Greg and Kartik have written a Viewpoint in Physics on a recent paper.
The work, by C. Panuski, et al., entitled "Fundamental Thermal Noise Limits for Optical Microcavities", discusses how thermorefractive noise can limit the quality factor (Q) achievable for small mode volume (V) photonic cavities. This may bound the ratio Q/V for a given cavity geometry, which can fundamentally limit the circulating intensity enhancement it provides. The viewpoint discusses the major implications of such result, especially in the context of reaching single photon non-linearities, if thermo-refractive noise is not countered, which may be possible through various techniques.
To read more, check out the viewpoint, or the original paper by C. Panuski et al on Physical Review X