Threshold reduction in a cyclic photonic molecule laser composed of identical microdisks with whispering-gallery modes

Lasing modes in cyclic photonic molecules (CPMs) composed of several identical thin semiconductor microdisks in free space are studied in a linear approximation. Maxwell's equations with exact boundary conditions and the radiation condition at infinity are considered as a specific eigenvalue problem that enables one to find natural frequencies and threshold gains. It is demonstrated that careful tuning of the distance between the disks in CPMs is able to drastically reduce the lasing thresholds of the whispering-gallery modes having small azimuth indices.     

Longitudinal whispering-gallery modes in metal microcavities

Longitudinal waves in microcavities emerging because of spatial dispersion are investigated. It is shown that longitudinal modes similar to whispering-gallery modes exist in microcavities of this kind. These longitudinal modes should exist in metal microcavities in the vicinity of the plasma frequency.     

Quality-factor enhancement of supermodes in coupled microdisks

We investigate the optical modes in a coupled pair of semiconductor microdisks in symmetric and asymmetric configurations both experimentally and theoretically. While the quality factors of coupled first- and second-order whispering gallery modes (WGMs) show a conventional crossing, the quality factors of the same-order WGMs reveal an interesting splitting behavior, leading to the formation of high- and low-quality supermodes. Our results are reproduced by numerical simulations, and an explanation based on optical interference is suggested. Quality-factor splitting is a subtle phenomenon that might help to design microarchitectures for efficient optical coupling in cavity quantum electrodynamic experiments.     

Optomechanical dynamics in detuned whispering-gallery modes cavity

The dynamics of a microresonator in detuned whispering-gallery modes (WGM) cavity opto-mechanical system are investigated by the quantum Langevin equation. A WGM cavity coupling to two parallel waveguides is devised to study the transmission and reflection of this system. In single mode WGM cavity, without optomechanical coupling, both the transmission and reflection of the cavity present a Lorentzian dip and peak. When the coupling between the cavity mode and mechanical mode is considered, the transmission and reflection of the optomechanical cavity show “W” and “M” shape mode splitting. Moreover, under the action of a controlling and a probe laser, the output field at the probe frequency presents electromagnetically induced transparency (EIT)-like spectrum in the system. We give the physical origin of EIT-like and the pump-probe response for the WGM shares all the features of the Λ system in atoms. Further, due to backscattering, the two traveling waves in WGM are coupled with a rate γ. The transmission and reflection of the optomechanical cavity display three modes splitting in the spectra with optomechanical coupling between the two cavity modes and the mechanical mode.     

Nonlinear whispering-gallery modes in a microellipse cavity

We study theoretically the effect of deformed microelliptical cavities on lasing characteristics. We show that the transition of stationary lasing states from unidirectional rotational waves to a mixture of clockwise and anticlockwise rotational waves occurs when the shape of a disk is deformed to that of an ellipse.     

Towards controlled coupling between a high-Q whispering-gallery mode and a single nanoparticle

We discuss our recent experiments that aim at the realization of coupling between a nano-emitter that is placed at the extremity of a sharp glass-fiber tip and a high-Q whispering-gallery mode. We quantify the influence of the tip using different probes and modes of a microsphere with different quality factors and mode extensions. Our measurements show that a micron-sized tip results in a substantial perturbation of the modes. On the contrary, by using a tip of diameter about 100 nm it should be possible to couple a nanoparticle to the most-confined modes of a microsphere without spoiling quality factors even as high as 10⁸. Received: 10 August 2001 / Revised version: 17 October 2001 / Published online: 23 November 2001     

High-Q exterior whispering-gallery modes in a metal-coated microresonator

We propose a kind of plasmonic whispering-gallery mode highly localized on the exterior surface of a metal-coated microresonator. This exterior (EX) surface mode possesses high quality factors at room temperature, and can be efficiently excited by a tapered fiber. The EX mode can couple to an interior (IN) mode and this coupling produces a strong anticrossing behavior, which not only allows conversion of IN to EX modes, but also forms a long-lived antisymmetric mode. As a potential application, the EX mode could be used for a biosensor with a sensitivity high of up to 500 nm per refraction index unit, a large figure of merit, and a wide detection range.     

Shift of whispering-gallery modes in microspheres by protein adsorption

Biosensors based on the shift of whispering-gallery modes in microspheres accompanying protein adsorption are described by use of a perturbation theory. For random spatial adsorption, theory predicts that the shift should be inversely proportional to microsphere radius R and proportional to protein surface density and excess polarizability. Measurements are found to be consistent with the theory, and the correspondence enables the average surface area occupied by a single protein to be estimated. These results are consistent with crystallographic data for bovine serum albumin. The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target region, with adsorption in the most sensitive region varying as 1/R(5/2). Specific parameters for single protein or virus particle detection are predicted.     

