Beam Pattern Measurements on Quantum Cascade Lasers Operating at 2.8 THz Aurèle J.L. Adam, J. Niels Hovenier, Irmantas Kasalynas, Tjeerd O. Klaassen, Jian-Rong Gao, Benjamin S. Williams, Sushil Kumar, Qing Hu, Ekaterina E. Orlova and John L. Reno Abstract—Quantum cascade lasers operating in the THz region are of great potential interest as local oscillators for THz heterodyne detection. To assess the applicability of this new source as LO, a study of the properties of the emitted radiation, like beam shape and optical phase front, is very important. In this paper we will present results of the beam profile of ‘metal- metal’ cavity QCL’s using the conical section method, designed for spherical antenna pattern measurements. It appears that, contrary to earlier waveguide based simulations, the beam patterns show strong angular intensity oscillations. Index Terms—Beam profile, Local oscillator, Quantum cascade laser, THz radiation I. INTRODUCTION T HE development of TeraHertz technology has been severely hindered by the lack of versatile sources. For long, one had to rely on bulky optically pumped far-infrared lasers or on complicated systems based on either frequency multiplication of high frequency microwave radiation (typically 80–100GHz) or on generation of radiation at the frequency difference of two (VIS/NIR) diode lasers. The quest for new sources has resulted recently in the development of the Quantum Cascade Laser (QCL) [1]. After the first demonstration of emission in the mid-infrared range (λ ≈ 4 µm; 75 THz)[2], now Quantum Cascade Lasers (QCL) are operating CW in the THz range, down to 2.1 THz (λ ≈ 143 µm)[3]. These sources are very promising as local oscillators for heterodyne detection, especially for frequencies above 2 THz where the usual frequency multiplier systems do not provide enough output power. In order to use the QCL for that purpose, apart from the frequency and power stability, one also needs to investigate the pattern of the emission beam. This latter issue is very important because, in order to keep the electrical dissipation of these cryogenic devices low, the dimensions of the active region should be reduced as much as possible. As a result, the dimensions of the waveguide cavity may turn out to be of the order of, or even smaller than, the emission wavelength. We will report for instance on the beam pattern of a QCL, emitting at λ = 107 µm, with a 650 x 25 x 10 µm 3 metal-metal cavity. A set-up has been build to measure the beam pattern of QCL’s in the near- and far field region. It consists of a Helium flow cryostat with optical windows, offering a wide angle of view (up to 120º full cone angle). The THz intensity is measured using a pyroelectric detector, mounted on a two-axis rotation system. We will report on beam patterns of QCL bars with a metal-metal cavity of various dimensions, emitting at 2.8 THz. II. THz. LASER SAMPLES A number of different design types for the basic heterostructure module as well as for the cavity of THz QCL’s exists. The heterostructure design employed for the THz QCL’s used in this research is based on resonant LO-phonon scattering to selectively depopulate the lower radiation level, while maintaining a long upper level lifetime, see fig. 1 [4,5]. The cavity of this QCL, operating at 2.8 THz, is of the metal- metal type, fabricated using a copper-to-copper thermo- compression bonding technique [5,6]. A.J.L. Adam, J.N. Hovenier, I. Kasalynas, T.O. Klaassen and J.R. Gao are with the Kavli Institute of NanoScience, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands. (phone:+31.15.2786136 ; fax:+31.15.2783251;e-mail: t.o.klaassen@tnw.tudelft.nl ) B.S. Williams, S. Kumar and Q. Hu are with the Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge MA 02139, U.S.A. Fig. 1. Energy level scheme of the AlGaAs/GaAs heterostructure module. Lasing at about 2.8 THz occurs between levels 5&4 E.E. Orlova is with the Institute for Physics of Microstrucutres, Russian Academy of Sciences, GSP-105, 603950 Nizhny Novgorod, Russia J. L. Reno is with the Sandia National Laboratories, Albuquerque, NM 87185-0601, U.S.A The advantage of such a metal-metal type of waveguide structure is two-fold. First of all, a better heat contact of the 16th International Symposium on Space Terahertz Technology 291