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Applied Physics Letters

Stimulated emission in vicinity of the critical angle

J. K. Asane, Md Golam Rabbani Chowdhury, Kanij Mehtanin Khabir, Viktor A. Podolskiy, Mikhail A. Noginov

Published Jul 19, 2021• 2 citations
Publisher

Abstract

We have demonstrated amplified spontaneous emission (ASE) propagating along the planar interface between two adjacent dielectrics with slightly different refractive indexes. This emission originates from the leaky mode, fueled by optical gain in the low-index dielectric, that is outcoupled to the high-index dielectric in vicinity of the critical angle for total internal reflection. This led us to the observation of spectacular concentric rings of ASE emission occurring above the low and soft stimulated emission threshold. The results of our study can be used to develop novel miniature low-threshold stimulated emission sources and photonic circuits operating at optical frequencies.

DOI
10.1063/5.0051901

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We have demonstrated amplified spontaneous emission (ASE) propagating along the planar interface between two adjacent dielectrics with slightly different refractive indexes. This emission originates from the leaky mode, fueled by optical gain in the low-index dielectric, that is outcoupled to the high-index dielectric in vicinity of the critical angle for total internal reflection. This led us to the observation of spectacular concentric rings of ASE emission occurring above the low and soft stimulated emission threshold. The results of our study can be used to develop novel miniature low-threshold stimulated emission sources and photonic circuits operating at optical frequencies.
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 physical nature of the backward-stimulated emission can be understood by considering the angular distribution of the total radiation from a set of atomic dipoles driven by an intense laser beam. From the classical point of view, the backward-stimulated emission is simply a reflection from the interface between a dielectric and a gain medium from the mismatch in the imaginary parts of their indices of refraction. For bulk gain structure, backward-stimulated emission is usually too weak to be noticed. However, in the case of a gain layer much thinner than a wavelength or a periodic thin gain structure with a spacing of λ/2, the backward-stimulated emission is enhanced and can be as large as the forward stimulated emission in terms of the amplitude of radiation field.1
Super-critical angle fluorescence (SAF) generated from an ensemble of emitters overlying a high-contrast dielectric-dielectric interface is discussed. At increased excitation, signatures of self-amplification of the SAF were observed, which differed from the emission properties of normal fluorescence generated within the critical angle. Contrasting behavior was observed between the SAF and normal fluorescence with respect to bandwidth narrowing for varying gain lengths. Power-law variation was recovered for the peak intensity as a function of excitation energy. Upon disturbance of pristine conditions of the interface, the signatures of the self-amplified SAF were lost.
Summary form only given. Over the last three decades of the 20 th century many experiments have demonstrated that the rate of spontaneous emission for an emitter is determined by its electromagnetic surrounding. This phenomenon has been usually studied for atoms or molecules placed in the vicinity of mirrors or in optical resonators. Furthermore, it is also well-known that the fluorescence lifetime of an emitter changes after being embedded in dielectric media where aside from a change in the refractive index, it experiences the local fields in the dielectric. For a spherical medium larger than the order of a wavelength it is known that the coupling to Mie modes of this dielectric resonator has a large influence on the emission rate of an emitter inside. However, when the medium becomes comparable or smaller than a wavelength one can no longer speak of resonator modes. Here we report on an experiment where we have studied the variation of the spontaneous emission rate as the dielectric medium surrounding an emitter is made smaller and smaller down to a few nanometers.
Introduction Most of the lightwave transmission systems operating today benefit fully from extensive deployment of optical amplifiers (OA). These components perform the amplification of optical intensity directly in the optical domain, without resorting to any electrical-domain regeneration. Optical amplifiers increase the average number of output photons per unit time by several orders of magnitude compared with the incoming signal intensity. However, as with every amplifying process, optical amplification cannot be achieved without paying back for the unavoidable noise power added to the signal intensity. As we will see in detail in Chapter 6, the optical amplification process comes with optical noise generated by spontaneous emission inside the active medium. The intensity of the optical signal is represented by the flux of the signal photon population. Conversely, the photons generated by the spontaneous emission process inside the optical cavity represent the noise photon population. The fundamental phenomenon supporting optical amplification is stimulated emission. The basic property of stimulated emission, making the great difference from spontaneous emission and enabling optical amplification, is the coherent nature of the photon generation process. Accordingly, both signal and noise photons go through the same multiplication process by means of stimulated emission. Unless the optical cavity where the stimulated emission takes place (the active cavity) presents selective features for enhancing the signal photon population relative to the spontaneous one, there is no way to avoid noise photons going through stimulated emission, filling most of the cavity population. As a consequence, both photon populations increase, leading respectively to amplified signal intensity and to amplified spontaneous emission intensity. It should be apparent that every noise photon can create a new identical noise photon as a result of stimulated emission, effectively increasing the noise intensity in the active optical cavity. This is the well-known mechanism of amplified spontaneous emission (ASE) noise in optical amplification. Any improvement in the optical amplifier structure devoted to increasing stimulated over spontaneous emission will probably increase the optical signal-to-noise ratio (OSNR) at the cavity output. The spectrum of gain characteristic of the optical active medium is usually wide enough to support an efficient amplification of the broadband spontaneous emission spectrum, leading to the broadband ASE noise power spectral density.