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Coherent polarizations in the band-to-band continuum of bulk GaAs are studied in pump-probe experiments with 20 fs pulses. For the first time, we observe quantum beats on a 100 fs time scale that are due to an impulsively excited quantum coherence between heavy and light hole states. The beat frequency is determined by the heavy-light hole energy splitting changing continuously with the energy separation between the laser and the band gap. Theoretical calculations of the coherent response based on the semiconductor Bloch equations for a three-band scheme account for the data.[S0031-9007(96)02256-9] PACS numbers: 78.47.+ p, 42.65.Re, 78.20.Ci The fundamental nonequilibrium dynamics of optically excited semiconductors occurs on ultrafast time scales. Spectroscopy with femtosecond laser pulses provides direct information on such phenomena and has identified a coherent regime of material response in which the nonlinear polarization of the material and the electric field of the pulse couple in a phase-coherent way. In semiconductors with a direct band gap, both excitonic and free carrier excitations give rise to coherent polarizations with distinctly different properties [1-9]. Excitonic polarizations have been studied in great detail and new phenomena like wave packet propagation [1] and/or beating phenomena [2-4] have been observed in femtosecond four-wavemixing experiments where the pulse spectrum overlaps with transitions of different frequency. For an ensemble with different transition frequencies, e.g., heavy hole excitons of different binding energy in quasi-two-dimensional (2D) structures, the oscillatory overall polarization originates from the contributions from the individual two-level constituents [2,4]. A different situation exists if the transitions are coupled via a common state and quantum interference in three-level systems causes polarization beats for heavy hole (HH) and light hole (LH) excitons [3][4][5]. In 2D systems, studies of the coherent response are facilitated by the-relatively slow-picosecond phase relaxation of excitonic polarizations.Much less is known on nonlinear polarizations in the band-to-band continuum of semiconductors, showing dephasing kinetics in the sub-100-fs regime [6]. Recent four-wave-mixing experiments using sub-20-fs pulses close to the band gap of bulk GaAs [7] and bulk CdSSe [8] gave evidence of oscillatory coherent polarizations. In Ref.[7], the oscillations were attributed to the coherent coupling of interband transitions and LO (longitudinal optical) phonons, giving insight into nonMarkovian quantum kinetics. The much higher oscillation frequency in CdSSe was ascribed to intervalence band quantum beats of free carriers. In such experiments, however, the pulse spectrum overlapped with the absorption edge of the material where (i) excitonic effects are important, and (ii) the strong variation of absorption across the pulse spectrum leads to nonlinear propagation effects [9] and/or detuning oscillations [10]. Thus, optical excitation well above the absorption edge i...

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Figure 2. a) SPEs appear as bright localized PL spots at edges and folds in a WSe 2 monolayer. Reproduced with permission. [4] Copyright 2015, Optical Society of America. b) Antibunching and photon cascade from an exciton-biexciton emitter system in GaSe. Reproduced with permission. [9] Copyright 2017, IOP Publishing Ltd. c) Photoluminescence of positioned emitters in MoS 2 . Adapted with permission. [113] Copyright 2021, American Chemical Society. d) Polarization scan of the PL spectrum of a WSe 2 emitter showing exciton and biexciton transitions. The energy differences provide the biexciton binding and fine structure splitting energies. Reproduced under the terms of a Creative Commons Attribution 4.0 International license. [33]