The mid-infrared (MIR) to terahertz (THz) spectral range has its unique scientific and technological significance, as it hosts the strongest and fingerprint-like absorption lines of countless molecular species, making it the ideal spectral range for developing sensing technologies with superior selectivity and sensitivity for a broad range of applications. Quantum cascade lasers (QCLs) are currently the preferred light sources for many applications in this spectral range, thanks to their compactness, convenient operation and high output power. However, in the THz region the performance of QCLs is still not sufficient for various real-world applications. THz QCLs have much lower performance than MIR QCLs, and the highest operating temperature of THz QCLs is still limited to ~200K. Furthermore, currently no QCL can operate within the 5 THz to 11THz range. In this exploratory project, we plan to develop a new type of compact, high-performance and room-temperature operating THz sources to cover this "gap" spectral range. The proposed devices are based on an untested but promising new operation principle, and the successful demonstration of such devices will bring transformative impacts to the research field of THz sources and enable various applications. Therefore, the proposed research is suitable for the NSF EAGER program. This project will allow graduate and undergraduate students to actively participate in cutting-edge research, and acquire the knowledge, skills, experiences and broad perspectives necessary for their future leadership in scientific research and technology development on the competitive global stage. Combining research with education and outreach activities will also be a focus of our work, aiming at benefiting students of all age-groups and backgrounds, including those from underrepresented groups.
The objective of this project is to systematically explore how to realize a new type of THz sources based on a fundamentally different device operation principle. The device operation principle consists of two key processes: (1) generating optical phonons by resonant inter-subband transitions in multiple-quantum-wells (MQWs), and (2) transferring the energy from the generated optical phonons to resonant THz antennae which then emit photons into free space. As the device operation principle is not sensitive to temperature, such THz sources should operate well at room temperature and above. Designs of the MQWs and the THz antennae will be optimized to make both processes efficient, leading to a high overall energy conversion efficiency which is potentially orders of magnitude higher than that of typical THz QCLs. Moreover, such THz sources have a surface-emitting configuration, so the output power scales up with the device area. The proposed research may also allow us to gain new and/or deeper insights into the interesting and complex physics underlying the interplay between inter-subband transitions in MQWs, optical phonons and electromagnetic resonances of photonic structures. Interactions involving all three excitations have not been systematically studied. A better understanding of the underlying physics will guide us to improve the device design, and may inspire us to pursue new possibilities of more advanced devices.