Multi-wavelength applicable waveguide vortex beam generator in lithium niobate processed by femtosecond laser direct writing

In this Letter, we report a micrometer-scale customized guiding device designed for generating vortex beams in lithium niobate (LN) crystals processed by femtosecond laser direct writing (FLDW) technology. Confocal-Raman microscope measurements were skillfully utilized to thoroughly clarify the microscopic mechanisms involved in the laser-induced alteration of the crystal lattice and the formation of various waveguide types. Finite-difference time-domain (FDTD) simulations confirm that the designed array of stable-optimized Type-I waveguides with stepped length gradients demonstrates selective orbital angular momentum (OAM) excitation under multi-wavelength illumination and the wavelength-OAM correlation governed by the waveguide length-dependent phase accumulation mechanism. The wavelength-dependent vortex generation was experimentally characterized using an end-coupled interrogation setup with continuous laser sources at 1064 nm and 532 nm, verifying the structural response to wavelength variations. Our results vividly demonstrate that the multi-waveguide hybrid structure within lithium niobate crystals has remarkable potential for applications in vortex beam generators functioning at different wavelengths.