ALBERT

Description: Reduced complexity in production of large-scale, flexible surface enhancement Raman spectroscopy (SERS) active substrate has been implemented at the water surface, when polydimethylsiloxane (PDMS) solution having lower density than the water meets with self-assembled polystyrene (PS) nanoparticles (NPs). Through tuning its flowability, the PDMS solution could effectively wet PS NPs, wherein the PS NPs can be embedded in the PDMS during the curing process. This technical innovation saves ill-posed transfer steps as present in traditional operations which may cause damaged nanostructures, and it could be beneficial for preparing a large scale, high quality, and flexible SERS active substrate. Field tests demonstrated that the Raman signal enhancement factor could reach up to the order of ∼10 7 with decent repeatability less than 10%.

Description: We report the large nonlinear response and ultrafast carrier relaxation dynamics of a graphene-Bi 2 Te 3 heterostructure produced by two-step chemical vapour deposition. The nonlinear refractive index reaches n 2  = 0.2 × 10 −7  cm 2 /W at the telecommunication wavelength of 1550 nm, which is almost seven orders of magnitude larger than that of the bulk Si material. Additionally, a pump-probe experiment is performed to investigate the ultrafast dynamic process (intraband relaxation time τ 1  = 270 ± 20 fs; interband relaxation time τ 2  = 3.6 ± 0.2 ps) of the graphene-Bi 2 Te 3 heterostructure. Then, based on the donor-acceptor structure model, we propose a theoretical model to explain the dynamic relaxation process. Our results show that the graphene-Bi 2 Te 3 heterostructure is a promising saturable absorber for ultrafast pulse laser applications at telecommunication wavelengths.

Description: Research on sheath expansion is critical to the understanding of the dielectric recovery process in a vacuum interrupter after interruption of vacuum arcs. In this paper, we investigated how residual plasma affects breakdown in the sheath expansion period after the current zero. To simulate sheath expansion and breakdown, we developed a fully kinetic particle-in-cell Monte Carlo collision model with one spatial dimension and three velocity dimensions. The model accounted for various collisions, including ionization, excitation, elastic collisions, charge exchange, and momentum exchange, and we added an external circuit to the model to make the calculations self-consistent. The existence of metal vapor slowed the sheath expansion in the gap and caused high electric field formation in front of the cathode surface. The initial residual plasma, which was at sufficiently low density, seemed to have a limited impact on breakdown, and the metal vapor dominated the breakdown in this case. Additionally, the breakdown probability was sensitive to the initial plasma density if the value exceeded a specific threshold, and plasma at sufficiently high density could mean that breakdown would occur more easily. We found that if the simulation does not take the residual plasma into account, it could overestimate the critical value of the metal vapor density, which is always used to describe the boundary of breakdown after interruption of vacuum arcs. We discussed the breakdown mechanism in sheath expansion, and the breakdown is determined by a combination of metal vapor, residual plasma, and the electric field in front of the cathode surface.