The Role of Photo-Striction in Tailoring the Nano-Scale Phase Changes in Amorphous Selenium Thin Films
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The Role of Photo-Striction in Tailoring the Nano-Scale Phase Changes in Amorphous Selenium Thin Films S. Gayathri1, G. Sreevidya Varma1, and S. Asokan1, 2 1
Dept. of Instrumentation & Applied Physics, Indian Institute of Science, Bangalore, KA 560012, India
2
Applied Photonics Initiative, Indian Institute of Science, Bangalore, KA 560012, India
ABSTRACT The photo-structural changes in a 2 μm thick amorphous selenium (a-Se) thin film, thermally evaporated on Si substrate, have been studied by micro-Raman spectrometer with 532 nm laser at different laser power densities (1 to 64 W/mm2) and exposure times (15 – 60 s). In addition, the nanoscale photo-structural changes/photo-thermodynamic phase changes are examined on 90 nm a-Se film coated on SiO2 cladding of the optical Fiber Bragg Grating (FBG) sensor on prolonged exposure to a low power (1 mW/mm2, 10 mW/mm2) 532 nm laser, which is measured by means of photostriction (light induced strain) in the material. INTRODUCTION Several photo-induced phenomena are observed in a-Se films when irradiated with bandgap and supra-bandgap light [1,2,3]. Yet our understanding of their origin and correlation between them is still ambiguous [1]. Many of the practical applications of a-Se in linear and non-linear optics are based on its interesting optical properties [4]. The a-Se thin film can be considered as an ideal test material for our study, since (i) the phase transformations can be induced relatively easier by supra-bandgap light illumination, and (ii) it can be prepared by relatively simple and inexpensive methods such as vacuum thermal evaporation with reproducible properties over large areas [1,5,6]. Micro-Raman spectroscopy is a powerful technique for observing structural changes qualitatively in chalcogenide glasses [7]. Here, the growth of photo-thermally induced crystallization (PC) in a-Se is examined using Raman spectroscopy with varying power density (P) and exposure time (t) with 532 nm laser. Generally, it is difficult to observe the in-situ growth of photo-crystallization process in a-Se films using Raman spectroscopy due to delay in data acquisition. Therefore, an in-situ monitoring technique has been proposed here, which interfaces 90 nm a-Se film with an FBG sensor for examining the time evolution of dynamic photo-structural changes in relation with photo-striction at lower powers (1 mW/mm2, and 10 mW/mm2). EXPERIMENT A Fiber Bragg Grating (FBG) is an intrinsic sensor fabricated in the core of an optical fiber. In the present work, FBGs are fabricated in a highly photo-sensitive optical fiber (SM1500 (4.2/125)) using the phase mask technique in which the core of the fiber is exposed to a KrF excimer UV laser operating at 248 nm wavelength with pulse energy of 2.56 mJ at 100 Hz. This method inscribes a refractive index grating over a length of 3 mm with a baseline Bragg wavelength around 1550 nm.
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