The ability to induce, observe, and control quantum coherent interactions in room-temperature, electrically driven optoelectronic devices is of utmost significance for advancing quantum science and engineering towards practical applications. We demonstrate here a coherent interference phenomena, Ramsey fringes, in an inhomogeneously broadened InAs/InP quantum-dot (QD) ensemble in the form of a 1.5-mm-long optical amplifier operating at room temperature. The observation of Ramsey fringes in semiconductor QDs was previously achieved only at cryogenic temperatures and only in isolated single-dot systems. A high-resolution pump-probe scheme where both pulses are characterized by cross-frequency-resolved optical gating reveals a clear oscillatory behavior both in the amplitude and the instantaneous frequency of the probe pulse with a period that equals one optical cycle at the operational wavelength. Using nominal input delays of 600–900 fs and scanning the separation around each delay in 1-fs steps, we map the evolution of the material decoherence and extract a coherence time of 340 fs. Moreover, we observed a unique phenomenon, which cannot be observed in single-dot systems, that the temporal position of the output probe pulse also oscillates with the same periodicity but with a quarter cycle delay relative to the intensity variations. The modulation of the pulse separation results from coupling between the real and imaginary parts of the susceptibility and the quadrature delay is the time domain manifestation of its complex nature.

• Received 5 December 2017
• Revised 1 June 2018

DOI:https://doi.org/10.1103/PhysRevB.97.241117