Integrated Raman lasers have been well explored using silica and silicon platforms for decades. A well‐known equation with negligible nonlinear losses is employed for predictions of Raman lasing threshold powers in critically coupled cavities. However, nonlinear losses are known to be highly detrimental to silicon devices. Herein, for the first time, including the effects of linear loss, nonlinear losses, and cavity design, a new general equation that predicts the onset of Raman lasing in a cavity is derived and validated experimentally. Generally, a cavity with a small effective area, a short length, and high quality factors (Qs) at both pump and Stokes wavelengths can lase at relatively low pump power. This theory is verified by the experimental results with sub‐milliwatt threshold powers in 2.8 mm long multimode cavities with different Qs at different pump wavelengths. The derived Raman gain coefficients from the measurements follow the scaling rules of Raman gain. This work advances the understanding of Raman lasing in high‐Q multimode cavities. It also shows that Raman lasing at O/S‐band is possible in racetrack resonators without needing any reverse bias and the broad operation wavelength is promising for single‐chip silicon devices operating at all communication bands.

Multimode silicon resonators with ultralow propagation losses for ultrahigh quality (Q) factors have been attracting attention recently. However, conventional multimode silicon resonators only have high Q factors at certain wavelengths because the Q factors are reduced at wavelengths where fundamental modes and higher-order modes are both near resonances. Here, by implementing a broadband pulley directional coupler and concentric racetracks, we present a broadband high-Q multimode silicon resonator with average loaded Q factors of 1.4 × 106 over a wavelength range of 440 nm (1240–1680 nm). The mutual coupling between the two multimode racetracks can lead to two supermodes that mitigate the reduction in Q factors caused by the mode coupling of the higher-order modes. Based on the broadband high-Q multimode resonator, we experimentally demonstrated a broadly tunable Raman silicon laser with over 516 nm wavelength tuning range (1325–1841 nm), a threshold power of (0.4 ± 0.1) mW and a slope efficiency of (8.5 ± 1.5) % at 25 V reverse bias.

We experimentally study the generation of optical frequency combs (OFCs) in dual-pumped high-quality factor (>106) multimode silicon racetrack resonators and show that sub-milliwatt (0.3 mW) input pump powers were sufficient to produce six-order OFC generation with eleven peaks, even in waveguides with normal dispersion. The low pump power and enhanced efficiency of the OFC generation can be attributed to mode coupling between two mode families of the multimode resonator, which acts to change the effective magnitude and the sign of the local dispersion of the resonator. We experimentally observed that the OFC generation had 3.6 times more peaks and 12.1 dB higher conversion efficiency than that without any bias. We compared the efficiencies of the OFC generation at different pump wavelengths within and beyond the mode coupling region. At low pump powers circulating in the resonator, pump wavelengths in the mode coupling regime produced 1.3 times more peaks and 8 dB enhancement in conversion efficiency than pumping beyond the mode coupling regime. The experimental results were consistent with the theoretical simulations by solving the modified Lugiato–Lefever equation.

As a typical wide bandgap semiconductor, ZnO has received a great deal of attention from researchers because of its strong physicochemical characteristics. During the past few years, great progress has been made in the optoelectronic applications of ZnO, particularly in the photocatalysis and photodetection fields. To enable further improvements in the material’s optoelectronic performance, construction of a variety of ZnO-based composite structures will be essential. In this paper, we review recent progress in the growth of different ZnO–graphene nanocomposite structures. The related band structures and photocatalysis and photoresponse properties of these nanocomposites are discussed. Additionally, specific examples of the materials are included to provide an insight into the common general physical properties and carrier transport characteristics involved in these unique nanocomposite structures. Finally, further directions for the development of ZnO–graphene nanocomposite materials are forecasted.

Here, the influence of pump powers for silicon microresonator-based Kerr comb generation (KCG) was investigated theoretically and experimentally in two types of dual-pumped KCG. One employs dual pumps at different free spectral range (FSR) spacings of the resonator for broadband comb spectrum and the other to produce comb lines at one FSR spacing for low pump powers. The KCG is modeled by including nonlinear absorption and mode interaction into the Lugiato–Lefever equation. Both the theoretical and the experimental results indicate that the KCG with pump powers inside the ring of tens of milliwatts would be limited by high nonlinear absorption even with a 25 V reverse bias to deplete free carriers. The most efficient comb spectrum occurs with recirculating pump power in the ring cavity of 13 mW (0.3 mW pump power in the bus waveguide) at 25 V reverse bias. Generally, for silicon KCG at the telecom wavelengths, the use of low pump powers and reverse bias is essential. At 25 V reverse bias, increasing the stored pump power in the ring to 160 mW would suppress the KCG and produce a similar number of comb lines as 1 mW recirculating pump power. This work advances our understandings of silicon KCG in the telecom band.