High-harmonic generation in submicron-thick chirped periodically poled thin-film lithium niobate

Submicron-thick thin-film lithium niobate (TFLN) has emerged as a promising platform for nonlinear integrated photonics. In this work, we demonstrate the efficient simultaneous generation of broadband 2nd–8th harmonics in chirped periodically poled (CPP) TFLN. This is achieved through the synergistic effects of cascaded χ (2) nonlinear up-conversion and χ (3) self-phase modulation, driven by near-infrared femtosecond pulses with a central wavelength of 2100 nm and a pulse energy of 1.2 μJ. Remarkably, the 7th and 8th harmonics extend into the deep ultraviolet (DUV) region, reaching wavelengths as short as 250 nm. The 3rd–8th harmonic spectra seamlessly connect, forming a broadband supercontinuum spanning from the DUV to the visible range (250–800 nm, −25 dB ), with an on-chip conversion efficiency of 19% (0.23 μJ). This achievement is attributed to the CPP-TFLN providing multiple broadband reciprocal lattice vector bands, enabling quasi-phase matching for a series of χ (2) nonlinear processes, including second harmonic generation (SHG), cascaded SHG, and third harmonic generation. Furthermore, we demonstrated the significant role of cascaded χ (2) phase-mismatched nonlinear processes in high-harmonic generation (HHG). Our work unveils the intricate and diverse nonlinear optical interactions in TFLN, offering a clear path toward efficient on-chip HHG and compact coherent white-light sources extending into the DUV.