We present a quantum perturbation theory on two-photon absorption (2PA) in monolayer and bilayer graphene which is Bernal-stacked. The theory shows that 2PA is significantly greater in bilayer graphene than monolayer graphene in the visible and infrared spectrum (up to 3 μm) with a resonant 2PA coefficient of up to ∼0.2 cm/W located at half of the bandgap energy, γ(1) = 0.4 eV. In the visible and terahertz region, 2PA exhibits a light frequency dependence of ω(-3) in bilayer graphene, while it is proportional to ω(-4) for monolayer graphene at all photon energies. Within the same order of magnitude, the 2PA theory is in agreement with our Z-scan measurements on high-quality epitaxial bilayer graphene deposited on SiC substrate at light wavelength of 780 and 1100 nm.

As one of the rising 2D materials, niobium‐carbide (Nb2C, well‐known as a member of MXene family) has attracted considerable attention owing to its unique physical and chemical properties. In this work, few‐layer Nb2C nanosheets (NSs) with large (≈255 nm) and small (≈48 nm) lateral dimensions are obtained via a combination of selective etching and liquid cascade centrifugation. Their relaxation time and photophysics process are systematically investigated by transient absorption spectroscopy, and the size effect is demonstrated by phonon‐bottleneck mechanism. Ultrafast fast relaxation time (37.43 fs) and slow relaxation time (0.5733 ps) are observed due to the symmetric structure and metallicity of Nb2C NSs. The nonlinear optical properties of Nb2C NSs are studied by Z‐scan technique, and both saturable absorption and reverse‐saturable absorption are observed. According to first principle calculations, these phenomena can be attributed to the special band structure of Nb2C near the Fermi level, where two‐photon absorption or multiphoton absorption may occur under the irradiation of long wavelength light. These intriguing results suggest that few‐layer Nb2C NSs can be used as building blocks for broadband ultrafast photonics and optoelectronic devices and also hold the potential for breakthrough developments in these fields.

We report on the observation of the spin separation of light in the plane of incidence when a linearly polarized beam is reflected or refracted at a planar dielectric interface. Remarkably, the in-plane spin separation reaches hundreds of nanometers, comparable with the transverse spin separation induced by the well-known spin Hall effect of light. The observation is properly explained by considering the in-plane spread of wave-vectors. This study thus offers new insights on the spinoptics and may provide a potential method to control light in optical nanodevices.

This paper presents a review of recent research on direct upgrading of coal/biomass volatiles into aromatics by catalytic pyrolysis and syngas by gasification with catalytic steam reforming. Coal/biomass valorization is considered an important part to fill up the depletion of modern fossil fuel resources. The catalytic pyrolysis process is a potential approach to improve coal tar/bio-oil quality by minimizing its undesirable properties (high viscosity, corrosivity, instability, etc.) and producing renewable fuels and high-value chemicals, such as aromatics (benzene, toluene, ethylbenzene, xylenes, etc.). Gasification reforming as a promising process for renewable energy utilization can produce H2-rich syngas. The produced syngas can be further synthesized to fuel and chemicals via Fischer–Tropsch synthesis. Thus, this study provides a comprehensive review of the research and development of conversion of coal and biomass volatiles in terms of technological types and catalysts. Aspects related to upgrading technology, the reactor type of catalytic pyrolysis and gasification, and the reaction mechanisms to specific products during the catalytic process are also discussed comprehensively. In particular, catalytic upgrading by fast pyrolysis involves a series of reactions, including deoxygenation, cracking, hydrocarbon pool mechanism, aromatization, and condensation, as well as desulfurization and denitrification in the gasification process. Some key points that are to be addressed for the established process of coal and biomass volatile upgrading may include finding multifunctional catalysts and reactor development for improving the efficiency. Expanding and enhancing knowledge about catalyst utilization and fundamental reaction mechanisms in the thermochemical catalytic conversion technologies of coal and biomass will play an important role in the generation of chemicals and carbon-neutral fuels.