Ultrasharp nonlinear photothermal and photoacoustic resonances and holes beyond the spectral limit.

High-resolution nonlinear laser spectroscopy based on absorption saturation, Lamb-dip and spectral hole-burning phenomena have contributed much to basic and applied photonics. Here, a laser spectroscopy based on nonlinear photothermal and photoacoustic phenomena is presented. It shows ultrasharp resonances and dips up to a few nanometres wide in broad plasmonic spectra of nanoparticles. It also demonstrates narrowing of absorption spectra of dyes and chromophores, as well as an increase in the sensitivity and resolution of the spectral hole-burning technique. This approach can permit the study of laser-nanoparticle interactions at a level of resolution beyond the spectral limits, identification of weakly absorbing spectral holes, spectral optimization of photothermal nanotherapy, measurements of tiny red and blue plasmon resonance shifts, multispectral imaging and multicolour cytometry.