Argonne National Laboratory Center for Nanoscale Materials U.S. Department of Energy

Light Scattering by Nanoparticles: Understanding Confinement of Light for Nanophotonics Applications through Near-Field Scanning Optical Microscopy

Schematic of the apertureless near-field optical microscope
Schematic of the apertureless near-field optical microscope. The optical scattering from the AFM probe tip provides the subdiffraction-limited optical field information.

One of the motivations of nanoscience is to achieve sufficient control over photon propagation in nanostructures so as to effectively replace the electron with the photon in all-optical integrated circuits. The much greater speed and bandwidth of light pulses versus electrons promise new capabilities and size reduction of photon based “electronics.” Arrays of metal nanoparticles are currently considered a leading candidate for photon propagation due to plasmons (free electron waves) that can produce extraordinarily intense optical fields very close to the nanoparticles’ surfaces. Closely spaced nanoparticles can lead to collective behavior and delocalization of the plasmon between nanoparticles, in principle leading to photon propagation with transverse widths approximately the size of the nanoparticles. However, the basic science of how to access and control sub-diffraction limited photon flow in such structures is in its infancy.

Using near-field scanning optical microscopy, CNM researchers have recently discovered several unusual aspects of the optical near-field response of illuminated silver and gold nanoparticles (20-40 nm in diameter) that may affect nanooptics research. First, the spatial extent of the optical near-field is strongly polarization sensitive. Only the polarization component perpendicular to the substrate surface shows confinement to within a few nanometers of the nanoparticles. If all polarizations are detected, the spatial extent of the field is over one micron. Second, the illumination must be resonant with the plasmon energy to show a confined optical field. Nonresonant excitation leads to scattering of the illumination field with a spatial profile again over one micron. Third, the illumination field scatters at the nanoparticle into the “far-field” at a 20-degree angle from the substrate surface. While this has been observed for nanometric defects on bulk metal films, this has not been observed before for isolated nanoparticles. This research provides insight into how to couple light into nanoparticle arrays because the reverse should also be true (i.e., an illumination angle of 20 degrees from the substrate will couple photons to the nanoparticle most effectively). Thus, these observations provide insight into the mechanism for coupling photons into arrays and the polarization of collective modes that are most likely to propagate in a sub-diffraction limited manner.

G.A. Wurtz, J. Hranisavljevic, J.S. Im, and G.P. Wiederrecht, submitted to J. Phys. Chem. B.

August 27, 2003

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