CONTRAST ENHANCEMENT USING POLARIZATION-MODULATION SCANNING NEAR-FIELD OPTICAL MICROSCOPY (PM-SNOM)

Polarization contrast scanning near-field optical microscopy (SNOM) pr ovides information on the orientation of molecules and molecular aggre gates on surfaces. Other than in transmission or fluorescence SNOM, th e control of polarization demands for SNOM probes having extinction ra tios better than 20:1 in at least two perpendicular polarization direc tions. Most common SNOM probes consist of tapered and metal-coated fib er tips. While the birefringence of fibers can be compensated, the tap ering often exhibits depolarizing effects that make good qualitative o r even quantitative measurements difficult. In an attempt to improve t he quality of the apertures, we have modified metal-coated fiber probe s with a focused ion beam (FIB). As an example of investigations where a very good polarization control is needed, we present and discuss po larization-modulation SNOM (PM-SNOM) measurements on dye crystals. The modulation of the direction of linearly polarized light and the use o f lock-in techniques allows the simultaneous detection of absorption t ogether with magnitude and orientation of optical anisotropy. (C) 1998 Elsevier Science B.V. All rights reserved.