Title: Two-dimensional Coherent Spectroscopy of Excitons in Semiconductor Quantum Wells

Speaker: Dr. Rohan Singh, LANL

Abstract: Ultrafast spectroscopy, with subpicosecond time resolution, is an important tool to study fundamental light-matter interactions. Coherent spectroscopy techniques, which measure the coherence decay, are especially useful for problems such as measuring homogeneous linewidths in the presence of inhomogeneous broadening, revealing couplings between multiple states, and revealing many-body interactions. The classic example of such an experiment is four-wave mixing (FWM). Two-dimensional coherent spectroscopy (2DCS) is a powerful extension of the FWM technique and has been used to study a variety of systems including semiconductor nanostructures [1]. The power of the technique lies in unraveling complex one-dimensional signal into two dimensions. While this is helpful to get a good qualitative picture of the light-matter interactions, quantifying these effects can be challenging. In this talk, after introducing the technique, I will discuss my work on quantifying exciton-exciton interactions in semiconductor quantum wells (QWs). Specifically, I will discuss experiments that were used to directly map redistribution of exciton energies during spectral diffusion of excitons in disordered semiconductor quantum wells. We directly measure a transition from regime where increase and decrease in exciton energy are equally probable at higher sample temperatures (> 20 K) [2] to preferential relaxation of excitons to lower-energy states at lower temperatures [3]. Later, I will discuss efforts to quantify exciton-exciton interactions by treating excitons as interacting boson [4].

[1] S. T. Cundiff and S. Mukamel, “Optical multidimensional coherent spectroscopy”, Phys. Today 66, 44 (2013).

[2] 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).

[3] R. Singh et al., “Localization dynamics of excitons in disordered semiconductor quantum wells”, Phys. Rev. B 95, 235307 (2017).

[4] R. Singh et al., “Polarization-dependent exciton linewidth in semiconductor quantum wells: A consequence of bosonic nature of excitons”, Phys. Rev. B 94, 081304(R) (2017).