Terahertz and mid-infrared photoexpansion nanospectroscopy

We report a method of taking mid-infrared and terahertz absorption spectra on nanoscale using compact mW-level sources, such as quantum cascade lasers, and a standard atomic force microscope (AFM). Light absorption is detected via deflection of an AFM cantilever due to local sample thermal expansion. To enable detection of minute sample expansion, the repetition frequency of the laser pulses is moved in resonance with the cantilever mechanical frequency. Plasmonic enhancement under the apex of a sharp metalized AFM tip is also utilized to further improve the detection sensitivity. As a result, we are able to obtain high-quality absorption spectra from samples thinner than 10 nm. The spatial resolution is principally determined by the diameter of the high-intensity spot in the vicinity the AFM tip, and is below 50nm. This technique requires no optical detectors.