A mirrorless photonic free-electron laser oscillator

Photonic crystals have been used to provide fundamental control over the interaction between light and matter, including stimulated emission. For example, in Bloch-mode lasers, the photonic crystal provides field enhancement through reduced group velocity and offers larger mode volumes through distributed feedback. In a photonic free-electron laser (pFEL), where electrons stream through a photonic crystal (see Fig.1), gain and coherent output is provided by free electrons through the emission of coherent Cherenkov radiation. The property of this radiation mechanism is that the optical gain can be scaled over a large range of the electromagnetic spectrum via selecting an appropriate spatial period of the photonic crystal, and be tuned continuously via the electron velocity. Furthermore, due to the periodic dispersion of the Bloch modes, the pFEL can be operated in the so-called backward wave regime where the group velocity is directed opposite to the phase velocity. In this regime, light generated downstream in the photonic crystal travels upstream, where it bunches the electron beam. The increased bunching subsequently increases the downstream emission. Consequently, the backward wave interaction provides a feedback mechanism and creates an oscillator without the need for external mirrors. This mirrorless oscillator can provide continuously tunable, narrow bandwidth light, for use in, e.g., spectroscopic applications.