Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants

• Main Authors: McLeod, Alexander S, Kelly, Priscilla, Goldflam, M. D, Gainsforth, Zack, Westphal, Andrew J, Dominguez, Gerardo, Thiemens, Mark, Fogler, Michael M, Basov, D. N
• Format: Journal Article
• Language: English
• Published: 30-06-2014
• Subjects: Physics - Materials Science; Physics - Optics
• DOI: 10.48550/arxiv.1308.1784

Phys. Rev. B 90, 085136 (2014) Near-field infrared spectroscopy by elastic scattering of light from a probe
tip resolves optical contrasts in materials at dramatically sub-wavelength
scales across a broad energy range, with the demonstrated capacity for chemical
identification at the nanoscale. However, current models of probe-sample
near-field interactions still cannot provide a sufficiently quantitatively
interpretation of measured near-field contrasts, especially in the case of
materials supporting strong surface phonons. We present a model of near-field
spectroscopy derived from basic principles and verified by finite-element
simulations, demonstrating superb predictive agreement both with tunable
quantum cascade laser near-field spectroscopy of SiO$_2$ thin films and with
newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of
crystalline SiC. We discuss the role of probe geometry, field retardation, and
surface mode dispersion in shaping the measured near-field response. This
treatment enables a route to quantitatively determine nano-resolved optical
constants, as we demonstrate by inverting newly presented nanoFTIR spectra of
an SiO$_2$ thin film into the frequency dependent dielectric function of its
mid-infrared optical phonon. Our formalism further enables tip-enhanced
spectroscopy as a potent diagnostic tool for quantitative nano-scale
spectroscopy.