Flattened broadband notch filters using guided-mode resonance associated with asymmetric binary gratings

In this letter, the broadband notch filter exhibiting a flattened bandstop spectral response is experimentally demonstrated for the first time by using the guided-mode resonance associated with the periodic subwavelength structure possessing asymmetric binary grating profiles. The proposed filter consisting of grating and waveguide structures is realized by utilizing a single-layer poly-silicon film deposited on a quartz substrate. The asymmetric grating profile is carried out by dividing the one-filling-factor binary structure into the one with two distinct filling factors within one identical period. A flat bandstop spectral response at the central wavelength of 1.22 mu m is obtained under normal incidence for transverse-electric polarization. Its transmittance of bandstop spectrum can be suppressed as low as -20 dB within a broad spectral range of 85 nm and its corresponding Delta lambda/lambda is larger than 6 %. The spectra under oblique incidence are demonstrated to verify the proposed filter has a high immunity for the angular deviation up to 20 degrees.