Light incident on nanoscale metal-insulator-metal (MIM) plasmonic gratings generates surface plasmon polaritons (SPPs) which resonate and propagate within the grating structure. The SPP resonant wavelength can be altered by introducing a gradient in the width of the MIM grooves. Specifically, a symmetrically graded grating with a narrow central groove leads to a gradient in the effective refractive index such that the index increases in the direction of the central groove. This index gradient guides non-localized SPP waves towards the grating center. This waveguiding of SPPs along with localized SPP modes within the narrow central grooves give rise to multi-wavelength electric field enhancement at the grating center. SPP coupling across the grooves leads to an increase in electromagnetic field strength with decreasing groove width. However, standard nanofabrication techniques limit the minimum width of the grooves to approximately 50nm, preventing maximum field enhancement. Herein we report on the development of a novel nanoplasmonic graded grating with a 10 nm central groove flanked by increasingly wider grooves on either side, which are fabricated using thin film RF magnetron sputter deposition technique. These structures are studied using COMSOL Multiphysics modelling in which we vary the groove width, groove separation and groove depth, and thus demonstrate localization of broadband incident light. Raman spectroscopy and fluorescence microscopy are used to demonstrate field enhancement at several visible wavelengths.
We report on a new class of plasmonic nanogratings in which the gradient in the groove-width enables facile fabrication of multiwavelength surface-enhanced Raman spectroscopy (SERS) substrates. These substrates have the potential of achieving unprecedented detection sensitivity, specificity and speed. The structure of these nano-gratings consist of metal-insulator-metal grooves with a 40 nm central groove width flanked by a series of grooves on either side with gradually increasing width. Groove widths increase in steps of 5 nm up to a maximum width of 200 nm positioned farthest from the central groove on either side. The gradient in groove width in turn produces a gradient in the effective refractive index of the grating determined by the groove width at each location. Together, multiple laser wavelengths can be simultaneously confined to the centrally situated narrow grooves, with the neighbouring larger grooves guiding the nonlocalized waves toward the grating center from both directions. This generates a maximally enhanced plasmonic field over a broad range of wavelengths on the surface of the nanograting which can be used to increase the Raman scattering efficiency of a sample molecule distributed over the structure. The structures were fabricated using electron beam lithography, reactive ion etching, and sputter-deposition techniques. Experimental results demonstrated up to four orders of magnitude enhancement in the SERS intensity of 1 mM phospholipid samples deposited over the graded nano-gratings. In addition, characterization of the phospholipids in aqueous phase flowing over the nano-gratings integrated within a microfluidic device revealed that the Raman peaks were only detectable with the enhancement introduced by the grating. These results were obtained using 532, 638, and 785 nm lasers, demonstrating the multispectral sensing capability of the graded gratings for static and dynamic characterization of low concentration species.