Credit: © 2008 OSA

A simple far-field technique that can generate quantitative two-dimensional maps of a structure's local electromagnetic density of states (LDOS) could benefit a number of applications. These include enhanced biolabelling and improved optical antennas for harvesting solar energy.

The LDOS is an important quantity that describes the available optical eigenmodes for a photon at a specific location within a material. As can be imagined, it is vital for the understanding and control of many quantum optical phenomena.

Typically, the LDOS is measured by scanning tunnelling microscopy or by scanning near-field optical microscopy operating in illumination mode. However, the images produced by these methods largely depend on the surface cleanliness and the quality of the near-field tip. Moreover, cross-coupling mechanisms between the tip and the structure itself are often neglected in the theoretical analysis of results.

Now, Caijin Huang and colleagues from Université de Bourgogne in France (Opt. Lett. 33, 300–302; 2008) have proposed an experimental method for determining the LDOS. The approach relies on Fourier filtering and integrating the differential scattering cross-section of the material's evanescent modes.

To perform the measurements, the researchers illuminate the structure under test using a 0.65-numerical-aperture lens and collect the scattered light, which contains both propagating and evanescent components, using a 1.45-numerical-aperture lens. The evanescent components are then selected using a beam stop, a pinhole and a photomultiplier tube. Laterally scanning the target structure enables the researchers to record the scattering response for each position so that they can reconstruct a two-dimensional image of the local scattering cross-section of the evanescent modes.

To validate the approach, the team measured the LDOS of two samples — an optical corral comprising 18 identical 50-nm-thick gold nanoparticles, and a 4 μm × 2 μm stadium shape comprising 34 gold nanoparticles.