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ANOMALOUS PHASE-TRANSITION AND PHOTON-LOCALIZATION IN STIMULATD-EMISSION IN A MICROSCOPIC OPTICAL CAVITY

F. De Martini

Published Jul 18, 1988• 0 citations
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Abstract

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.

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Vacuum Confinement Effects on Molecular Dynamics in a Microscopic Cavity

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Several experimental schemes of vacuum-confinement in electromagnetic-bounded structures are presented together with related results involving Spontaneously Emitting (SE) molecules. Striking anomalies in the SE spatial-distribution and decay-times are detected. In addition, an anomalous "zero-threshold-laser" behaviour of molecular Stimulated Emission (StE) in a single-mode Casimir-type microscopic cavity is reported. This novel effect is due to an unexpected collective molecular process taking place at an exceedingly low excitation level in the optical-pumping scheme. In the framework of "Phase-transition" theory this effect corresponds to an order-disorder transition taking place at a critical-temperature T c which is virtually infinite.

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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.

Journal of the Optical Society of America B•Feb 1, 1993•42 citations
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.
Several experimental schemes of vacuum-confinement in electromagnetic-bounded structures are presented together with related results involving Spontaneously Emitting (SE) molecules. Striking anomalies in the SE spatial-distribution and decay-times are detected. In addition, an anomalous "zero-threshold-laser" behaviour of molecular Stimulated Emission (StE) in a single-mode Casimir-type microscopic cavity is reported. This novel effect is due to an unexpected collective molecular process taking place at an exceedingly low excitation level in the optical-pumping scheme. In the framework of "Phase-transition" theory this effect corresponds to an order-disorder transition taking place at a critical-temperature T c which is virtually infinite.
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.
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.
The near-zero threshold and the very high gain of the active optical microlaser are investigated both theoretically and experimentally. These properties are found to be determined by the synergy of several quantum-statistical processes taking 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 merging of the spontaneous emission with stimulated emission (StE) and of the consequent anomalous onset of the collective atomic behavior at very low excitation levels. An excitation threshold of about 50 pJ has been determined experimentally with a molecular oxazine microlaser excited by a femtosecond-pulse source. The relevance within the overall StE dynamics of the processes of mode-competition fluorescence loss, interatom transverse Bose correlations, and periodic excitation are 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 microcavity dynamics to quantum-dynamical systems such as the ``microscopic'' parametric oscillator and to Raman and Compton scattering is also considered.
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.