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The temperature increase and temperature gradients induced by mid-infraredlaser illumination of vertical gold nanoantenna arrays embedded into polymerlayers was measured directly with a photothermal expansion nanoscope. Nanoscalethermal hotspot images and local temperature increase spectra were bothobtained, the latter by broadly tuning the emission wavelength of a quantumcascade laser. The spectral analysis indicates that plasmon-enhancedmid-infrared vibrations of molecules located in the antenna hotspots areresponsible for some of the thermoplasmonic resonances, while Joule heating ingold is responsible for the remaining resonances. In particular, plasmonic darkmodes with low scattering cross-section mostly produce surface-enhancedinfrared absorption (SEIRA), while bright modes with strong radiation couplingproduce Joule heating. The dark modes do not modify the molecular absorptionlineshape and the related temperature increase is chemically triggered by thepresence of molecules with vibrational fingerprints resonant with the plasmonicdark modes. The bright modes, instead, are prone to Fano interference, displayan asymmetric molecular absorption lineshape and generate heat also atfrequencies far from molecular vibrations, insofar lacking chemicalspecificity. For focused mid-infrared laser power of 50 mW, the measurednanoscale temperature increases are in the range of 10 K and temperaturegradients reach 5 K/$\mu$m in the case of dark modes resonating with stronginfrared vibrations such as the C=O bond of poly-methylmethacrylate at 1730cm$^{-1}$.

Thermoplasmonic Effect of Surface Enhanced Infrared Absorption in Vertical Nanoantenna Arrays