  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.
