Passive plasmonic nanoantenna arrays for high-efficiency optical-to-terahertz conversion
Photoconductive antennas are extensively used for the generation of terahertz radiation. They generally consist of an antenna fabricated on a photo-absorbing semiconductor substrate. When illuminated by an optical pump beam with terahertz frequency components, photocarriers are generated in the substrate. The photogenerated carriers drift to the antenna under an external bias voltage to induce a terahertz photocurrent that drives the antenna to generate terahertz radiation.
One of the major limitations of existing photoconductive antennas, especially those operating at telecommunication optical wavelengths (~1550 nm), is their large dark current level, which degrades device reliability due to excessive Joule heating. To address this limitation, a new class of bias-free terahertz emitters was recently introduced, which utilizes the naturally induced built-in electric field at the surface of semiconductors to drift the photo-generated carriers. By eliminating the bias voltage and dark current, a highly reliable optical-to-terahertz conversion is achieved.
The first generation of these bias-free terahertz emitters utilized an array of plasmonic nanoantennas in the form of gratings fabricated on epitaxially-grown undoped and p+ doped InAs layers on a semi-insulating GaAs substrate. Using this bias-free terahertz emitter, record-high optical-to-terahertz conversion efficiencies were achieved compared to other bias-free terahertz emitters based on nonlinear optical process, photo-Dember effect, and spintronics. Despite its superior efficiency compared to the state-of-the art, the optical-to-terahertz conversion efficiency of this emitter is limited by leakage of the terahertz current from the terahertz radiating elements to the highly doped substrate and destructive interference of the photocurrent components injected to the terahertz radiating elements. This research is focused on the design and optimization of a new generation of plasmonic nanoantenna arrays to address these efficiency limitations and further enhance the optical-terahertz conversion efficiency of bias-free terahertz emitters.