Ultrafast Spectroscopy of Semiconductor Nanostructures
by Dr. Rohan Singh (Post-doctoral Research Associate, Physical Chemistry and Applied Spectroscopy Chemistry Division, Los Alamos National Laboratory)
Friday, January 11, 2019 from to (Asia/Kolkata)
Semiconductor nanostructures are an important class of materials for optoelectronic applications. Ultrafast spectroscopy is commonly used to probe light-matter interaction in these materials in the picosecond timescale. The optical response in such materials is often dominated by excitons, which are pseudoparticles comprising of an electron-hole pair. After a brief introduction of exciton physics,in this talk I will present results from a couple of studies probing exciton dynamics in semiconductor nanostructures. First, I will present experiments revealing sub-picosecond energy transfer between dopant and the semiconductor host in Mn-doped CdSe colloidal quantum dots. We utilize these unusually fast interactions to ionize a hot exciton in these nanostructures, which has possible applications in photon upconversion . Later, I will discuss two-dimensional spectroscopy experiments that were used to directly map redistribution of exciton energies during spectral diffusion of excitons in disordered semiconductor quantum wells. We see a transition from spectral diffusion regime where increase and decrease in exciton energy are equally probable at higher sample temperatures (> 20 K)  to preferential relaxation of excitons to lower-energy states at lower temperatures . These results were used to demonstrate deconvolution of the exciton dynamics from the optical response .  R. Singh et al., “Unusually Fast Auger Excitation of Hot Electrons due to Spin-Exchange Interactions in Mn-doped Quantum Dots” (in preparation)  R. Singh et al., “Quantifying spectral diffusion by the direct measurement of the correlation function for excitons in semiconductor quantum wells”, J. Opt. Soc. Am. B 33, C137 (2016).  R. Singh et al., “Localization dynamics of excitons in disordered semiconductor quantum wells”, Phys. Rev. B 95, 235307 (2017).  M. Richter et al., “Deconvolution of optical multidimensional coherent spectra”, Sci. Adv. 4, eaar7697 (2018).