Fig. 1. (a) Schematic of the PT-symmetric circular Bragg laser. (b) Schematic of the azimuthally modulated circular Bragg laser with radial PT symmetry. (c) Radial profile of the refractive index of the two types of lasers, showing the real (Δnr) and imaginary (Δni) refractive index modulations that satisfy the PT symmetry requirement.

Fig. 2. (a) Normalized threshold gain (αr0) for even-order azimuthal modes of PT-symmetric and conventional radially chirped circular Bragg lasers. Inset shows the E field profile of the targeted 8th-order azimuthal mode. (b) Effect of a small deviation away from the exceptional point on the normalized threshold gain values for the PT-symmetric radially chirped circular Bragg lasers.

Fig. 3. (a) Output spectra for two circular Bragg laser structures with radial PT symmetry. The upper corresponds to the one without azimuthal modulation, while the lower corresponds to the one with azimuthal modulation. (b) E field profiles of the modes at the three peaks in the laser output spectra. Modes (i) and (ii) from the upper spectrum are of the 8th and 1st azimuthal order, respectively. Mode (iii) from the lower spectrum is of the 8th azimuthal order.