Optical microscopy can be used to image nanostructures, but the technique invariably relies on fluorescence light for the readout. Now, a team led by Ji-Xin Cheng has put forward a new technique to image non-fluorescent objects using a so-called "pump-probe" process in which an initial pump laser beam perturbs the charge carrier density in a sample and so modulates the transmission of a subsequently applied probe light beam.

In their experiments, Cheng and colleagues pump a sample of graphite nanoplatelets (each of which measures around 100 nm in diameter) with a pulse of 1064 nm-wavelength light. The sample continues to emit light for several femtoseconds even after the pulse is switched off. Next, the researchers fire a probe pulse of light at 830 nm at the sample during this time, which stimulates the emission of light from the sample.

Finally, they employ a doughnut-shaped laser beam to saturate electronic transitions in the area of the sample surrounding the focus of the laser beam. This ensures that the probe pulse is only modulated at the focus centre.

Sub-diffraction resolution at high speeds

The team says that it can then use the three aligned laser beams to image objects with sub-diffraction resolution at high speeds. This is possible thanks to the fact that charge carriers (electrons and holes) that have been excited by the pump field recombine quickly (in 20–30 femtoseconds) and then interact with optical photons more slowly (in 100 femtoseconds to a few picoseconds).

This technique, termed saturated transient absorption microscopy, temporally "bleaches" the ground state of the sample, so that the absorption signals only arise from the very centre of the focus," explains Cheng

Although the researchers have only used their method to image graphite nanoplatelets for now, there is nothing to stop them applying the technique to other nanostructures that also strongly absorb light – such as zinc oxide and iron oxide.

"Our work is a nice way to characterize morphological and carrier dynamic properties of nanomaterials such as nanotubes and graphene," team member Pu Wang told nanotechweb.org. It may also be good for studying nanostructures in biological environments or inside functional materials.

The team says that it is now looking to push the spatial resolution of its method down to 50 nm, to study nanodomains in graphene, for example.

The present work is detailed in Nature Photonics.