Anti-reflection coating design for metallic terahertz meta-materials

TL;DR

This paper presents a silicon-based single-layer anti-reflection coating that reduces metallic reflectivity at near-infrared frequencies, enabling enhanced optical probing of nano-structures in highly reflective environments without disrupting terahertz interactions.

Contribution

The design maintains terahertz near-field enhancement while suppressing reflectivity, verified through calculations, measurements, and fabrication of a terahertz meta-material unit cell.

Findings

Reflectivity of infrared and terahertz radiation is reduced by the coating.

Near-field enhancement in terahertz meta-materials is preserved with the coating.

Magneto-optical response of a nano-wire is clearly detected despite small area coverage.

Abstract

We demonstrate a silicon-based, single-layer anti-reflection coating that suppresses the reflectivity of metals at near-infrared frequencies, enabling optical probing of nano-scale structures embedded in highly reflective surroundings. Our design does not affect the interaction of terahertz radiation with metallic structures that can be used to achieve terahertz near-field enhancement. We have verified the functionality of the design by calculating and measuring the reflectivity of both infrared and terahertz radiation from a silicon/gold double layer as a function of the silicon thickness. We have also fabricated the unit cell of a terahertz meta-material, a dipole antenna comprising two 20-nm thick extended gold plates separated by a 2 $\mu$m gap, where the terahertz field is locally enhanced. We used the time-domain finite element method to demonstrate that such near-field enhancement is preserved in the presence of the anti-reflection coating. Finally, we performed magneto-optical Kerr effect measurements on a single 3-nm thick, 1-$\mu$m wide magnetic wire placed in the gap of such a dipole antenna. The wire only occupies 2\% of the area probed by the laser beam, but its magneto-optical response can be clearly detected. Our design paves the way for ultrafast time-resolved studies, using table-top femtosecond near-infrared lasers, of dynamics in nano-structures driven by strong terahertz radiation.