Extracavity-generated femtosecond near-UV vortex beams and their use for filament inscription in bulk polymethylpentene

Abstract

This study demonstrated that a femtosecond near-ultraviolet (NUV) vortex beam—generated by first modulating a near-infrared (NIR) laser beam with a liquid crystal spatial light modulator (SLM) and then converting the modulated NIR beam into the NUV via second-harmonic generation (SHG)—enabled in situ direct modification of polymethylpentene (PMP) transparent from MIR to the NUV. This capability arises from the beam’s helical-phase-controlled asymmetric energy flow, spatial field redistribution, and tunable orbital angular momentum (OAM). Theoretical analysis showed that the pattern of a vortex beam can be controllably adjusted using the SLM, and this spatially asymmetric electric field distribution can be exploited to spatially modulate the Kerr-induced refractive index change to achieve controlled nonlinear polarization with a topology-dependent spatial profile. Experimentally, SHG was found to conserve OAM: integer topological charges convert into symmetric ring modes, while fractional charges yield asymmetric C-shaped patterns. The controlled energy flux was encoded in the topological-charge-dependent ring diameter and pattern asymmetry. For direct inscription in the polymer, precise management of energy flux—achieved by tailoring the topological charge (m2ω387 = 1–6 for linear scaling; m2ω387 = 10 with 1–5μJ)—enabled predictable control over filament inner diameter and the transition to χ(3)-mediated multi filamentation. Bulk PMP inscription further showed that OAM femtosecond laser direct writing represents a transformative shift: from a nonlinear physical phenomenon to a low-complexity, topologically designable engineering method for fabricating millimeter-scale micro-photonic devices with ~1μm features. This method offers linear diameter control via topological charge, low-loss ring-core waveguides, and efficient broadband guidance.