Ultrahigh Figure-of-Merit in Metal-Insulator-Metal Magnetoplasmonic Sensors Using Low Loss Magneto-optical Oxide Thin Films

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1 Supplementary Information Ultrahigh Figure-of-Merit in Metal-Insulator-Metal Magnetoplasmonic Sensors Using Low Loss Magneto-optical Oxide Thin Films Jun Qin,, Yan Zhang,, Xiao Liang,, Chuan Liu,, Chuangtang Wang,, Tongtong Kang,, Haipeng Lu,, Li Zhang,,, Peiheng Zhou,, Xin Wang,, Bo Peng,, Juejun Hu, Longjiang Deng *,, and Lei Bi *,, National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu , China, State Key Laboratory of Electronic Thin-Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, , China, Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA *Corresponding authors: and Supporting Information 1. Optical loss and figure of merit of CeYIG films compared to those of Au and ferromagnetic metals We compare the extinction ratio (the imaginary part of the refractive index) and magneto-optical figure of merit of CeYIG and other ferromagnetic and noble metals as shown in Figure S1. The extinction ratio of CeYIG is much lower than the metals, and the magneto-optical material figure of merit of CeYIG is two orders higher than those of other ferromagnetic metals in the visible to near infrared wavelength range

2 Figure S1. (a) Extinction coefficients (K) of CeYIG, Co, Au, Fe and Ni in the visible to near infrared wavelength range. 1-2 (b) The magneto-optical material figures of merit (defined as Faraday rotation per length (FR) divided by K) for several ferromagnetic metals and the CeYIG material in the visible to near infrared. Supporting Information 2. Material and Device structures The crystal structure, magnetic/moke hysteresis, cross-sectional element mapping of the MIM device stack is shown in figure S3. Clear diffraction peaks from Au, TiN, CeYIG and YIG are observed in figure S3a, indicating that all layers are well crystallized. Split of CeYIG diffraction peaks, e.g. peak (640), is due to diffraction from both the YIG and CeYIG layers with different lattice constants. In figure S3b, the magnetization is normalized to the total thickness of the CeYIG and YIG layers. The saturation magnetization of 102 emu/cm 3 is close to that of phase pure CeYIG films. 8,9 Both magneto-optical Kerr, Faraday effect and Faraday ellipticity are characterized on the multilayer thin film. The Faraday rotation of CeYIG is measured to be deg/cm at 650 nm wavelength. In figure S3c, the sharp interfaces between different layers suggest that little interdiffusion takes place during the annealing process. Homogeneity of the element distribution can also be observed in the EDS line plots shown in the lower panel of this figure. 2

3 Figure S2. Structure of the MIM magneto-plasmonic transducer. (a) XRD spectrum of the multilayer thin films in the sensor device. (b) Room temperature magnetic hysteresis loop of the sensor device. The inset shows the longitudinal Kerr effect hysteresis measured on the device at a wavelength of 633 nm. (c) Cross-sectional elemental mapping by TEM for all elements. Supporting Information 3. Mode evolution with changing Au thickness Figure S3. E x field profile in the x-z plane with Au layer thickness changing from 10 nm to 200 nm, and keeping all other layer thicknesses constant (same as Figure 1b) at the wavelength of 650 nm. When the Au film is thinner than 50 nm, the WG mode and SPR mode can strongly hybridize. When Au is thick enough ( 200 nm), only the SPR mode can be excited, which is consistent with the dispersion relation in Figure 3b. 3

4 Supporting Information 4. NRPS and absorption loss of the MIM waveguide The waveguide nonreciprocal phase shift, propagation loss and figure of merit defined as NRPS divided by the propagation loss of the MIM waveguide is shown in figure S2. A maximum NRPS is obtained when the CeYIG layer is 80 nm thick. Note that the highest NRPS is not achieved when the entire dielectric layer is CeYIG. This is because according to eq. (2), the NRPS of the dielectric mode considers the integral of the H y field gradient in the dielectric layer. By introducing a SiO 2 and YIG buffer layer, the NRPS can be maximized. 7 The optical loss increases steadily with the CeYIG thickness because CeYIG has a higher index (2.42) compared to YIG (2.221). The mode shifts toward the MIM WG side. TiN contributes to most of the loss (>70%) for all CeYIG thicknesses. Therefore, by choosing an intermediate CeYIG thickness of ~50 nm as demonstrated in this paper, high NRPS and relatively low optical loss can be simultaneously achieved. Figure S4. NRPS, propagation loss α and NRPS/ α as a function of the CeYIG layer thickness. The TiN, Au and SiO 2 thicknesses are fixed to 44 nm, 10 nm and 15 nm, respectively. The total thickness of YIG and CeYIG layer is fixed as 100 nm. The NRPS and absorption loss of the waveguide is then simulated as a function of the CeYIG layer thickness changing from 0 to 100 nm, i.e. changing the ratio of YIG to CeYIG thicknesses. The red curve is the NRPS and the blue curve is the absorption coefficients in cm -1. Supporting Information 5. Fitting of the TMOKE spectrum by a Fano line shape 4

5 Figure S5. The Fano spectrum is defined by TMOKE( θ ) = A + B( qγ / 2 + θ θ ) / (( Γ / 2) + ( θ θ ) ), where Γ is the line width, θ 0 is the resonant angle, q is the Fano parameter, and A and B are constants describing the background and the overall peak height, respectively. 10 (a), (b) The fitting curves match well with both simulated and experimental spectra. The fitted line width Γ is used for FOM calculations in this paper. Supporting Information 6. Linearity and stability of the sensor device Figure S6. (a) Reflectivity dip positions as a function of sensing indices from 1.33 to The device showed good linearity for a wide index range. (b) Reflectance spectrum before and after oxygen plasma cleaning. The peak position and line shape is almost identical, indicating good stability of the device. The minimum reflectance varies only slightly from measurements to measurements, possibly due to the slight variation of the laser spot position on the sensor surface after each sample loading process and temperature variation. 5

6 Supporting Information 7. Gas sensing performance with the sensing medium being air We show the gas sensing performance of our MIM MOSPR device in air as shown in figure S7. The device sensitivity is obtained by numerical simulations, whereas the SPR and MOSPR spectra width are obtained both by simulations and experiment characterizations. The thicknesses of TiN, SiO 2 and YIG layers are fixed as 44 nm, 15 nm and 52 nm respectively. High FoM can be achieved in a wide range of thickness combinations. The optimum FoM of 7619 RIU -1 is obtained at a CeYIG thickness of 108 nm and Au layer thickness of 5 nm. In this case, the sensor shows a sensitivity S=62.4 degriu -1 and a FWHM line width of Γ= Figure S7. (a) Simulated FoM contour plot as function of Au and CeYIG layer thicknesses. (b) Experimental and simulated reflectance spectra of the same sensor device used in Fig. 2, with the sensing medium being air. The simulated spectrum for this structure is also shown for comparison. Since this device structure is optimized for the best FoM in water, the mode is mostly localized in the MIM WG side. (c) Normalized experimental and simulated TMOKE spectrums as a function of incident angle. Although the device is far from optimum geometries in air, a narrow FWHM of 0.1 is still observed experimentally. The simulated device FoM is 595 RIU -1. REFERENCES (1) Goto, T.; Onbaşlı, M. C.; Ross, C. A. Magneto-optical Properties of Cerium Substituted Yttrium Iron Garnet Films with Reduced Thermal Budget for Monolithic Photonic 6

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