Silicon nitride based medium contrast gratings for generating longitudinally polarized resonant fields

We discuss the design of partially etched 1D silicon nitride medium-index contrast sub-wavelength grating structures for generating strong longitudinally polarized resonant field and compare this with focal-field simulations obtained for radially and linearly polarized incident light. Normal incident plane-wave polarized perpendicular to the silicon nitride grating lines (TM polarization) generates both in-plane and out-of-plane field components relative to the grating plane, also here called as field depolarization, the ratio of which can be engineered by varying the grating dimensions. To have maximum depolarization above the structure, the etch depth of the gratings was fixed low at 30 nm, keeping the pitch and duty cycle fixed at 1 𝜇m and 50% respectively and the unetched thickness was varied. Maximum depolarization ratio, defined as the ratio of maximum longitudinal to transverse electric field intensity above the structure was observed to be 1.2 for an unetched thickness of 300 nm at resonant wavelength of 1489 nm. With thicker unetched thickness of 1000 nm, the depolarization within the structure can be maximized to 4.8 with resonance at 1826 nm. Often the generation of strong longitudinally polarized focal-fields relies on the use of tightly focused radially polarized incident light and imposes restrictions on the specimen due to the use of high-index immersion media. Such depolarization ratios are typically achieved with high numerical aperture (NA) focusing objective lens with NA greater than 1.25. Furthermore, we also report a simulation-based study of these structures for enhancing dark excitonic photoluminescence from Tungsten Diselenide(WSe2) monolayer integrated with these structures and observed the photoluminescence in presence of grating to be 20 times enhanced than that off grating.