Photoinduced refractive index change in As2Se3 by 633nm illumination A. C. van Popta, R. G. DeCorby, C. J. Haugen, T. Robinson, and J. N. McMullin ECE Department and TRLabs, University of Alberta, ECERF Edmonton AB,CANADA, T6G 2V4
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D. Tonchev and S. O. Kasap EE Department, University of Saskatchewan, Saskatoon, SK, CANADA S7N 5A9
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Abstract: Photodarkening of amorphous As2Se3 thin films was generated by a 633-nm HeNe laser. The refractive index and absorption coefficient of the chalcogenide glass was determined, both before and after exposure, by analyzing the material’s transmission spectrum. In order to accurately determine the optical constants, the thin film’s non-uniform thickness was accounted for. The increase in the refractive index and the coefficient of absorption was investigated and was found to demonstrate saturation with increased exposure time. Index changes as high as 0.05, or 2%, were obtained in As2Se3, a promising glass for all-optical switching. © 2002 Optical Society of America OCIS codes: (120.5710) Refraction; (120.7000) Transmission; (160.2750) Glass and other amorphous materials; (300.1030) Absorption; (310.6860) Thin films, optical properties
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1. Introduction Chalcogenide glasses exhibit many attractive optical properties, including a high refractive index, large nonlinearities, and excellent transmission at infrared wavelengths [1,2]. Consequently, chalcogenides are very promising glasses for ultrafast all-optical switching [3,4]. In particular, As2Se3 has a Kerr index approximately 1000x that of silica glass [3]. Chalcogenide glasses also exhibit many photoinduced and electron beam induced phenomena [5,6], including photoexpansion [7,8] and reversible photodarkening [9]. Illuminating amorphous chalcogenide with near band-gap light will cause a red shift of the optical absorption edge and a corresponding increase in the refractive index. The initial state can be recovered by annealing near the glass transition temperature. The exact mechanisms involved are not fully understood and further study is required. The optical properties of amorphous thin films can be measured by analyzing the material’s transmission spectrum. This analysis was pioneered by J. C. Manificer, et al. [10] and extended by R. Swanepoel [11], and since then has been successfully applied to several chalcogenide glasses, including As2Se3 [12]. Swanepoel’s original work [11] assumed a film of uniform thickness. However, it is common for thin films to exhibit a wedge-shaped profile, which may lead to errors in the analysis if left unaccounted for. Fortunately, Swanepoel has also published methods describing how to determine the optical properties of such non-ideal films [13], and these too have been applied to chalcogenide glass [14-16]. Swanepoel’s methods are advantageous because they are non-destructive and yield the dispersion relation over a large range of wavelengths without any prior knowledge of the film’s thickness. In order to use chalcogenide glasses in integrated optics, including wavelength selective devices for WDM networks [17], it is important to know the optical constants and how they can be changed with band-gap illumination. We report on the exposure-dependent change in optical constants of As2Se3 across a broad spectral range. 2. Theory Fig. 1 is a model of a thin film deposited on a transparent substrate. The film and substrate are surrounded by air of index no=1, and the incident light from the spectrophotometer (used to measure transmittance) is normal to the substrate. The film has a refractive index n=n-ik and a coefficient of absorption α = 4πk/λ. The substrate has a refractive index s, and must be thick enough to eliminate any resonant modes apart from those within the film. The film is
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assumed to have a wedge-shaped profile so that the area under illumination varies linearly in thickness according to: (1) d = d ± ∆d The transmission spectrum will contain interference fringes that obey the basic formula: mλ = 2nd
(2)
where: m = 1, 2, 3, … at maximum points in the transmission spectrum. m = 1/2, 3/2, 5/2, … at minimum points in the transmission spectrum. Swanepoel’s method [11, 13] requires envelopes to be constructed through the peaks and troughs of the transmission spectrum. Let TM (λ) describe an envelope containing all the maxima in the transmission spectrum and let Tm (λ) describe an envelope containing all the minima; both are considered to be continuous functions of λ. Restricting the analysis to the highly transparent region (n2 >> k2) and setting α = 0, two transcendental equations result:
TM (λ) =
λ a 1+ b 2πn∆d tan −1 tan 1/ 2 1/ 2 2 2 2πn∆d (1 − b ) λ − 1 b ( )
Tm (λ) =
λ a 1− b 2πn∆d tan −1 tan 1 − b 2 1/ 2 λ 2πn∆d (1 − b 2 )1/ 2 ( )
(3)
(4)
where a = A / (B+D) and b = C / (B+D). Furthermore, A = 16 n2s, B = (n+1)3 (n+s2 ), C = 2 (n2 - 1) (n2 - s2 ), and D = (n – 1)3 (n – s2 ). Eq. (3) and (4) will yield a unique solution for both n and ∆d in the region 0
