Dynamic Tuning and Symmetry Lowering of Fano Resonance in Plasmonic Nanostructure

We present dynamic tuning and symmetry lowering of Fano resonances in gold heptamers accomplished by applying uniaxial mechanical stress. The flexible heptamer structure was obtained by embedding the seven-gold-nanocylinder complex in a polydimethylsiloxane membrane. Under uniaxial stress, the Fano resonance exhibited opposite spectral shifts for the two orthogonal polarizations parallel and perpendicular to the mechanical stress. Furthermore, a new resonance was observed for polarization parallel to the mechanical stress but not for the perpendicular polarization. The experimental results showed good agreement with the numerical simulations. A detailed group theoretical analysis showed that the symmetry lowering caused by the mechanical stress not only splits the originally degenerate mode but also modifies the originally optically inactive mode into an optically active mode, which then interacts strongly with a closely spaced mode and exhibits anticrossing behavior. The symmetry tuning enabled by applying mechanical stress is a simple and efficient way to engineer the nature of coupled plasmon resonances in complex nanostructures. The mechanically tunable plasmonic nanostructures also provide an excellent platform for dynamically tunable nanophotonic devices such as tunable filters and sensors.