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Semiconductor Lasers and Laser Dynamics

How short time scales substitute for cryogenic cooling: quantum coherent effect in room temperature QD amplifiers (Conference Presentation)

G. Eisenstein

Published May 23, 2018• 0 citations
Publisher

Abstract

How short time scales substitute for cryogenic cooling: Quantum Coherent Effect in Room Temperature QD Amplifiers Gadi Eisenstein1 and Johann Peter Reithmaier2 1Electrical Engineering Dept. and Russell Berrie Nanotechnology Institute Technion – Israel Institute of Technology, Haifa 32000 Israel 2Technological Physics, Institute of Nanostructure Technologies and Analytics, CINSaT, University of Kassel, 34132 Kassel, Germany Abstract Semiconductor quantum dots (QDs) serve often as a two-level system for studying quantum coherent phenomena. Observation of quantum coherent effects requires observation times that are shorter than the coherence time. In room temperature semiconductor QDs, those times are of the order of 350 fs and therefore it is common to cool the QDs to cryogenic temperatures. A few years ago, we have introduced an alternative approach by which the cooling is substituted for by operation with ultra-short pulses having durations of 100-200 fs. The platform we employ is a 1.5 mm long InAs/InP QD optical amplifier. Measurements use the cross frequency resolved gating (X-FROG) technique and the experiments are accompanied by a comprehensive model that treats the QD amplifier as a cascade of two level system and also considers the QD gain inhomogeneity as well as non-resonant interactions such as two photon absorption and an accompanying Kerr-like effect. Several quantum coherent effects have been demonstrated including Rabi oscillations and self- induced transparency as well as coherent control of the Rabi oscillations achieved by shaping the pulses spectral phase. A modification of the experimental set up by which two pulses in a pump probe configuration are used with each pulse being separately measured using the X-FROG technique enables to demonstrate the optical analog of Ramsey-Resonances. Setting a nominal delay between the input pulses and changing the delay in 1 fs steps, we observe clear Ramsey fringes in the probe amplitude, instantaneous frequency and surprisingly also the separation between the two output pulses. The latter stems from the coupling between the real and imaginary parts of the susceptibility. It is unique to our system as it a cumulative effect that requires a reasonably long propagation distanced and can therefore only be seen in a waveguide configuration and never in a single QD which was used in all previous Ramsey experiments. The last effect is photon echo which was only studied numerically up to now. The talk will survey the various experimental results and will highlight the essence of operating with short pulses to induce and observe quantum coherent effects in room temperature QD amplifiers.

