The AFM is interfaced with an infrared spectrometer. For work using Fourier transform infrared spectroscopy (FTIR), the spectrometer is equipped with a conventional black body infrared source. A particular region of the sample may first be chosen on the basis of the image obtained using the AFM imaging mode of operation. Then, when material at this location absorbs the electromagnetic radiation, heat is generated, which diffuses, giving rise to a decaying temperature profile. The thermal probe then detects the photothermal response of this region of the sample. The resultant measured temperature fluctuations provide an interferogram that replaces the interferogram obtained by a conventional FTIR setup, e.g., by direct detection of the radiation transmitted by a sample. The temperature profile can be made sharp by modulating the excitation beam. This results in the generation of thermal waves whose diffusion length is inversely proportional to the root of the modulation frequency. An important advantage of the thermal approach is that it permits to obtain depth-sensitive subsurface information from surface measurement, thanks to the dependence of thermal diffusion length on modulation frequency.