A better shot in the dark

Nano Lett. doi:10.1021/nl1033304 (2010)

Dark-field microscopy relies on keeping radiation that has not been scattered by the sample of interest away from the optics that are used to form the final image. This approach offers improved sensitivity and signal-to-noise ratio, but it also reduces resolution. Now Lukas Novotny and colleagues at the University of Rochester and two research centres in Spain — the ICFO and the ICREA, both in Barcelona — have shown how this problem can be overcome through the use of nonlinear optics.

The new approach relies on a process called four-wave mixing in which two lasers of different frequencies, ω₁ and ω₂, interact with each other to produce an electromagnetic field with a frequency 2ω₁ – ω₂. When the angles of incidence for the two laser beams have certain values, the four-wave mixing field is evanescent — that is, it decays with distance rather than propagating through space. However, when it is scattered by the sample, it is converted into a propagating field that can be collected by an objective lens and used to form a dark-field image of the sample.

Novotny and colleagues used their approach to image patterns made by depositing titanium dioxide on a gold substrate to spell '4WM', and by scratching a silicon surface with an atomic force microscope. They also found that the contrast of the images depended on the relative alignment of the features in the sample and the plane defined by the two laser beams.