Princeton University

School of Engineering & Applied Science

High performance THz quantum cascade lasers based on novel materials and designs

Professor Karl Unterrainer, Photonics Institute, Technische Universitat Wien
Engineering Quadrangle, B327
Wednesday, February 24, 2016 - 11:00am

Quantum cascade lasers (QCLs) are most successful quantum devices which cover a broad spectral range from the infrared to the THz. However, their operation is still limited in terms of operating temperature, output power and beam properties. Therefore, we investigate new materials system and active regions designs. We use InGaAs/GaAsSb due to the lower effective mass and lower conduction band offset. We have achieved record operating temperatures despite the large interface roughness asymmetries in this material. We compare this to InGaAs/InAlAs structures which show even higher operating temperatures. For InAs/AlAsSb quantum cascade structures lasing is observed in moderate magnetic fields. Another strategy to improve the performance is to increase the number of cascades in the active region. However, due to the epitaxial growth the maximum active region thickness is limited to around 10–15 µm. To circumvent this growth problem we developed a direct wafer-bonding technique to increase the thickness of the active region by stacking the same active region. With these devices we reach record output power levels of almost 1 Watt and a significantly improved far field. This performance allows real time THz imaging using micro bolometer cameras. The unique properties of QCLs allow the fabrication of photonic crystals, micro pillar arrays as well as of micro-cavities. They show very low threshold currents and coupling between them allows the realization of circuits. We show that quantum cascade structures can also work as detector, modulator and amplifier.
C. Deutsch et al., “High performance InGaAs/GaAsSb terahertz quantum cascade lasers operating up to 142 K”, Applied Physics Letters 101, 211117 (2012).
M. Brandstetter et al., “High power terahertz quantum cascade lasers with symmetric wafer bonded active regions”, Applied Physics Letters 103, 171113 (2013).
D. Bachmann et al., “Broadband terahertz amplification in a heterogeneous quantum cascade laser”, Opt. Express 23, 3117 (2015).

Karl Unterrainer received the MS degree in Physics 1986, and his Ph.D. degree from the Universität Innsbruck in 1989. Subsequently, he worked on development of a far infrared tunable cyclotron resonance laser as assistant professor at the Technical University Vienna. In 1994 and 1995 he worked as a visiting researcher at the Quantum Institute, University of California, Santa Barbara. Since 2004 he his full professor at the Photonics Institute, Technische Universität Wien. Since 2006 he is coordinating the FWF research cluster “Infrared Optical Nanostructures IR-ON”. His main research areas are THz spectroscopy of semiconductor nanostructures and the development of THz devices.