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Soliton microcombs, which require the hosting cavity to operate in an anomalous dispersion regime, are essential to integrate photonic systems. In the past, soliton microcombs were generated on cavity whispering gallery modes (WGMs), and the anomalous dispersion requirement of the cavity made by normal dispersion material was achieved through structural dispersion engineering. This inevitably degrades the cavity optical quality factor (Q) and increases pump threshold power for soliton comb generation. To overcome the challenges, here, we report a soliton microcomb excited by cavity polygon modes. These modes display anomalous dispersion at near-infrared while optical Q factors exceeding 4×106 are maintained. Consequently, a soliton comb spanning from 1450 nm to 1620 nm with a record low pump power of 11 mW is demonstrated, a three-fold improvement compared to the state of the art on the same material platform.

Microresonators of ultrahigh quality ( Q ) factors represent a crucial type of photonic devices aiming at ultra-high spectral resolution, ultra-high sensitivity to the environmental perturbations, and efficient nonlinear wavelength conversions at low threshold pump powers. Lithium niobate on insulator (LNOI) microdisks of high Q factors are particularly attractive due to its large second-order nonlinear coefficient and strong electro-optic property. In this letter, we break through the long standing bottleneck in achieving the Q factors of LNOI microresonators beyond 10 ^8 , which approaches the intrinsic material absorption limit of lithium niobate (LN). The ultra-high Q factors give rise to a rich family of nonlinear optical phenomena from optical parametric oscillation (OPO) to harmonics generation with unprecedented characteristics including ultra-low pump threshold, high wavelength conversion efficiency, and ultra-broad operation bandwidth. Specifically, the threshold of OPO is measured to be only 19.6 μ W, and the absolute conversion efficiency observed in the second harmonic generation reaches 23%. The record-breaking performances of the on-chip ultra-high Q LNOI microresonators will have profound implication for both photonic research and industry.

We report the fabrication of crystalline microresonators of high quality (Q) factors with a controllable wedge angle on lithium niobate on insulator (LNOI). Our technique relies on a femtosecond laser assisted chemo-mechanical polish, which allows us to achieve ultrahigh surface smoothness as critically demanded by high Q microresonator applications. We show that by refining the polish parameters, Q factors as high as 4.7 × 107 can be obtained and the wedge angle of the LNOI can be continuously tuned from 9° to 51°.