Highly-twisted states of light from a high quality factor photonic crystal ring

Twisted light in the form of orbital angular momentum (OAM) can provide an additional spatial dimension for information transmission. Here, the authors demonstrate a prescription for OAM generation using photonic crystal ring resonators, in which high cavity quality factors (up to 10(6)) are retained. Twisted light with orbital angular momentum (OAM) has been extensively studied for applications in quantum and classical communications, microscopy, and optical micromanipulation. Ejecting high angular momentum states of a whispering gallery mode (WGM) microresonator through a grating-assisted mechanism provides a scalable, chip-integrated solution for OAM generation. However, demonstrated OAM microresonators have exhibited a much lower quality factor (Q) than conventional WGM resonators (by >100x), and an understanding of the limits on Q has been lacking. This is crucial given the importance of Q in enhancing light-matter interactions. Moreover, though high-OAM states are often desirable, the limits on what is achievable in a microresonator are not well understood. Here, we provide insight on these two questions, through understanding OAM from the perspective of mode coupling in a photonic crystal ring and linking it to coherent backscattering between counter-propagating WGMs. In addition to demonstrating high-Q (10(5) to 10(6)), a high estimated upper bound on OAM ejection efficiency (up to 90%), and high-OAM number (up to l = 60), our empirical model is supported by experiments and provides a quantitative explanation for the behavior of Q and the upper bound of OAM ejection efficiency with l. The state-of-the-art performance and understanding of microresonator OAM generation opens opportunities for OAM applications using chip-integrated technologies.

Automated summary

The authors demonstrate a method for generating orbital angular momentum (OAM) using photonic crystal ring resonators, while maintaining high cavity quality factors (up to 10^6). By ejecting high angular momentum states of a whispering gallery mode (WGM) microresonator through a grating-assisted mechanism, a scalable and chip-integrated solution for OAM generation is achieved.