Nonlinear optical spectroscopy of dipole and electron spin coherences in semiconductor nanostructures

Vadim G. Lyssenko, John Erland Østergaard

I. Ya. Gerlovin, V. V. Ovsyankin, Boris V. Stroganov +1 more

D. Golde, T. Meier, S. W. Koch

Gaël Nardin

Abstract Multidimensional coherent optical spectroscopy (MDCS) is an elegant and versatile tool to measure the ultrafast nonlinear optical response of materials. Of particular interest for semiconductor nanostructures, MDCS enables the separation of homogeneous and inhomogeneous linewidths, reveals the nature of coupling between resonances, and is able to identify the signatures of many-body interactions. As an extension of transient four-wave mixing (FWM) experiments, MDCS can be implemented in various geometries, in which different strategies can be used to isolate the FWM signal and measure its phase. I review and compare different practical implementations of MDCS experiments adapted to the study of semiconductor materials. The power of MDCS is illustrated by discussing experimental results obtained on semiconductor nanostructures such as quantum dots, quantum wells, microcavities, and layered semiconductors.

Meenakshi Dutt

In this work, we study the resonant nonlinear optical response obtained when optical fields are used to excite charged semiconductor quantum dots (QDs). The basic physics of charged excitons (trions) created by optical excitation of charged QDs is of interest from the point of view of optically driven spin based quantum computing (QC). Stimulated Raman excitation, resonantly enhanced by the optical dipole coupling to the trion state, was used to optically generate electron spin coherence in the ground state of charged QDs. The evolution of the spin coherence was monitored through quantum beats in the phase-sensitively detected four wave mixing signal. The decay of the beats is a measure of the spin coherence time, which was found to be at least an order of magnitude greater than either the trion dipole coherence, or the Raman coherence time between excitons in a single neutral QD. A fascinating outcome of the experiment was the first observation of a contribution to the spin coherence induced by the vacuum field, which is also responsible for spontaneous emission from the trion state, known as spontaneously generated coherence (SGC). QC demands that coherent optical manipulations should be performed within the spin coherence time. We performed coherent optical control experiments, with a pair of phase-locked pump pulses, that showed the quantum nature of the coherence through interference of different quantum mechanical pathways. We showed both in experiment and theory that we can manipulate the spin coherence on the time scale of the Larmor frequency, as well as on an ultrafast femtosecond time scale, corresponding to the optical frequency of the trion state. Finally, resonant optical excitation of both the bright and nominally forbidden transitions from a single electron spin in a QD to the trion state were demonstrated. This allows for direct optical access to all the transitions required for optically driven QC with QD electron spins. We used frequency-domain nonlinear spectroscopy measurements on single QD trions to obtain the trion dipole decoherence and decay rates. Further, we demonstrated that the single electron spin coherence could be generated and detected optically, and found that the spin coherence time was significantly greater than in ensemble measurements at the same magnetic field.

Gabriela M. Slavcheva, Philippe Roussignol