Nonlinear photothermal imaging

A new technique is described whereby one can extract the nonlinear photothermal properties of a sample in photothermal microscopy. The basic scheme relies on chopping a pump heating beam at ω and detecting the temperature variation of the sample using a linear temperature sensor (in our case a mirage detector) tuned to 2ω. Harmonic images of cracks and other samples show very high contrast when compared with the fundamental image.