We present the microfabrication of a solid immersion lens from silicon for
scanning near-field optical microscopy. The solid immersion lens (SIL) achi
eves spatial resolution better than the diffraction limit in air without th
e losses associated with tapered optical fibers. A 15-mum-diameter SIL is f
ormed by reflowing photoresist in acetone vapor and transferring the shape
into single-crystal Si with reactive ion etching. The lens is integrated on
to a cantilever for scanning, and a tip is fabricated opposite the lens to
localize lens-sample contact. Using the Si SIL, we show that microfabricate
d lenses have greater optical transparency and less aberration than convent
ional lenses by focusing a plane wave of 633-nm light to a spot close to a
wavelength in diameter. Microlenses made from absorbing materials can be us
ed when the lens thickness is comparable to the penetration depth of the li
ght. Tolerance to errors in curvature and thickness is improved in micromac
hined lenses, because spherical aberrations decrease with lens diameter. We
demonstrate scanning near-field optical microscopy with the Si SIL and ach
ieve spatial resolution below the diffraction limit in air by resolving 200
-mn lines with 633-nm light.