Intense laser-driven nonlinear optical phenomena in two-dimensional nodal line semimetals (NLS) exhibit complex mechanisms, particularly in the ${\mathrm{Nb}\mathrm{Si}}_x{\mathrm{Te}}_2$ material systems characterized by nonsymmorphic symmetry-protected band degeneracy. Our findings reveal how nonsymmorphic symmetry-protected band degeneracy fundamentally influences a material’s nonlinear optical responses. Notably, the nonsymmorphic glide-mirror symmetry leads to the exclusive generation of odd-order harmonics from inversion-symmetry-broken NLS. Moreover, harmonics are emitted parallel and perpendicular to the driving laser’s polarization. We demonstrate that distinct generation mechanisms arise from intrachain and interchain processes, with their relative contributions varying significantly with the polarization of the driving laser pulse. The polarization dependence exhibits twofold anisotropy, with each harmonic order showing characteristic angular distributions of maximum yield. Additionally, our analysis of the ellipticity dependence reveals an intricate interplay between interband and intraband mechanisms. These insights open possibilities for controlling harmonic generation through precise tuning of parameters of the driving laser and highlight the potentials of NLS materials for the fabrication of lightwave-based photonic, optoelectronic, and quantum devices operating on ultrafast timescales.

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• Nonlinear optics
• Optoelectronics
• Polarization of light
• Nodal-line semimetals
• Transition metal dichalcogenides
• Symmetries
• High-harmonic generation
• Optical spectroscopy