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Physical Review A

Route toward the ideal thresholdless laser

S. M. Dutra, J. P. Woerdman, Jorrit Visser, Gerard Nienhuis

Published Feb 26, 2002• 2 citations
Publisher

Abstract

We consider what happens to a laser when all incoherent processes are reduced to the minimum needed to keep emission irreversible. Specifically, we investigate the case where the vacuum Rabi frequency is larger than any decay rate in the laser except for the atomic polarization decay rate. Using a fully quantum description, we show that this laser can be made to go continuously from a regime with a well-defined threshold to the ideal thresholdless regime, where the photon statistics is always Poissonian even for arbitrarily small pump powers. We suggest how a proof-of-principles experiment can be realized in the microwave domain.

DOI
10.1103/physreva.65.033824

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We consider what happens to a laser when all incoherent processes are reduced to the minimum needed to keep emission irreversible. Specifically, we investigate the case where the vacuum Rabi frequency is larger than any decay rate in the laser except for the atomic polarization decay rate. Using a fully quantum description, we show that this laser can be made to go continuously from a regime with a well-defined threshold to the ideal thresholdless regime, where the photon statistics is always Poissonian even for arbitrarily small pump powers. We suggest how a proof-of-principles experiment can be realized in the microwave domain.
An almost ideal thresholdless laser can be realized in the strong-coupling regime of the light-matter interaction, with Poissonian fluctuations of the field at all pumping powers. Here, we show that this ideal scenario is thwarted by quantum nonlinearities when crossing from the linear to the stimulated emission regime. A universal jump in the normalized intensity correlation function is predicted to occur, the measurement of which could be used to establish a standard fingerprint of the onset of lasing in the strong coupling regime.
Lasing threshold in the conventional lasers is the minimum input power required to initiate laser oscillation. It has been widely accepted that the conventional laser threshold occurring around a unity intracavity photon number can be eliminated in the input-output curve by making the so-called β parameter approach unity. The recent experiments, however, have revealed that even in this case the photon statistics still undergo a transition from coherent to thermal statistics when the intracavity mean photon number is decreased below unity. Since the coherent output is only available above the diminished threshold, the long-sought promise of thresholdless lasers to produce always coherent light has become questionable. Here, we present an always-coherent thresholdless laser based on superradiance by two-level atoms in a quantum superposition state with the same phase traversing a high-Q cavity. Superradiant lasing was observed without the conventional lasing threshold around the unity photon number and the photon statistics remained near coherent even below it. The coherence was improved by reducing the coupling constant as well as the excited-state amplitude in the superposition state. Our results pave a way toward always-coherent thresholdless lasers with more practical media such as quantum dots, nitrogen-vacancy centers and doped ions in crystals.
We solve the problem of a laser with variable pumping statistics for any relative magnitude of the atomic- and cavity-decay constants, and obtain a different regime of sub-Poissonian light generation. We show that, even for Poissonian pumping, the noise in the amplitude quadrature outside the cavity can be reduced up to 50% below the shot-noise level when the polarization but not the populations can be adiabatically eliminated. Maximum noise reduction in this case is obtained when the lower level decays much faster than the upper one and occurs at a frequency given by the geometrical mean of the decay rates of the field and the lower-level population. Furthermore, the full consideration of atomic memory effects leads to a generalization of previous results on regularly pumped lasers. We find that, for regular pumping, maximum amplitude-noise reduction (up to complete quieting) still occurs at zero frequency in all cases. However, a long-living polarizaton leads to an increase in amplitude noise and may even eliminate the dip at zero frequency, at the same time leading to a significant quenching of the laser linewidth in the bad-cavity limit.
Although conventional lasers operate with a large number of intracavity atoms, the lasing properties of a single atom in a resonant cavity have been theoretically investigated for more than a decade. Here we report the experimental realization of such a one-atom laser operated in a regime of strong coupling. Our experiment exploits recent advances in cavity quantum electrodynamics that allow one atom to be isolated in an optical cavity in a regime for which one photon is sufficient to saturate the atomic transition. In this regime the observed characteristics of the atom-cavity system are qualitatively different from those of the familiar many atom case. Specifically, we present measurements of intracavity photon number versus pump intensity that exhibit "thresholdless" behavior, and infer that the output flux from the cavity mode exceeds that from atomic fluorescence by more than tenfold. Observations of the second-order intensity correlation function demonstrate that our one-atom laser generates manifestly nonclassical light that exhibits both photon antibunching and sub-Poissonian photon statistics.