Detection and Signal Processing for Near-Field Nanoscale Fourier Transform Infrared Spectroscopy

TL;DR

This paper details the detection and processing methods for near-field nanoscale Fourier transform infrared spectroscopy, clarifying spectrum generation, interpretation, and providing new equations for data analysis.

Contribution

It introduces a comprehensive explanation of nano-FTIR spectrum generation, a new equation for scattering process modeling, and an algebraic method to extract local extinction coefficients.

Findings

Validated the algebraic expression with nano-FTIR and ATR-FTIR spectra.

Provided a new equation for modeling scattering processes.

Clarified the relationship between spectrum components and material properties.

Abstract

Researchers from a broad spectrum of scientific and engineering disciplines are increasingly using near-field infrared spectroscopic techniques to characterize materials nondestructively and with nanoscale spatial resolution. However, sub-optimal understanding of a technique's implementation can complicate data interpretation and even act as a barrier to enter the field. Here we outline the key detection and processing steps involved in producing scattering-type near-field nanoscale Fourier transform infrared spectra (nano-FTIR). The largely self-contained work (i) explains how normalized complex-valued nano-FTIR spectra are generated, (ii) rationalizes how the real and imaginary components of spectra relate to dispersion and absorption respectively, (iii) derives a new and generally valid equation for spectra which can be used as a springboard for additional modeling of the scattering processes, and (iv) provides an algebraic expression that can be used to extract the sample's local extinction coefficient from nano-FTIR. The algebraic expression is validated with nano-FTIR and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra on samples of polystyrene and Kapton.