# Aluminum-Based Plasmonic Photodetector for Sensing Applications
## 4 Results and Discussion
The photocurrent value is normalized to the power of the incident radiation. To verify our theoretical model and analyze the underlying physical mechanisms, we compared the experimental results of the photocurrent with the angular and spectral dependences of the light absorption in Si calculated using the RCWA method for a trapezoidal profile (Figure 4) with discretization into N = 10 layers. The calculations were performed for a range of wavelengths and incidence angles corresponding to the experimental conditions. Our electrodynamic model accounts for all light absorbed within the Si substrate. However, electron-hole pairs generated far from the p-n junction contributed minimally to the photocurrent due to recombination. To address this limitation, we introduced an effective active layer depth, \(d_{a}\), into the model. This parameter limits the integration range in Equation (4) to \(d_{a}\). Since it is difficult to derive a heuristic estimate for \(d_{a}\), we treated it as a free parameter when fitting our model to the experimental data. The optimal value obtained for \(d_{a}\) was 9.2 \(\upmu\)m. The above-mentioned optimal parameters of the Al grating were selected as the geometric parameters of the trapezoidal profile. A direct comparison of the experimental photocurrent spectra (Figure 9a) with the theoretically calculated Si substrate absorption spectra (Figure 9b) reveals a qualitative agreement in the main (1-st order) resonance peak positions and their evolution with the incidence angle. Also, there is a good agreement with respect to the half-width and general shape of the peaks: both the experimental and theoretical first-order resonances have almost identical half-widths. The tendency to the monotonic decrease in the relative peak intensity with the increase in the incidence angle is also reflected in the theoretical calculations. On the other hand, there are some minor discrepancies, namely, in the relative intensity of the second-order (short-wavelength) resonance peaks. These are far more pronounced in the theoretical curves than in the experimental ones. Moreover, the half-width of the second-order resonances is significantly higher. These discrepancies can be related to several factors. First, the conducted electrodynamics simulations provided only the power of absorbed light. Therefore, one should be cautious when comparing these results to the experimentally measured photocurrents. Purely electrodynamics models cannot account for recombination processes; moreover, these processes are spatially dependent because of doping and surface impact, and possible material inhomogeneity. Second, the trapezoidal profile model and the RCWA-layered discretization of the structure profile may also be imperfect approximations. The actual structure profile was much smoother, but such kinds of profiles are much more difficult to model using the RCWA because it requires not only extremely precise profile investigation but also profile discretization with very small steps, which leads to a significant increase in computation time. A similar comparison of the measured angular photocurrent dependences with the results of the theoretical simulations of absorption in the active Si layer is presented in Figure 10. These curves correspond to three different wavelengths of p-polarized light. Thus, the evolution of the SPP resonance maximum with the frequency/wavelength shift is demonstrated. The structure of the angular absorption dependence is relatively simple: a single asymmetric SPP-related maximum with a dip on the left wing is related to the emergence of the first diffraction order. It is evident in Figure 10 that the Si absorption curves repeat the shape of the experimental photocurrent dependences with some small deviations, probably due to the profile approximation as mentioned above. The close agreement between the experimental and theoretical results demonstrates validity of the RCWA method in describing the optical response of PPPDs.