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

One of the most important challenges of terahertz time-domain spectroscopy THz-TDS) systems is the low sensitivity and narrow bandwidth of photoconductive terahertz detectors used in these systems. The performance limitations of photoconductive terahertz detectors are mainly due to the inherent tradeoff between high quantum efficiency and ultrafast operation of photoconductors. This tradeoff restricts the number of photogenerated carriers that drift to the contact electrodes in a sub-picosecond time scale; thus, strength of the induced ultrafast photocurrent that is directly proportional to the incident terahertz field on the detector. Light concentrators, such as metasurfaces and plasmonic structures, can mitigate this tradeoff and offer high quantum efficiency and ultrafast operation simultaneously. Lookin, Inc. proposes to develop a novel photoconductive terahertz detector based on plasmonic nanocavities, where light can be trapped in a thin, high-mobility photoconductor that is right below the photoconductor contact electrodes. The plasmonic nanocavity structure is designed to maximize the photogenerated carrier concentration at locations where the induced electric field by the incident terahertz radiation is maximum. Therefore, a significant increase in optical photon-to-collected electron efficiency can be observed, which allows the detector to offer high-performance even at very low optical power levels. The design can be also optimized to offer a broad detection bandwidth and high switching contrast. Our preliminary theoretical and experimental studies predict that the proposed detector can offer more than 109 dB SNR with a data acquisition time of 1s over a 0.1-5.5 THz frequency range, a 3-dB bandwidth of >1 THz, and a switching contrast of more than 107 at a 0.1 mW optical power level. One of the major advantages of the proposed detector is that it consists of terahertz nanoantenna arrays with a scalable active area and, thus, ideally suited for use in a terahertz focal plane array THz-FPA). Our company plans to use the high-performance photoconductive detectors developed during this Phase I project to realize a THz-FPA consisting of more than 1 kpixels for use in THz-TDS systems. During Phase I, Lookin, Inc. will conduct extensive theoretical and experimental studies on the electrical and optical characteristics of plasmonic nanocavities to identify optimized semiconductor superlattice and geometric parameters that enable high-performance photoconductive terahertz detection in THz-TDS systems. After the successful demonstration of the optimized terahertz detector prototypes, the company will build proof-of-concept 3×3 THz-FPAs and explore various data readout approaches to record the FPA output data with a high data acquisition rate, while minimizing the electrical, optical, and packaging complexity of the system through a cost effective solution. The proposed terahertz detector and FPA based on plasmonic nanocavities will have a transformative impact on terahertz science and technology. Although THz-TDS systems offer many unique functionalities for various chemical identification, material characterization, and biomedical imaging applications, their practical utilization for solving real-world problems has been extremely limited because of the lack of high-performance, multi-pixel detectors that can offer both high data quality and fast data acquisition over a broad frequency range. The proposed terahertz detector addresses all of these problems and enables THz-TDS systems to fulfil their true potential to solve many real-world problems of the society.