Abstract
Transmission of Terahertz (THz) electromagnetic wave through a substrate is encumbered because of scattering, multiple reflections, absorption, and Fabry–Perot effects when the wave interacts with the substrate. We present the experimental realization of nonresonant electromagnetic field enhancement by a factor of almost 104 in substrate-free 5-nm gold nanoslits. Our nanoslits yielded greater than 90% normalized electric field transmission in the low-frequency THz region; the slit width was 5 nm, and the gap coverage ratio was 10−4 of the entire membrane, 0.42 mm2. This large field enhancement was attributed to gap plasmons generated by the THz wave, which squeezes the charge cross-section, thus enabling very highly dense oscillating charges and strong THz field transmission from the nanoslits.
Highlights
Transmission of Terahertz (THz) electromagnetic wave through a substrate is encumbered because of scattering, multiple reflections, absorption, and Fabry–Perot effects when the wave interacts with the substrate
Field enhancement has received widespread attention since the first report, in 1998, of extraordinary optical transmission (EOT) from a subwavelength aperture in a metal based on surface plasmon polaritons (SPP)[1], because of its many potential applications, including nonlinear plasmonics[2,3], optical antennas[4,5], high harmonic electromagnetic wave generation[6,7], optical rectification[8], surface enhanced Raman scattering[9] and metamaterial sensors[10,11,12], nonlinear optics[2,13,14], and superlenses and cloaking[15]
The optical transmission of white light through the nanogap indicated the clear formation of the nanogap, and the diffracted light of the longer wavelength was transmitted, whereas the transmission was null when the light was polarized along the slit direction
Summary
For the experimental realization of the very large field enhancement by using subwavelength nanoslits, we fabricated 5-nm slits in 100-nm thick gold film on a 1-μm SiNx membrane and performed THz time domain spectroscopy (TDS) over the frequency range 0.05–1.7 THz (wavelength 6–0.175 mm) in transmission mode. Some irregularities in transmission curve was observed because of the grating modes when the slit array diffracted the incident radiation into the sample plane This results can be explained by poor coupling of THz wave among the evanescent modes because of higher ( ) refractive index of substrate[29]. We kept the robust nitride membrane to support the metallic nanoslits and increase the stability of the nanogap for the THz experiments
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