Photothermal heterodyne imaging and absorption spectroscopy of individual nonfluorescent nano-objects

In the fast evolving field of nanoscience, where size is crucial for the properties of the objects, simple and sensitive methods for the detection and characterization of single nano-objects are needed. We recently developped an all-optical method called Photothermal Heterodyne Imaging (PHI) that allows for the unprecedented detection of individual nano-objects such as gold nanoparticles with diameter down to 1.4 nm (67 atoms) as well as non-luminescent semiconductor nanocrystals [1]. This method relies on the absorptive properties of the nano-object and does riot suffer from the drawbacks of luminescence-based methods (e.g. rapid photobleaching or blinking). The experimental PHI setup consists in a combination of two laser beams: an intensity-modulated heating beam, close to resonance, and a cw off-resonance probe beam. Absorption of the heating beam by a nanoparticle induces a time-modulated increase of the temperature in the vicinity of the nanoparticle, Propagation of the probe beam through the resulting time-modulated index of retraction profile, produces a frequency shifted scattered field which is detected by its beatnote with the probe field on a fast photodiode. The measured photothermal signal is directly proportional to the nanoparticle absorption cross section and in good agreement with an electrodynamical calculation based on the scattered field from a time-modulated index of refraction profile. PHI opens new pathways for quantitative specroscopic measurements on individual non-luminescent nano-objects. We took advantage of the high sensitivity of this method to study for the first time the absorption spectra of individual gold nanoparticles with diameter down to 5 nm [2]. Sensitivity at the single particle level allowed us to access the dispersions of the SPR resonant peak energy E_R and homogeneous red half width at half maximum Γ_1/2. We found that a very small ellipticity in the nanoparticle shape results in a significant dispersion of E_R. We shown that for nanoparticles smaller than 20 nm, Γ_1/2 is significantly broadened because of surface damping mechanisms leading to reduced SPR decoherence times. This observation of intrinsic size effects in the optical response of gold nanoparticles is analysed within the frame of Mie theory using a size-dependent surface damping term in the dielectric constant.