THz Radiation Efficiency Enhancement from Metal–ITO Nonlinear Metasurfaces

Abstract

Strong single-cycle THz emission has been demonstrated from nonlinear plasmonic metasurfaces, when excited by femtosecond laser pulses. In order to invoke a higher nonlinear response, such metasurfaces have been coupled to thin indium-tin-oxide (ITO) films, which exhibit an epsilon-near zero (ENZ) behavior in the excitation wavelength range and enhance the nonlinear conversion. However, the THz conductivity of the ITO film also reduces the radiation efficiency of the meta-atoms constituting the metasurface. To overcome this, we etch the ITO layer around the plasmonic meta-atoms, which allows harnessing of the enhanced localized fields due to the ENZ behavior of the remaining ITO film, while improving the THz radiation efficiency. We report an increase of more than 1 order of magnitude in the emitted THz spectral power density, while the energy conversion efficiency approaches 10–6. This simple yet very effective fabrication scheme provides important progress toward increasing the range of applications of nonlinear plasmonic metasurface THz emitters.

Generating Angular-Varying Time Delays of THz Pulses via Direct Space-to-Time Mapping of Metasurface Structures

“…All the regions of the metasurface consist of gold meta-atoms with C3 rotational symmetry grown on a glass substrate coated with 20 nm thick ITO film, where the ITO surrounding the meta-atoms was etched in the last step of the fabrication process in order to increase the radiation efficiency of the emitters. 39 As was previously shown, 37 each of these uniform regions generates single-cycle broadband THz radiation upon incidence with a femtosecond NIR pulse. In addition, due to an accumulated P–B phase, for any pump polarization angle ϕ relative to the principal axis of the meta-atom, the generated THz polarization angle is rotated by 3ϕ relative to the pump.…”

“…We fabricated symmetric and antisymmetric double-pulse generators as shown schematically in Figure b. All the regions of the metasurface consist of gold meta-atoms with C3 rotational symmetry grown on a glass substrate coated with 20 nm thick ITO film, where the ITO surrounding the meta-atoms was etched in the last step of the fabrication process in order to increase the radiation efficiency of the emitters . As was previously shown, each of these uniform regions generates single-cycle broadband THz radiation upon incidence with a femtosecond NIR pulse.…”