Modeling the effect of different dimensions in high contrast grating mirror

Abstract

The reflectivity spectrum in Vertical-cavity surface-emitting lasers (VCSELs) is investigated by replacing High Contrast Grating (HCG) layer instead of distributed Bragg reflectors (DBR) as a top mirror. In addition, the scalability feature in the HCG layer is used to design a reflector for λ=1,300 nm. High reflectivity (>99 %) mirrors in VCSELs are mandatory because of the short gain length. In conventional VCSELs, distributed Bragg reflectors (DBR) are used to compensate the weak gain. Recently, the Hasnain group presented High Contrast Grating (HCG) layer as a high reflectivity monolayer (Huang et al., IEEE J Sel Top Quantum Electron 15(5):1485–1499, 2009). Not only is the HCG layer considerably thinner than a conventional DBR, but it also provides a much broader reflectivity spectral width. Furthermore, the HCG layer is polarization sensitive so that using correct dimensions it can be made to reflect the TE-mode or the TM-mode. Here, we compare the reflectivity spectrum from a HCG monolayer with a DBR. In HCG, there are three important factors that determine the spectrum range that can be used for tuning: thickness (t), periodicity (Δ) and duty-cycle (η) (Fig. 51.1). The structure is shown in Fig. 51.1. Two structures are investigated in which the bottom mirror in both of them is the same and is composed of alternating layers of n-type AlAs and AlGaAs with quarter wavelength thickness and a 27.5-period. The top mirror in DBR case (conventional VCSEL) is a 19.5-period p-type AlAs and AlGaAs DBR mirror (Aghaeipour et al., Opt Quantum Electron 45:115–126, 2013). In the HCG case (HCG-VCSEL), a layer of AlGaAs/air HCG is used instead of p-DBRs. In the following, we compare reflectivity spectrum in conventional VCSEL andHCG-one for λ=850 nm (Fig. 51.2). The reflectivity spectrum in HCG-VCSEL is much broader than that of the convention DBR one. In the next step, we change the dimensions of HCG and investigate the reflectivity spectrum at λ= 1,300 nm. One interesting point about HCG refers to its scalability (Huang et al., IEEE J Sel Top Quantum Electron 15(5):1485–1499, 2009). In the other word, using the same material we reach to a broader spectral range by changing the dimension of HCGlayer. In Fig. 51.3, the three parameters (t, Λ and η) have been tuned for λ= 1,300 nm for TE-mode and TM-mode. In summary, a HCG layer is a good candidate for replacing DBR reflectors in VCSELs, on the one hand to reduce the dimension of the laser on the other hand to broaden the reflectivity spectrum.In addition to that, a HCG layer which is a 1D-Photonic Crystal plays an important role to control the output polarization.