Using a laser with adjustable pulse repetition rate, the mechanical resonances of c) In the excitation process, temperature and expansion increase during the pulse time (t p ) and repeat the same process with the repetition frequency (f rep ). The signal in frequency domain will only present at the f rep and it's higher order modes . While operating AFM-IR at contact mode, in the resonance-enhanced operation d), f rep matches the selected resonance frequency of the cantilever to resonantly exciting it, thereafter, the enhanced AFM-IR signal is proportional to the mode's quality factor.