Second-harmonic generation from a single core/shell quantum dot.

Nanoparticles emitting two-photon luminescence are broadly used as photostable emitters for nonlinear microscopy. Second-harmonic generation (SHG) as another two-photon mechanism offers complementary optical properties but the reported sizes of nanoparticles are still large, of a few tens of nanometers. Herein, coherent SHG from single core/shell CdTe/CdS nanocrystals with a diameter of 10 to 15 nm is reported. The nanocrystal excitation spectrum reveals resonances in the nonlinear efficiency with an overall maximum at about 970 nm. Polarization analysis of the second-harmonic emission confirms the expected zinc blende symmetry, and allows extraction of the three-dimensional nanocrystal orientation. The small size of these nonlinearly active quantum dots, together with the intrinsic coherence and orientation sensitivity of the SHG process, are well adapted for ultrafast probing of optical near-fields with high resolution as well as for orientation tracking for bioimaging applications.