Polariscopy with optical near-fields

Area of Science:

• Optics and Photonics
• Materials Science
• Spectroscopy

Background:

• Polarization analysis is crucial for understanding light-matter interactions in propagating optical fields.
• Evanescent fields, confined to surfaces, offer unique probing capabilities but are less explored for polarization analysis.
• Terahertz (THz) frequencies provide insights into molecular vibrations and material structures.

Purpose of the Study:

• To extend polarization analysis from propagating optical far-fields to evanescent fields.
• To develop a method for probing and mapping anisotropy in material absorption-dispersion properties using evanescent THz waves.
• To investigate the potential for 3D anisotropy mapping (polarization tomography) in complex polymer samples.

Main Methods:

• Utilized an attenuated total reflection (ATR) setup with a synchrotron source for THz frequency measurements.
• Employed scalar intensity (E²) for absorbance analysis of s- and p-polarized incident light components.
• Developed a four-polarization method to map absorbance and retrieve retardance spectra from transmittance measurements.

Main Results:

• Successfully demonstrated polarization analysis in the evanescent field, revealing non-accessible sample anisotropy.
• Validated mapping of sample anisotropy perpendicular to the surface, observing a cos²(θ) absorbance dependence.
• Obtained absorbance maps and retardance spectra for poly-hydroxybutyrate (PHB) and poly-L-lactic acid (PLLA) samples.

Conclusions:

• Evanescent field polarization analysis provides new insights into material anisotropy, complementing far-field studies.
• The developed THz-ATR method enables detailed characterization of complex polymer structures.
• The study outlines a pathway towards 3D polarization tomography for comprehensive material analysis.