Refractive-index sensor and method

Abstract

A method for measuring a refractive index of a medium includes exciting a first antisymmetric resonance of a first metasurface, including a first periodic array of resonators formed on a substrate surface, with illumination incident on the first metasurface at a non-normal incidence angle with respect to the substrate surface, the first metasurface including the medium encapsulating the first periodic array of resonators. The method also includes determining a refractive index of the medium from a first amplitude of a first transmitted signal that includes a portion of the illumination transmitted through the first metasurface.

What is claimed is: 
     
         1 . A method for measuring a refractive index of a medium, comprising:
 exciting a first anti symmetric resonance of a first metasurface, including a first periodic array of resonators formed on a substrate surface, with illumination incident on the first metasurface at a non-normal incidence angle with respect to the substrate surface, the first metasurface including the medium encapsulating the first periodic array of resonators; and   determining a refractive index of the medium from a first amplitude of a first transmitted signal that includes a portion of the illumination transmitted through the first metasurface.   
     
     
         2 . The method of  claim 1 , a spectrum of the first transmitted signal having a stopband at a center wavelength λ 1  with a linewidth δλ 1 , center wavelength λ 1  exceeding both a maximum spatial dimension of each resonator of the first periodic array of resonators and a unit cell size of the first periodic array, wherein, when the illumination has a spectral bandwidth exceeding linewidth δλ 1  and including center wavelength λ 1 , determining the refractive index comprising:
 determining center wavelength λ 1  from the first transmitted signal; and 
 determining the refractive index at center wavelength λ 1  according to a lookup table that maps each of a plurality of stopband center-wavelengths to a respective one of a plurality of candidate refractive indices of the medium. 
 
     
     
         3 . The method of  claim 1 , a spectrum of the first transmitted signal having a stopband with a linewidth δλ 1  wherein, when the illumination has a center wavelength λ 0  and a spectral bandwidth less than linewidth δλ 1 , determining the refractive index comprising:
 determining the refractive index from the first transmitted signal according to a lookup table that maps each of a plurality of numerically-simulated transmittances of the medium, at center wavelength λ 0 , to a respective one of a plurality of refractive indices of the medium. 
 
     
     
         4 . The method of  claim 1 , when exciting the first antisymmetric resonance, the non-normal incidence angle being between two degrees and ten degrees. 
     
     
         5 . The method of  claim 1 , further comprising:
 exciting, with the illumination, a second antisymmetric resonance of a second metasurface including a second periodic array of resonators formed on a substrate surface, the second periodic array geometrically differing from the first periodic array; and   determining the refractive index of the medium from a second amplitude of a second transmitted signal and the first amplitude, the second transmitted signal including a portion of the illumination transmitted through the second metasurface.   
     
     
         6 . The method of  claim 1 , further comprising:
 exciting, with the illumination, a second antisymmetric resonance of a second metasurface including a second periodic array of resonators formed on a substrate surface, the second periodic array geometrically differing from the first periodic array; and   determining whether a change in the first amplitude is caused by a change in center wavelength of the illumination or a change in the refractive index of the medium by comparing the change in the first amplitude to a change in a second amplitude of a second transmitted signal, the second transmitted signal including a portion of the illumination transmitted through the second metasurface.