Efficient THz Emission Using Thin Black Phosphorus Photoconductive Absorber and Loss-free Dielectric Light Trapping

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The proposed project aims at advancing the technology of antennas at high frequencies in the terahertz band from 100 GHz to 4000 GHz. The proposed research will use new materials, design and fabrication tools to deliver advanced and more efficient terahertz antennas. The anticipated new antennas will be capable of providing increased radiated power that will advance several applications of significant importance to society, such as cancer detection, homeland security, communications, and education. The new terahertz antennas will likely advance the imaging of breast cancer tumor margins that remains clinically limited due to the lack of adequate radiated power from terahertz antennas. The new antennas will potentially impact homeland security by providing powerful antennas for the detection of explosives, non-metallic weapons, and drugs that could be hidden in clothing. The proposed terahertz antennas will also likely impact future 5G wireless communications systems where antennas are envisioned to operate at frequencies greater than 100 GHz to support higher data demands from a vast number of applications. For education, the proposed research will offer unique training opportunities to prepare the next generation of leading scientists and engineers, including minority and first-generation college students. Interactive laboratory demonstrations will be developed based on several scientific efforts involved in the proposed research. Furthermore, outreach activities on antenna education will be engaged in the project to target school districts and other educational programs serving underrepresented and minority students in Arkansas. Terahertz (THz) photoconductive antennas (PCAs) are devices with the attractive capability to emit broadband pulses that provide frequencies up to 6 THz but suffer a primary challenge of low power conversion efficiency of 10-5 from pump laser to terahertz emission. The proposed research aims at advancing this technology by replacing the low temperature gallium arsenide (GaAs) semiconductor with thin layer of black phosphorus (BP) covered with loss-free nanospheres acting as a light trapping layer. The goal is to increase the radiated terahertz average power by a factor of ten and bandwidth by a factor of two over the conventional PCA technology. The first task is to fabricate and model PCAs utilizing the thin multi-layered semiconductor BP, which is strongly light absorbing and has a high saturation velocity. This new material has a potential to increase the carrier generation in the device and has not been investigated in THz thin film emitters yet. The second task is to incorporate low-loss dielectric nanophotonic structures to provide a performance boost beyond what has been achieved with lossy plasmonic elements. The idea is to avoid parasitic absorption or reflection in metallic layers by using transparent dielectric structures to engineer light transport into thin layers via evanescent waves or other photonic waveguiding. The third task is to measure the broadband terahertz emission spectrum of the fabricated devices, iteratively optimize their performance through modeling, and benchmark the new platform against conventional technologies. The measurements will be conducted on a THz time-domain spectroscopy system in which the emitter is the proposed device while the detector and the rest of the system will be obtained from a commercial system. This task will also test the transfer of the developed BP-PCA device into commercial systems in the future. The research results will be disseminated in archived papers and conference presentations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.