Near-field scanning optical microscopes (NSOMs) beat the diffraction limit by confining light to sub-wavelength dimensions. Resolutions down to around 50 nm are possible.

Active optical tips can achieve even better resolutions thanks to an optically active (for example, fluorescent) nano-object "glued" onto the tip apex. The object is optically excited using laser light injected into the tip and optical imaging or spectroscopy is performed with the light the object emits. If incorporated in a NSOM, the spatial resolution could reach the size of the nano-object itself.

Strong graft
Serge Huant's team at the Institut Néel in Grenoble, CNRS (French National Centre for Scientific Research) and Joseph Fourier University, in collaboration with the Laboratory for Quantum and Molecular Photonics (LPQM) at the Ecole Normale Supérieure (ENS) in Cachan, have now grafted a fluorescent diamond nanocrystal that measures just 20 nm across onto the tip of a NSOM. The diamond nanocrystal, which is a quantum emitter, hosts a (nitrogen-vacancy) "colour-centre" and acts as single-photon source. The graft is strong enough to use the resulting active tip as a nanosource of light in a near-field microscopy experiment.

What's more, the process to attach the tip is an all-optical one and there is no need to employ external manipulators or an electron microscope as in previous work by other groups, explains Huant.

To show that the modified NSOM worked, the researchers used it to image metallic nanostructures. Only single photons – that is, photons emitted one at a time – illuminate the imaged structure. This is confirmed by the small scanning area and the quantum emitter's excited lifetime of just a few nanoseconds.

According to the team, the device might find applications in various areas of nanoscience – for example, for accurate optical positioning of a quantum object in all three dimensions as in high-resolution magnetometry. "The scanning single-photon source should also be able to launch single plasmons (single photons interacting with the free electrons in a metal) at well-defined positions in metallic nanostructures," team members Aurélien Cuche and Aurélien Drezet told "This should lead to innovative studies in quantum optics with single plasmons – a new field known as quantum plasmonics."

The scientists now plan to improve the strength of the active tip by reinforcing the nanodiamond-optical tip interface. They would also like to try even smaller nanodiamonds on the tip. Indeed, 5 nm fluorescent diamonds have recently been developed by other research groups (for example, Small doi:10.1002/smll.200801802 and Nanotechnology doi: 10.1088/0957-4484/20/23/235602).

The work will appear in Optics Express.