Vortex and Anti-Vortex Lasing Modes in Disclination States

In photonics, the ability to control the angular momentum of light has long been of significant interest. For the purpose of creating different degrees of freedom in spatially distinct channels for data transmission, the orbital angular momentum (OAM) is particularly useful. Although the directional output and generation efficiency of OAM beams in previous works are impressive, considerable scattering loss and large energy consumption are unavoidable when constructing ultra-small optical devices due to the absence of mechanisms related to high-quality light confinement. Furthermore, additional work is required to tune the pump beam when the emission chirality of vortex microlasers depends on circularly polarized optical pumping. Realizing ultra-compact laser systems that can self-configure vortex modes with OAM properties and reliably localize resonant modes is therefore still challenging.<br/>A more recent development is the introduction of photonic topological insulators (PTIs), which are attractive tools for the localization of topological states and robust light manipulation. PTI research has expanded to include topological defects such as dislocations and disclinations, which typically break the symmetric geometries of topological crystalline insulators (TCIs) [1,2]. Notably, the presence of a fractional charge in the disclination leads in topological bound states in TCIs. [3]. Disclination defects, however, have not yet been used in the realization of ultra-small light sources with quantized OAM states. Here, we report on two novel topological lasing modes with OAM states found in disclination defect nanocavities: vortex and anti-vortex modes.<br/>The photonic disclination structure was constructed by cutting the crystal into four identical sectors, applying the Frank angle of Ω = +90°, and adding one sector to a square-lattice photonic crystal structure. Different disclination defects were examined by shifting the air holes in the C5 symmetric core and the nearest layer to the core. Numerical simulations using finite-element method (FEM) were performed to optimize the structural parameters. Then, the localized in-gap states were opened at the disclination defect, and vortex and anti-vortex modes appeared in the gap, showing different polarization distributions. For the experimental demonstration of the vortex and anti-vortex nanolasers, we fabricated pentagon-shaped photonic crystal structures based on the FEM simulation results in a InGaAsP slab incorporating three quantum wells. We demonstrated the vortex and anti-vortex lasing modes in the disclination defects of topological photonic crystal structures. The OAM states of these modes were identified by measuring the polarization-dependent spatial intensity profiles and interferences patterns. The realization of the photonic disclination states may open up new opportunities for creating distinctive OAM vector laser beams for the next-generation optical communication.<br/><br/>[1] Y. Liu et al., “Bulk–disclination correspondence in topological crystalline insulators,” Nature <b>589</b>, 381-385 (2021).<br/>[2] C. W. Peterson et al., “Trapped fractional charges at bulk defects in topological insulators,” Nature <b>589</b>, 376-380 (2021).<br/>[3] W. A. Benalcazar, T. Li, and T. L. Hughes, “Quantization of fractional corner charge in <i>C_n</i>-symmetric higher-order topological crystalline insulators,” Phys. Rev. B <b>99</b>, 245151 (2019).