In this paper, we present our investigations of the effects of vertical-sidewall roughness (VSR) on guided-mode resonance (GMR) filters made of subwavelength grating for applications to ultrasensitive biosensors operated under IR illumination. We designed the spectral FWHM of the grating filter to be as narrow as possible in order to emphasize the sensitivity and VSR effects. Three types of VSR morphologies on the grating--in terms of the correlation length xi and the rms of the maximum roughness deviation sigma--were considered and evaluated. Rigorous coupled-wave analysis was then implemented to quantify the shifts in the reflective resonance peak wavelength value (PWV) of the grating filter. Our simulations show that for specific xi values, the PWVs remain constant even if sigma becomes as large as 10 nm; this indicates dramatic bandgaplike stripes, which are similar to the bandgaps observed in the band diagrams of photonic crystals in the xi-sigma diagram that we have proposed in this study. In other words, the effects of VSR on the GMR biosensor performance are insignificant when xi is located at certain bands; therefore, this type of roughness is highly tolerable even if the linewidth of the filter is decreased to only a few tens of nanometers.
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