Laser cavities are open systems, in that energy can leak to the outside via output coupling. The ‘‘normal modes’’ are therefore quasinormal modes, with eigenvalues that are complex and eigenfunctions that extend outside the cavity, such that any normalization integral is dominated by the region outside; in short, such systems are non-Hermitian. This paper addresses the question: How is the complex eigenvalue (i.e., the mode frequency) changed when the cavity is perturbed by a small change of dielectric constant? The usual time-independent perturbation theory fails because of non-Hermiticity. By generalizing the work of Zeldovich [Sov. Phys.—JETP 12, 542 (1961)] for scalar fields in one dimension, we express the change of frequency in terms of matrix elements involving the unperturbed eigenfunctions, so that the problem is reduced to quadrature. We then apply the formalism to shape perturbations of a dielectric microdroplet, and give analytic formulas for the frequency shifts of the morphology-dependent resonances. These results are, surprisingly, independent of the radial wave function, so that all integrals can be performed and explicit algebraic expressions are given for axially symmetric perturbations.
"Resonant modes are central in nanophotonics and quantum optics and pave the way for enhanced lightmatter interactions with potential applications in energy efficient photovoltaics, integrated photonic circuits and quantum information technology. Examples of resonant modes include the well-known Mie resonances of spherical objects   and localized surface plasmons of plasmonic nanoparticles, with applications in photovoltaics , surface-enhanced Raman scattering  or as " plasmon rulers " . Likewise, the optical modes of microcavities in micropillars or photonic crystals (PhCs) have been used for enhancement of the Purcell effect of quantum emitters  and for realizing cavity quantum electrodynamics experiments  and single-photon emission   as well as for demonstrating nanolasers  and optical switching . "
[Show abstract][Hide abstract] ABSTRACT: We present a numerical method for calculating quasi-normal modes of open nanophotonic structures. The method is based on scattering matrices and a unity eigenvalue of the roundtrip matrix of an internal cavity, and we develop it in detail with electromagnetic fields expanded on Bloch modes of periodic structures. This procedure is simpler to implement numerically and more intuitive than previous scattering matrix methods, and any routine based on scattering matrices can benefit from the method. We demonstrate the calculation of quasi-normal modes for two dimensional photonic crystals where cavities are side-coupled and in-line-coupled to an infinite waveguide.
Journal of the Optical Society of America A 04/2014; 31(10). DOI:10.1364/JOSAA.31.002142 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Temperature measurements were made using a novel microoptical sensor based on dielectric microspheres that are excited by coupling light from optical fibers. The technique exploits the morphology-dependent shifts in resonant frequencies that are commonly referred to as the whispering gallery modes. A change in the temperature of the microsphere leads to a change in both the size and the index of refraction of the sphere which results in a shift of the resonant frequency. By monitoring this shift, the temperature of the environment surrounding the sphere can be determined. The whispering gallery mode shifts are observed by scanning a tunable diode laser that is coupled into the optical fiber on one end and monitoring the transmission spectrum by a photodiode on the other. When the microsphere is in contact with a bare section of the fiber, the optical modes are observed as dips in the intensity of the light transmitted through the fiber. Temperature measurements were made in both air and water using this novel technique. Measurements by the microoptical sensor were compared to those by thermocouples with good agreement between the two sets of results.
"Deviation of the resonator shape from an ideal sphere results in removal of degeneracy by m and other effects. For a spheroid (ellipsoid) characterized with small deviation from an ideal sphere, the frequency shift of each WGM is given by  "
[Show abstract][Hide abstract] ABSTRACT: We review recent advances in the application of dielectric whispering-gallery (WG) resonators in optics and photonics, tracing the growth of the technology from the experiments with freely flying spherical droplets of transparent liquids to integrated on-chip microresonators. Both passive (such as filters) and active (such as lasers) whispering-gallery-mode (WGM)-based devices are discussed. Problems and possible future developments in the field are outlined.
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