The objective of this program is to develop room-temperature terahertz semiconductor Raman lasers based on giant Raman nonlinearities associated with intersubband transitions in semiconductor superlattices. The new devices are expected to operate at room temperature because the Raman gain does require population inversion across the low-energy terahertz transition. Giant values of Raman nonlinearity will allow using midinfrared quantum cascade laser butt-joined to the Raman section for pumping. Such device configuration will result in millimeter-sized electrically-pumped terahertz semiconductor laser sources.
The intellectual merit of this research is to explore a novel and highly promising approach to produce the first room-temperature terahertz semiconductor laser source. This work will enhance our understanding of optical nonlinearities and intersubband electron dynamics in semiconductor nanostructures. This project promises significant advances in state of the art terahertz sources, yielding compact semiconductor devices operating at room temperature, at higher power, and with new functionalities such as broadband electric tunability.
The broader impacts are also significant. The proposed research lies at the intersection of the optoelectronics, nonlinear optics, and physics of semiconductors. This combination of disciplines offers a unique educational environment for the students involved in the project. Knowledge and techniques developed during research will be incorporated into graduate- and undergraduate-level courses, disseminated through publications, technology transfer, and the research groups? websites. Room-temperature terahertz semiconductor lasers developed as a result of this program are expected to transform existing terahertz instrumentation with applications ranging from high-resolution spectroscopy and local oscillators for radio astronomy to terahertz remote sensing and imaging.
The project aimed to produce room-temperature sources in 1-5 THz that are compact, broadly-tunable, and suitable for a production in large quantities. THz quantum cascade lasers (QCLs) are a promising technology for this spectral range; however, their maximum operating temperature in pulsed mode is still below 200° K, due to difficulty of maintaining sufficient population inversion between closely-spaced upper and lower energy levels in a semiconductor multi-quantum-well system under bias voltage. Our proposed goal was to investigate the possibility of creating a THz intersubband Raman laser. Such devices would not require population inversion to produce lasing and therefore are expected to operate at room temperature. However, we could not achieve THz Raman lasing in any of our device designs, even at cryogenic temperatures. Detailed theoretical analysis, performed in collaboration with Alexey Belyanin’s group (the co-PI on the project), demonstrated that THz Raman gain is significantly reduced by pump intensity saturation (compared to what was originally expected by a simple model) and the THz Raman lasing, while potentially possible using complex multi-quantum-well level arrangements, is extremely difficult to achieve in practice, see Vijaraghavan et al., Proceedings of IRMMW-THz Conference, pp. 590-591 (2011). As a result of this negative outcome, we had to investigate other approaches to produce room-temperature THz sources that meet the requirements of being compact, broadly-tunable, and mass-producible. We initially focused on sources based on nonlinear THz generation in mid-IR QCLs. Free carriers limit the absorption length for THz radiation in mid-IR QCLs to 20-100 μm. One way to circumvent this problem is to ‘concentrate’ optical nonlinearity near the output facet of the laser. We investigated the performance of mid-IR QCLs with passive multi-quantum-well sections designed for giant nonlinearity for difference-frequency generation (DFG) and integrated near the output facet, see Adams et al., Appl. Phys. Lett. 98,151114 (2011). However, the power output of these devices was in the nW level and the lasers only operated at cryogenic temperatures. Theoretical analysis performed in collaboration with Alexey Belyanin’s group (the co-PI on the project) showed that due to intensity saturation of intersubband transitions THz power output of these devices is very limited, see Cho et al.,Proc. SPIE 7953, 79530U, San Francisco, CA (2011). The breakthrough came when we realized that the InGaAs/AlInAs/InP materials system used for state-of-the-art mid-IR QCLs is ideal for implementing QCL sources based on Cherenkov DFG scheme in which optical nonlinearity with population inversion may be integrated into the whole QCL active region and THz radiation is extracted into an undoped InP substrate along the whole length of the laser waveguide, see Vijayraghavanet al., Appl. Phys. Lett. 100, 251104 (2012). Further optimization of the active region and waveguide design in these devices resulted in room-temperature QCL sources providing record 120 μW of peak power output at 4 THz and tunable in the1.7-5.3 THz range, see Vijayraghavan et al., Nature Comm. 4, 2021 (2013). These devices represent the first widely-tunable room-temperature THz sources that are similar in compactness, operation simplicity, and mass producibility to near IR diode lasers. Further investigation of the design space of these devices and performance improvement is now continued.
