A team at Nanyang Technological University (NTU) developed an electrically pumped, topological, bulk quantum cascade laser (QCL) that operates in the terahertz (THz) frequency range. The THz QCL is governed by a mechanism known as bound states in the continuum (BICs). BICs are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. BICs can confine light with a theoretically infinite Q factor, which can improve light-matter interaction. The researchers demonstrated a miniaturized, electrically pumped, topological, bulk THz QCL that showed single-mode lasing with a side-mode suppression ratio around 20 dB. They also observed a cylindrical vector beam for the far-field emission, which is evidence for topological, bulk BICs lasers. The miniaturization of single-mode, beam-engineered THz lasers is promising for many applications, including imaging, sensing, and communications. Top left: The numerical calculations of the far-field beam at different polarizations ranging from 0° to 135°. Bottom left: The schematic view of the QCL device under pulsed electrical pump and the emitted laser beam. Top right: The measured laser spectrum at its maximum output. The inset shows the spectrum in dB scale, and the side-mode suppression ratio is around 20 dB. Bottom right: The probed far-field beam profile and its polarization-resolved beam profiles. Courtesy of S. Han et al. Until now, topological lasers have focused primarily on lasing from topological edge states. In earlier work, the NTU team demonstrated that topological edge states make THz QCLs exceptionally robust. The team’s current research focuses on the topological bulk band edges that reflect the topological bulk-edge correspondence. The NTU team showed that the band edges of topological bulk lasers are recognized as BICs due to their nonradiative characteristics and robust topological polarization charges in the momentum space. The lasing modes of these lasers show both in-plane and out-of-plane tight confinements in a compact laser cavity. The topological, bulk BICs engages both vertical (out-of-plane) and lateral (in-plane) confinement, which enhances coherent emission. The topological band, inversion-induced reflection further strengthens lateral confinement for the topological, bulk BICs mode. The topological, bulk, THz QCL provides an optical mode in the topological domain that cannot propagate to the trivial domain in the lateral direction, due to the distinct topological phases. Therefore, the desired optical modes are laterally confined in the topological domain. Because the band edge modes have symmetry incompatibility for infinite Q factors and far-field polarization singularity in the momentum space, they are considered BICs, and they provide vertical confinement. Both lateral and vertical confinement mechanisms are desirable for laser device miniaturization and single-mode laser emission. “The development of electrically pumped, topological insulator lasers has drawn a lot of research interest recently, because it is considered as one promising direction with great prospects in pushing the boundaries of topological photonics into practical applications,” the researchers said. The researchers showed that the band-inverted, topological band edges could support high-quality factors, making them advantageous in gain competition compared to other photonic modes. “By properly constructing a topological photonic cavity and integrating it with gain materials, we can make it lase,” the team said. “Meanwhile, the photonic band topology allows the optical modes to be robustly protected against defects, disorders, and sharp bends.” The researchers used a QCL wafer cladded by double metal layers for the electrically pumped, compact, topological, bulk THz QCL. The topological photonic cavity, in which the nontrivial domain is surrounded by the trivial domain, is etched air cylinder holes from the surface emission facet. Electrically pumped semiconductor lasers offer high efficiency, compactness, and solid-state stability. For the mid-infrared and THz radiations, QCLs are considered the most important light sources operating under electrical pumping. “The demonstration of a monolithic terahertz vector beam laser will attract a lot of interest from different research fields,” the researchers said. “Basically, our laser cavity design only relies on the dielectric refractive index modulation. Therefore, it can be easily scaled to other wavelength regimes, such as the mid-IR, the near-IR, and the visible regions, which could be developed for more potential applications.” The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-023-01200-8).