Three-octave-spanning supercontinuum generation in z-cut quasi-phase matching thin-film lithium niobate

The generation of supercontinuum spectra on integrated nonlinear optical platforms opens up new possibilities for applications in spectroscopy, metrology, signal processing, optical communications, and more. As an extremely promising low-loss nonlinear integrated photonics platform, there is an urgent need to achieve efficient and broadband supercontinuum generation (SCG) on thin-film lithium niobate (TFLN). However, most advancements in this field have not fully exploited the potential of χ(2) nonlinearity, leading to constraints in both spectral width and flatness. In this work, we introduce broadband quasi-phase matching into TFLN, effectively leveraging the synergistic effects of χ(2) and χ(3) nonlinearities. By combining cascaded χ(2) frequency upconversion, self-phase modulation (SPM), and dispersive wave (DW) generation, we have achieved, for the first time, a broadband, flat, three-octave-spanning SCG covering the ultraviolet–visible–NIR region (320–2500 nm, −30 dB) with microjoule-level energy in z-cut quasi-phase matching TFLN. The ultraviolet–visible spectral region consists of second–fourth harmonics, while the NIR region is composed of the spectrally broadened pump wave by SPM and DW generated by soliton fission, with a conversion efficiency of up to 31% from the pump to the ultraviolet–visible region. We conducted a comprehensive study on the critical roles of χ(2) and χ(3) nonlinearities in SCG and provided a systematic, in-depth analysis of the spectral evolution process. The experimental results show excellent agreement with numerical simulations. Our work paves the way for SCG across the full transparency window of LN.