In this Letter, we report phase-matched four-wave mixing separated by over one-octave in a dispersion engineered crystalline microresonator. Experimental and numerical results presented here confirm that primary sidebands were generated with a frequency shift up to 140 THz, and that secondary sidebands formed a localized comb structure, known as a clustered comb in the vicinity of the primary sidebands. A theoretical analysis of the phase-matching condition validated our experimental observations, and our results good agree well with numerical simulations. These results offer the potential to realize a frequency tunable comb cluster generator operating from 1 µm to mid-infrared wavelengths with a single and compact device.Phase-matched four-wave mixing (FWM) in a whispering gallery mode (WGM) microresonator driven by a continuous wave (CW) laser have been studied for decades [1]. In particular, a microresonator frequency comb (Kerr comb) realized via a cascade FWM process makes it possible to achieve broadband optical frequency comb sources characterized as having high repetition rates, compactness and low-energy consumption [2]. They provide fascinating applications including precise spectroscopy [3] and coherent data transmission [4]. A Kerr comb, and particularly a dissipative Kerr soliton (DKS), which is a stable low noise state, features a coherent broad comb spectrum with a smooth envelope [5]. However, spectral broadening over one octave remains a challenge mainly because of the limitation imposed by group velocity dispersion (GVD). Spectral broadening utilizing dispersion engineering has been achieved with Cherenkov radiation, which can be understood in terms of the coherent dispersive wave in the frequency domain emitted from a soliton propagating along a resonator [6]. A dispersive wave induces asymmetrical spectral profiles and occurs at the point where a simple phase-matching condition is satisfied, and it is determined by the signs and values of higher-order (i.e., third-, fourth-and fifthorder) dispersion parameters.Higher-order dispersion plays important roles not only as regards a Kerr soliton in an anomalous GVD regime, but also in a weak normal GVD regime. Since a normal dispersion usually does not allow modulation instability (MI) near the pump, phase-matched FWM may be considered to occur only in an anomalous dispersion resonator. However, higher-order dispersion, particularly even orders of dispersion, enables a resonant MI process, called clustered frequency combs, to occur far from the pump mode [7][8][9]. Clustered combs, characterized by FWM generation with parametric sidebands that have a * takasumi@elec.keio.ac.jp large-frequency shift, have been reported using an MgF 2 microresonator [7,9] and silica microtoroids [8]. The fact that clustered combs have the potential to utilize microcomb source emitting in the 1.0 to 3.5 µm wavelength region with just a near-infrared pump indicates interesting potential applications, for example laser processing and optical communication. In particular,...

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Optical microcavities, which can confine light spatially, are important devices in the field of optics. Singlecrystal calcium fluoride (CaF2) is one of the most suitable materials for optical microcavities. To manufacture a cavity made from CaF2, ultra-precision machining is an appropriate process. In the previous study, we have investigated the machinability of CaF2 and successfully manufactured a whispering gallery mode (WGM) CaF2 cavity. Although the manufactured cavity can confine light for a certain amount of time, it was found that the cavity performance could be unstable due to its negative thermo-optic coefficient. In this study, a novel CaF2brass hybrid WGM microcavity was proposed and manufactured, which could stabilize the cavity performance. By using finite element method (FEM) simulations, it was shown that the proposed microcavity structure facilitates rapid heat diffusion from the cavity. A hybrid microcavity without a large brittle fracture was successfully manufactured by ultra-precision machining. Thermo-optic (TO) effects that cause an instability of the cavity performance are assumed to be suppressed during resonance.