Excitonics

Diffraction based pulse shaping, developed by the Nelson group, can produce multiple and arbitrarily shaped coherent femtosecond pulses, with excellent phase stability and high temporal resolution. The spatial and temporal control afforded by this method provides a unique platform for staging phase-matched degenerate four-, six-, and eight-wave mixing experiments; as well as two- and three-dimensional Fourier Transform optical spectroscopy (FTOPT).

We are currently utilizing this novel pulse shaping scheme for χ(3), χ(5), and χ(7) multi-dimensional optical spectroscopy experiments of semiconductor quantum wells embedded in an optical microcavity and organic semiconductor J-aggregate double-walled nanotubes and thin films. The excellent phase stability allows us to probe the coupling and dynamics of multiple electronic states, including those of single- and multi-exciton states. Moreover, the nonlinear signal of interest is detected in the rotating frame, analogous to 2D FT NMR methods developed for radio-frequency pulses.

J-aggregate thin films -- J-aggregates are a class of molecular aggregates that exhibit a wide variety of enhanced nonlinear properties due to very strong intermolecular coupling. They can self-assemble into a variety of nanometer-scale structures, depending on the properties of the molecule. We've begun studies on BIC molecular J-aggregates, which are thought to assemble into two-dimensional brickwork structures (see below). Due to the strong coupling, an optical excitation is coherently shared among many molecules forming a delocalized exciton. The size of the exciton depends on the strength of the coupling and disruptions to the coupling due to disorder. 2D FTOPT can elucidate the mechanisms behind disorder because it separates homogeneous dephasing (dynamic disorder, i.e. exciton-phonon scattering) from inhomogeneous dephasing (static disorder, i.e. energetic disorder). By studying the temperature dependence of the disorder, we can determine which type of disorder controls the delocalization for different temperatures.

All-optical manipulation of exciton-polariton condensates -- Exciton-polaritons are composite particles resulting from strong interactions between exciton states in quantum wells and photon modes in the semiconductor microcavity (see below). They have already been used to successfully demonstrate Bose-Einstein condensation and superfluidity in the past decade. We are currently working on the all-optical manipulation of polariton condensates by using novel two-dimensional potentials. The potential profiles are generated from the high-density exciton reservoir by exploiting the repulsive interactions between excitons. Future work will concentrate on the all-optical dynamic modulation and control of polariton condensates.

Future projects -- Other topics we are interested in investigating in the future include interactions among coupled electronic states in inorganic semiconductor nanocrystals, organic molecular crystals, hybrid J-aggregate/nanocrystal systems, and Bose-Einsten condensation of exciton-polaritons.