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Optics Letters

Enhancement of spontaneous and stimulated emission in the microlaser by standing-wave resonant excitation

F. De Martini, Luca Crescentini, Paolo Mataloni

Published Oct 1, 1992• 10 citations
Publisher

Abstract

By a new method of momentum–space excitation an increase of the spontaneous emission rate and a large reduction of the microlaser threshold are obtained. The method is demonstrated by an optically excited micro-laser filled with europium atoms. The method is general and may be extended to active cavities with any dimension and geometry. Our results on spontaneous emission are interpreted as a demonstration of the first-order interference of the vacuum field.

DOI
10.1364/ol.17.001370

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Annual Meeting Optical Society of America•Jan 1, 1990•0 citations
By a new method of momentum–space excitation an increase of the spontaneous emission rate and a large reduction of the microlaser threshold are obtained. The method is demonstrated by an optically excited micro-laser filled with europium atoms. The method is general and may be extended to active cavities with any dimension and geometry. Our results on spontaneous emission are interpreted as a demonstration of the first-order interference of the vacuum field.
In an active ``microcavity'' the condition of maximum ``enhancement'' of spontaneous emission corresponds to resonant coupling of atoms with a single field mode. We demonstrate that this exceptional condition results in the effective cancellation of spontaneous emission, in the onset of stimulated emission at exceedingly low excitation levels, and in an anomalously high stimulated-emission gain. In the context of phase-transition theory the active microcavity behaves as a statistical ensemble undergoing an order-disorder transition at an extremely high value of the critical temperature.
High gain and virtual zero threshold have been recognized to be distinctive properties of the microlaser since its original proposal and realization by our laboratory in 1988. These properties are investigated both theoretically and experimentally. They are found to be determined by the synergy of several quantum-statistical processes that take place in the condition of extreme field confinement provided by the peculiar Casimir-type topology of the optical microcavity. The determination of the microcavity mode structure leads to a detailed study of the process of spontaneous emission (SpE), its merging with stimulated emission, and the consequent anomalous onset of the collective atomic behavior at low excitation levels. A microlaser excitation threshold of ~50 pJ has been determined experimentally with a molecular Oxazine microlaser excited by a femtosecond source. The relevance in atomic dynamics of the processes of SpE inhibition—enhancement, mode competition, fluorescence loss, interatomic transverse Bose correlations, and periodic excitation—is investigated both theoretically and experimentally. A discussion of the overall process in terms of a second-order phase transition in a nonequilibrium statistical problem is given. The extension of the microlaser dynamics to other quantum systems, such as the microscopic parametric oscillator, and to Raman and Compton scattering is considered.
In recent papers we have reported the first proposal and realization of the “microscopic cavity” (or “microcavity”) i.e. with relevant dimension d~d = (λ/2) where λ is the wavelength of the cavity-confined radiation (1). By this apparatus the Spontaneous-Emission (SE) inhibition-enhancement process in molecular physics (“Purcell effect”) has been demonstrated at optical frequencies (1). In the present work we report the demonstration that the phase of (SE) “enhancement” (i.e. d=d) is actually nonexistent in a microcavity as (SE) is overcome and merges into Stimulated-Emission (StE) at exceedingly low levels of molecular excitation, thus realizing, for the first time in Optics, a “Zero-Threshold-Laser” action. This is a direct consequence of the peculiar and novel condition of Quantutm-Statistical-Electrodynamics offered by the microcavity and consisiting of a resonant single-mode field-atom interaction. Assume that an atom or molecule with an homogeneous-broadened resonance at λ and linewidth Δλ = (λ·Δv/v), placed in a microcavity, emits a fluorescence photon at time t=to, and suppose that this is the only photon present in the cavity at that time.
Recently, we observed enhancement and inhibition of visible spontaneous emission by atoms in a confocal resonator.1 In these experiments, the partial emission rate of the atom into the resonator modes is modified by a factor of 1/(1 − R), where R is the mirror reflectivity. The change in the total spontaneous emission rate is proportional to the solid angle ΔΩ subtended by the mirrors; the change was small (+1.6%, −0.5%) because ΔΩ ≈ 0.04 sr was small. Currently we are conducting experiments in which atoms are placed in a concentric resonator of large solid angle ΔΩ ≈ 3.6 sr and finesse of ~10. We estimate that a 30% decrease and 300% increase should be observable in the total spontaneous emission rate, when the resonator is tuned off or on-resonance, respectively. Results of this new experiment are presented. In addition to studying spontaneous processes, one can also consider the stimulated emission which occurs when a cavity field builds up which contains more than one photon. We have formulated a model which describes both the spontaneous and stimulated emission by the atom into the cavity and clarifies the relationship between enhanced spontaneous emission and stimulated emission.
The method of "Periodic-Pumping" for Spontaneous-Emission (SpE) investigation has been first introduced by our laboratory.1 This method is applied here to vacuum-confinement structures [i.e., the "microcavity"2,3 and single-mirror confinement devices]. The method consists of the detection of the effect of spatial displacement of the excitation-region in the vacuum-confined active-zone on the SpE average probability: Γ = (1/T). The method is illustrated with reference to the Fabry-Perot microcavity with spacing d = d ≡ λ/2, i.e., in "SpE enhancement" condition. There the injection-angle θ of the well-collimated coherent pumping beam (SHG by an unstable-cavity Nd-YaG laser) which interferes only once in the microcavity by singlemirror reflection, is related to the position of the center-of-mass of the active-zone. A strong effect of SpE lifetime change under θ-variation enables the study of the distribution of the self-interfering vacuum field in the microcavity.