Cascaded nonlinear down-conversion in poling-free lithium niobate nanophotonic waveguides.

While cascaded second-order nonlinear processes have revolutionized shorter-wavelength light generation through up-conversion, their potential for longer-wavelength emission via parametric down-conversion remains fundamentally unexplored - constrained by narrow phase-matching bandwidths, insufficient parametric gain and stringent requirements on complex poling. Here, we break this paradigm through a reverse-polarization dual-layer lithium niobate nanophotonic waveguide that achieves broadband phase matching and record 15.3% conversion efficiency for mid-infrared (MIR) generation. Pumped at 1.03 μm, the cascaded optical parametric generation and difference frequency generation processes produce dynamically tunable MIR emissions spanning 2.06-3.09 μm - a relative bandwidth of 40%. By strategically engineering dispersion characteristics, we further extend the versatility of this platform: pumping at telecommunications-compatible wavelengths (1.3-1.6 μm) generates discrete MIR lines at 3.59, 3.93, 4.31, and 4.6 μm, penetrating the crucial molecular fingerprint region. This work not only establishes cascaded down-conversion as a viable strategy for efficient long-wavelength generation but also provides a blueprint for designing reconfigurable nonlinear photonic systems, with transformative implications for spectroscopy, sensing, and wavelength-division multiplexing technologies.