  We investigate local heat generation by molecules at the apex of
polymer-embedded vertical antennas excited at resonant mid-infrared
wavelengths, exploiting the surface enhanced infrared absorption (SEIRA)
effect. The embedding of vertical nanoantennas in a non-absorbing polymer
creates thermal isolation between the apical hotspot, the locus of heat
generation, and the heat sink represented by the substrate. Vibrational
mid-infrared absorption by strongly absorbing molecules located at the antenna
apex then generates nanoscale temperature gradients at the surface. We imaged
the thermal gradients by using a nano-photothermal expansion microscope, and we
found values up to 10 K/microm in conditions where the radiation wavelength
resonates with both the molecule vibrations and the plasmonic mode of the
antennas. Values up to 1000 K/microm can be foreseen at maximum quantum cascade
laser power. The presented system provides a promising thermoplasmonic platform
for antenna-assisted thermophoresis and resonant mid-infrared photocatalysis.
