Efficient Transverse Multiwave Interactions up to Six-Wave Mixing in a High-Q Lithium Niobate Microresonator.

High-order nonlinear optical processes beyond four-wave mixing (χ^{(3)}) are crucial for advancing ultraviolet (UV) light sources and quantum technologies, yet their practical implementation remains challenging due to inherently weak high-order nonlinear susceptibilities and stringent broadband phase-matching requirements-limitations that persist even in state-of-the-art high-Q microresonators. A breakthrough transverse multiwave mixing scheme is proposed in a high-Q lithium niobate microresonator, only under a single continuous-wave (cw) laser pump. Our approach leverages self-organized subwavelength photorefractive gratings (SPGs) generated through a bidirectional stimulated Raman scattering (SRS) process in the microresonator, without using two external counterpropagating lasers. Under single-wavelength pumping (1546 nm), bidirectional SRS (1713 nm) spontaneously generates dynamic SPGs that enable dual-function control: (1) broadband momentum compensation (Δk≈2π/Λ, Λ=grating period) to resolve broadband phase-matching challenges in high-order nonlinear interactions and (2) preservation of ultrahigh-quality factor (Q>7×10^{6}) for enhanced nonlinear conversion. Moreover, unlike conventional longitudinal configurations requiring more rigid multiwavelength resonance alignment, our transverse architecture decouples nonlinear processes from cavity-mode constraints through SPG-mediated momentum engineering, enabling simultaneous support for sum-frequency generation to six-wave mixing processes across 500-nm bandwidth with high conversion efficiencies. Furthermore, the cascaded SRS process is simultaneously activated to generate a light signal for subsequent nonlinear interactions. Consequently, this novel approach enables the first demonstration of single-pump phase-matched transverse SFG with record conversion efficiency (590%/W), and transverse multiwave mixing processes from χ^{(3)} (four-wave) to χ^{(5)} (six-wave) processes are achieved for the first time using only the single cw pump, leading to efficient visible and UV light generation, representing a notable advance in nonlinear integration.