Universal spectral purification of ultrahigh-Q microresonators for ideal soliton microcombs

Soliton microcombs offer a compact means to generate equally spaced spectral lines via a delicate balance of Kerr nonlinearity and anomalous dispersion in nonlinear microresonators. However, the simultaneous excitation of multiple transverse mode families can disrupt soliton formation and degrade spectral uniformity. Here, we demonstrate universal spectral purification of microresonators with ultrahigh intrinsic Q factors exceeding 10⁸. An aluminum ring is deposited onto a silica microdisk to eliminate high-order transverse modes selectively by introducing additional losses. The resulting soliton microcombs exhibit an ideal sech² spectral envelope and enable continuous tuning of the soliton repetition frequency over a 300 kHz range without compromising phase noise performance. Our approach can be universally applied to integrated photonic platforms to reduce transverse modes crowding in high-Q resonators, facilitating the generation of broadband classical and quantum light with ideal performance.