High-Sensitivity and Broadband Terahertz Focal Plane Arrays Based on Plasmonic Photoconductive Nanocavities

Performance of existing terahertz time-domain spectroscopy (THz-TDS) systems is still limited by the low sensitivity of photoconductive switches used as terahertz detectors. The sensitivity of conventional photoconductive terahertz detectors is limited due to the trade-off between ultrafast operation and quantum efficiency, which restricts the number of photogenerated carriers that can efficiently contribute to terahertz detection. Another problem of conventional photoconductive terahertz detectors is the necessity of an extremely precise optical alignment. This tight optical alignment requirement also prevents researchers from realizing terahertz detector arrays since it is very challenging to align the optical beam on multiple detectors simultaneously. As a result, conventional THz-TDS systems use single-pixel detectors and raster scanning to capture the image/spectrum of an object, which requires significantly long measurement times. To address all of the limitations of conventional photoconductive terahertz detectors, Lookin, Inc. proposes to develop terahertz focal plane arrays (THz-FPAs) that offer large field-of-view and high-sensitivity operation. The company plans to build two different THz-FPAs; (1) a one-dimensional THz-FPA consisting of 1024 pixels for line scanning, (2) a 256×256 THz-FPA for two-dimensional scanning. To realize these products and demonstrate their capabilities, Lookin, Inc. will adapt new nanofabrication techniques, design read-out circuits/boards, and take terahertz images of various objects with the fabricated THz-FPA prototypes at high scan speeds. Lookin, Inc. will also develop a software that can process the output of the FPA pixels in real time to generate terahertz images and videos. The building block of the proposed THz-FPAs will be the high-performance photoconductive terahertz detector, which was developed by Lookin during the Phase I SBIR program. The detector uses an array of photoconductive nanoantennas integrated with a plasmonic nanocavity, which is specifically designed to increase the optical absorption around the nanoantennas. This approach enabled boosting the efficiency of photoconductive terahertz detectors by three orders of magnitude. As a result, high-SNR THz-TDS operation was achieved even at very low optical pump power levels. In addition, by using nanoantenna arrays and distributing them over a large area, Lookin’s detector requires a much less demanding optical alignment compared to conventional photoconductive detectors. The area of this novel detector based on nanoantenna arrays can be easily scalable to build multi-pixel terahertz detector arrays. Broadband THz-FPAs with high sensitivity and high efficiency would revolutionize the terahertz technology market by addressing the most crucial needs of practical THz-TDS systems. They not only improve the performance of THz-TDS systems in their current applications, but they also enable new applications. Such THz-FPAs would transform THz-TDS systems from a metrology tool with a slow scan speed and limited field-of-view to a high-throughput instrument that can be used in industrial settings for various quality control applications.