientation of the electric vector E is either vertical (V) or horizontal (H) and the polarization

Results

The measured photoinduced optical anisotropy was determined as the polarized degree of transmission P defined as P=(II)/(I+I) where I and I are the intensities of transmitted light through the sample, polarized parallel (‖) or orthogonal (⊥) to the polarization state of the inducing light [1]. To avoid the influence of scalar photodarkening, samples were initially irradiated with polarized light changed at regular intervals (10 min) for about 3 h at room temperature, until the

Discussion

Recently, a reversible athermal photo-induced amorphous state of a crystalline chalcogenide alloy (As50Se50) has been observed [5]. The photo-produced amorphous film was subsequently crystallised by annealing below the glass-transition temperature. Such a process can be repeated apparently indefinitely [5]. It has been suggested 6, 7, 8that during the process of crystallisation, amorphous As50Se50 forms molecular units isomorphous with that of realgar, the naturally occurring form of As4S4, and

Conclusions

(1) We have observed `reversed' PA in amorphous As50Se50 with exponential growth kinetics but opposite `sign' and about three times larger amplitude of dichroism compared to room-temperature PA. (Fig. 1b).

(2) We have measured `giant' PA of amorphous As50Se50 during the period when the thin film is crystallising, reaching magnitudes five times larger compared with room-temperature PA.

(3) Such `giant' or high-temperature PA has different kinetics, being `delayed' after the change of the plane of

Acknowledgements

It is a pleasure for P.K. to acknowledge financial support of the Wolfson Foundation.

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