Chalcogenide Glass Thin-Film and Fiber Structures for Chemical and Biological Sensing

10.1 INTRODUCTION Chalcogenide glasses (ChGs) are well known for their high infrared (IR) transparency and amenability to fabrication in fiber and thin film forms, which makes them attractive candidates for mid-IR optical chemical and biological sensors. This chapter describes recent advances in the production of sensing devices from a variety of thin film and fiber structures, highlighting the critical materials parameters needed for achieving low loss, high detectivity operation. Because the sensitivity of chemical sensors in thin-film and fiber form is typically directly related to some physical characteristic, such as optical path length, which is much higher in these low-dimension structures than that of the associated bulk glass optics, the physical form of the sensor ultimately dictates or influences the required material properties such as optical loss, viscosity, or coefficient of thermal expansion. Sections 10.2 and 10.3 will review the functionality of ChGs in thin film and fiber sensing applications, with a focus on how this functionality is impacted by the choice of ChG constituents. The strengths and drawbacks of these sensing approaches will be discussed, as some ChG compositions are well-known for limited chemical and temperature stability, as well as high photosensitivity, which can make them excellent candidate material systems for use in some sensing regimes and environments, but preclude their use in other applications. Having outlined the required material characteristics necessary for these sensing applications, the discussion in Section 10.4 will then turn to compositional design approaches used to obtain these characteristics using multi-component ChGs. Varying elemental compositions can be used to tailor glass optical and thermal properties to meet design challenges; however these properties are tightly correlated and so the problem becomes one of global, rather than individual, optimization in the parameter space of interest. This chapter concludes with a brief discussion of the conclusions of the present review, as well as the future directions the authors see this exciting field expanding to include.