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 has contributed much to basic and applied photonics. Here, a laser spectroscopy based on nonlinear nanobubble-related 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 cellular chromophores, as well as an increase in the sensitivity and resolution of the spectral hole-burning technique. This approach can permits the study of nonlinear plasmonics at a level of resolution beyond the spectral limits, the identification of weakly absorbing spectral holes, spectral optimization of photothermal nanotherapy, measurements of tiny red and blue resonance shifts in nanoplasmonic sensors, the use of negative contrast in photoacoustic technique, multispectral imaging and multicolour cytometry.