Infrared Spectroscopy and Imaging at Nanometer Scale

Infrared (IR) vibrational spectroscopy is one of the oldest and the most widely used analytical tool for the characterization of chemical species in samples. Spatially resolved IR spectroscopy, the IR spectro-microscopy, may offer information on the spatial arrangement of chemical species on sample surfaces as well as the chemical identity of the species. However, conventional far-field IR microscopy offers spatial resolution of $5{\mu}m$, which is not nearly enough for fully characterizing the sample structures in many cases. In this article, we review the recently developed spectro-microscopy methods that can overcome the diffraction limit of light and offer chemical maps of samples at nanometric scales. We mainly focus on IR near-field microscopy, IR photothermal microscopy, IR photo-induced force microscopy, and finally the IR-Visible photothermal lensing microscopy. The key principles of each technique, practical merits, and limitations are described.