Second-harmonic nanoprobes

Abstract

Inorganic non-centrosymmetric nanocrystals — subwavelength structures capable of supporting the second-harmonic generation of light — are receiving growing attention as useful imaging probes for bioimaging or applications in nonlinear optics and micromanipulation. They can be excited at any wavelength — unlike fluorescent molecules or quantum dots — and do not suffer from blinking or bleaching.Scientists in Germany have now dispersed nanocrystals of potassium titanyl phosphate (KTP, a common nonlinear optical material) over a planar optical waveguide and shown that the evanescent field from a guided mode in the waveguide can generate second-harmonic light (Opt. Express 18, 23218–23225; 2010). Furthermore, generation of the second harmonic was shown to be coherent with the signals from adjacent nanoparticles, thereby forming interference patterns (see image). The discovery of a convenient and practical means of exciting such nanocrystals without the need for direct scanning illumination suggests that they may become useful for probing biological samples deposited on a planar optical chip.