Contrast enhancement using polarization-modulation scanning near-field optical microscopy (PM-SNOM)

Abstract

Polarization contrast scanning near-field optical microscopy (SNOM) provides information on the orientation of molecules and molecular aggregates on surfaces. Other than in transmission or fluorescence SNOM, the control of polarization demands for SNOM probes having extinction ratios better than 20 : 1 in at least two perpendicular polarization directions. Most common SNOM probes consist of tapered and metal-coated fiber tips. While the birefringence of fibers can be compensated, the tapering often exhibits depolarizing effects that make good qualitative or even quantitative measurements difficult. In an attempt to improve the quality of the apertures, we have modified metal-coated fiber probes with a focused ion beam (FIB). As an example of investigations where a very good polarization control is needed, we present and discuss polarization-modulation SNOM (PM-SNOM) measurements on dye crystals. The modulation of the direction of linearly polarized light and the use of lock-in techniques allows the simultaneous detection of absorption together with magnitude and orientation of optical anisotropy.