Large Q-Factors in Small WGM Resonators

Thanks to progress in fabrication techniques, quality factors (Q) of whispering gallery mode (WGM) resonators are now well above one million with materials that are relevant to Photonic Integrated Circuits (PIC) [1,2]. However, so far only cavities with radii much larger than the operating wavelength λ reach such performances. In order to fully exploit the potential of the available materials, it is desirable to reduce the mode volume V, and so increase the ratio Q/V of the cavity. However while Q is limited by material impurities and parasitic absorption for large resonators, bending losses dominate for small radii. Thus, bending losses represent a fundamental obstacle to the reduction of V.