Emission properties of nanolasers during the transition to lasing

Abstract

This review addresses ongoing discussions involving nanolaser experiments, particularly those related to thresholdless lasing or few-emitter devices. A quantum-optical (quantum-mechanical active medium and radiation field) theory is used to examine the emission properties of nanolasers under different experimental configurations. The active medium is treated as inhomogeneously broadened semiconductor quantum dots embedded in a quantum well, where carriers are introduced via current injection. Comparisons are made between a conventional laser and a nanolaser with a spontaneous emission factor of unity, as well as a laser with only a few quantum dots providing the gain. It is found that the combined exploration of intensity, coherence time, photon autocorrelation function and carrier spectral hole burning can provide a unique and consistent picture of nanolasers in the new regimes of laser operation during the transition from thermal to coherent emission. Furthermore, by reducing the number of quantum dots in the optical cavity, a clear indication of non-classical photon statistics is observed before the single-quantum-dot limit is reached. Nanolasers operate in a regime distinctly different to that of conventional lasers, and a consistent model of nanolasing physics is needed. Whereas conventional lasers are characterized by a marked intensity jump at the onset of lasing, micro- and nanocavity lasers with well-developed three-dimensional optical mode confinement can have a vanishing small intensity jump approaching thresholdless behavior. Weng Chow from Sandia National Laboratories in the United States, with colleagues Frank Jahnke and Christopher Gies from the University of Bremen in Germany, has reviewed recent nanolaser experiments and developed a model based on quantum-optical theory to examine photon statistics in the different operational regime of nanolasers compared with conventional lasers. The results dispute the notation of thresholdless lasing and suggests the emergence of non-classical photon statistics for few-atom or few-quantum-dot active regions.