59 Bulgarian Chemical Communications, Volume 45, Special Issue B (pp. 59-62) 2013 Sensor properties of asymmetric Bragg stack from chalcogenide glass and PMMA A. Lalova * , R. Todorov Institute of Optical Materials and Technologies “Acad. J. Malinowski”, Bulgarian Academy of Sciences, Acad. G. BonchevStr., bl. 109, 1113 Sofia, Bulgaria. Received October 17, 2013; Revised November 25, 2013 Asymmetric Bragg stack with one defect layer was designed and prepared through layer-by-layer deposition of spin coated Poly (methyl methacrylate) (PMMA) and vacuum deposited chalcogenide film. In this work, first the thickness changes, Δd of the thin films from poly (methyl methacrylate) following exposure to chloroform vapors in the concentration range 1000 - 9000 ppm were determined. Two layered structure from PMMA and vacuum deposited chalcogenide film was consecutively prepared. The ability of the thin chalcogenide films to let the chloroform vapors passing through was investigated. The asymmetric photonic structure consists of 11 alternating layers from As 30 Ge 10 S 60 chalcogenide glass and PMMA. The defect layer from PMMA is situated before the last high refractive index film from chalcogenide glass. The thickness of the defect layer from poly (methyl methacrylate) was determined so that the pass band to be centered at wavelength of 520 nm. It was observed an offset of the position of the pass band to larger wavelengths after exposure to chloroform vapors. The proposed multilayered structure exhibits potential for applications as optical sensor. Keywords: Bragg stack, chalcogenide glass, PMMA, thin films, sensor properties INTRODUCTION Photonic crystals can be defined as structures in which the dielectric constant (refractive index) shows a periodic variation in one, two or in all three orthogonal directions [1]. 1D photonic crystals, or the so-called multilayer structures consist of alternating layers of two materials with different refractive indices resulting in a periodically varying refractive index in one direction but homogeneous in the other two directions. The thickness of the layers in the Bragg stack is determined by the following equation: nd = λ 0 /4 (1) where n is the refractive index of the layer, d is the thickness and λ 0 is the wavelength of the center of the fundamental reflection band of the Bragg stack. It is known that gases possess a refractive index close to that of air, differing of the order of 10 -4 [2]. It is easily estimated then from formula (1) that different gases permeating into the structure cannot cause a significant change in the position, λ 0 of the fundamental reflection band. Therefore, the manufacture of a gas sensor based on the changes of the refractive index is not possible and materials must be sought that change their volume under the influence of the gas that would be the object of detection. From the literature it is known that upon contact with chloroform thin PMMA (poly methyl methacrylate) films increase their thickness by Δd = 13.6-19.7% [3]. In previous works [4, 5] the possibility was shown for preparation of a Bragg stack from As 30 Ge 10 S 60 /PMMA for the infrared spectral range and the potential for gas sensing application. In the present paper we demonstrate modeling and deposition of multilayered coating from chalcogenide glass and polymer working as Bragg stack in the visible spectral range. The potential for gas sensor application is demonstrated. The chalcogenide glass composition was chosen such that the material would be transparent in the larger part of the visible region and at the same time would possess sufficiently high refractive index. Our previous studies [6] have shown that thin films of this composition have a band gap of 2.45eV (506 nm). EXPERIMENTAL DETAILS Thin films from As 30 Ge 10 S 60 were deposited in high vacuum of 10 -3 Pa by thermal evaporation of previously weighted quantities of the bulk material. Bulk glasses from As 30 Ge 10 S 60 were synthesized in a quartz ampoule from elements of purity 99,999 % by the method of melt quenching [7, 8]. The deposition rate was 0.4 nm/s, and it was monitored * To whom all correspondence should be sent: E-mail: alalova@iomt.bas.bg © 2013 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria