The d=20nm punched spot illuminated by a Dvd laser diode shouldn't overheat.
Back to (thicker) gold, deposited on thick silica - for instance because this silica makes the lens. It should withstand 300K heating. The silica (1.3 W.m-1.K-1) hemisphere conducts 370µW from a light spot of D=300nm.
Gold reflects >99% (...at least in air! Gold has an index) so the incoming light power can be 37mW. Let's take 20mW as I believe it's a common power for DVD burners; if yours is stronger, reduce the current and gain lifespan; if needed, use a duty cycle at some MHz.
The d=20nm hole at the D=300nm spot lets <90µW pass through, which is half the light of an old green LED. No adaptive filter, no multiple sources, no integration time - my preferred version. This power leaves adjustment margin if, for instance, light reflection is weaker than expected.
Marc Schaefer, aka Enthalpy