Coupled microcavities with unidirectional single mode via femtosecond laser direct-writing

Abstract

Optical microcavities play a key role in both fundamental research on light-matter interaction and also applications such as integrated optics and sensors. Among them, whisper gallery mode (WGM) microcavity outstands itself by low loss, high Q-factor and high sensitivity to their dielectric environment. It can be found to have a variety of applications, including nonlinear optics, quantum electrodynamics, bio-sensors, low-threshold lasers, etc. However, the multi-mode nature of WGM microcavity is inconsistent with the basic requirements for these applications, i.e., a single-mode output and tunable wavelength. Therefore, the modulation of whisper gallery mode towards a unidirectional single-mode output is meaningful for both studying cavity dynamics and developing the above-mentioned applications. Here in this paper a brief review is carried out on the study of coupled dye-doped polymer microcavity processed by femtosecond laser direct-writing (FSLDW). The content covers fabrication, microcavity structure design, lasing and coupling mechanism study. The powerful patterning ability of FSLDW can realize complex three-dimensional microcavity structure design, which follows two schemes. One is to integrate a filter port to a microcavity. The other is to bring two or more microcavities in close proximity to each other for coupling. Based on such schemes, three kinds of microcavity structures, which are stacked microdisks, a microdisk integrated with gratings and stacked spiral-ring and circular-ring microcavity, are developed for the mode modulation. It is shown that all the three kinds of structures support unidirectional single-mode emissions with low lasing threshold. For the case of the stacked microdisks, the coupling can have a vernier effect among their modes and hence the mode selection. For the case of the microdisk cavity integrated with gratings, the gratings work as a filter port to select a certain mode according to their own period. For the case of the stacked spiral-ring and circular-ring microcavities, it is the structure asymmetry of the former that leads to the single-mode output. The mode modulations based on the mentioned microcavity structures have successfully maintained the high Q-factor of WGMs, which makes these cavities promising unidirectional single-mode microlasers. Combining with theoretical simulations, it is confirmed that the mode coupling between the microcavities (or between gratings and a microcavity) is responsible for the mode selection. Moreover, the unique structure design can break the rotational symmetry of the microcavity and hence achieve unidirectional laser emission. By careful designing and processing, successful modulationscan be achieved on a series of polymer microcavities. With both high Q-factor and good lasing directionality, these microcavity lasers could be well explored in integrated optical systems and organic optoelectronic devices.