Optical landscape models will enhance microscopy and biosensing

17 Jun 2002

French researchers have created mathematical models of the nanoscale patterns of light that appear above the surface of materials.

A new model of the electromagnetic field that emanates a small distance above a material's surface is set to have implications for optical microscopy and biosensing. Christian Girard of the French National Centre for Scientific Research in Toulouse (together with colleagues there and at the University of Bourgogne in Dijon and the Laboratory for Microstructural Research in Bagneux, France) decided to model the electromagnetic field patterns found above different atomic-surface configurations. The scientists were partly motivated by the fact that the fields cause interference to the signal from a scanning near-field optical microscope (SNOM) as the probe approaches the sample. Gaining a picture of these evanescent light fields, suggests Girard, will greatly improve topographical maps of surfaces when combined with the SNOM data and information from a scanning tunnelling microscope.

The scientists used their model to investigate the electromagnetic fields that are formed by certain arrangements of nanoscale "posts", that were just 100 nm tall. They used massive parallel computing facilities at Toulouse to calculate a function called the photonic local density of states (LDOS) for increasingly symmetrical arrangements. The photonic LDOS describes surface optical phenomena, which is related to the probability of finding a photon in a particular location. Visualizations of their results for a circular arrangement of posts produced circular patterns of photon waves at selected frequencies that concurred neatly with previous work performed by scientists at IBM. (The IBM work had resulted in pictures of electron waves from circles of atoms, or "quantum corrals".)

Girard says that current lithography techniques make the actual construction of these nanostructures - "optical corrals" - entirely feasible. He is confident that it won't be long before a laboratory produces a nanostructure with properties that match the predictions. Not only that, the calculations suggest that an optical corral would exert influence over the fluorescence spectrum of a molecule placed within it, i.e., that it would force the molecule to radiate at a wavelength that is favoured by the arrangement of posts. This points to potential applications for biosensing because much brighter fluorescent probes would be available for the investigation of biomolecules.