Coupling of whispering-gallery modes in size-mismatched microdisk photonic molecules

Mechanisms of whispering-gallery (WG)-mode coupling in microdisk photonic molecules (PMs) with slight and significant size mismatches are numerically investigated. The results reveal two different scenarios of modes interaction depending on the degree of this mismatch and offer new insight into how PM parameters can be tuned to control and modify WG-mode wavelengths and Q factors. From a practical point of view, these findings offer a way to fabricate PM microlaser structures that exhibit low thresholds and directional emission and at the same time are more tolerant to fabrication errors than previously explored coupled-cavity structures composed of identical microresonators.     

Directional emission from whispering-gallery modes in deformed fused-silica microspheres

Highly directional emissions from whispering-gallery modes are demonstrated in deformed nonaxisymmetric fused-silica microspheres. Ray trajectory analysis indicates that ray dynamics in a three-dimensional reso-nator differs qualitatively from that of a two-dimensional system, with Arnol'd diffusion playing an essential role in the three-dimensional system.     

Microkinetics of the first- and second-order phase transitions

A model of the phase transition in a lattice of interacting nodes, in which each node is a statistical system with internal structure, is introduced. Configuration entropy of microscopic states of the node is defined as a basic parameter of the model. In the frame of the model the first- and second-order phase transitions are considered in details. The distinction between them on the microscopic level is analyzed. Phase diagrams have been calculated in the mean-field approximation. Changes of the phase diagrams and modifications of phase transitions under external pressure and irradiation are investigated in the frame of the microkinetic approach. Results are referred to real systems.     

Observation of second-order hyper-Raman generation in LiNbO3 whispering-gallery mode disk resonators

In this Letter we report experimental demonstration of nonlinear frequency conversion at several optical frequencies in a whispering-gallery mode resonator (WGMR). Because of the enhancement of nonlinear interactions inside a WGMR, interaction of a 1064 nm pump field with a LiNbO(3) disk produced a weak but measurable non-phase-matched 532 nm second-harmonic field at room temperature (>100 °C below the phase-matching temperature) for pump powers of a few tens of milliwatts. For higher pump powers, we observed the generation of four additional fields at 545, 559, 573, and 587 nm. The relative spectral shift between two consecutive fields corresponds to a 455 cm(-1) vibrational mode in LiNbO(3) crystal. Our preliminary analysis indicates that these fields are the result of a multiphonon hyper-Raman scattering in which two photons of the pump field are converted into one photon of a higher-frequency field and one or several optical phonons.     

Identification of first- and second-order magnetic phase transitions in ferromagnetic perovskites

A criterion used for the determination of first- and second-order magnetic phase transitions from purely magnetic methods is applied to manganese perovskites of formula La_2/3(Ca_1−xSr_x)_1/3MnO₃. A crossover from first- to second-order character at a tolerance factor t=0.92 is found, which also brings about several variations in other physical properties. At t=0.92 a change from orthorhombic to rhombohedral symmetry also takes place. The impossibility of establishing static cooperative Jahn–Teller distortions in the rhombohedral symmetry is suggested as being responsible for the observed behaviour.     

Mode manipulation in system of coupled microcavities with whispering gallery modes

In recent years the studies of electromagnetic modes in solid spherical microcavities have been of great interest both for their potential applications and fundamental optical properties. A system of coherently coupled microcavities may be called a “photonic molecule” and can be employed in the tight-binding device in order to manipulate photons in micrometer length scale. In this work we demonstrate the possibility of mode manipulation in systems of symmetric photonic molecules formed by placing several high-Q micro-spheres in contact. We observe photonic nanojets that reflect the symmetry of the photonic molecule, with 3 jets located at 120 degrees with respect to each other for the triangular photonic molecule. A benzene molecule-like structure consisting of a 7-microspheres cyclic photonic molecule shows a field emission pattern similar to the spatial distribution of the orbitals of the benzene molecule. We also present some results showing the coexistence of whispering gallery modes and photonic nanojets in the same structure.     

Controlled coupling of counterpropagating whispering-gallery modes by a single Rayleigh scatterer: a classical problem in a quantum optical light

We present experiments where a single subwavelength scatterer is used to examine and control the backscattering induced coupling between counterpropagating high-Q modes of a microsphere resonator. Our measurements reveal the standing wave character of the resulting symmetric and antisymmetric eigenmodes, their unbalanced intensity distributions, and the coherent nature of their coupling. We discuss our findings and the underlying classical physics in the framework common to quantum optics and provide a particularly intuitive explanation of the central processes.     

Inequivalence of first- and second-order formulations in D=2 gravity models

The usual equivalence between the Palalini and metric (or affinity and vielbein) formulations of Einstein theory fails in two spacetime dimensions for its Kaluza-Klein reduced (as well as for its standard) version. Among the differences is the necessary vanishing of the cosmological constant in the first-order forms. The purely affine Eddington formulation of Einstein theory also fails here.The present results were reported in the Proceedings of the Markov Memorial Quantum Gravity Seminar.This work was supported bt the NSF under grant #PHY-9315811.     

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ～ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (T _C1), to the crystallization temperature (T _ac), and to the Curie temperature in the crystalline phase (T _C2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH～1?12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ～ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ～ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.