Stimulated effects and optical resonances of microspheres

Abstract

The peculiar optics of transparent dielectric micrometer sized droplets is a topic of current interest. Key aspects of their behavior are related to the presence of resonances occurring when the particle acts as an electromagnetic cavity. Since these resonances are functions of particle size, shape and refractive index, they are often called morphology dependent resonances (MDRs). Lorentz-Mie scattering theory for spheres predicts a dense spectrum of high Q (>106) resonances throughout the visible. They provide a convenient feedback mechanism for coherent processes such as lasing and stimulated Raman scattering. MDRs also account for the ripple structure observed in elastic scattering. Recent droplet lasing and stimulated Raman scattering experiments in our laboratory using a novel stabilized vibrating orifice aerosol generator and a high resolution spectrograph reveal new insights into the properties of the MDRs. We have been able to identify specific resonances, to measure the experimental Q and compare observed behavior with Mie theory. Anomalous dispersion is found to play a significant role in droplets both through strong frequency pulling effects and in influencing the MDR Q. Multiple radial order MDRs are observed simultaneously in the lasing and SRS spectra consistent with a simple theory we present to account for the relative intensities of the various orders. For methanol droplets <20-µm in diameter, the spectral spacing of MDRs becomes comparable to the Raman gain width and a slow programmed variation of particle size yields a unique size resonance spectrum in the SRS output as various order MDRs are tuned into and out of resonance.