Fig. 1. Sketch of the enhanced light-absorption structure that comprises a cavity-embedding Si layer, two GLs enclosing the cavity, and a substrate. The coordinate system is shown, where the grating periodicity and grooves/lines are along the x and y axes, respectively, and the layer stacking and light impinging directions are along the z axis.

Fig. 2. Sketchy flowchart of the optimization process. The GUI Module inputs the trial parameters, assessed as described in the text, fixed parameters, and constraints to the Optimization Module. There, the trial-and-error multi-start algorithm generates the next start points and inputs them into the Simulation Module, which feeds the algorithm back and loops until attaining an optimum.

Fig. 3. Efficiency spectra of the designed structures with a 0.15 μm thick Si layer. Broken lines: red, 2GL−TE1; green, 2GL−TM1; full blue line, 2DBR1. The structural parameters are found in Table 2.

Fig. 4. Efficiency spectra of the designed structures with a 0.025 μm thick Si layer. Broken lines: red, 2GL−TE2; green, 2GL−TM2; full blue line, 2DBR2. The structural parameters are found in Table 2.

Fig. 5. Electric and magnetic fields’ amplitudes squared (left ordinate) and Poynting vector modulus (right ordinate), normalized to the input |Sz|, across the cavity–absorber range of the dual-DBR structures: (a) 2DBR1; (b) 2DBR2. The intersections of the dashed lines with the abscissa mark the Si layer ends. Simulated at the CDW.

Fig. 6. Normalized to the input values, squared amplitudes of the EM fields versus 0≤x/Λ≤1 and z as defined in the text. (a), (c), (e) For 2GL−TE1: 0.22  μm≤z≤0.671  μm; x/Λ=0.426 is the internal grating lamella-groove interface derived line; (0,1)×(0.35  μm,0.5  μm) is the absorber cut within the microcavity. (b), (d), (f) For 2GL−TE2: 0.209  μm≤z≤0.679  μm; x/Λ=0.433 is the internal grating lamella-groove interface derived line; (0,1)×(0.412  μm,0.437  μm) is the absorber cut within the microcavity. Simulated at the CDW.

Fig. 7. Normalized squared amplitudes of the (a), (b) electric and (c) magnetic fields’ components for the 2GL−TM1 structure shown versus 0≤x/Λ≤1 and 0.273  μm≤z≤0.631  μm as defined in the text. For a simulation convenience, the grating cell was inverted so that x/Λ=0.396 is now the grating lamella-groove interface derived line; (0,1)×(0.305  μm,0.455  μm) is the absorber cut within the microcavity. The normalization and simulation wavelengths are the same as in Fig. 6.

Fig. 8. Maps of the EM power flow at the CDW in the structures (a) 2GL−TE1 and (b) 2GL−TE2 (Table 2). The quiver ranges are defined in Section 4, and the horizontal dashed lines show the material interfaces. The structure cut is shown in the inset to (a).

Fig. 9. Maps of the EM power flow at the CDW in the structures (a) 2GL−TM1 and (b) 2GL−TM2 (Table 2). The quiver ranges are defined in Section 4, and the horizontal dashed lines show the material interfaces.

Fig. 10. Possible placement of electrical contacts to the photoactive Si layer in the PD applications of the dual-GL structure, sketched in Fig. 1. The placement is appropriate for the BSI considered in this study as described in the text.

Fig. 11. Efficiency spectra θ dependence of the 2GL−TM2 structure. The structural parameters are found in Table 2.

Fig. 12. Front standalone GL: (a) the original; (b) an effective-medium bilayer substitute.

Fig. 13. Back standalone GL: (a) the original; (b) an effective-medium bilayer substitute.