Figure 3.

(a) Curve of the zeroth order reflectance as a function of displacement of the silicon grating along the x-axis for the optimal optical design; (b) reflective mode when the reflectance was the maximum in one period; the lateral shift of the upper grating was around 300 nm; (c) transmitted mode when the reflectance was the minimum in one period, the lateral shift of the upper grating was around 390 nm.