Content area
Full Text
Received: 24 April 2019
Accepted: 9 December 2019
Published online: 12 February 2020
Quantum cascade lasers are compact, electrically pumped light sources in the technologically important mid-infrared and terahertz region of the electromagnetic spectrum1,2. Recently, the concept of topology3 has been expanded from condensed matter physics into photonics4, giving rise to a new type of lasing5-8 using topologically protected photonic modes that can efficiently bypass corners and defects4. Previous demonstrations of topological lasers have required an external laser source for optical pumping and have operated in the conventional optical frequency regime5-8. Here we demonstrate an electrically pumped terahertz quantum cascade laser based on topologically protected valley edge states9-11. Unlike topological lasers that rely on large-scale features to impart topological protection, our compact design makes use of the valley degree of freedom in photonic crystals10,11, analogous to two-dimensional gapped valleytronic materials12. Lasing with regularly spaced emission peaks occurs in a sharp-cornered triangular cavity, even if perturbations are introduced into the underlying structure, owing to the existence of topologically protected valley edge states that circulate around the cavity without experiencing localization. We probe the properties of the topological lasing modes by adding different outcouplers to the topological cavity. The laser based on valley edge states may open routes to the practical use of topological protection in electrically driven laser sources.
(ProQuest: ... denotes formulae omitted.)
Quantum cascade lasers (QCLs) are electrically pumped semiconductor lasers based on intersubband electron transitions within multiple quantum wells in semiconductors1,2. They are among the most important sources of mid-infrared and terahertz (THz) radiation owing to their compactness, electrical pumping performance and high efficiency13. Their practical applications include telecommunication14, THz signal processing15, imaging16, sensing and spectroscopy. As with any laser, the emission characteristics of a THz QCL depend on the design of the photonic cavity and are generally strongly affected by the cavity shape17,18. One promising design is the use of topological edge states, which form running-wave modes that are robust against perturbations to the underlying structure5-8 and can efficiently bypass defects (which may arise during fabrication and packaging) and sharp corners. Unlike conventional waves, topological edge states resist the formation of localized standing-wave modes, which is helpful for suppressing the spatial hole-burning effect19,20. This is a particularly important consideration for QCLs...