Self-Induced Transmission on a Free Exciton Resonance in a Semiconductor

Abstract

We observe coherent long-distance propagation of an optical pulse in resonance with the free exciton at high light intensities in an optically thick semiconductor. The experiments show pulse reshaping, pulse breakup, and a high degree of transmission. Microscopic calculations using the semiconductor Maxwell-Bloch equations yield good agreement with the experimental data. [S0031-9007 (98)07576-0] PACS numbers: 78.20.Bh, 71.35.GgPropagation experiments with short light pulses are crucial for the simultaneous study of temporal and spatial coherence in matter. In semiconductors, as opposed to atomic systems which can be modeled by noninteracting two-level systems [1], spatial dispersion [2] and excitationinduced nonlinearities of the excitonic resonance [3][4][5] will result in remarkable modifications of the pulse propagation features. At low light intensities, temporal polariton beating of the transmitted pulse has been observed and shows excellent agreement with linear dispersion theory [6]. In nonlinear experiments, however, many-body effects (such as exciton-exciton collisions), unknown in atomic systems, will destroy the polariton beating after the pulse by interaction-induced dephasing [7-9]. Let us recall the situation in idealized two-level systems: Rabi oscillations (coherent electron density oscillations) lead to lossless soliton propagation of 2p pulses-so-called self-induced transparency (SIT)-and pulse breakup for input pulse areas larger than 3p [10]. Following this area theorem [11], SIT was pioneered in atomic vapor about 25 years ago [12]. In semiconductors, SIT has been demonstrated on bound excitons in CdS [13], which resemble ideal noninteracting two-level systems due to the restricted wave function overlap between different excitons. So far, SIT had not been found on free excitons and the chances for its discovery are low because theoretical investigations [14,15] came to the conclusion that interaction-induced dephasing between excitons with overlapping wave functions will reduce the polarization coherence necessary for the establishment of complete pulse transmission. On the other hand, the appearance of Rabi oscillations in semiconductors [14,[16][17][18], as well as propagation over long distances [19], has been predicted at light intensities larger than p. Thus at least partially coherent propagation should be feasible. In this Letter, we will show experimentally that coherent long-distance propagation and multiple-pulse breakup on a free exciton resonance in a semiconductor at high light intensities is possible. Because many observed features resemble SIT in atomic systems, we will call this newly observed phenomenon self-induced transmission in semiconductors.We used 50-80 fs pulses around 680 nm with pulse energies of about 100 nJ from an optical parametric amplifier [20] pumped by a regenerative Ti:sapphire amplifier (COHERENT REGA) at 200 kHz. Careful alignment using FROG [21] made sure that the pulses were chirp-free, which is essential for the experiment. The pulses were focused onto...

Nonlinear pulse propagation in an intersubband resonance multiple quantum wells

“…Different from the atomic systems which could be modeled as nonlinear two-level systems [5], the many-body effects arising from the macroscopic carrier density in semiconductors [6] may prevent the establishment of complete selfinduced transparency(SIT) [7,8,9]. Nevertheless, the phenomenon self-induced transmission [10,11] is observed in semiconductors, which has many observed features resemble SIT in atomic systems, such as Rabi flopping of the carrier density, coherent nonlinear long-distance propagation, and a high degree of transmission.…”

Optical solitons in semiconductor quantum dot waveguides

“…When the area of the pulse Θ > π, a soliton (2π pulse) is generated. It was shown in reference [10] that solitons can form in waveguides covered with thin films of homogeneously broadened two-level atoms at exact resonance.In semiconductors, experimental observations of selfinduced transmission on a free exciton resonance in CdSe [11] and in InGaAs quantum dot waveguides [8] have been reported. Theoretically, the effect of SIT for solitons in an inhomogeneously broadened SQD ensemble in the presence of single-excitonic and biexcitonic transitions has been investigated numerically in reference [12].…”

“…In semiconductors, experimental observations of selfinduced transmission on a free exciton resonance in CdSe [11] and in InGaAs quantum dot waveguides [8] have been reported. Theoretically, the effect of SIT for solitons in an inhomogeneously broadened SQD ensemble in the presence of single-excitonic and biexcitonic transitions has been investigated numerically in reference [12].…”

Signatures of carrier-wave Rabi flopping in GaAs

“…Illuminating a semiconductor with a constant light intensity can lead to a periodic oscillation of the inversion, a phenomenon which is known as Rabi flopping [1]. Using pulsed excitation, Rabi flopping has been observed on semiconductors [2][3][4][5][6][7][8]. These Rabi floppings exhibited periods in the range from 50 fs to 1 ps.…”

“…However, using the optical Bloch equations in the context of semiconductors has several shortcomings: (i) For envelope Rabi flopping, it has been shown several times [2][3][4][5][6][7][8] within the framework of the semiconductor Bloch equations that the Coulomb interaction of carriers in the bands gives rise to an internal field which adds to the external laser field. This leads to an enhancement of the envelope pulse area Q by as much as a factor of 2 [2].…”