Microfabricated silicon solid immersion lens

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We present the microfabrication of a solid immersion lens from silicon for scanning near-field optical microscopy. The solid immersion lens (SIL) achieves spatial resolution better than the diffraction limit in air without the losses associated with tapered optical fibers. A 15-μm-diameter SIL is formed by reflowing photoresist in acetone vapor and transferring the shape into single-crystal Si with reactive ion etching. The lens is integrated onto a cantilever for scanning, and a tip is fabricated opposite the lens to localize lens-sample contact. Using the Si SIL, we show that microfabricated lenses have greater optical transparency and less aberration than conventional 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 used when the lens thickness is comparable to the penetration depth of the light. Tolerance to errors in curvature and thickness is improved in micromachined lenses, because spherical aberrations decrease with lens diameter. We demonstrate scanning near-field optical microscopy with the Si SIL and achieve spatial resolution below the diffraction limit in air by resolving 200-nm lines with 633-nm light.