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
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.