Infrared refractive index dispersion of PMMA spheres from synchrotron extinction spectra

TL;DR

This study uses high-resolution FTIR spectroscopy of PMMA spheres to analyze electromagnetic resonances, enabling determination of the sphere's radius and the material's refractive index dispersion in the near-infrared region.

Contribution

The paper introduces a method to extract the refractive index and its dispersion from synchrotron IR extinction spectra by analyzing ripple structures caused by electromagnetic resonances.

Findings

Refractive index of PMMA shows strong frequency dependence in near-infrared.

Ripple analysis allows accurate determination of sphere radius and refractive index.

Method is applicable to other transparent micron-sized spherical materials.

Abstract

We performed high-resolution Fourier-transform infrared (FTIR) spectroscopy of a polymethyl methacrylate (PMMA) sphere of unknown size in the Mie scattering region. Apart from a slow, oscillatory structure (wiggles), which is due to an interference effect, the measured FTIR extinction spectrum exhibits a ripple structure, which is due to electromagnetic resonances. We fully characterize the underlying electromagnetic mode structure of the spectrum by assigning two mode numbers to each of the ripples in the measured spectrum. We show that analyzing the ripple structure in the spectrum in the wavenumber region from about $3000\,$cm$^{-1}$ to $8000\,$cm$^{-1}$ allows us to both determine the unknown radius of the sphere and the PMMA index of refraction, which shows a strong frequency dependence in this near-infrared spectral region. While in this paper we focus on examining a PMMA sphere as an example, our method of determining the refractive index and its dispersion from synchrotron infrared extinction spectra is generally applicable for the determination of the index of refraction of any transparent substance that can be shaped into micron-sized spheres.