Nonlinearity and photostructural changes in glassy As2S3 thin films

Non-linear absorption of chalcogenide thin films has been investigated with femtosecond pump-probe experiments. Different peculiarities of femtosecond pulse absorption have been found in the case of on- and off-resonance pumping and probing, due to either intraband or interband transitions. The physical mechanisms which can explain such different behaviours are discussed.

On-off resonance femtosecond non-linear absorption of chalcogenide glassy films

Professor B.T. Kolomiets was one of the authors of the discovery of electronical semiconductor properties of chalcogenide glasses. He developed a complex investigations of the structure and physical properties of a large group of chalcogenide glasses. He often underlined the importance of this class of materials for understanding of non-crystalline solids physics. Such appreciation can be found, for example, in one of the last of his summary papers ”Chalcogenide glasses” written by him in 1981 on the occasion of the jubilee of Saint-Petersburg Physical-Technical Institute. In that paper the main attention was paid to describing the unique features of the glassy semiconductors, the the new phenomena, first theoretical notions and potentialities for practical applications were mentioned. Prof. Kolomiets was very reminiscent and noted in that article the contribution of Prof. A.F. Ioffe and Prof. N.A. Goriunova in the discovering of the new group of semiconductors — glassy semiconductors. I am happy that my scientific activity began in the famous laboratory of Prof. B.T. Kolomiets in 1959 while the first researches of the chalcogenide glasses were developed, especially researches concerning optical, electrical, photoelectrical phenomena, etc. I remain very obliged to Prof. B.T. Kolomiets for the possibility to work in his laboratory and to deal with new class of non-crystalline semiconductors which became then materials of great interest in many laboratories of the world. This is why it is a particular pleasure for me to contribute with a special article in honour of Professor B.T. Kolomiets ninetieth birthday. First of all, it is worthy to note what is the reason of interest for mentioning that class of matirials. The main reason of interest of the scientific community to this class of materials was the fact that this class of glassy semiconductors successfully complements the properties of well known groups of crystalline semiconductors. Really of an insignificant change of the composition or the content in the crystals is quite sufficient for achieving a sharp change of physical parameters, then chalcogenide glasses admit smooth change of properties even in cases of considerable changes of the composition or the strong alloy. If many electronic processes in the crystals are controlled by the redistribution of charge carriers on the separate discrete local levels, then in the chalcogenide semiconductors a quasicontinous distribution of localized states is discovered, which generates a great peculiarity of electronic processes, in particular, in the transport phenomenon, in optical and photoelectric processes. For example, in the chalcogenide glass semiconductors the mobility of the charge carriers may differ from the mobility of crystals by million and more orders of magnitude. If the crystals having hard crystal structures do not permit removing of atoms from their fixed position in the network, the chalcogenide glasses, as well as other disordered materials having a more labile structure, permit changing of the position of the neighbor atoms already on the insignificant energetic influence by means of optical and X-ray radiation or electron beam. Comparing the properties of crystals and of glasses, the enumeration of these peculiarities of the chalcogenide glasses may be continued, but probably, even those mentioned above are quite sufficient for stressing the originality of the properties of chalcogenide glasses, allowing us to propose the above-mentioned class of materials for wide application. We will confine ourselves to presenting only a short information concerning the peculiarities of energetical spectrum and its contribution to the photoinduced phenomena. Some results of application of this group of materials in optoelectronics will be also presented.

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Chalcogenide glasses in optoelectronics

Two aspects of photoinduced phenomena in oxide and chalcogenide glasses have been studied. One is a comparison of irreversible, reversible, and transitory changes in As2S3, which gives insights into universal features of the photoinduced phenomenon. The other is composition dependence of structures and photodarkening in GeO2–GeS2 glasses. The dependence suggests that percolating GeS2 structures are required for the photodarkening.

Photoinduced phenomena in group VIb glasses

We report Er-doped Ge-Ga-Se films and waveguides deposited using co-thermal evaporation and patterned with plasma etching. Strong photoluminescence at 1.54 µm with intrinsic lifetime of 1 ms was obtained from deposited films with 1490 nm excitation. Erbium population inversion up to 50% was achieved, with a maximum of ~55% possible at saturation for the first time to the author’s knowledge, approaching the theoretical maximum of 65%. Whilst gain was not achieved due to the presence of upconversion pumped photoinduced absorption, this nonetheless represents a further important step towards the realization of future chalcogenide Erbium doped waveguide amplifiers at 1550 nm and in the Mid-infrared.

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Greater than 50% inversion in Erbium doped Chalcogenide waveguides.

Measurements are reported on photoinduced optical absorption (PA) in chalcogenide glass fibres. Experimental results on kinetics and stationary state characteristics are presented. The analysis of experimental results in terms of a multiple trapping model enables the authors to evaluate the characteristic energy of localized states distribution.

I P Culeac

Papers

Photoinduced changes of optical absorption in chalcogenide glass fibres