The application of femtosecond spectroscopy to the study of ultrafast dynamics in semiconductor materials and nanostructures is reviewed with particular emphasis on the physics that can be learned from it. Excitation with ultrashort optical pulses in general results in the creation of coherent superpositions and correlated many-particle states. The review comprises a discussion of the dynamics of this correlated many-body system during and after pulsed excitation as well as its analysis by means of refined measurements and advanced theories. After an introduction of basic concepts—such as coherence, correlation and quantum kinetics—a brief overview of the most important experimental techniques and theoretical approaches is given. The remainder of this paper is devoted to specific results selected in order to highlight how femtosecond spectroscopy gives access to the physics of coherences, correlations and quantum kinetics involving charge, spin and lattice degrees of freedom.First examples deal with the dynamics of basic laser-induced coherences that can be observed, e.g. in quantum beat spectroscopy, in coherent control measurements or in experiments using few-cycle pulses. The phenomena discussed here are basic in the sense that they can be understood to a large extent on the mean-field level of the theory. Nevertheless, already on this level it is found that semiconductors behave substantially differently from atomic systems. Subsequent sections report on the occurrence of coherences and correlations beyond the mean-field level that are mediated either by carrier–phonon or carrier–carrier interactions. The corresponding analysis gives deep insight into fundamental issues such as the energy–time uncertainty, pure dephasing in quantum dot structures, the role of two-pair or even higher correlations and the build-up of screening. Finally results are presented concerning the ultrafast dynamics of resonantly coupled excitations, where a combination of different interaction mechanisms is involved in forming new types of correlations. Examples are coupled plasmon–phonon and Bloch–phonon oscillations.The results reviewed in this paper clearly reveal the central role of many-particle correlations and coherences for the ultrafast dynamics of dense semiconductor systems. Both the presence of strong correlation effects and the formation of coherences in a genuine many-particle system have important implications for the controllability of optical signals from this class of materials, which is of utmost importance for applications in present-day and future optoelectronic devices.

https://iopscience.iop.org/article/10.1088/0034-4885/67/4/R01/pdf

Femtosecond spectroscopy in semiconductors: a key to coherences, correlations and quantum kinetics

Upon femtosecond laser irradiation, a bright, broadband photoluminescence is observed from graphene at frequencies well above the excitation frequency. Analyses show that it arises from radiative recombination of a broad distribution of nonequilibrium electrons and holes, generated by rapid scattering between photoexcited carriers within tens of femtoseconds after the optical excitation. Its highly unusual characteristics come from the unique electronic and structural properties of graphene.

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Nonlinear broadband photoluminescence of graphene induced by femtosecond laser irradiation

Nonlinear broadband photoluminescence of graphene induced byfemtosecond laser irradiation

Using two-dimensional spectroscopy, we resolve multi-polariton coherences in quantum wells embedded inside a semiconductor microcavity and elucidate how multi-exciton correlations mediate polariton nonlinear dynamics. We find that polariton correlation strengths depend on spectral overlap with the biexciton resonance and that up to at least four polaritons can be correlated, a higher-order correlation than observed to date among excitons in bare quantum wells. The high-order correlations can be attributed to coupling through the cavity mode, although the role of high-order Coulomb correlations cannot be excluded.

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Influence of multi-exciton correlations on nonlinear polariton dynamics in semiconductor microcavities

Summary form only given. Due to the possibility of realizing strong exciton-photon coupling in semiconductor microcavities, one could expect that also biexcitons are strongly modified by polariton effects, leading to formation of bipolaritons. In this work, we have investigated the nonlinear optical properties of a high-quality quantum well (QW) microcavity in time-resolved pump-probe spectroscopy at T=5 K. The sample consists of a 25 nm GaAs single QW in the center of a wedged /spl lambda/-cavity with AlAs-AlGaAs Bragg reflectors.

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Biexcitons or bipolaritons in a semiconductor microcavity

Summary form only given. Recent pump-and-probe measurements performed on the exciton-polariton in GaAs-based microcavities have been interpreted in terms of contributions due to biexcitons. Inspired by these initial hints we investigated a microcavity containing a single 5 nm ZnSe quantum well which provides large biexciton binding energies and small exciton linewidths. As a result we report here the first direct observation of a spectrally isolated, pump-induced resonance due to polariton-biexciton transitions in a microcavity.

U. Neukirch

Papers

Polariton-biexciton transitions in a semiconductor microcavity