We report a method of taking mid-infrared and terahertz 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. The spatial resolution is principally determined by the diameter of the high-intensity spot in the vicinity of a sharp metalized AFM tip, and is below 50nm. To enable detection of minute sample expansion, the repetition rate of the laser pulses is moved in resonance with the cantilever mechanical frequency. The technique requires no optical detectors.
We report diffuse reflection imaging of concealed powdered samples in atmospheric air using a quantum cascade laser
operating at 2.83 THz. The imaging system uses a helium-cooled silicon bolometer for mapping radiation diffusely
reflected and scattered from samples, and a room-temperature pyroelectric sensor for simultaneously acquiring a
specular image. A range of powders concealed within plastic packaging and standard FedEx envelopes was imaged
with a resolution of better than 0.5 mm, and it was possible to detect powdered samples concealed within packaging
from which there was a strong component of surface reflection. The feasibility of performing dual-wavelength diffuse
reflection imaging for identification of illicit drugs and explosives is discussed.