ABSTRACT
This chapter describes the use of a family of dielectric metal oxide nanoparticles (NPs), harmonic NPs, characterized by a noncentrosymmetric crystal structure and high nonlinear optical efficiencies, and displaying a very rich nonlinear response. The selectivity in the retrieval of harmonic NPs in optically congested environments, besides the aforementioned minimization of fluorescence hindrance by wavelength selection, can be further improved using a multiorder harmonic approach. To date, the most successful and widespread ones are those based on quantum dots and upconversion NPs for imaging, because of their bright luminescence emission, and those using metal particles for sensing, thanks to the sensitivity of surface plasmon scattering to the local environment. The simultaneous collection and analysis of these signals allow developing refined detection approaches for increased sensitivity and selectivity in demanding imaging applications. Nonlinear optical observables present various advantages, including the possibility to detect background-free parametric signals at excitation wavelengths which depose no or minimal energy on the sample under investigation.
