Fig. 2: Wavelength conversion through plasmon-coupled surface states.

a Schematic of a nanoantenna array on an InAs semiconductor substrate, which is designed to couple photo-excited surface plasmons to the surface states where a built-in electric field drifts the photo-induced electron gas to the nanoantennas to generate radiation at the optical beat frequencies. The nanoantenna geometry and semiconductor structure are chosen to maximize the spatial overlap between the built-in electric field and photoabsorption profiles. Three-dimensional computer aided design (CAD) drawing of the fiber connector (item no: 30125D2) provided as compliments by Thorlabs, Inc. b Photograph, microscopy, and scanning electron microscopy images of a fabricated nanoantenna array on a substrate consisting of a 100-nm-thick undoped InAs layer grown on a 500-nm-thick InAs epilayer with a p-type doping of 10¹⁹ cm⁻³ grown on a semi-insulating GaAs substrate. c Measured terahertz radiation (in blue) and noise (in red) spectra generated from the nanoantenna array when pumped by 3.68 nJ optical pulses at a 1550 nm center wavelength. The time-domain radiated terahertz pulse is shown in the inset. A total of 3200 time-domain traces are captured and averaged to resolve this spectrum. d Measured terahertz pulse energy/power from the fabricated nanoantenna array as a function of the optical pulse energy/power (inset) in comparison with other passive optical-to-terahertz converters reported in the literature. The optical pulse energy/power range of the available laser for these measurements was ~10 nJ/1 W. A linear dependence between the terahertz and optical pulse energy/power levels is expected at higher pulse energy/power levels (Supplementary Fig. 4). A comparison with active optical-to-terahertz converters reported in the literature is provided in Supplementary Fig. 5.