Quasi-periodic distributed feedback laser

Although lasers have found numerous applications, their design is often still based on the concept of a gain medium within a mirror cavity. Exceptions to this are distributed feedback lasers(1), in which feedback develops along a periodic structure, or random lasers, which do not require any form of cavity(2). Random lasers have very rich emission spectra, but are difficult to control. Distributed feedback devices, conversely, have the same limited design possibilities of regular lasers. We show, by making use of a quasi-crystalline structure in an electrically pumped device, that several advantages of a random laser can be combined with the predictability of a distributed feedback resonator. We have constructed a terahertz quantum cascade laser based on a Fibonacci distributed feedback sequence, and show that engineering of the self-similar spectrum of the grating allows features beyond those possible with traditional periodic resonators, such as directional output independent of the emission frequency and multicolour operation.