Abstract

The high-index-contrast grating (HCG) as shown in Fig. 1(a) is a thin near-subwavelength grating layer surrounded by lower refractive index materials [1]. The capability of providing high reflectivity over a broad wavelength range makes the HCG an attractive alternative to distributed Bragg reflectors (DBRs) in vertical-cavity surface-emitting lasers (VCSELs) [2]. In order to achieve a broad bandwidth with high reflectivity, it was previously considered that the grating needs to be surrounded directly by lower index materials as shown in Fig. 1(a). Recently, we have proposed a new type of broadband grating reflector that has a high-refractive-index cap layer next to the grating layer as shown in Fig. 1(b), and have shown that this new reflector can provide a high reflectivity over a broader wavelength range than the HCG, see Figs. 1(c) and 1(d) [3]. This new reflector is denoted as a hybrid grating (HG) since the grating and the gap layer are in general made of different materials. It is noteworthy that the field intensity in the cap layer is similar or higher than the incident field intensity. Thus, a very compact vertical-cavity laser structure consisting of an active HG reflector with a gain material in the cap layer, a short passive cavity made of air or other low index material, and a second reflector can be realized. This brings great advantages to the hybrid III-V/SOI vertical-cavity laser of our interest such as simplification of fabrication process and significantly improvement of heat dissipation. The physics of the broadband reflection process in the HG reflector can be understood by two supplementary pictures i.e., the leaky mode picture and the Bloch mode picture and will be discussed in detail in the presentation. Also the effect of finite size of HG structure will be investigated by using our in-house developed three-dimensional simulator.

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