Cavities of monolithic subwavelength grating VCSELs

Monolithic subwavelength gratings (MSGs) can be implemented in any material of a real refractive index larger than 1.75 without the need of the combination of low and high refractive index materials as it is in high-contrast... [ view full abstract ]

Monolithic subwavelength gratings (MSGs) can be implemented in any material of a real refractive index larger than 1.75 without the need of the combination of low and high refractive index materials as it is in high-contrast gratings (HCGs). Recently, we achieved the first continuous-wave lasing of electrically-injected vertical-cavity surface-emitting lasers (VCSELs) that use a GaAs MSG mirror. VCSELs with one or both MSG mirrors have great prospects in application to monolithically integrated phosphide- and nitride-based VCSELs that lack monolithically integrated materials of high refractive index contrast. MSG replacing top DBR in arsenide-based VCSELs results in reduced size of the VCSEL by 30-50 % and two MSGs replacing both DBRs would enable a radical 90% or greater reduction thus potentially reducing the VCSEL manufacturing costs.

MSG however provides phase change of reflected light being depended on geometrical parameters of MSG. On the one hand side, makes the fabrication control of this type of structures more complicated, and on the other hand side, it provides additional degree of freedom enabling tuning the properties of the resonating light. We perform numerical analysis based on plane wave admittance method of the resonant cavities embedded between mirrors in the configurations: MSG – MSG and MSG – DBR. Quality (Q) factor of MSG VCSELs is not related to the power reflectance of the mirrors with simple relationship as it is in conventional VCSELs. As an example dependence of Q-factor (Fig. 1), power reflectance (Fig. 2) and resonant wavelength (Fig. 3) of VCSEL composed of top and bottom 20 stripes MSGs of the same parameters: L – period and F – duty cycle. For the sake of comparison Q-factor of standard VCSEL with 21 pairs of top DBR is ~10^4.5, corresponding power reflectance of top DBR is 0.9993. Such high Q-factor of MSG VCSEL can be achieved in broad range of MSG parameters (green region in Fig. 1) which relates to significantly lower (~0.99) power reflectance of MSG mirrors (Fig. 2). Additionally MSG VCSELs offer resonant wavelength tuning by modification of lateral parameters of MHCG (Fig. 3) in the range of 40 nm for Q > 10^4.5.