# Presentation: 716857

## CAREER: Microscopic Study of Light Localization Hui Cao, Northwestern University,  DMR-0093949

- Surface plasmons represent collective excitations of electrons in metals. Since they act as X-ray waves with optical frequencies, they play an important role in nano-optics. Using the near-field scanning optical microscopy, we have observed for the first time the coexistence of localized and delocalized surface plasmon modes in percolating metal films. The localization lengths of surface plasmon modes vary from the size of single metal grain to that of the entire system. The physical origin for this inhomogeneous localization lies in the short-range correlations among elements of the governing  Kirchhoff Hamiltonian.

- 1m  1m scanning electron micrograph of percolating silver film on a glass substrate.

- High-order moments of near-field intensity enhancement as a function of silver concentration p.

**Notes:**
Surface plasmon represents a collective excitation of electrons that is driven by external light. Its frequency is equal to that of incident light, but its wavelength can be as short as that of X-ray.  Therefore, surface plasmon plays an important role in the development of nano-optics. We studied the surface plasmon in metal-dielectric nano-composites. 
Using a near-field scanning optical microscopy, we investigated the near-field intensity statistics in semicontinuous silver films over a wide range of surface coverage. The variance of intensity fluctuations and the high-order moments of intensity enhancement exhibit local minima at the percolation threshold. This reduction in local field fluctuations results from resonant excitation of delocalized surface plasmon modes. By probing the modification of the critical indices for high-order moments of intensity enhancement caused by the delocalized states, we provide the first experimental evidence for the coexistence of localized and delocalized surface plasmon modes in percolating metal films.

## CAREER: Microscopic Study of Light Localization Hui Cao, Northwestern University,  DMR-0093949

- One undergraduate Olivia Tang, two graduate students H. Noh and X. Wu, one postodoc fellow Katyayani Seal (female) have contributed to this work. Because of her excellent work, Olivia is selected as the member of Phi Beta Kappa, and one of the 2006 Nanoscience Scholars by the NSF-funded National Center for Learning and Teaching in Nanoscale Science and Engineering. She has also successfully obtained the Northwestern University Undergraduate Research Grant.

**Societal Impact:**
- The ability to localize and delocalize optical excitations in metal-dielectric nano-composites is very important to the development of nano-scale photonic devices.  Strong localization of electromagnetic energy in the nano-scale can enhance various linear and nonlinear optical processes, while the delocalization allows the buildup of spatial correlations over large distance.

**CAREER: Microscopic Study of Light Localization **Hui Cao, Northwestern University,  DMR-0093949

**Notes:**
One undergraduate Olivia Tang, two graduate students H. Noh and X. Wu, one postodoc fellow Katyayani Seal (female) have contributed to this work. Because of her excellent work, Olivia was selected as the member of Phi Beta Kappa, and one of the 2006 Nanoscience Scholars by the NSF-funded National Center for Learning and Teaching in Nanoscale Science and Engineering. She also successfully obtained the Northwestern University Undergraduate Research Grant. 

The ability to localize light in a microscopic region is very important to the development of nano-scale photonic devices.  Strong buildup of electromagnetic energy in the nano-scale enhances various linear and nonlinear optical processes that have been used for sensors,  and enables ultra-sensitive detection of single molecules in biomedical sensing.