Near-field scanning optical microscopes (NSOMs) allow scientists to beat the diffraction limit by confining light to subwavelength dimensions. In the so-called aperture configuration, an optical tip (obtained by etching and coating an optical fibre with a metal) is used. This tip has a subwavelength aperture at its apex and resolutions down to around 50 nm can be achieved.
Active optical tips can achieve even better resolutions. Here, an optically active (for example, fluorescent) nano-object is grafted onto the tip apex and the object optically excited using laser light injected into the tip. Researchers then perform optical imaging or spectroscopy using the light emitted by the nano-object. If implemented in a NSOM, the spatial resolution could reach the size of the nano-object itself – around 10 nm.
The new nanoparticles, which were produced by Bruno Masenelli and co-workers at the University of Lyon, are made by ablating a target of cerium-doped YAG with a pulsed laser beam. They are about 10 nm across or smaller, have short radiative times (that is, they efficiently emit light) and have good stoichiometry (they are only slightly different from the target).
The nanoparticles can also be modified to emit light at various wavelengths from the infrared to the ultraviolet by replacing the cerium ions with other rare-earth ions. And thanks to their biocompatibility, they can be used in biology applications.
The team, which includes researchers from the Institut Néel, Grenoble, CNRS (French National Centre for Scientific Research) and the University Joseph Fourier; now plans to reduce the size of the active tip from the 400 nm range to the targeted 10 or 20 nm. One way of doing this would be to combine particle-deposition techniques on tips with shape control of the initial optical tip via milling.
"We will also improve the particle properties, in particular their stoichiometry and cerium doping levels," team leader Serge Huant and doctoral student Aurélien Cuche told nanotechweb.org.
The researchers have submitted their work to Nanotechnology.