Polarization-modulation near-field optical microscope for quantitative local dichroism mapping

Abstract

A couple of experimental techniques have been implemented to an aperture near-field scanning optical microscopy (NSOM) to obtain reliable measurement of sample dichroism on the local scale. First, a method to test NSOM tapered fiber probes toward polarization conservation into the near optical field is reported. The probes are characterized in terms of the in-plane polarization of the near field emerging from their aperture, by using a thin dichroic layer of chromophore molecules, structured along stretched polymeric chains, to probe such polarization when approached in the near-field region of the probe. Second, to assure that the light intensity coupled in the fiber is polarization independent, an active system operating in real time has been realized. Such combination of techniques allowed quantitative imaging of local dichroism degree and average orientation by means of dual-phase lock-in demodulation of the optical signal. Translation of the coupled light polarization state in the near field has been observed for one-half of the tested probes. For the others, the tip acts as a polarizer, and therefore showed it was not suitable for polarization modulation NSOM measurements

Near‐field vs. far‐field polarization properties of hollow pyramid SNOM tips

“…When we rotate the sample under the tip, as an analyzer, the detected fluorescence intensity is given by the convolution between the dichroic absorption of the sample and the NF polarization pattern out of the tip. If (θ S -θ OUT ) is the angle between the sample strain axis and the NF polarization axis, the fluorescence signal can be written as [3] ( )…”

“…In particular we demonstrate that for hollow pyramid SNOM tips the input polarization is always preserved in the NF, at variance with the FF behavior. Therefore, our results show that in every SNOM application involving the use of polarized light the properties of the tip should be checked by experimental methods able to single the NF contribution out of the global signal [3].…”

“…In this way, dichroic chromophore molecules are concentrated in less then 500 nm thickness at the upper film surface and the stretching aligns the molecules giving rise to a strong absorption dichroism. The fluorescence signal can thus be used to probe the inplane NF polarization without artefacts coming from FF scattering [3]: the excitation wavelength lies within the absorption region of the cromophore, while fluorescence is selected by means of a suitable spectral filter. The collected signal is larger when the linear polarization incident on the sample is parallel to the strain axis.…”

“…The extension of these techniques to Scanning Near-Field Optical Microscopy (SNOM) allows to overcome the diffraction limit and therefore meets the growing demand for high-resolution imaging of nanostructured materials. In this context, the characterization and control of light polarization out of a subwavelength aperture (tapered fiber or micromachined tip) is a crucial and not yet fully understood issue [1][2][3][4][5]. Moreover, the polarization state in the Near-Field (NF) plays a fundamental role also in determining the contrast and resolution properties for pure imaging of simple metallic or dielectric structures [1,2].…”

Polarization-Modulation Techniques in Near-Field Optical Microscopy for Imaging of Polarization Anisotropy in Photonic Nanostructures

“…The respective Jones matrices are reported in Table 10.2. A less expensive scheme, employing a rotating linear polarizer [79], will be described in Sect. 10.4.3.…”