Coexistence of the Radial-Guided Mode and WGM in Azimuthal-Grating-Integrated Microring Lasers

Abstract

Whispering-gallery mode (WGM) resonators, renowned for their high Q-factors and narrow line widths, are widely utilized in integrated photonics. Integrating diffraction gratings onto WGM cavities has gained significant attention because these gratings function as azimuthal refractive index modulators, enabling single-mode WGM emissions and supporting beams with orbital angular momentum (OAM). The introduction of curved grating structures facilitates guided mode resonances by coupling high-order diffracted waves with leaking modes from the waveguide. These gratings act as wavelength-selective mirrors and support concentric circular radial-guided modes. This study investigates the coexistence and interaction between OAM-carrying WGMs and radial-guided modes with Bessel beam characteristics in an active cladding grating-integrated microring laser. These phenomena are examined through both three-dimensional simulations and experiments. The active layer enhances the radial-guided modes at the microring's center, where cylindrical waves from the active cladding produce strong guided mode resonance at specific wavelengths corresponding to radial modes. Additionally, general WGMs are formed and confined within the microring. The effects of grating depth and microring size on radial-guided mode resonance are evaluated through two-dimensional simulations and experiments. These insights pave the way for integrating functional lasers into photonic circuits and advancing technologies for topological optical vortex emission and manipulation.

Figure 1

(A) Spectrum of the active cladding microring laser with R = 25 μm, showing radial modes (magenta triangle), WGMs (blue dot). (B) Schematic of the coexistence of radial modes and WGMs. (C)–(J) Simulation results on active cladding microring, where (C)–(F) for pure ring: (C) dependence of reflectivity on wavelength, (D) intensity distribution of the light source formed by the active layer in the xy-plane at 572.25 nm, (E) intensity distribution of fundamental radial mode at 572.51 nm, and (F) intensity distribution of WGM at 602.25 nm. And (G)–(J) for grating integrated ring: (G) dependence of reflectivity on wavelength, (H) intensity distribution of the light source formed by the active layer in the xy-plane at 587.92 nm, (I) intensity distribution of fundamental radial mode at 587.79 nm, and (J) intensity distribution of WGM at 602.24 nm.

Figure 2

Effect of the grating. (A, B) SEM image of the grating-integrated microring with R = 25 μm: (A) total image of microring, and (B) the grating structure with depth d_g = −85 nm, period Λ = 294.16 nm, and duty cycle σ = 0.2. (C) Schematic of the effect of the grating on the radial waveguide mode and the WGM. (D) Excitation image with R = 10 μm and d_g = −25 nm with focus risen in z-axis.

Figure 3

(A)–(D) Spectra of grating-integrated microring lasers: (A) R = 40 μm, d_g = −25 nm; (B) R = 40 μm, d_g = −85 nm; (C) R = 10 μm, d_g = −25 nm; (D) R = 10 μm, d_g = −85 nm. Radial modes: magenta triangles; WGMs: blue dots; λ_GMR: orange dashed lines. (E)–(G) Simulation results on radial guide mode with different parameters: (E) Λ varied, R = 5 μm, d_g = 40 nm, (F) R varied, Λ = 314 nm, d_g = 80 nm, and (G) d_g varied, R = 5 μm, Λ = 314 nm.