Nonlinear-optical response and Raman signals of nanocrystalline lithium niobate

This paper discusses the fabrication of LiNbO₃ nanocrystals of controlled size by combining a double-alkoxide sol-gel bottom-up synthesis route with subsequent wet-chemical etching using hydrofluoric acid. This route produces LiNbO₃ nanocrystals with mean diameters between 11 and 30 nm, and with individual diameters between 5 and 50 nm. The nonlinear optical response of single nanocrystals of these batches is examined by illuminating the nanocrystals with light pulses at 1064 nm wavelength. The generated second harmonic is measured varying the pump-pulse energy, the light polarisation, and the crystal diameter. By comparing the measured dependences with a straightforward model, it is shown that the lithium niobate nanocrystals exhibit the nonlinear optical coefficients known from the bulk material, at mean crystal sizes of about 11 nm as well as for individual crystal sizes of about 5 nm. Further knowledge about the properties and crystallinity of the nanocrystals is gained by Raman spectroscopy.