Micro-optical resonators for microlasers and integrated optoelectronics:   recent advances and future challenges
  Optical microcavities trap light in compact volumes by the mechanisms of almost total internal reflection or distributed Bragg reflection, enable light amplification, and select out specific (resonant) frequencies of light that can be emitted or coupled into optical guides, and lower the thresholds of lasing. Such resonators have radii from 1 to 100 mkm and can be fabricated in a wide range of materials. Devices based on optical resonators are essential for cavity quantum-electro-dynamic experiments, frequency stabilization, optical filtering and switching, light generation, biosensing, and nonlinear optics. 
Trevor M. Benson, Svetlana V. Boriskina, Phillip Sewell, Ana Vukovic,   Stephen C. Greedy, and Alexander I. Nosich