  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$^{-1}$.
