Quantitative absorption imaging of single nanoparticles by widefield interferometric photothermal microscopy

We demonstrate widefield interferometric photothermal microscopy to detect the local refractive index changes of single nanoparticles induced by the dissipated heat due to light absorption (known as the thermal lens effect). The sensitivity of our system is sufficient for visualizing very small single gold nanoparticles, as small as 5 nm, with a dissipated heat of a few nanowatts. We establish a model to explain the generation mechanism of the photothermal signal. For small particles (< 40 nm), the photothermal signal is contributed by the scattered field of the thermal lens, independent of the particle size and materials. For large particles, we find that the strong scattered field of the sample would interfere with that of the thermal lens, leading to an enhancement of the photothermal contrast. This work shows that the complete scattering property of the sample is required for accurate absorption quantification through photothermal measurement.