New features of HCG reflectivity under finite excitation

The use of vertical-cavity surface-emitting lasers based on high-contrast gratings (HCG-VCSELs) as light sources is getting diffused over a very broad range of applications. For this reason, nowadays, their design-oriented modeling is a very hot research topic. Usually HCGs are designed on the basis of their plane-wave response. On the other hand, their reflection performance has been recently characterized for an incident Gaussian beam, to account for the source finiteness. This leads to an excitation similar to the VCSEL mode, without using the entire, complicated 3-D model. It has been observed that reflectivity is much lower compared to the normal-incident plane-wave case. Moreover, as reported in Fig. 1, it has been shown that the plane-wave reflectivity exhibits a very strong angular dependence, causing a degeneration of the well-known reflectivity peak to a trench. This mechanism impacts strongly on the reflection performance for a finite beam, as shown in Fig. 2, where reflectivity is reported as a function of the Gaussian beam waist, for two operating wavelengths. In this work the reflectivity degradation is investigated through theoretical analyses that will be discussed during the presentation. These results allow to obtain additional design guidelines.