We demonstrate numerically and experimentally the generation of powerful supercontinuum vortices from femtosecond vortex beams by using multiple thin fused silica plates. The supercontinuum vortices are shown to preserve the vortex phase profile of the initial beam for spectral components ranging from 500 nm to 1200 nm. The transfer of the vortex phase profile results from the inhibition of multiple filamentation and the preservation of the vortex ring with relatively uniform intensity distribution by means of the thin-plate scheme, where the supercontinuum is mainly generated from the self-phase modulation and self-steepening effects. Our scheme works for vortex beams with different topological charges, which provides a simple and effective method to generate supercontinuum vortices with high power.
The filamentation of the femtosecond laser pulse in air with a preformed density hole is studied numerically. The result shows that density-hole-induced defocusing effect can relieve the self-focusing of the pulse, and by changing the length of the density hole and relative delay time, the filamentation length, intensity, spectral energy density and broaden region can be effectively controlled. When a short density hole with millisecond delay time is introduced, a significant elongation of the filamentation and enhancement of supercontinuum intensity can be obtained. This study provides a new method to control filamentation by pulse sequence.
We demonstrate theoretically that few-cycle vortex beam with subterawatt peak power can be generated by self-compression of mid-infrared femtosecond vortex beam using the thin-plate scheme. The 3 𝜇m femtosecond vortex beam with input duration of 90 fs is compressed to 15.1 fs with the vortex characteristics preserved. The conversion efficiency is as high as 91.5% and the peak power reaches 0.18 TW. The generation of the high-peak-power few-cycle vortex beam is owing to the proper spatiotemporal match by this novel scheme, where the spectrum is broadened enough, the negative group velocity dispersion can compensate the positive chirp induced by nonlinear effects, and multiple filamentation is inhibited for the keeping of the vortex characteristics. Our work will help to generate isolated attosecond vortices, opening a new perspective in ultrafast science.
Aiming at the dry pulverized biochar fuel refined by thermochemical conversion technology, its properties were obtained through experiment. By blending 5% additives to improve the ash fusion characteristics of biomass particles and considering the viscosity-temperature characteristics and the matching principle of CO2 reaction activity, the entrained flow gasification technology scheme of biomass particles was formulated. Based on the gasification reaction kinetics of dry pulverized biochar, a numerical simulation model was established to predict the flow characteristics, syngas composition, and particle conversion process in the gasifier. The results show that under the swirling entrainment and centrifugal force, the oxidizing reaction of particles is completed in the upper part of the gasifier quickly after mixing with oxygen. And the gasification reduction reaction occurs in the recirculating area and the pipe flow area. These effectively complete the conversion of char. The simulation results are in good agreement with the equilibrium calculation values, which proves the feasibility and rationality of the biomass particle entrained flow gasification technology scheme based on the HT-L.
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