Self-Organized Nanogratings for Large-Area Surface Plasmon Polariton Excitation and Surface-Enhanced Raman Spectroscopy Sensing

Abstract

Surface Plasmon Polaritons (SPP) are exploited due to their intriguing properties for photonic circuits fabrication and miniaturization, for surface enhanced spectroscopies and imaging beyond the diffraction limit. However, the excitation of these plasmonic modes by direct illumination is forbidden by energy/momentum conservation rules. One strategy to overcome this limitation relies on diffraction gratings to match the wavevector of the incoming photons with that of propagating SPP excitations. The main limit of the approaches so far reported in literature is that they rely on highly ordered diffraction gratings fabricated by means of demanding nano-lithographic processes. In this work we demonstrate that an innovative, fully self-organized method based on wrinkling-assisted Ion Beam Sputtering can be exploited to fabricate large area (cm^2 scale) nano-rippled soda-lime templates which conformally support ultrathin Au films deposited by physical deposition. The self-organized patterns act as quasi-1D gratings characterized by a remarkably high spatial order which matches properly the transverse photon coherence length. The gratings can thus enable the excitation of hybrid SPP modes confined at the Au/dielectric interfaces, with a resonant wavelength which can be tuned either by modifying the grating period, photon incidence angle or, potentially, the choice of the thin film conductive material. Surface Enhanced Raman Scattering experiments show promising gains in the range of 10^3 which are competitive, even before a systematic optimization of the sample fabrication parameters, with state-of-the art lithographic systems, demonstrating the potential of such templates for a broad range of optoelectronic applications aiming at plasmon-enhanced photon harvesting for molecular or bio-sensing.