Abstract
Surface plasmon resonance sensors based on grating coupling exhibiting two resonant wavelengths are less observed because usually thick metal films are used. In this paper, we propose gold gratings on a gold film with two narrow reflection dips. One is sensitive to the refractive index (RI) of the cover media with a sensitivity of 720 nm/RIU. However, the other is sensitive to the RI of the substrate, and hence, it can be used as a reference dip. This self-referenced sensor significantly reduces the error contributions of the light intensity fluctuations and temperature variation. Moreover, the electric field is stronger with less decay length than that of gold gratings without gold film. Therefore, gold gratings with a gold film are more suitable to be a biosensor. Besides, we also investigate the structure parameters (the thickness of the gold film, the thickness and width of gold gratings, and the RI of the substrate) on the reflection spectra.
Highlights
Over the last two decades, surface plasmonic sensors have been demonstrated in the area of medical diagnostics, environmental monitoring, and food safety.1–4 There are several methods to excite surface plasmonic resonance (SPR), but the two main ones are using a prism (Kretschmann–Raether5,6) and grating coupling7,8 to match momentum because the propagation constant of surface plasmon wave is always higher than that of the optical wave propagating in free space
(2) figure of merit (FOM) defined by the wavelength shift caused by a 10 nm cover analyte divided by the full width at half maximum (FWHM) is largest
As the refractive index (RI) of the analyte increases, the peak-to-valley ratio of the reflection dip caused by SPR propagated at the interface of gold and the substrate becomes larger, which is good to be distinguished as a reference dip, while the peak-to-valley ratio of the dip caused by SPR at the surface of gold grating and analyte remains large up to 90% when the RI of the analyte changes from 1 to 1.33, which is good for sensing with a large signal to noise
Summary
Over the last two decades, surface plasmonic sensors have been demonstrated in the area of medical diagnostics, environmental monitoring, and food safety. There are several methods to excite surface plasmonic resonance (SPR), but the two main ones are using a prism (Kretschmann–Raether5,6) and grating coupling to match momentum because the propagation constant of surface plasmon wave is always higher than that of the optical wave propagating in free space. In the grating coupling method, there are several structures: (1) the period gold nanostructure, (2) the metal grating on top of the metal film, and (3) the dielectric grating on top of the metal film.. In the grating coupling method, there are several structures: (1) the period gold nanostructure, (2) the metal grating on top of the metal film, and (3) the dielectric grating on top of the metal film.12,13 A structure consisting of a periodic nano-plasmonic grating on top of a metallic plasmonic thin film sandwiched in between a dielectric was numerically evaluated with a very high Q of 143 at the self-reference peak.. With thicker gold film and less difference of RI between the analyte and the substrate, the self-referenced dip induced by SPR excited at the interface of gold and the substrate degenerates
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