Thermoplasmonic Effect of Surface Enhanced Infrared Absorption in Vertical Nanoantenna Arrays

TL;DR

This study investigates how vertical gold nanoantenna arrays enhance mid-infrared absorption and induce localized heating through plasmonic effects, revealing different behaviors of dark and bright modes in thermoplasmonic processes.

Contribution

It provides direct nanoscale measurements of thermoplasmonic effects in nanoantenna arrays and distinguishes the roles of dark and bright plasmonic modes in infrared absorption and heating.

Findings

Dark modes produce surface-enhanced infrared absorption without altering molecular lineshape.

Bright modes cause Fano interference and generate heat with less chemical specificity.

Temperature increases of up to 10 K and gradients of 5 K/μm were observed at specific vibrational resonances.

Abstract

The temperature increase and temperature gradients induced by mid-infrared laser illumination of vertical gold nanoantenna arrays embedded into polymer layers was measured directly with a photothermal expansion nanoscope. Nanoscale thermal hotspot images and local temperature increase spectra were both obtained, the latter by broadly tuning the emission wavelength of a quantum cascade laser. The spectral analysis indicates that plasmon-enhanced mid-infrared vibrations of molecules located in the antenna hotspots are responsible for some of the thermoplasmonic resonances, while Joule heating in gold is responsible for the remaining resonances. In particular, plasmonic dark modes with low scattering cross-section mostly produce surface-enhanced infrared absorption (SEIRA), while bright modes with strong radiation coupling produce Joule heating. The dark modes do not modify the molecular absorption lineshape and the related temperature increase is chemically triggered by the presence of molecules with vibrational fingerprints resonant with the plasmonic dark modes. The bright modes, instead, are prone to Fano interference, display an asymmetric molecular absorption lineshape and generate heat also at frequencies far from molecular vibrations, insofar lacking chemical specificity. For focused mid-infrared laser power of 50 mW, the measured nanoscale temperature increases are in the range of 10 K and temperature gradients reach 5 K/$\mu$m in the case of dark modes resonating with strong infrared vibrations such as the C=O bond of poly-methylmethacrylate at 1730 cm$^{-1}$.