Anti-reflection coating design for metallic THz meta-materials.

We developed a silicon-based, single-layer anti-reflection coating that suppresses the reflectivity of metals at near-infrared wavelengths, enabling optical probing of nano-scale structures embedded in highly reflective surroundings. Our design does not affect the interaction of terahertz (THz) radiation with these metallic structures that can be used to achieve THz near-field enhancement. We demonstrated the functionality of the design by calculating and measuring the reflectivity of both infrared and THz radiation from a silicon/gold double layer as a function of the silicon thickness. We also fabricated the unit cell of a THz meta-material, a dipole antenna comprising two 20-nm thick extended gold plates separated by a 2 $\mu$m gap, where the THz 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 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 of strong THz-driven dynamics in nano-structures using table-top femtosecond near-infrared lasers.