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arXiv:2010.15198 · Physical Review Research

Room-temperature coherent revival in an ensemble of quantum dots

Igor Khanonkin, Ori Eyal, Johann Peter Reithmaier, Gadi Eisenstein

Published Jul 20, 2021• 10 citations
arXivHTMLPDF

Abstract

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 ${T}_{2}$ longer than 5 ps is extracted. This is a record long ${T}_{2}$ 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 (${T}_{2}$ and ${T}_{2}^{*}$) enable the determination of the two main decoherence mechanisms: carrier-carrier and carrier-phonon scatterings.

DOI
10.1103/physrevresearch.3.033073

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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.
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.
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.
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