Incident Angle-Tuning of Infrared Antenna Array Resonances for Molecular Sensing

Nanoantenna arrays with resonances in the mid-infrared spectral range enable a high sensitivity in surface-enhanced infrared absorption spectroscopy. Commonly, multiple antenna arrays with different geometries or surrounding materials have to be fabricated in order to match and enhance different absorption bands of interest. Here, we demonstrate that, by simply changing the angle of incidence, the near-field enhancement of the antenna arrays can be spectrally tuned for maximizing sensitivity for different vibrational modes of surface molecules. Varying the incident angle spectrally shifts the rayleigh anomalies and collective excitation and the peak field enhancement of the antennas occur. This allows us to tune the antenna array resonance to two adjacent molecular absorption bands without changing the geometry or surrounding material of the antennas. Characteristic Fano lineshapes that alter upon changing the incident angle are observed, and the angle-dependent signal enhancement is analyzed. We gain an improvement of the absorption enhancement by a factor of up to 1.75 compared to the usual angle-averaged measurements.