Infrared Sensitivity of Plasmonic Metal Films with Hole Arrays to Microspheres In and Out of the Holes

Both experiments and finite difference time domain (FDTD) calculations show that infrared (IR) absorption spectroscopy and IR transmission resonances of metal films with arrays of subwavelength holes (meshes) are more sensitive to material in the holes of the mesh than to material on the front or back surfaces of the mesh. IR extinction spectra of isolated, individual latex (polystyrene) spheres inside or outside of a hole on plasmonic Ni mesh are compared to similar isolated microspheres on a ZnSe substrate. Although vibrational spectra of single microspheres on ZnSe are dominated by scattering, vibrational spectra of microspheres in microholes of metal mesh are dominated by absorption. The IR absorption spectrum of a latex microsphere with mesh is much stronger when the microsphere is inside a mesh hole rather than on the front or back surface, which pertains to the nature of spectroscopic enhancements on the plasmonic mesh. These results are supported and enhanced by FDTD Simulations showing changes in the primary infrared transmission resonance of the mesh when latex spheres are placed in different positions. The simulations demonstrate how interaction with a plasmonic resonance changes the intensity and line shape of a vibration in the dielectric material.