Terahertz Injection Locking of Quantum Cascade Lasers using Modified Uni-Travelling Carrier Photodiodes (UTC4QCL)

Within the framework of the priority program INtegrated TERahErtz sySTems Enabling Novel Functionality (INTEREST) funded by the German Research Foundation (DFG), terahertz injection locking of Quantum Cascade Lasers (QCLs) with novel, modified Uni-Travelling Carrier (MUTC) photodiodes (PDs) will be developed in cooperation with Universität Duisburg-Essen (UDE, Duisburg), and the Institute of Optical Sensor Systems of the German Aerospace Center (HU/DLR, Berlin).

This project aims at phase locking of QCLs for frequencies between 2 and 5 THz using a  local oscillator (LO) injection lock signal generated by a high-saturation output power MUTC-PD. An interconnect technology suitable for cryogenic application will be developed to efficiently couple the THz output signal from the MUTC-PD into the QCL chip with both components being mounted in the cryogenic chamber. The PDI team focuses on the development of QCLs for frequencies below 2.5 THz, which is challenging since the injection selectivity of carriers into the upper laser level decreases with decreasing lasing frequency. The development includes the design of active regions, the growth of the heterostructures by molecular beam epitaxy, the fabrication of lasers, and the determination of their operating properties. One of the scientific challenges is to determine which processes lead to leakage currents and how the material composition of the injection barrier affects them. Finally, QCLs in the range between 2 and 5 THz will be fabricated and provided to our project partners. We aim at single-mode QCLs with tuning ranges of a few GHz, which can be operated in a mechanical cooler. The injection-locked QCL using MUTC-PDs will be integrated in a THz absorption spectrometer for high-resolution molecular spectroscopy. The HU/DLR team will determine the performance of the injection-locked QCL by direct mixing its emission with that from a multiplied millimeter-wave source in a Schottky diode mixer.