Highly Efficient Multichromatic Raman Microlasers from Cavity Polygon Modes on Thin-Film Lithium Niobate Platforms

The integration of stimulated Raman scattering (SRS) and second order nonlinearity (χ⁽²⁾) in noncentrosymmetric photonic microresonators presents a highly promising solution for developing on-chip coherent light sources with exceptional bandwidth and flexible tunability, which are crucial for precision metrology and coherent communication. However, such systems frequently face challenges including limited conversion efficiency and restricted bandwidth, despite employing high quality-factor (Q > 10⁶) whispering gallery modes (WGMs) in microresonators for dramatically enhancing light-matter interaction. In this work, in contrast to using WGMs, we introduce a novel methodology leveraging cavity polygon modes within an X-cut thin-film lithium niobate microdisk to achieve highly efficient multichromatic Raman microlasers. Specifically, high-Q square modes characterized by two parallel sides oriented perpendicularly relative to the optical axis of lithium niobate crystal were excited. These modes offer distinct advantages, including enhancing both mode-field overlap (>80%) and improved phase matching by utilizing the largest second-order susceptibility component (d₃₃), which is critical for efficient Raman-quadratic interactions. Experimental results demonstrate significant advancements in multiwavelength laser generation. Forward and backward stimulated Raman microlasers are simultaneously demonstrated at 1624 nm with high conversion efficiencies of 32.4% and 50.2%, respectively, corresponding to a total conversion efficiency of 47% at 2.73 mW pump power. And a 1 ms short-term integral linewidth of the forward Raman microlasers reaches 5.2 kHz. Meanwhile, our system enables the generation of multiwavelength Raman-quadratic laser signals across the ∼800 nm and ∼530 nm spectral bands.