Imaging single metal nanoparticles in scattering media by photothermal interference contrast

We have developed a photothermal method for far-field optical detection of nanometer-sized metal particles, combining high-frequency modulation and polarization interference contrast. We can image gold colloids down to View the MathML source in diameter, with a signal-to-noise ratio higher than 10. This is a considerable improvement over commonly used optical methods based on resonance plasmon scattering which, for background reasons, are limited to particles of more than about View the MathML source in diameter. By adding View the MathML source latex spheres in the sample, we also show that in addition to its intrinsic sensitivity, our photothermal method is totally insensitive to non-absorbing scatterers.