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Annual Meeting Optical Society of America

Photon localization in microcavity stimulated emission

F. De Martini, Michele Marrocco, D. Murra

Published Jan 1, 1989• 0 citations
Publisher

Abstract

The process of stimulated emission from two nearby microlasers excited in an active microscopic cavity has been adopted to investigate the onset of the Bose-Einstein correlation established over very short distances d of order λ from the fluorescence source of stimulated radiation.1,2 A marked effect of photon localization, which is unexpected according to standard QED, has been experimentally found. The effect is relevant for distances of the order bλ, for b ranging between 10 and 100, for a microcavity spacing equal to λ/2, i.e., in the cavity single resonant mode condition.2 The effect tends to disappear for macroscopic cavity spacings, reproducing in this case the expected QED behavior, i.e., the one which is well known in standard laser physics. The dependence of the effect on the microcavity finesse and on the mirror reflectivities has been investigated.

DOI
10.1364/oam.1989.wl39

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Physical Review Letters•Oct 1, 1990•83 citations
The process of stimulated emission from two nearby microlasers excited in an active microscopic cavity has been adopted to investigate the onset of the Bose-Einstein correlation established over very short distances d of order λ from the fluorescence source of stimulated radiation.1,2 A marked effect of photon localization, which is unexpected according to standard QED, has been experimentally found. The effect is relevant for distances of the order bλ, for b ranging between 10 and 100, for a microcavity spacing equal to λ/2, i.e., in the cavity single resonant mode condition.2 The effect tends to disappear for macroscopic cavity spacings, reproducing in this case the expected QED behavior, i.e., the one which is well known in standard laser physics. The dependence of the effect on the microcavity finesse and on the mirror reflectivities has been investigated.
The process of zero threshold laser action in stimulated emission (StE) has been first reported by our laboratory, together with spontaneous emission (SpE) anomalies.1,2 In this work a new microcavity technique aimed at the investigation of the Bose-Einstein quantum correlations between two microlasers coupled to the same quasi plane-wave microcavity k-mode, orthogonal to the microcavity mirrors and excited by two independent beams at λ = 0.53 μm SHG by a common TEM00 Nd:YAG unstable-cavity laser. In the experiment the micrometrically controlled transverse distance s is the relevant variable of the experiment. The StE radiation at λ = 0.632 μm emerging from the microcavity on the k-mode is detected through an efficient spatial filter. The measurement of the exponential thresholdless-laser gain at various coherent-pump intensities of the overall laser system leads to the functional s-dependence of a relevant quantity: the "degree of Bose-Einstein correlation" a (s, d) among the active spots1,2 excited in the microcavity. The behavior of this quantity for microcavity spacings d = [d ≡ (λ/2, 5d, 10d], demonstrates that the rapid quantum decorrelation taking place in the condition of maximum confinement, i.e., at d = d, decreases at large d values approaching asymptotically, for a macroscopic-cavity d > d, the full correlation over the entire plane-wave phase surface expected according to the common QED notion of full photon freespace delocalization. An extended theoretical account of this novel effect of fundamental atom-field interaction physics and laser physics is given on the basis of the quantum interference taking place within the StE process among the plane waves belonging to the modal k-distribution for a microcavity of finite finesse. A region in the microcavity of interaction coherence (referred to as a photon-localization: effect) is established, giving rise to a strong dependence of the quantum-mechanical interatom correlation on the parameters of the microcavity. The theoretical results are found to be in agreement with our experimental findings. General considerations on the QED concept of transversal photon localization will be given on the basis of our results. These are apparently the first to be concerned with such a fundamental topological problem. This one is found to involve, in a new manner, relativistic retardation effects as well as a new EPR-type experimental configuration. Experimental results on the process of field interference by the partially interacting microlasers1,2 will also be reported. This topic raises (and our results answer) additional questions regarding the quantum structure of light. The QED theory of the interference process will also be given with reference to our results.
Anomalous quantum processes in the context of spontaneous emission (SpE)1,2 and stimulated emission (StE)3,4, which are closely related to the fundamental dynamics of photon emission and to the related plane-wave quantum photon delocalization process4,5 were studied by our laboratory by use of the optical microcavity. The most efficient method we adopt in our present studies consists of a detailed investigation of the Bose-Einstein (BE) transverse quantum correlation process established within the microcavity active plane between two identical cylindrical microlasers (with diameter δ≈30 μm) excited by two identical focused pump beams (at wavelength λ p = 0.53 μm when working within nanosecond exciting pulse-duration (Δτ) conditions, SHG by an unstable-cavity Nd-YAG laser) and coupled to the same and common microcavity plane-wave, forward k-mode. Our investigation of the quantum coupling process consists of the determination of the thresholdless laser-gain (g) of the overall active device as a function of the radial distance s between the microlasers and determined by the position of the pump focused beams. The solution of the detailed QED intermicrolaser coupling equations leads to the determination of the s-dependence of the relevant dynamical quantity α(s), the degree of BE transverse quantum correlation by assuming the microcavity spacing d = d ¯ N , d ¯ ≡ λ / 2 as a relevant parameter. The overall laser output radiation is detected by a high quantum efficiency, cooled photomultiplier by previous focusing onto a 20-μm pinhole, making a spatial filter that dis criminates efficiently against detection of radiation emitted on FP-cavity modes other than the forward-mode.4
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.
An extended theoretical and experimental analysis is reported of the classic problem, first discussed by E Fermi, of the interaction between two quantum systems A and B. In our case, the two systems are confined within an optical microcavity and the interaction takes place in the transverse direction, i.e. orthogonal to the only k-wavevector allowed by the cavity with the minimum spacing d= 1/2 lambda 0 . In these conditions, our theory analyses in detail the process of two-atom super-radiance. The experiment consists of an investigation, by the use of a femtosecond laser, of the process of coupling of two microlasers active over the same transverse mode of an optical microcavity.
A microcavity technique is adopted to investigate, we believe for the first time, the process of transverse interatom quantum correlations taking place after photon emission, according to the plane-wave QED photon-delocalization model. We find that the QED model is indeed verified, but that it is generally limited by the nonideal cavity properties. The concept introduced here of ``transverse quantum-correlation length'' is of fundamental relevance to all kinds of Casimir-type vacuum-confinement processes, including ``anomalous'' QED effects in spontaneous emission. It also sets a fundamental limitation on the performance of the ``thresholdless laser,'' a device of actual scientific and technological interest.