The resonant plasmonic properties of metallic nanostructures depend strongly on charge carrier density. While researchers have reported shifts of the resonant absorption frequency of plasmonic nanostructures due to electrostatically induced changes of charge density, the converse —the dependence of charge density and electrostatic potential on optical absorption— has been largely overlooked. Recently, we have reported a theoretical framework and provided experimental evidence for a ‘plasmoelectric effect’, a newly described mechanism for generating electrochemical potentials in plasmonic nanostructures via narrowband absorption.

Our initial work with Au colloid nanoparticles has shown that, unlike the more familiar thermoelectric or photovoltaic effects, the magnitude and sign of the plasmoelectric potential depends on the frequency difference between the plasmon resonance and incident radiation [1]. Radiation at shorter wavelength induces an increase of electron density that blue-shifts the plasmon resonance. This response is driven by the increased heat that accompanies increased absorption. Similarly, radiation at longer wavelengths decreases electron density to induce a red-shift of the absorption maximum.

Here, we report measurements on lithographically patterned samples with a design that has been optimized via iterative full wave simulations (FDTD method) to maximize plasmoelectric potentials, and to further distinguish the phenomenon from the thermoelectric effect. The structure consists of an electrically grounded 20 nm Au thin film on glass with lithographically defined square-lattice arrays of 100 nm diameter holes. The spectral position of the plasmonic resonance of the hole arrays is very sensitive to the inter-hole spacing, allowing sharp and highly tunable absorption peaks to be defined across the visible spectrum, between 550 nm and 700 nm, by varying the pitch between 150 to 300 nm in fabricated devices.

Scanning Kelvin probe force microscopy (KPFM) determined the surface potential of device structures while varying the wavelength of incident radiation near the plasmon resonance. Under 30 mW cm-2 monochromatic illumination, we measure induced potentials of Â± 150 mV from hole arrays, with a characteristic sign change for illumination blue or red of the absorption maximum. This is a 2-order of magnitude increase of measured plasmoelectric potentials compared with the response of Au colloid particles. By comparing devices with plasmonic resonances spanning the optical spectrum, we observe clear evidence for the pitch-dependent and wavelength-dependent trends consistent with our theoretical framework. Our findings guide the development of solid-state power conversion devices based on the plasmoelectric effect, as our devices generate driving electrochemical potentials 1000x larger than comparable thermocouples under equivalent optical power.

[1] Sheldon, et al. (2012). Plasmoelectric Potentials in Metal Nanostructures, in review

Matthew Sheldon

California Institute of Technology

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