The terahertz (THz) radiation in the frequency range of ~1-5 THz has mainly unique properties and applications in imaging, sensing, spectroscopy, communication, and medical diagnosis. Most of these applications requires a sufficient THz light power density so that the light can pass through materials and able to be detected for analysis. This has fueled intense studies on the generation of high power THz light in the past decades. The recent demonstrated THz source based on nonlinear generation inside a mid-IR quantum cascade laser (QCL) is proved to be the only room temperature semiconductor light source that delivers mW-level power at room temperature in the 1-5 THz range. The main challenge for this type of sources is to generate higher THz power up to tens of milliwatts with good beam quality which is required by most of the real-world application. A novel approach is proposed to develop a chip-based THz source with high power output by extracting THz light from the entire cavity. This simple to use and compact source will be an enabling technology which will allow easy access to THz spectroscopy/ imaging for the broader scientific community.
The objective of the proposed research is to demonstrate a room temperature, monolithic THz source with sub-milliwatt level output power in continuous wave operation and tens of milliwatt level in pulsed mode operation. In the previous demonstrations, edge-emitting THz sources based on nonlinear generation inside a mid-IR QCL have limited outcoupling efficiency (~6-10%) due to the limited outcoupling aperture size regarding to the entire cavity. The approach in this proposed project is to use a surface-emission scheme based on THz diffraction grating defined in the semi-insulating InP substrate of a epi-down bonded THz QCL source for high-power and efficient THz outcoupling. Unlike the traditional distributed-feedback (DFB) grating which interacts with the guiding laser modes by providing the optical feedback, the proposed THz diffraction grating simply diffract the incident Cerenkov emission cone into the discrete directions. With an optimized grating structure, surface emission from the entire cavity with diffraction efficiency up to 90% is achievable. This will drastically increase the THz outcoupling efficiency and power. In contrast to a THz sources based gas lasers, which is bulky and expensive, the proposed THz QCL source offers a monolithic solution that, once developed, has potential for cost-effective mass production using the existing semiconductor laser fabrication infrastructure. This project is an excellent example of a multidisciplinary approach used to circumvent existing technological limitations. Solid state physics, material science, nonlinear optics, and laser physics are all major components of the research plan, which are all supported generally by NSF, and are used, in this case, together to combine multiple functional elements into a single, compact, high power device.
