Anomalous extinction in index-matched terahertz nanogaps

Abstract

Slot-type nanogaps have been widely utilized in transmission geometry because of their advantages of exclusive light funneling and exact quantification of near-field enhancement at the gap. For further application of the nanogaps in electromagnetic interactions with various target materials, complementary studies on both transmission and reflection properties of the nanogaps are necessary. Here, we observe an anomalous extinction of terahertz waves interacting with rectangular ring-shaped sub-30 nm wide gaps. Substrate works as an index matching layer for the nanogaps, leading to a stronger field enhancement and increased nonlinearity at the gap under substrate-side illumination. This effect is expressed in reflection as a larger dip at the resonance, caused by destructive interference of the diffracted field from the gap with the reflected beam from the metal. The resulting extinction at the resonance is larger than 60% of the incident power, even without any absorbing material in the whole nanogap structure. The extinction even decreases in the presence of an absorbing medium on top of the nanogaps, suggesting that transmission and reflection from nanogaps might not necessarily represent the absorption of the whole structure.

Terahertz-driven polymerization of resists in nanoantennas

“…In this experiment, the incoming THz wave illuminates the substrate side first. Although the direction of the incident THz wave has changed, the transmission characteristics do not change, as previously reported 37 . The transmitted signal from the sample is normalized to the signal from the substrate.…”

A Transformative Metasurface Based on Zerogap Embedded Template

“…Metallic nano‐trenches, or nano‐slots, are a viable solution to solving this issue, as fine tuning of the gap width leads to a very large change in the optical properties of the sample due to gap plasmon modes within the gap. [ 21–23 ] Most importantly, in these structures the gap width and the operating wavelengths are completely decoupled, which means that changing the gap width by a few nanometers can significantly change the optical properties of the slits or slots operating at millimeter or longer wavelengths. This has been partially demonstrated in active slot antennas where gap widths modulated from 5 to 2 nm by thermal expansion of metallic films caused 30% modulation of transmitted terahertz waves.…”

Ultra-Narrow Metallic Nano-Trenches Realized by Wet Etching and Critical Point Drying

“…This indicates that the gap-filling dielectric has been removed successfully without any apparent damage to the nano-trenches, leading to a decrease in the permittivity inside the gap and a subsequent decrease in effective indices within the gap. Figure 4b shows the theoretical spectra calculated with the coupled mode method [31,32], which have been successfully implemented in simulating nanostructures in long-wavelength regimes [33][34][35]. We modeled collapsing of the gap as an increase in imaginary permittivity of the gap material, i.e., an increase in conductivity, due to formation of metallic bridges in the gap.…”

“…Figure 4 b shows the theoretical spectra calculated with the coupled mode method [ 31 , 32 ], which have been successfully implemented in simulating nanostructures in long-wavelength regimes [ 33 , 34 , 35 ]. We modeled collapsing of the gap as an increase in imaginary permittivity of the gap material, i.e., an increase in conductivity, due to formation of metallic bridges in the gap.…”