# Large Q factor with very small Whispering Gallery Modes resonators

## TL;DR

This paper introduces a method to significantly enhance the quality factor of small whispering gallery mode resonators by using concentric dielectric rings to reduce bending losses and radiation leakage.

## Contribution

The paper proposes a novel scheme using external dielectric rings to control radiation and greatly increase the Q factor of small micro-resonators.

## Key findings

- Q factor increased by several orders of magnitude
- Bending losses effectively reduced with concentric rings
- Small volume resonators achieve high Q performance

## Abstract

Efficient micro-resonators simultaneously require a large quality factor $Q$ and a small volume $V$. However, the former is ultimately limited by bending losses, the unavoidable radiation of energy of a wave upon changing direction of propagation. Such bending losses increase exponentially as $V$ decreases and eventually result in a drop of $Q$. Therefore, circular cavities are generally designed with radii that are much larger than the optical wavelength. The same leakage of energy by radiation limits the sharpness of bends in photonic integrated circuits. In this article, we present a way to reduce bending losses in circular micro-resonators. The proposed scheme consists of one or more external dielectric rings that are concentric with the cavity. These rings alter the field outside the cavity where radial oscillations set in, and thus control the far field radiation. As a result, the $Q$ factor can be increased by several orders of magnitude while keeping a small cavity volume.