Confocal Microscopy Enables Direct Observation of Photonic Nanojets

Lynn M. Savage

If you shine a beam of light onto a sphere just a few microns in diameter, an unusual effect takes place: On the side opposite where the beam strikes, a local field enhancement shaped like a tiny jet tail appears. Called photonic nanojets, these objects have waist diameters smaller than the diffraction limit as well as high intensity and low divergence. But although some researchers believe that nanojets may provide the means to detect and even image particles smaller than the diffraction limit using basic optical microscopes, the effect thus far has been hard to study because it has been observed only indirectly.

Now Patrick Ferrand and his colleagues at the Fresnel Institute in Marseille, France, have observed the nanojets directly, using confocal microscopy to record stacked images of 1-, 3- and 5-μm latex beads that were illuminated with a collimated beam.

On the left is a raw stack of images acquired for a 5-μm sphere illuminated at 520 nm. The detection plane moves upward (towards the bead) in 500-nm steps between each 2-D scan. On the right is a reconstruction of the photonic jet along the optical axis. The location of the microsphere is indicated by the white circle. The effects of the observation volume of the system have been corrected by numerical deconvolution. Reprinted with permission of Optics Express.

For each particle size, the investigators dispersed solutions containing the beads onto borosilicate glass coverslips. The concentration of the particles was deliberately low to prevent light scattering between adjacent beads. The researchers sent collimated unpolarized quasimonochromatic light onto the sample through a microscope condenser set in Köhler illumination with a minimum aperture diaphragm opening. Using the scanning detection mode of a custom-made laser-scanning confocal microscope system, they acquired the 3-D light intensity distribution as stacks comprising 41 frames of 100 × 100 pixels taken at 500-nm steps through the focal range.

The confocal system included an avalanche photodiode made by PerkinElmer Inc. of Fremont, Calif.; galvanometers made by Cambridge Technology Inc. of Lexington, Mass.; a high-speed analog voltage output card made by National Instruments Corp. of Austin, Texas; and an inverted microscope stand and high-numerical-aperture objective from Carl Zeiss of Jena, Germany. The scientists used Molecular Probes’ 20-nm fluorescent spheres with a Zeiss reflector cube to help characterize the observation volume defined by the confocal system.

The images in each stack showed a cross section of the jet formed when the microbeads were illuminated at 520 nm. The scientists observed that some of the image planes exhibited localized increases in intensity and that other planes showed the formation of concentric rings. The surrounding area remained at a constant level of intensity, which they believe can provide a reference for quantifying local intensity enhancements.

Data reconstruction provided views of the nanojets as they would be seen alongside the optical axis. The 5-μm beads created the widest jets — the 3-μm beads were about 15 percent narrower, for example — and had the highest increase in intensity compared with the incident beam. The 1-μm beads provided the weakest focusing effect of the sizes tested.

The investigators also tested groups of similarly sized beads that were in contact with each other and found that each sphere produced independent jets that had negligible influence on jets from nearby beads.

Optics Express, May 12, 2008, pp. 6930-6940.