Photosensitive Polymer Takes a Near-Field Picture

Hank Hogan

Near-field optical phenomena have attracted attention because they promise to enable finer resolution in microscopy than is otherwise possible. Consequently, the optical near-fields of nanostructures have been a subject of both theoretical and experimental research.

Researchers have used a photosensitive polymer to create a negative image of the optical field near silver and gold nanoparticles exposed to linearly or circularly polarized irradiation (left and right, respectively). The results from atomic forcemicroscopy display good agreement between theory and experiment. The technique offers an approach for high-resolution near-field microscopy.

Researchers from Université de Technologie de Troyes in France and at Argonne National Laboratory and Northwestern University in Evanston, both in Illinois, have demonstrated a way to detect such optical near-fields without the disturbance typically caused by optical probes. Instead, they take pictures of near-fields using a photosensitive polymer and a laser.

The investigators have used the method to image the optical near-field around silver and gold nanostructures. Such noble metal nanoparticles have enhanced optical near-fields under illumination resulting from plasmons, or collective electron excitations.

The researchers first fabricated nanostructures using electron-beam lithography, resulting in particles that were 75 nm across and about 50 nm high, and that were spaced about 500 nm apart in a regular array. They coated the array with an 80-nm-thick polymer layer, which they constructed by grafting the azobenzene dye Dispersed Red 1 to the backbone of polymethyl methacrylate, a transparent rubber.

Studies have shown that azo dyes act like molecular motors in the presence of polarized light. After photoexcitation, the dye molecules push and pull their polymeric host. This photoinduced movement transports the polymer away from the light, shifting mass from high- to low-intensity regions. The movement of the polymer is not due to swelling or ablation, and the process does not require the use of a developer or other chemical, which could alter the relief image.

The investigators used the 514-nm line of an argon-ion laser or the 532-nm line of a frequency-doubled diode-pumped Nd:YAG to illuminate the nanostructures embedded in the film. They carefully controlled the beam’s intensity and either linearly or circularly polarized it. They used atomic force microscopy to read the polymer’s shape after approximately 20 minutes of irradiation.

“The result is an effective negative image of the near-field,” said Christophe Hubert, a member of the research team and a staffer with Laboratoire de Nanotechnologie et d’Instrumentation Optique at Troyes. “The thickness of the polymer layer is smaller where the fields are higher,” he added.

To validate their results, the researchers used electrodynamic modeling methods to compute what the optical fields should be. These computations yielded results that were qualitatively similar to those observed in the polymer, with matching hills and valleys, but there were some discrepancies between the experiment and the theory. “The polymer has a slightly broader response than the predicted near-fields,” Hubert said.

He pointed out that the new method offers another approach for high-resolution, near-field microscopy. The polymer response could be optimized to further improve the quantitative measurement of optical near-fields. The technique also could be used to move small amounts of material with light.