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Calculated reflectivity spectrum for TM modes in a planar MIM structure. The solid black lines indicate light line with an incident angle of 32°, without a grating and with single and multiple scattering from a grating with . The MIM geometry is Au and tabulated values were used for the frequency dielectric constants (Ref. 11). First order diffraction into plasmon modes occurs at points A and B, while second order coupling occurs at C and D. The inset shows a schematic of the simulated device and the field distribution for the MIM mode (surface modes are omitted for clarity).
Scanning electron micrograph of grating couplers after reactive ion etching. (a) grating depth of . (b) Grating depth of . Inset shows a cross section of a completed grating coupler with a grating depth of and insulator thickness of .
(a) Reflectance change measured by subtracting the reflectivity of a grating device from that of MIM structure without a grating coupler, using TM (-polarized) light for a Au device with a grating period of and depth of . Analytic results using the theory from Ref. 12 are shown both with and without the correction due to a finite grating depth of calculated with a FDFD model. (b) Line sections from (a) showing reflectivity as a function of wavelength for several angles. The dashed lines denote the maximum mode intensity at 32° for the three modes. The line sections have been offset for clarity.
Results of FDFD simulations. (a) Shift in optimal coupling wavelength as a function of grating depth, for a plane wave incident at 32°. (b) component of the electric field normalized by the incident field for the first order diffracted mode . (c) Same as (b) but for the second order diffracted mode . The device geometry is the same as the inset in Fig. 2 with a grating depth of and a grating period of .
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