Tailoring high-order harmonics spectral and spatial properties via a multimodal regime

We studied high-order harmonic generation (HHG) inside a microfluidic device fed with helium by analyzing the propagation of the fundamental field and the evolution of emerging XUV radiation in a multimode regime. The low-ionization regime investigated here is identified as a particularly straightforward framework for isolating pure mode-beating-induced effects, enabling a direct assessment of the role of modal dynamics in shaping the harmonic generation process. By varying the modal population through the manipulation of the coupling conditions, we demonstrated the possibility of optimizing the intensity and spectral content of the emitted harmonics, achieving an extension of the cutoff energy by more than 30 eV. Furthermore, we modeled the spatial evolution of the XUV field inside the waveguide, revealing the emergence of mode-beating-induced phase-matching channels that give rise to complex harmonic spatial profiles. This work demonstrates the potential of HHG inside waveguides for all-optical, on-site shaping of the generation process via modal-based spatial modulation of the driving field.