Objective Vortex beams, characterized by a spiral phase wavefront and a central phase singularity, carry orbital angular momentum (OAM). The infinite orthogonal states provided by OAM offer a new degree of freedom for manipulating optical fields, showing tremendous potential in high-capacity spatial division multiplexing communications, optical micromanipulation, and quantum information processing. Compared to continuous-wave (CW) vortex beams, pulsed vortex lasers with high peak power hold significantly greater application value in nonlinear optical frequency conversion and laser micromachining. However, traditional pulsed vortex lasers generally rely on bulky solid-state crystal resonators or complex spatial optical paths, making it difficult to meet the urgent demands of modern photonics for device miniaturization and high stability. Furthermore, high-power pumping in bulk crystals induces severe thermal lensing effects, which drastically distort the fragile spatial phase of vortex beams. To achieve compact and highly stable pulsed vortex sources, dielectric optical waveguides act as an ideal platform. The micrometer-scale geometric structure of the waveguide endows the device with an extremely high surface-to-volume ratio, which significantly enhances the heat dissipation capability of the system. In terms of pulse generation, passive Q-switching using two-dimensional saturable absorbers (SAs) is highly favored due to its compact footprint. Topological insulators, particularly bismuth selenide (Bi₂Se₃), feature a Dirac-cone electronic band structure and exhibit remarkable broadband nonlinear optical responses with large modulation depths. This study aims to propose and demonstrate a highly stable, compact passively Q-switched waveguide vortex laser by combining a femtosecond laser direct written (FsLDW) Nd:YAG waveguide with a high-quality Bi₂Se₃ SA grown by chemical vapor deposition (CVD).

Methods A circular depressed-cladding waveguide was fabricated in a 1 at.% Nd³⁺-doped YAG crystal using the FsLDW technique. The fabrication utilized a Ti:sapphire regenerative amplifier delivering 120 fs pulses at a central wavelength of 795 nm with a repetition rate of 1 kHz. Parallel low-refractive-index damage tracks were inscribed to form a circular cladding boundary with a radius of 15 μm, perfectly preserving the original luminescence properties and lattice quality of the crystalline core. The geometric morphology and guiding characteristics were verified via optical microscopy and micro-Raman spectroscopy. Concurrently, a high-quality Bi₂Se₃ thin film was synthesized on a CVD method. Atomic force microscopy (AFM) and Raman spectroscopy were employed to characterize its morphology and structural properties. The nonlinear optical absorption parameters of the Bi₂Se₃ film were quantitatively evaluated using an open-aperture Z-scan technique driven by a 1030 nm femtosecond fiber laser. A compact laser resonator was subsequently constructed. An 808 nm continuous-wave Ti:sapphire laser served as the pump source, coupled into the waveguide via a spherical plano-convex lens. The Bi₂Se₃ film was inserted into the cavity as the SA. Notably, a spiral phase plate (SPP) with a topological charge of 1 was utilized as the output coupler. Compared to spatial light modulators or liquid crystal q-plates, the pure dielectric SPP possesses an exceptionally high laser damage threshold, directly generating the vortex beam while ensuring the robustness of the cavity.

Results and Discussions The AFM measurements indicate that the average thickness of the synthesized Bi₂Se₃ film is approximately 16 nm. The Z-scan experimental results and subsequent nonlinear transmission fitting reveal that the Bi₂Se₃ SA exhibits an excellent modulation depth of 8.6% and a saturation intensity of 26.25 GW/cm² at 1 μm, indicating its strong capability for pulse modulation. In the laser performance evaluation, stable CW and passive Q-switching operations around 1064 nm were systematically compared under both TE and TM polarized pumping. After inserting the Bi₂Se₃ SA, stable Q-switched pulse trains were generated. The pulse temporal characteristics evolved typically with the increase of absorbed pump power. Specifically, at an absorbed pump power of 850 mW, the laser yielded the optimal pulsed performance: the shortest pulse width compressed to 39 ns, and the highest repetition rate reached 9.1 MHz. The corresponding maximum single-pulse energy was calculated to be 4.42 nJ. The recorded pulse trains exhibited uniform amplitude and high temporal stability without distinct amplitude fluctuations. Furthermore, the near-field intensity distributions captured by a CCD camera demonstrated a typical annular (donut-shaped) hollow profile, confirming the high generation quality of the vortex beam. More importantly, comparing the beam profiles at low and high pump powers, the annular contour showed no obvious distortion or degradation. This phenomenon provides strong evidence that the unique geometric structure of the circular waveguide effectively mitigates the thermal lensing effect through rapid transverse heat dissipation. Theoretical estimations further confirm that the maximum intracavity peak pulse energy fluence (1.3 mJ/cm²) is well below the physical damage threshold of the Bi₂Se₃ film (3.18 mJ/cm²).

Conclusions A compact, highly thermally stable, passively Q-switched Nd:YAG waveguide vortex laser was successfully demonstrated. The integration of the FsLDW-customized circular depressed-cladding waveguide and the CVD-grown Bi₂Se₃ topological insulator effectively overcomes the thermal and physical bulkiness bottlenecks of traditional solid-state vortex lasers. The robust thermal management capability of the waveguide physically locks the high-order transverse mode, ensuring distortion-free annular vortex beam output even under intense pumping conditions. The experimental results verify the excellent nonlinear optical modulation properties and impressive thermal damage resistance of the Bi₂Se₃ film. This research provides a novel experimental paradigm and solid foundation for the development of miniaturized, highly stable all-solid-state pulsed vortex sources, paving the way for advanced on-chip photonic integrated circuits and optical manipulation systems.