Ultra-low Threshold Current THz Quantum Cascade Lasers With Buried Double-metal Waveguide Structures
An Anglo-French team composed of TeraView Ltd., Cambridge University, the University of Paris 7, and Thales R&T, has reported the operation of quantum cascade lasers (QCLs) emitting at 2.8 Terahertz (THz) with a threshold current of 19 mA at 4K (see S. Dhillon et al., Applied Physics Letters, August 1, 2005 issue), the lowest threshold ever measured without the help of intense magnetic field.
Under a bias voltage of 5 V, these devices yield a dissipated power at a threshold of just 100 mW, thus offering a concrete solution for operation with lightweighted and portable closed cycle Stirling cryocoolers. According to the researchers this represents an important technological breakthrough, which is foreseen to positively impact on the commercialization of THz QCLs. Indeed, thanks to their high power, compactness and low cost, these far infrared semiconductor lasers could become the next generation of commercial THz sources. Promising applications include security screening, namely the detection of plastic explosives and other chemical and biological agents, and medical imaging. TeraView Ltd., the world’s first company solely devoted to the commercial exploitation of THz light, has led pioneering research in these fields developing state of the art imaging systems based on photoconductive THz generation.
Ultra-low threshold current operation was obtained by embedding the active region of a recently demonstrated QCL (S. Barbieri et al., Applied Physics Letters, vol. 85, p. 1674, 2004) into a novel THz buried waveguide scheme. Such a scheme is based on surface plasmon waves, formed at the interface between a metal and a semiconductor. By exploiting this concept the research team has recently demonstrated (J. Alton et al., Applied Physics Letters, vol. 86, p. 71109, 2005) that two-dimensional optical confinement, i.e. in the directions parallel and perpendicular to the surface, can be obtained via the simple evaporation of a metal stripe on top of a semiconductor (see the left panel of Fig. 1, the shaded area represents the computed optical mode intensity). This technique allows for the realization of ridge-free, buried waveguide-cavities, where the active region is completely surrounded by the semiconductor material. This improves the device thermal conductivity, increasing the maximum operating temperature in continuous wave.
In the right panel of Fig. 1, a schematic layout of the buried waveguide realized by the authors is reported. It is based on a double plasmonic confinement obtained by sandwiching the 11.5 microns thick active region between two metal layers, also providing the device top and bottom electrical contacts. This so-called “double-metal” geometry was initially applied to THz QCLs by Williams and co-workers in 2003 (B. S. Williams et al., Applied Physics Letters, vol. 83, p. 5142, 2003).
Presently the team of researchers is concentrating on improving the wafer bonding technique used to position the active region on top of the host n+ substrate (see Fig. 1). Recently, by using a gold-gold bonding, the operating voltage dropped from 5 to 2V, reducing the dissipated power at threshold by more than a factor of two.