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Conference on Lasers and Electro-Optics

The excitonic ac Stark splitting in quantum wells

Michele Saba, Francesco Quochi, Cristiano Ciuti, Denis Martin, Jean Louis Staehli, B. Deveaud, Giovanni Bongiovanni, Andrea Mura

Published Aug 29, 2005• 0 citations
Publisher

Abstract

Summary form only given. The occurrence of Rabi flopping in excitonic transitions of semiconductors demonstrate that an intense light field can induce a non-perturbative ac Stark effect despite the very fast excitation-induced dephasing. However the homogeneous broadening induced by the strong excitation prevented the observation in quantum wells of the spectral counterpart of Rabi oscillations in time, the ac Stark splitting. We report the evolution of the absorption spectrum of a specially designed single InGaAs quantum well as a function of the growing excitation density. The excitation is provided by a pulsed Ti:sapphire laser (100 fs of pulse duration) and the measurements are taken in a pump-probe geometry.

DOI
10.1109/iqec.2000.907871

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Summary form only given. The occurrence of Rabi flopping in excitonic transitions of semiconductors demonstrate that an intense light field can induce a non-perturbative ac Stark effect despite the very fast excitation-induced dephasing. However the homogeneous broadening induced by the strong excitation prevented the observation in quantum wells of the spectral counterpart of Rabi oscillations in time, the ac Stark splitting. We report the evolution of the absorption spectrum of a specially designed single InGaAs quantum well as a function of the growing excitation density. The excitation is provided by a pulsed Ti:sapphire laser (100 fs of pulse duration) and the measurements are taken in a pump-probe geometry.
Ultrafast pump-probe measurements evidence the dynamical Stark splitting in the exciton absorption spectrum of a single semiconductor quantum well. The intensity of the Stark sidebands in the spectrum is comparable to the linear absorption and their separation scales as the pump field amplitude. In the time domain the Rabi oscillations are observed. The semiconductor Bloch equations solved in the Hartree-Fock approximation demonstrate that the coherent exciton-light coupling dominates over the many-body dephasing.
Summary form only given. In the excitonic ac Stark shift in quantum wells the excitonic absorption line is blueshifted by shining on the sample an intense laser pulse, whose frequency is lower than the exciton transition frequency. The effect is bigger when the laser spectrum approaches the resonance with the excitonic absorption. But if the laser and excitonic absorption spectra sensibly overlap, a great amount of electronic population is excited and consequently the Coulomb nonlinearities due to the interaction between excited carries or excitons dramatically increase. One prominent effect under these conditions is the excitation- induced dephasing which can destroy the coherence of the electronic system and therefore the coupling between excitons and light, which is inherently coherent. Some evidence for the resonant Stark effect was already reported, but the direct observation of the spectral splitting was still missing. This observation demonstrates that the coherent exciton light coupling in semiconductors can be strong enough to survive the fast dephasing, and that therefore an efficient ultrafast manipulation of the exciton absorption in the coherent transient is possible. In order to magnify the ac Stark effect and its visibility a special sample was designed: a single In/sub 0.04/Ga/sub 0.96/As quantum well is grown on the top of an AlAs/AlGaAs distributed Bragg reflector; two /spl lambda//2 spacers separate the quantum well from the mirror and from the air. In this configuration the light field incident on the sample and the one reflected by the mirror constructively interfere at the quantum well position, giving a high effective absorption.
The Rabi splitting of the negatively charged exciton in a single InGaAs quantum dot is observed in resonance transmission spectroscopy. We use a pump laser excitation to drive strongly the unpolarized trion transition in a quantum dot and detect its modified absorption spectrum with a second weak probe laser. By tuning the pump laser near resonance, we observe an ac-Stark effect dispersion, with a power dependent Rabi splitting on resonance, both signatures of a strongly coupled two level system. Although the pump and probe laser fields are resonant with the same transition, we do not observe all features in the Mollow spectrum. We combine the results of pump probe with saturation spectroscopy data to deduce the individual contributions to the low power linewidth.
The coherent optical response of high quality microcavities containing quantum wells is investigated. The dynamics of the ac Stark effect is investigated by degenerate pump–probe experiments, by exciting the quantum well excitons at various intensities and detunings. The evolution from a simple dynamical excitonic Stark shift to a splitting is clearly observed, even when the excitation is degenerate with the continuum states. The experimental results agree well with the predictions of a theoretical model based on the semiconductor Maxwell-Bloch equations.
Recently the shortest pulses directly out of a Ti:sapphire laser as short as 6.5fs has been generated by use of single quantum well (SQW) GaAs absorber [1]. Since this laser produces pulses with durations shorter than or comparable to the polarization dephasing time and sufficient power for Rabi flopping of the excited carrier density, the noninstantaneous response of the absorber polarization has to be taken into account. We develope the theory of coherent absorber mode locking taking into account also inhomogeneous broadening of the semiconductor transition by freely movable carriers and Coulomb renormalization of the transition frequency in the scope of semiconductor Bloch equations. Photo-echo measurements in highly excited GaAs yield a decreasing of the effective dephasing with increasing of the laser excited carrier density as T2 ∝ N-1/3. In the SQW absorber the center wavelength of 750nm is well above the bandedge of the GaAs SQW at 870nm and the broad pulse spectrum excites mainly 3D carriers above the confinement depth. We have shown that Rabi-flopping is self-consistently produced due to sub10fs pulse formation at TW/cm2 intensities corresponding to an excited carrier density up to 1020cm-3.
Summary form only given. We report studies of exciton Rabi splitting where spectrally narrow pump and probe pulses were used. In this case, coherent spectral oscillations in pump-probe spectra can be avoided by tuning the central frequency of the probe pulse. The intensity dependence of the Rabi splitting obtained from these studies reveals a distinctly nonlinear relationship between the Rabi frequency and the applied electric field, in contrast to the linear dependence expected from mean-field based theoretical models. Pump-probe experiments were performed on a 130 /spl Aring/ GaAs/AlGaAs multiple QW sample at 10 K.