Imaging the Engineered Polarization States of Surface Plasmon Polaritons at Visible Wavelengths

We experimentally demonstrate and image the hybrid nature of surface plasmon polaritons (SPPs) at visible wavelengths excited in low aspect ratio 1D and 2D periodic plasmonic structures with periods nearly equal to the resonance wavelengths, fabricated on thin homogeneous metal-coated glass substrates. A thin homogeneous metal layer was sandwiched between the substrate and the periodic metal patterns to out-couple the SPPs as leakage radiation through the substrate. This resulted in the detection of SPP excitation as transmission peaks in a dark background, as opposed to transmission dips in a bright background in the structures without the metal layer reported earlier. Due to the periods being almost equal to the resonance wavelengths, the transmission peaks were not due to extraordinary transmission phenomenon, but because of the radiative leakage of wavelength selective SPPs excited on the surface. A cross-axis polarizer-analyzer was used in broadband leakage radiation microscopy to diminish the direct zeroth -order transmission and image the real and Fourier plane characteristics of the SPP transmission. The bright emission of different colors against a dark background corresponding to the transmission plasmonic resonances for different periods, in both real and Fourier plane revealed the hybrid nature of excited SPPs, when the polarizer was positioned at 45$^\circ$ with respect to the grating vector. The fabricated plasmonic substrates present interesting opportunities for imaging and sensing applications.