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Previous year- Opto-Electronics Workshop September 2009

The Use of Selenium-Based Glasses in High Resolution X-Ray Image Detectors: From Conception to Commercialization
Safa Kasap

Selenium rich glasses are x-ray photoconductors. They can absorb x-rays and generate electrons and holes that can be drifted and collected. The importance of the x-ray photoconductivity of stabilized amorphous selenium (a-Se alloyed with As to stabilize it) was well recognized by Xerox during the fifties and sixties, and Xerox Medical Systems marketed a-Se based xeroradiographic x-ray imaging systems from 1970s to 1990s, when eventually it was closed. The advent of active matrix array (AMA) flat panels based on a-Si:H TFTs during the 1980s and 1990s not only ushered in the ubiquitous flat panel displays but also played a crucial role in the development of flat panel x-ray image detectors. By the end of 1990s, researchers had already demonstrated that a thick a-Se layer could be deposited on a flat panel AMA and then electroded to construct an x-ray image detector [1]. It turned out that these detectors possessed unmatched resolution; unmatched by detectors based on using an x-ray scintillator. Today there are several companies marketing a-Se based large area x-ray image detector for digital x-ray imaging. The road from the initial conception and the demonstration of a prototype to commercialization, however, had a number of very difficult technological obstacles, which had to be overcome before commercial products could be sold (and approved by national health control agencies). The actual detector is not a simple stabilized a-Se layer but a special multilayer structure consisting of three types of a-Se alloys. The present talk will highlight the initial concepts, the realization of the prototype and the technological problems that had to be overcome: The fabrication of thick selenium glass layers over large areas, adhesion of the thick a-Se glass layer to the AMA substrate, the reduction of the dark current, the prevention of crystallization, and the enhancement of electrical properties to achieve the required performance are some of the problems that had to be overcome. Some of these problems have not been totally eliminated. Sound scientific principles and a basic understanding of glass science and technology played an important role in solving the problems and delivering a commercial product. [1] John Rowlands and Safa Kasap, Physics Today, 50, 24 (November 1997).

Rare Earth Doped Chalcogenide Glass: Past Success and Future Prospects
D. W. Hewak, University of Southampton

In the quest for an efficient optical fibre amplifier, rare earth doped chalcogenide glasses experienced a rebirth of interest in the early 1990’s, when these materials were revealed as a candidate for an efficient optical fibre amplifier operating around 1300 nm. This research spawned a wide range of related activities including light sources further into the infrared, targeting the 2 – 5 micron mid-infrared region, chalcogenide optical integrated circuits and other device geometries. In this talk we describe our work with gallium lanthanum sulphide based glasses encompassing over fifteen years of research. Our initial success with praseodymium and dysprosium doped glass and fibre, new transitions in the mid-infrared, the first laser demonstrations in a chalcogenide glass host and most recently glass microsphere based devices will be described. Driven by applications in telecommunications, aerospace, medicine and sensing, research continues with renewed interest in the mid-infrared, where efficient, compact solid state laser sources are lacking. We conclude this talk with an overview of our current activities and future directions

THz Spectroscopy of Photomodified Chalcogenide Glasses
Dr S. K. Sundaram, Pacific Northwest National Laboratory Richland

Chalcogenides glasses are well-known for their remarkable complexity of photosensitive mechanisms, namely, photocrystallization, photodarkening, photodissolution, photoexpansion, photomelting, photovitrification, and photostimulated changes in solubility.  Our objective was to modify glasses in the AsxS100-x system using a HeNe laser and track the changes using a terahertz (THz) spectrometer over a frequency range of about 180-500 GHz.   We processed this family of glasses using established methods for non-oxide glasses and measured THz transmission. Then, we laser-irradiated the glasses and measured THz transmission post-mortem.  We have demonstrated that photomodification affects the THz optical properties in selected chalcogenide glasses.  In two compositions, the refractive index in THz regime has been altered in the same manner (+ 0.06) with equivalent exposures to a 23 mW HeNe (632.8nm) laser.  Though these glasses show promise for application in THz regime, the structural origin of the property changes is not presently fully understood.