"With our higher-sensitivity terahertz system, you could see deeper into tissues or sense small quantities of illegal drugs and explosives from a farther distance. That's why it's important," said Mona Jarrahi, U-M assistant professor of electrical engineering and computer science and leader of the project.
Jarrahi's research team accomplished this by funneling the laser light to specifically selected locations near the device's electrode that feeds the antenna that transmits and receives the terahertz signal.
Their approach enables light to hitch a ride with free electrons on the surface of the metallic electrodes to form a class of surface waves called surface plasmon waves. By coupling the beam of light with surface plasmon waves, the researchers created a funnel to carry light into nanoscale regions near device electrodes.
Nature Communications - Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes
The output power of the terahertz sources and the sensitivity of the terahertz detectors can be increased even further by designing optical funnels with tighter focusing capabilities.
"This is a fantastic piece of engineering," said Ted Norris, U-M professor of electrical engineering and computer science. "It gets right to the central issue in photoconductive terahertz devices, which is collecting all the charge. Since every application benefits from increased sensitivity, for example, reduced data acquisition time or increased standoff distance, I expect this approach to be implemented widely."
Norris, an expert on terahertz technology, is director of the U-M Center for Photonic and Multiscale Nanomaterials.
ABSTRACT - Even though the terahertz spectrum is well suited for chemical identification, material characterization, biological sensing and medical imaging, practical development of these applications has been hindered by attributes of existing terahertz optoelectronics. Here we demonstrate that the use of plasmonic contact electrodes can significantly mitigate the low-quantum efficiency performance of photoconductive terahertz optoelectronics. The use of plasmonic contact electrodes offers nanoscale carrier transport path lengths for the majority of photocarriers, increasing the number of collected photocarriers in a subpicosecond timescale and, thus, enhancing the optical-to-terahertz conversion efficiency of photoconductive terahertz emitters and the detection sensitivity of photoconductive terahertz detectors. We experimentally demonstrate 50 times higher terahertz radiation powers from a plasmonic photoconductive emitter in comparison with a similar photoconductive emitter with non-plasmonic contact electrodes, as well as 30 times higher terahertz detection sensitivities from a plasmonic photoconductive detector in comparison with a similar photoconductive detector with non-plasmonic contact electrodes.
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