Towards SNIM: Scanning Near-Field Microscopy in the Infrared

Abstract

The SNOM has successfully replaced the wavelength by an aperture in determining an optical microscope’s ultimate resolution. Thus it appears now feasible that also long-wavelength infrared radiation is exploited for microscopy. The expected benefit is primarily an extension of available contrast mechanisms, especially to include “fingerprint” vibrational absorption specific to the infrared which can identify an object’s chemical composition.

Of the problems to attain a high-resolution SNIM we discuss specifically the cutoff and skin depth effects of metal-coated lightguides. We find that proper choices of tip materials and tip geometries can fully circumvent both obstacles, resulting in the prediction that the SNIM will challenge the SNOM’s 20 nm spatial resolution.

Furthermore we show that the SNIM’s resolution should be improvable even beyond 20 nm by implementing antenna tips as already demonstrated in “apertureless” SNOMs. As an extreme example of going beyond the wavelength limit, we report preliminary scale experiments with an apertureless radiowave-SNIM which has already resolved sub-micrometer features.