Detection limits in photothermal microscopy

We show how to push the detection limits in photothermal microscopy towards weaker single absorbers, by a systematic optimization of signal and noise sources. In particular, we (i) maximize the power of the probe laser beam, (ii) select optimal optical and thermal properties for the medium embedding the absorber, and (iii) thermally isolate the absorber from the glass substrate. These different experimental conditions are optimized in turn with single immobilized gold nanoparticles. We demonstrate the detection of a dissipated power of 3 nW with a signal-to-noise ratio of 8, and an integration time of 10 ms. This corresponds to a less than 0.1 K surface temperature rise for a 20 nm-diameter gold nanosphere (0.4 K for 5 nm). As an example of the achieved detection sensitivity, we show simultaneous photothermal and fluorescence detection of individual 20 nm fluorescent beads, each containing about 20 Nile red dye molecules.