Polarization-independent visible wavelength filter incorporating a symmetric metal-dielectric resonant structure

A nanophotonic polarization-independent visible wavelength filter is presented, incorporating a symmetric metal-dielectric resonant structure on quartz substrate, where a sub-wavelength grating, made up of a two-dimensional array of Al square sheets, is integrated with a Si(3)N(4) slab waveguide via an oxide layer. Incident light is orthogonally diffracted by the symmetric grating towards two directions of the grating groove, and then resonantly coupled to both transverse electric and transverse magnetic guided modes associated with the underlying waveguide, irrespective of light polarization. Polarization independent bandpass filtering was thus achieved around specific wavelengths, determined by the grating pitch and the effective index of the waveguide. Three devices, operating in the blue, green and red spectral bands, were built through design and analysis drawing upon the finite-difference time-domain method. The devices, DEV I, II, and III, were constructed with grating pitches of 285, 355 and 395 nm, respectively, while the core was 100 nm thick. They were inspected to function as an efficient bandpass filter, centered at 460, 560 and 610 nm, with bandwidths of about 13, 14 and 17 nm, respectively; the peak transmission efficiencies were consistently over 85%. Furthermore, the transfer characteristics, insensitive to light polarization, were satisfactorily confirmed for normal incidence.