Appl. Phys. Lett. 102, 031119 (2013)

Credit: © 2013 AIP

Terahertz (THz) quantum cascade lasers (QCLs) are promising light sources for THz applications such as spectroscopy, heterodyne detection and imaging. However, conventional edge-emitting ridge-waveguide THz QCLs suffer from multimode operation, extremely wide beam divergence and low output power. To overcome these drawbacks, Guozhen Liang and co-workers in Singapore, the UK and China have now demonstrated single-mode surface-emitting THz QCLs that use second-order concentric circular gratings (CCGs). The gold CCGs were fabricated on top of a conventional THz QCL mounted on a copper substrate. The CCGs were designed for single-mode emission at around 3.8 THz and surface emission for efficient and directional optical power out-coupling. The light–current–voltage characteristics of the device were compared with those of a similar device without CCGs. Although the threshold current density did not change greatly, the output power of the device with CCGs was about three times larger for the same level of electrical pumping. Another advantage of the CCG-based device was that its emission frequency was stable at all drive currents.