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Abstract The current morphology of Earth’s time-averaged magnetic field can be approximated to a geocentric axial dipole (GAD), but whether such an approximation remains valid in deep time needs to be investigated. Studies have used paleomagnetic data to reconstruct the ancient field and generally support a GAD morphology since 2 Ga. Recently, the GAD model for mid-Proterozoic time has been challenged, and an alternative model was proposed wherein the mid-Proterozoic field was dominated by a normal-tesseral quadrupole (NTQ) with spherical harmonics of degree l = 2 and order m = 1. We performed forward modeling to quantitatively compare whether a GAD or an NTQ could provide a better fit to mid-Proterozoic paleomagnetic directions. To deal with the ambiguity in plate reconstruction, we first considered data only from Laurentia, and then we expanded the analysis to Baltica by reconstructing its position relative to Laurentia using the geologically based Northern Europe–North America (NENA) configuration. Finally, we included data from Siberia using two reconstruction models. Results showed that in three mid-Proterozoic intervals (1790–1740 Ma, 1485–1425 Ma, 1095–1080 Ma), a GAD morphology gives better, or equally good, fits compared to the NTQ morphology. In addition, a stable NTQ that persisted for hundreds of millions of years is disfavored from a geodynamic perspective. Overall, mid-Proterozoic paleomagnetic directions are more consistent with a dipolar field. We suggest that the GAD remains the most parsimonious model to describe the morphology of the mid-Proterozoic magnetic field. 

Materials with strong second-order ( ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$) optical nonlinearity, especially lithium niobate, play a critical role in building optical parametric oscillators (OPOs). However, chip-scale integration of low-loss ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$materials remains challenging and limits the threshold power of on-chip ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$OPO. Here we report an on-chip lithium niobate optical parametric oscillator at the telecom wavelengths using a quasi-phase-matched, high-quality microring resonator, whose threshold power ( $\sim <\#comment/>30\text{µ<\#comment/>}\mathrm{W}$) is 400 times lower than that in previous ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$integrated photonics platforms. An on-chip power conversion efficiency of 11% is obtained from pump to signal and idler fields at a pump power of 93 µW. The OPO wavelength tuning is achieved by varying the pump frequency and chip temperature. With the lowest power threshold among all on-chip OPOs demonstrated so far, as well as advantages including high conversion efficiency, flexibility in quasi-phase-matching, and device scalability, the thin-film lithium niobate OPO opens new opportunities for chip-based tunable classical and quantum light sources and provides a potential platform for realizing photonic neural networks. 

Abstract The location of the West African craton (WAC) has been poorly constrained in the Paleoproterozoic–Mesoproterozoic supercontinent Nuna (also known as Columbia). Previous Nuna reconstruction models suggested that the WAC was connected to Amazonia in a way similar to their relative position in Gondwana. By an integrated paleomagnetic and geochronological study of the Proterozoic mafic dikes in the Anti-Atlas Belt, Morocco, we provide two reliable paleomagnetic poles to test this connection. Incorporating our new poles with quality-filtered poles from the neighboring cratons of the WAC, we propose an inverted WAC-Amazonia connection, with the northern WAC attached to northeastern Amazonia, as well as a refined configuration of Nuna. Global large igneous province records also conform to our new reconstruction. The inverted WAC-Amazonia connection suggests a substantial change in their relative orientation from Nuna to Gondwana, providing an additional example of large-magnitude cumulative azimuthal rotations between adjacent continental blocks over supercontinental cycles. 

Here, we report ${\mathrm{\chi <\#comment/>}}^{\left(3\right)}$-based optical parametric oscillation (OPO) with widely separated signal–idler frequencies from crystalline aluminum nitride microrings pumped at $2\text{µ<\#comment/>}\mathrm{m}$. By tailoring the width of the microring, OPO reaching toward the telecom and mid-infrared bands with a frequency separation of 64.2 THz is achieved. While dispersion engineering through changing the microring width is capable of shifting the OPO sideband by $><\#comment/>9\mathrm{T}\mathrm{H}\mathrm{z}$, the OPO frequency can also be agilely tuned in the ranges of 1 and 0.1 THz, respectively, by shifting the pump wavelength and controlling the chip’s temperature. At high pump powers, the OPO sidebands further evolve into localized frequency comb lines. Such large-frequency-shift OPO with flexible wavelength tunability will lead to enhanced chip-scale light sources.