Fig. 1. Schematic drawing of the magneto-plasmonic sensor based on nanodisk arrays. The device’s geometrical parameters include diameter D, Au-nanodisk thickness t1, Co-nanodisk thickness t2, and period P. The p-polarized illumination source propagates in the y−z plane, and the magnetization is along x direction. The Au-substrate thickness is 150 nm.

Fig. 2. Optical and MO responses of the proposed nanostructure. (a) Reflection spectra for the sample with different direction of magnetization. The FWHM of these two reflection curves is 7.748 nm. For clarity of the shift in the reflected light, the spectral range is zoomed in within a 4 nm wavelength window in the inset. (b) Corresponding TMOKE spectrum calculated by Eq. (1). The FWHM is 0.125 nm. (c) FEM simulated reflection spectra of the demagnetized sample as a function of incident angle and wavelength. The dashed lines indicate (1,0) and (−1,0) plasmon modes, which are calculated by the k-conservation relationship. (d) Electric field distributions in the x−y and y−z planes for four unit cells of the structure. They share the same legend.

Fig. 3. Influence of incident angle and period on optical responses for the magneto-plasmonic system. (a) Reflection spectra as a function of incident angle while the period P=400  nm. (b) Reflection spectra as a function of period while the incident angle θ=50°. Contrast ratio and FWHM of reflection spectra as a function of (c) incident angle and (d) period, respectively. The other geometrical parameters are the same as those discussed in Fig. 2.

Fig. 4. Dependence of optical and MO responses on geometrical parameters. Reflection of the hybrid nanostructure where the Co layer is demagnetized as a function of (a) nanodisk diameter D (t1=20  nm and t2=13  nm), (b) Au-disk thickness (D=200  nm and t2=13  nm), and (c) Co-disk thickness (D=200  nm and t1=20  nm), respectively. (d)–(f) Corresponding change of reflection with the reversal of Co magnetization. (g)–(i) Corresponding TMOKE spectra.

Fig. 5. Refractive index sensitivity of Au/Co nanodisk arrays on top of Au film. (a) TMOKE spectrum for different dielectric environments as a function of wavelength. The refractive indices are air n=1, water n=1.33, butanol n=1.397, and chloroform n=1.4458. (b) Peak position of TMOKE curves as a function of refractive index. The black line indicates a linear fit to the dots, and the slope defines the sensitivity. (c) Calculated FOM for different embedding medium as a function of wavelength.

Fig. 6. Sensing-performance comparison of MO-SPR and SPR sensors. The red and green spheres represent the FOM for the sensors based on an array of Co-Au nanodisks and based an array of Au nanodisks, respectively, in media with same gradient refractive index (n=1, 1.33, 1.397, 1.4458). The black spheres indicate the ultimate value of FOM in propagating SPR sensors, which is calculated by Eq. (4).