Fast Photodarkening in Amorphous and Liquid Chalcogenide
- PDF / 139,515 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 76 Downloads / 221 Views
0918-H03-03
Fast Photodarkening in Amorphous and Liquid Chalcogenide Yoshifumi Sakaguchi, and Kozaburo Tamura Graduate School of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, Kyoto, 606-8501, Japan ABSTRACT Transient photo-induced optical absorption in amorphous and liquid As2Se3 was investigated using a nanosecond pulsed laser and a specially designed optical cell. The measurements have been performed in the second and minute domain and the nanosecond and microsecond domain. From the measurements in the second and minute domain using repeated illumination, we observed the accumulation of photo-induced absorption and the decay after stopping the illumination. The durable photodarkening was also observed after the decay. The accumulated photoinduced change becomes smaller with increasing temperature and approaches to be zero around the glass transition temperature. From the measurements in the nanosecond and microsecond domain, the transient photodarkening in the time domain responded for each pulsed laser was observed. This fast photodarkening was observed even in the liquid state. This is contrast to the cases of the durable photodarkening and the transient photodarkening in the second and minute domain. The origin of the observed photoinduced changes is discussed. INTRODUCTION Chalcogenide glasses exhibit reversible photodarkening, which is characterized by a shift of absorption edge to lower energy after prolonged illumination of band-gap light [1]. The photodarkening effect recovers by annealing near and below the glass transition temperature, Tg [1]. The optical change is accompanied by the structural change as the volume change [2] and the change of x-ray diffraction pattern [3] suggest. The structural change is not the local one such as the creation of defects. The whole bulk of the sample is involved in the structural change. In amorphous materials, there are various metastable states, which originate from the structural randomness of the materials. The state changes from one metastable state to another by the prolonged band-gap illumination. The photodarkening effect can be understood by the trapping of a state in a certain metastable state after stopping the illumination. It is pointed out that the excitation of lone-pair electrons plays a crucial role in the occurrence of the photodarkening [1]. It is expected that the excitation of lone-pair electrons induces a structural change raising the state to be a certain higher photo-excited state. After experiencing some relaxation processes, the state is finally trapped in a certain metastable state. To clarify the effect of the excitation of lone-pair electrons on the photodarkening, it is important to examine the photo-induced changes throughout the time range from the excitation of lone-pair electrons to the final state. Especially, an immediate response should be investigated. In this paper, we report the results of the measurements of transient photo-induced optical absorption in amorphous (a-) and liquid (l-) As2Se3. The absorption me
Data Loading...
