Nonlinear optical microscopy of single nanostructures

We review recent advances in nonlinear optical (NLO) microscopy studies of single nanostructures. NLO signals are intrinsically sensitive to the electronic, vibrational, and structural properties of such nanostructures. Ultrafast excitation allows for mapping of energy relaxation pathways at the single-particle level. The strong nonlinear response of nanostructures makes them highly attractive for applications as novel NLO imaging agents in biological and biomedical research. NLO modalities based on harmonic generation, multiphoton photoluminescence, four-wave mixing, and pump-probe processes are discussed in detail.