Antenna-mediated back-scattering efficiency in infrared near-field microscopy

Area of Science:

• Near-field optics
• Spectroscopy
• Nanophotonics

Background:

• Atomic Force Microscopy (AFM) probes are crucial for nanoscale imaging and spectroscopy.
• Understanding light-matter interactions at the AFM tip-sample interface is key for advanced optical techniques.
• Previous studies identified Fresnel reflectivity and SiC Reststrahlen edge effects in AFM back-scattering.

Purpose of the Study:

• To quantitatively evaluate the back-scattering efficiency and phase from a standard AFM probe interacting with a flat sample.
• To identify and characterize spectral features beyond Fresnel reflectivity, attributing them to antenna resonances.
• To elucidate the antenna properties of the AFM probe through numerical simulations.

Main Methods:

• Coherent frequency-comb Fourier-transform spectroscopy (c-FTIR) was used to measure back-scattering over a 9-12 µm wavelength range.
• Spectroscopic determination of back-scattering efficiency, eta(B), and phase.
• Numerical simulations of near fields, radiation zone fields, and far-field scattering distributions for realistic tip/sample configurations.

Main Results:

• Observed strong resonant back-scattering (eta(B) = 13%) at 955 cm(-1), suggesting surface-phonon-polariton excitation near SiC/Au boundaries.
• Identified spectral effects attributed to antenna resonances involving the probe shaft, cantilever, and sample.
• Simulations revealed standing-surface-plasmon-polariton patterns along the probe shaft and sample-dependent far-field antenna lobes.

Conclusions:

• AFM probe back-scattering is influenced by antenna resonances beyond simple Fresnel reflectivity.
• The probe acts as an antenna, with its scattering properties tunable by the sample's dielectric properties.
• Surface-phonon-polariton excitation plays a significant role in resonant scattering phenomena at specific frequencies.