Photothermal or thermal lens microscopy is based on the slight change of refractive index of a material through the heat released by optical absorption. The method involves two lasers which can easily be separated by good optical filters. The heating beam is absorbed by the objects to be detected, whereas the probing beam serves to detect and measure the local refractive index changes. The scattered probe beam interferes with transmitted or reflected probe beams, giving rise to a small modification of the probe intensities which are proportional to the field scattered, therefore to the absorbed energy. The modulation of the heating beam at a high frequency (MHz) allows a background-free detection of absorption via modulation of the detected probe beam. The advantage of this method is that photon noise can be much reduced when the probe beam has a sufficiently high intensity. This is possible whenever the probe wavelength is chosen outside of the absorption spectral range of the objects to be detected.
We have set up a photothermal microscope enabling simultaneous fluorescence measurements. The correlation of absorption and fluorescence for the same object simultaneously can bring precious information, for example about the mechanisms of photoblinking.