Abstract
Metallic gratings can support Fano resonances when illuminated with EM radiation, and their characteristic reflectivity versus incident angle lineshape can be greatly affected by the surrounding dielectric environment and the grating geometry. By using conformal oblique incidence thin film deposition onto an optical grating substrate, it is possible to increase the grating amplitude due to shadowing effects, thereby enabling tailoring of the damping processes and electromagnetic field couplings of the Fano resonances, hence optimizing the associated localized electric field intensity. To investigate these effects we compare the optical reflectivity under resonance excitation in samples prepared by oblique angle deposition (OAD) and under normal deposition (ND) onto the same patterned surfaces. We observe that by applying OAD method, the sample exhibits a deeper and narrower reflectivity dip at resonance than that obtained under ND. This can be explained in terms of a lower damping of Fano resonance on obliquely deposited sample and leads to a stronger localized electric field. This approach opens a fabrication path for applications where tailoring the electromagnetic field induced by Fano resonance can improve the figure of merit of specific device characteristics, e.g. quantum efficiency (QE) in grating-based metallic photocathodes.
Highlights
For the OAD sample, the grating amplitude of the deposited Ag thin film (~80 nm) is 15 nm higher than that of the sample grown under normal incidence geometry (~65 nm)
To better analyze the physics mechanism of this Fano resonance, we propose a pseudo frequency ωpsd and a pseudo incident angle θpsd, and use ωpsd to represent the effect of incident angle on the k// after setting a θpsd
We have compared the experimental surface topography and reflectivity curves for samples fabricated under OAD and ND geometries
Summary
For the OAD sample, the grating amplitude of the deposited Ag thin film (~80 nm) is 15 nm higher than that of the sample grown under normal incidence geometry (~65 nm). We want to point out that the simulation model used for the OAD sample is justified because the actual profile of the grating substrate should allow full coverage of the Ag thin film coating even if not uniform., This is expected because reflections of incident atoms from the adjacent grating groove can occur, allowing coating of the shadowed side of the groove considered.
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