Antireflection Coating for External-Cavity Quantum Cascade Laser Near 5 THz

We report a simple and practical method to design and fabricate antireflection (AR) coatings for the emission facet of GaAs-based laser chips suited to operating in the very far infrared (IR) or terahertz (THz) spectral region. Vacuum-deposited silica films about 8 μm thick serve as a single layer, quarter-wave AR and with reflectivities measured below 0.5% over ~10 cm range in the far IR. Quantum cascade lasers (QCL) thus coated function well at 8-10 K coupled to a tunable, external cavity optical system at ~158 cm (~4.75 THz). INTRODUCTION The practical utility of a quantum cascade laser (QCL) operating in the very far infrared will be enhanced by development of methods for tuning over wider bandwidths than is generally available at present. Gas spectroscopy is a well known application in the THz [1]. The atomic oxygen line at λ ~63 μm wavelength (~158 cm) is of particular interest. For this application, agile tuning of a coherent, narrow-band source over a range of order 0.5-1.0 cm would serve well. Tuning a QCL by an external cavity (i.e., EC-QCL) could provide such a λ range. While existing QCL devices based on GaAs can be designed for emission in the range of 1-5 THz [1], implementation of EC-QCL tuning methods would advance their utility. In particular, fabrication of EC-QCL will require development of low-loss emission of THz radiation from one facet of the QCL device in order that the optical cavity couple with an external mirror. Various strategies for AR on high-index materials such as GaAs in the THz have been reported, such as adhesive-bonded elements of silicon, quartz, or parylene [2-4]. Conventional AR coatings of deposited quarter-wave films are a mature technology in the visible and mid-tolongwave IR but present optical-materials challenges in the THz range and for cryogenic QCL operation [3]. Selection of a coating material for a simple, single-layer AR design must not only provide good transparency over the wavelength range of interest but also its index of refraction (n) should fall close to √ns, for ns the substrate index. Further complicating the optical AR design are wavelength and temperature dependences of n and ns. Dispersion in GaAs is reported to be ∆ns/ns ~0.4% over the range of 1-2 THz [5], and the temperature-dependence of the index ∆ns/ns ~0.8% integrated from 300 K to 8 K [6]. Thus, it is expected that the AR must maintain the desired low reflectivity under conditions which cause up to 1% changes in ns. Here, a coating was designed for AR on a GaAs-based QCL to function as a narrow-band tunable source for spectroscopy of the atomic oxygen line at ~158 cm. It was estimated that tuning of the EC-QCL over ~0.1 cm would serve well for spectroscopic purposes. Tuning for Mater. Res. Soc. Symp. Proc. Vol. 1016 © 2007 Materials Research Society 1016-CC07-03