Comparison Between Planar and Plano‐Concave Cavities in GaN‐Based VCSELs

ABSTRACT

We fabricated optically pumped GaN‐based VCSELs and systematically analyzed and compared the quality factor ( Q ), angle‐resolved characteristics, lasing threshold, emission spectra, and near‐field patterns between the planar and plano‐concave cavities. The plano‐concave cavity achieved Q factor >20 000 due to strong 3D optical confinement, whereas the planar cavity yielded Q values of only a few hundred. The planar cavity exhibits a single longitudinal mode lasing without high‐order modes and a single bright spot of the near‐field pattern image. On the other hand, the plano‐concave cavity enables both single/multiple longitudinal mode(s) lasing depending on cavity length ( L ), consistently accompanied by multiple high‐order lasing modes. Its near‐field pattern shows a circular Gaussian spot. Angle‐resolved results demonstrate the characteristics of 3D optical confinement in a plano‐concave cavity and 1D confinement in the planar cavity. The threshold of a plano‐concave cavity depended weakly on cavity length, increasing the threshold from 2.5 mJ cm ^{− 2} at L = 1 µm to 9.7 mJ cm ^{− 2} at L = 31 µm. By comparison, the planar cavity shows a dramatic threshold increase due to sharply rising internal losses, from 4.4 mJ cm ^{− 2} at L = 1 µm to more than 25 mJ cm ^{− 2} at L = 4 µm. These results provide valuable guidance for the future research of VCSELs.