Annular photonic crystal defect mode for operation of single-mode laser

In analogy to the impurity levels of semiconductors, optical cavities are created if irregular regions are introduced into the perfect photonic crystals. Photonic crystal single defect cavity is expected to obtain nearly thresholdless laser because it can create wavelength-scale small resonant mode with high-quality factors and high spontaneous emission coupling. In this paper, we have studied a new type of defect called annular microcavity created by drilling a circular hole in a usual defect cylinder in two-dimensional triangular photonic crystal by using the supercell method. The hole within the rod greatly modifies the defect modes by controlling filling fraction of dielectric material from the inner of the defect and lowering symmetry of the defect. It is found that the annular defect provides extra two more variables due to the hole (radius and refractive index) that can be utilized to alter the properties of the defect. And this opens up a new method to achieve and tune acceptor-mode cavity. Moving the hole can split the degenerate modes. The departure displacement determines the splitting space and the departure angle determines symmetry of distributions of electric fields. These results exhibit more advantages over the conventional defects and may be utilized to single-mode operation of microcavity laser.