Molten-alkali etching has been widely used to reveal dislocations in 4H silicon carbide (4H-SiC), which has promoted the identification and statistics of dislocation density in 4H-SiC single crystals. However, the etching mechanism of 4H-SiC is limited misunderstood. In this letter, we reveal the anisotropic etching mechanism of the Si face and C face of 4H-SiC by combining molten-KOH etching, X-ray photoelectron spectroscopy (XPS) and first-principles investigations. The activation energies for the molten-KOH etching of the C face and Si face of 4H-SiC are calculated to be 25.09 and 35.75 kcal/mol, respectively. The molten-KOH etching rate of the C face is higher than the Si face. Combining XPS analysis and first-principles calculations, we find that the molten-KOH etching of 4H-SiC is proceeded by the cycling of the oxidation of 4H-SiC by the dissolved oxygen and the removal of oxides by molten KOH. The faster etching rate of the C face is caused by the fact that the oxides on the C face are unstable, and easier to be removed with molten alkali, rather than the C face being easier to be oxidized.

Optical clock networks have distinct advantages for the dissemination of time/frequency, geodesy, and fundamental research. To realize such a network, the telecom band and optical atomic clocks have to be coherently bridged. Since the telecom band and optical atomic clocks reside in a distinct spectral region, second-harmonic generation is usually introduced to bridge the large frequency gap. In this paper, we introduce a new method to coherently link a 1550 nm continuous wave laser with a Ti:sapphire mode-locked laser-based optical frequency comb. By coupling the 1550 nm continuous wave laser light and the Ti:sapphire comb light together into a photonic crystal fiber, nonlinear interaction takes place, and new comblike frequency components related to the 1550 nm laser frequency are generated in the visible region. Consequently, we can detect beat notes between two combs in the visible region with a signal-to-noise ratio of more than 40 dB in a resolution bandwidth of 300 kHz. With this signal, we realize an optical frequency divider for converting the frequency of optical clocks in the visible region to the telecom band at 1.55 μm. An out-of-loop measurement shows that the additional noise and uncertainty induced in optical frequency conversion are 5×10-18 at 1 s averaging time and 2.2×10-19, respectively, which are limited by the uncompensated light path fluctuation but fulfill precision measurement using state-of-the-art optical clocks.