Linear and nonlinear chiral responses from arrays of gold nanoparticles

Considerable attention is devoted to determining and refining the optical properties of metal nanoparticle arrays. The evolution of nanofabrication techniques towards miniaturizing optoelectronic devices naturally suggests the possibility of using such arrays in nanoscale optical components. However, small-scale defects (tens of nanometers or less) in individual particles themselves may exert a significant influence on the overall optical responses of the array, especially when the particles (and/or arrays) appear symmetric on the scale of the particle (and/or array). We have observed strong linear and nonlinear chiral responses from regular arrays of lithographically-designed, low-symmetry, L-shaped gold nanoparticles (~ 200 nm arm lengths) through polarization azimuth rotation and circular difference measurements. Second-harmonic generation measurements exhibit much larger circular difference responses, being more sensitive to symmetry. Comparisons between arrays of symmetric and asymmetric particles imply that the small defects may be the primary source of broken symmetry and hence chirality.