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.

Credit: © 2010 OSA

The research was carried out by Ronja Bäumner and co-workers from Laser Laboratorium Göttingen, the University of Geneva and Georg August University. They first fabricated a 159-nm-thick planar waveguide of Ta2O5 on a 0.7-mm-thick glass substrate. Pulses from a frequency-doubled erbium-doped fibre laser (central wavelength of 780 nm and pulse duration of 150 fs) incident on the waveguide were coupled to a guided mode using a grating coupler. KTP nanoparticles with an average size of 185 nm were dispersed on the waveguide surface a few millimetres from the grating coupler. Light emitted from the nanoparticles was collected by an objective lens, passed through a polarizer plate and then detected by a CCD camera.