DOI
10.1117/12.2306218

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How short time scales substitute for cryogenic cooling: Quantum Coherent Effect in Room Temperature QD Amplifiers Gadi Eisenstein1 and Johann Peter Reithmaier2 1Electrical Engineering Dept. and Russell Berrie Nanotechnology Institute Technion – Israel Institute of Technology, Haifa 32000 Israel 2Technological Physics, Institute of Nanostructure Technologies and Analytics, CINSaT, University of Kassel, 34132 Kassel, Germany Abstract Semiconductor quantum dots (QDs) serve often as a two-level system for studying quantum coherent phenomena. Observation of quantum coherent effects requires observation times that are shorter than the coherence time. In room temperature semiconductor QDs, those times are of the order of 350 fs and therefore it is common to cool the QDs to cryogenic temperatures. A few years ago, we have introduced an alternative approach by which the cooling is substituted for by operation with ultra-short pulses having durations of 100-200 fs. The platform we employ is a 1.5 mm long InAs/InP QD optical amplifier. Measurements use the cross frequency resolved gating (X-FROG) technique and the experiments are accompanied by a comprehensive model that treats the QD amplifier as a cascade of two level system and also considers the QD gain inhomogeneity as well as non-resonant interactions such as two photon absorption and an accompanying Kerr-like effect. Several quantum coherent effects have been demonstrated including Rabi oscillations and self- induced transparency as well as coherent control of the Rabi oscillations achieved by shaping the pulses spectral phase. A modification of the experimental set up by which two pulses in a pump probe configuration are used with each pulse being separately measured using the X-FROG technique enables to demonstrate the optical analog of Ramsey-Resonances. Setting a nominal delay between the input pulses and changing the delay in 1 fs steps, we observe clear Ramsey fringes in the probe amplitude, instantaneous frequency and surprisingly also the separation between the two output pulses. The latter stems from the coupling between the real and imaginary parts of the susceptibility. It is unique to our system as it a cumulative effect that requires a reasonably long propagation distanced and can therefore only be seen in a waveguide configuration and never in a single QD which was used in all previous Ramsey experiments. The last effect is photon echo which was only studied numerically up to now. The talk will survey the various experimental results and will highlight the essence of operating with short pulses to induce and observe quantum coherent effects in room temperature QD amplifiers.
Light matter coherent interactions require that the coherent state induced in the matter be maintained for the duration of the observation. The only way to induce and observe such interactions in room temperature semiconductors, where the coherence time is of the order of a few hundred femtoseconds, is to use ultrashort pulse excitations and an ultrafast characterization technique. For media comprising an ensemble of nanostructure semiconductors such as self-assembled quantum dots, the gain broadening inhomogeneity also affects the interaction. Moreover, when gain media in the form of an active waveguide, such as optical amplifiers, are used, the interaction is distributed and includes nonresonant incoherent phenomena that occur simultaneously with the coherent effects. Such a complex system can exhibit, nevertheless, clear coherent interactions even at room temperature. Using InAs/InP quantum dot and wirelike quantum dash amplifiers, Rabi oscillations, self-induced transparency, coherent control using spectral pulse shaping, Ramsey interference, and photon echo have been demonstrated. The characterization employed cross frequency resolved optical gating, and the experiments were accompanied by a comprehensive finite difference time domain model that solves the Maxwell and Lindblad equations. This work has major implications on the understanding of the details of dynamical processes in active semiconductor devices, on short pulse generation from semiconductor lasers, and on various future quantum devices.
The use of ultrashort pulses allows to induce and observe quantum coherent effects in optical gain media where the coherence time is longer than the excitation pulses namely, room temperature quantum dot ensembles. We use InAs/InP quantum dot optical amplifiers as the experimental platform and have demonstrated effects such as coherently controlled Rabi Oscillations, Ramsey Fringes, Photon`Echoes, Prolongation of dephasing time via induced three pulse photon echoes. Recently we have advanced the field by demonstrating a hallmark experiment of quantum optics: Quantum Coherent collapse and revival which was never observed in a solid let alone at room temperature.
We demonstrate the hallmark concept of periodic collapse and revival of coherence in a room-temperature ensemble of quantum dots (QDs) in the form of a 1.5-mm-long optical amplifier. Femtosecond excitation pulses induce coherent interactions with a number of discrete homogeneous QD subgroups within an inhomogeneously broadened ensemble, which interfere constructively to induce coherent revival (CR). The amplitude decay of CR is dictated by the QD homogeneous linewidth, thus enabling its extraction in a double-pulse Ramsey-type experiment. The more common photon echo technique is also invoked and yields the same linewidth provided that both experiments use the same bias. A dephasing time T2{T}_{2}T2​ longer than 5 ps is extracted. This is a record long T2{T}_{2}T2​ for a room-temperature QD ensemble and testifies to the high quality of the InAs/InP QDs used in the experiments. Measured electrical bias and temperature dependencies of the transverse relaxation times (T2{T}_{2}T2​ and T2∗{T}_{2}^{*}T2∗​) enable the determination of the two main decoherence mechanisms: carrier-carrier and carrier-phonon scatterings.
We report direct observations of Rabi oscillations and self-induced transparency in a quantum dot optical amplifier operating at room temperature. The experiments make use of pulses whose durations are shorter than the coherence time which are characterized using Cross-Frequency-Resolved Optical Gating. A numerical model which solves the Maxwell and Schrödinger equations and accounts for the inhomogeneously broadened nature of the quantum dot gain medium confirms the experimental results. The model is also used to explain the relationship between the observability of Rabi oscillations, the pulse duration and the homogeneous and inhomogeneous spectral widths of the semiconductor.
We study experimentally and numerically the changes in pulse shape a Gaussian laser pulse undergoes when propagating through an electrically pumped quantum dot semiconductor optical amplifier (QD-SOA). The pulse energy is set to be resonant to the quantum dot ground state transition, and the pulse shape is analyzed in phase and amplitude in a heterodyne cross-correlation experiment. For the case of an inverted system, we observe the appearance of a periodic modulation of the temporal pulse profile above a critical pulse power. Similar signatures of pulse break-up have been observed at low temperature in quantum dot ensembles without electrical pumping and were linked to optically induced Rabi oscillations imprinting the coherent switching between absorption and stimulated emission of photons by the material system.
We present a numerical study of coherent control in a room temperature InAs/InP quantum dot (QD) semiconductor optical amplifier (SOA) using shaped ultra-short pulses. Both the gain and absorption regimes were analyzed for pulses with central wavelengths lying on either side of the inhomogeneously broadened gain spectrum. The numerical experiments predict that in the gain regime the coherent interactions between a QD SOA and a pulse can be controlled by incorporating a quadratic spectral phase (QSP) in the pulse profile. The sequential interaction with the gain medium of different spectral components of the pulse results in either suppression or enhancement of the coherent signatures on the pulse profile depending upon their proximity to the gain spectrum peak. In the absorption regime, positive QSP induces a negative chirp that adds up to that of a two photon absorption induced Kerr-like effect resulting in pulse compression while negative QSP enhances dispersive broadening of the pulse.