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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Coherent quantum-well absorber mode locking for excitation dependent dephasing

Vladimir P. Kalosha, Michael Müller, J. Herrmann

Published Jan 1, 1998• 0 citations
Publisher

Abstract

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.

DOI
10.1364/cleo_europe.1998.ctui59

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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. 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.
The regime of passive mode locking in a Ti:sapphire laser with a coherent absorber resulting in the emission of pulses with a duration considerably shorter than the absorber relaxation times is demonstrated experimentally. A cell filled with rubidium vapor placed in the laser cavity serves as a coherent absorber. Mode locking arises when the laser oscillation frequency is tuned to the resonance transitions in rubidium. In our opinion, 2π pulses of self-induced transparency appear in the cavity, and the reported results represent the first experimental demonstration of passive mode locking caused by the effect of self-induced transparency, which has previously been discussed only theoretically.
Intracavity self-induced transparency of a three-level absorber is studied in the scope of solid-state laser generation of an ultrabroadband electromagnetic pulse that drives the population of all absorber levels through complete Rabi flopping. We show that at sufficient pump rates a Ti:sapphire laser forces an intracavity GaAs single quantum-well absorber, which provides an inter-valence-band transition in the THz domain in addition to two direct optical interband transitions, into the self-induced transparency regime and acts as an all-solid-state ultrabroadband pulse emitter. In dependence on the resonator bandwidth, the intracavity pulse energy and the absorber dipole moments we obtain a multilevel self-induced transparency pulse spectrum which extends from the THz domain up to the ultraviolet. The steady-state sub-10-fs pulse consists of only a few optical cycles with the high-frequency components at its leading edge and a single to subcyclic THz component at its trailing edge.
Recently, a dependence of Rabi flopping on the carrier-envelope phase of the exciting laser pulses was predicted theoretically [Phys. Rev. Lett. 89, 127401 (2002)] for excitation of a thin semiconductor film with intense few-cycle pulses. Here, we report corresponding experiments on 50-100-nm thin GaAs films excited with 5-fs pulses. We find a dependence on the carrier-envelope phase arising from the interference of sidebands from the fundamental or the third-harmonic Mollow triplet, respectively, with surface second-harmonic generation.
Summary form only given. High-intensity pulse propagation of femtosecond laser pulses in optically thick semiconductor crystals can lead to self-induced transmission when the laser is tuned to a free exciton resonance. This effect means that at intensities on the order of MW/cm/sup 2/, pulses in the subpicosecond range lead to Rabi flopping of the carrier density. This causes the pulses to break up temporally after long-distance coherent propagation. The effect depends strongly on the coherence of the incident light field with the polarization of the exciton transition created in the semiconductor. Despite the expected excitation-induced dephasing, a large amount of coherent nonlinear transmission and a high contrast ratio of the Rabi flops are found at sample temperatures /spl les/10 K. In order to study the dynamics of the coherently driven polarization and the dependence on external dephasing, we introduced phonon-scattering by raising the temperature of our samples. The experiments were conducted with 800 fs pulses of an optical parametric amplifier in epitaxially grown CdSe. Our experiments clearly demonstrate that the coherent coupling of the free carrier polarization to the driving field of a high-intensity subpicosecond pulse with an area larger than /spl pi/ effectively prevents the expected dephasing by carrier-carrier scattering. Opening an additional dephasing channel by introducing phonon-scattering gradually weakens the coupling and suppresses characteristic features of self-induced transmission.
Summary form only given. In our recent experiments on the model semiconductor GaAs using 5 fs pulses, the Rabi period became as short as 3 fs, which is comparable to the period of light of 2.9 fs at the GaAs band edge. Hence, the medium's dynamics significantly distort the optical polarization within one optical cycle, which can be measured by a characteristic shape of the harmonics of the emitted light. In this paper we summarize our efforts to obtain a detailed understanding of these experiments by simulations based on a finite difference time-domain algorithm, solving the coupled Maxwell-Bloch equations in one dimension without using either the rotating wave approximation or the slowly varying envelope approximation. We consider not only transitions close to the band edge but also transitions up to 5.8 eV photon energy in a realistic fashion, i.e. comprising the known E/sub 1/ and E/sub 2/ resonances in GaAs which are due to band structure effects.