A heterodyne detection technique for highly localized IR spectroscopy based on an AFM. A pulsed IR source illuminates a sample and causes contact resonance of an AFM probe, which is a function of localized absorption. The probe is operated in intermittent contact mode and is therefore oscillated at a resonance frequency. A secondary oscillation is mixed in to the probe oscillation such that the sum of the secondary oscillation and the IR source pulse frequency is near another harmonic of the probe. A mixing effect causes measurable probe response at the other harmonic allowing data to be taken away from the pulse frequency, resulting in background effect rejection and improved spatial resolution.