报告题目：Tailoring between network rigidity and light induced effects in a-GexAs35-xSe65 thin films: femtoseconds to seconds

报告人：Pritam Khan

报告时间：5月5日（周四）下午4:30

报告地点：物理学院新楼五楼多功能厅

报告摘要：Chalcogenide glasses (ChG) are based on the chalcogen element S, Se and Te in combination with group IV or V elements of the periodical table. One of the unique and striking properties of ChG is the exhibition of a wide variety of lightinduced effects. Such effects have various applications ranging from holographic recording, waveguide designing, making diffraction elements and fabricating nano-antenna. Lightinduced effects in ChG are instigated from the photo-generation of electron-hole pairs which lead to the formation of exciton by the mutual Columbic interaction. The excitons are highly unstable and consequently interact with the deformable lattice of ChG and become self-trapped. The exciton self-trapping is associated with the structural rearrangement in the lattice that accounts for the various lightinduced optical properties in ChG. The lightinduced effects depend strongly on ChG structure, i.e. rigidity of the network because the extent of self-trapping will be different in floppy and rigid network. Till date, there is no direct relationship between the lightinduced effects and the rigidity of ChG network. The primary aim of the present thesis is to study the tailoring between network rigidity and lightinduced effects in a-GexAs35-xSe65 (0 ≤ x ≤ 25) thin films in a time domain that extends from nanoseconds (ns) to seconds. The thesis addresses another independent problem: kinetics of Ag photo-dissolution and photo-diffusion in As2S3 matrix as a function of the thickness of Ag and As2S3 layer. We have used time resolved pump-probe optical absorption spectroscopy to study the lightinduced effects.

Continuous wave (cw) pump-probe optical absorption study in a-GexAs35-xSe65 thin films reveals that metastable lightinduced effects do not exhibit a regular trend with network rigidity, rather Ge:As ratio plays the major role in determining the lightinduced response. Composition dependent light-induced responses of these samples indicate that there exist two parallel competing mechanisms of instantaneous photodarkening (PD) and slower photobleaching (PB). Subsequent Raman analysis on as-prepared and illuminated sample provides the direct evidence of structural rearrangements. For example, lightinduced enhancement in AsSe3/2 pyramidal unit gives rise to PD, whereas photo-oxidation of Ge together with decrease in edge sharing GeSe4/2 tetrahedra lead to PB. In a stark contrast, nanosecond (ns) laser induced transient absorption (TA) decreases dramatically from floppy to rigid system in consistent with network rigidity theory. Our results provide the direct experimental evidence of a self-trapped exciton.

报告人简介：2011-Present Doctorate of Philosophy, Department of Physics, Indian Institute of Science Education and Research Bhopal, MP, India

Supervisor: Dr. K. V. Adarsh

2009-2011 Master of Science, Department of Physics, University of Calcutta, India (Award Date: July 2011); Grade – “A”, 4.200 on a scale of 5, First Class, 64 %

2006-2009 Bachelor of Science, Department of Physics, Presidency College, Kolkata, India (Award Date July 2009), First class, 71.875%

May 2010 – July 2010，Summer Intern Fellow, Department of Physics, S. N. Bose National Centre for Basic Sciences, Saltlake, Kolkata

April 2015-August 2015，Visiting Scientist, Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, USA

邀请人：徐红星教授