Vladimir S. Ilchenko

ULTIMATE Q OF OPTICAL MICROSPHERE RESONATORS

We demonstrate the quality factor Q - (0.8 +/- 0.1) x 10(10) of whispering-gallery modes in fused-silica microspheres at 633 nm, close to the ultimate level determined by fundamental material attenuation as measured in optical fibers. The lifetime of ultimate Q is limited by adsorption of atmospheric water. Monitoring of adsorption kinetics with submonolayer sensitivity by Q factors and frequencies of whispering-gallery modes is demonstrated. The possibility of supermaterial Qs owing to intrinsic suppression of scattering losses in micropheres is discussed.

Quality-factor and nonlinear properties of optical whispering-gallery modes

Abstract The properties of optical resonators with quality-factor Q⩾108, effective volume of e.m. field localization Veff≈10−9 cm3 and threshold power of optical bistability Wbist≈10−5 W are described. The prospects to reduce Veff and Wbist are discussed. With possible reduction of controlling energy of optical switching down to a single quantum and employment of the monophotonic states of light, the whispering-gallery microresonators can open the way to realize Feynmans quantum-mechanical computer.

Optical resonators with whispering-gallery modes-part I: basics

We briefly review basic properties of dielectric whispering gallery mode resonators that are important for applications of the resonators in optics and photonics.

Optical microsphere resonators: optimal coupling to high-Q whispering-gallery modes

A general model is presented for coupling of high-Q whispering-gallery modes in optical microsphere resonators with coupler devices that possess a discrete and continuous spectrum of propagating modes. By contrast to conventional high-Q optical cavities, in microspheres the independence of high intrinsic quality-factor and controllable parameters of coupling via an evanescent field offer a variety of regimes similar to those that are already available in rf devices. The theory is applied to data reported earlier on different types of couplers to microsphere resonators and is complemented by the experimental demonstration of enhanced coupling efficiency (∼80%) and variable loading regimes with Q>108 fused-silica microspheres.

Optical resonators with whispering-gallery modes-part II: applications

We review photonic applications of dielectric whispering-gallery mode (WGM) resonators-tracing the growth of the technology from experiments with levitating droplets of aerosols to ultrahigh-Q solid state crystalline and integrated on-chip microresonators.

High-Q measurements of fused-silica microspheres in the near infrared

Measurements of the quality factor Q approximately 8x10(9) are reported for the whispering-gallery modes (WGMs) of quartz microspheres for the wavelengths 670, 780, and 850 nm; these results correspond to finesse f approximately 2.2x10(6) . The observed independence of Q from wavelength indicates that losses for the WGMs are dominated by a mechanism other than bulk absorption in fused silica in the near infrared. Data obtained by atomic force microscopy combined with a simple model for surface scattering suggest that Q can be limited by residual surface inhomogeneities. Absorption by absorbed water can also explain why the material limit is not reached at longer wavelengths in the near infrared.

Rayleigh scattering in high-Q microspheres

The Rayleigh scattering has to be largely suppressed in high-Q whispering-gallery modes in microspheres because of restrictions imposed on scattering angles by cavity confinement. Earlier estimates of the fundamental limit for the quality factor in fused-silica microspheres are revisited, and Q≃1012 is predicted in few-millimeter-size fused-silica spheres, if the surface hydration problem is ovecome. Particular effects of surface scattering losses are analyzed, and the manifestation of scattering in the form of intermode coupling is calculated. The predominant effect of counterpropagating mode coupling (intracavity backscattering) is analyzed in the presence of a mode-matched traveling-wave coupler. As much as 100% resonance reflection regime is shown to be feasible.

Tunable Optical Frequency Comb with a Crystalline Whispering Gallery Mode Resonator

We report on the experimental demonstration of a tunable monolithic optical frequency comb generator. The device is based on four-wave mixing in a crystalline calcium fluoride whispering gallery mode resonator. The frequency spacing of the comb is given by an integer number of the free spectral range of the resonator. We select the desired number by tuning the frequency of the pumping laser with respect to the corresponding resonator mode. We also observe a rich variety of optical combs and high-frequency hyperparametric oscillation, depending on the experimental conditions. A potential application of the comb for generating tunable narrow band frequency microwave signals is demonstrated.

Pigtailing the high-Q microsphere cavity: a simple fiber coupler for optical whispering-gallery modes.

We demonstrate a simple method for efficient coupling of standard single-mode optical fibers to a high- Q optical microsphere cavity. Phase-matched excitation of whispering-gallery modes is provided by an angle-polished fiber tip in which the core-guided wave undergoes total internal reflection. In the experimental setup, which included a microsphere with both an input and an output coupler, the total fiber-to-fiber transmission at resonance reached 23% (total insertion loss, 6.3 dB), with loaded quality factor Q> or =3 x 10(7) and unloaded Q approximately 1.2 x 10(8) at 1550 nm. A simple pigtailing method for microspheres permits their wider use in fiber optics and photonics devices.

Experimental observation of fundamental microwave absorption in high-quality dielectric crystals

Abstract This paper is devoted to the experimental study of small microwave losses (tan δ ≈ 10−4−10−9) in high-quality dielectric single crystals. Wide-temperature-range measurements of the loss tangent at frequencies 9–72 GHz have indicated the existence of fundamental (lattice) and extrinsic (due to imperfections) microwave absorption in sapphire and YAG crystals. Strong distinctive temperature dependence of the lattice absorption (tan δ ∼ Tα, α = 4 or α = 5 for different crystal symmetries) allows to identify it with the “photon+phonon→phonon” interaction due to lattice anharmonicity.