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We demonstrated a straightforward approach to generate red and near-infrared laser emissions by a Raman-assisted four wave mixing (FWM) process in a nonlinear Yb-doped fiber amplifier, delivering 342 fs pulses of 241 nJ at 864 nm, 834 fs pulses of 21 nJ at 751 nm, and 1.9 ps pulses of 3.8 &mu, J at 1030 nm. A pair of gratings was employed as the pre-compressor to promote the intensity of the fundamental wave in the main amplifier. Multiple wavelengths from 751 to 1273 nm resulted due to cascaded-Raman-assisted FWM. The pre-compression also underlay the achievement of 25.1, 701, and 2000 kW peak power for the red (751 nm), near-infrared (864 nm), and fundamental (1030 nm) components respectively, which restrained the gain narrowing effect during the amplification. It finally led to shorter pulse duration under increased power.

The filament length is increased by more than ten times and the refractive index is enhanced by six times by initiating filamentation in gold-nanoparticle-doped water. Strong supercontinuum and a rainbow like filament path are observed. © 2020 The Authors

We investigate the femtosecond laser filamentation in nanoparticle colloidal solutions. Colloidal solutions of water doped with gold (Au), silver (Ag), copper (Cu), and calcium carbonate (CaCO3) nanospheres under different sizes and concentrations were experimentally studied. A cyan–red light path was visualized, and the generated supercontinuum spectra spanning from 400 nm to 950 nm were obtained. It was found that the energy losses and frequency-dependent scattering are necessary prerequisites for generating a colorful light path.

We report a universal approach based on the surface plasmon resonances (SPRs) attained in filamentation in water doped with gold nanoparticles for enhancing the nonlinear refractive index. The filament-induced supercontinuum spectrum in water overlaps with SPRs of gold nanospheres, which further leads to a modification on the Kerr nonlinear refractive index. In our experiment, the measured nonlinear refractive index (n2) in water doped with gold nanoparticles increases by six times, as compared with that in pure water. Such enhancement may be useful for filament-induced nonlinear applications with modest incident intensity.

Beam dynamics and RF design have been performed of a new type trapezoidal IH-RFQ operating at 104 MHz for acceleration of 14C+ in the framework of RFQ based 14C AMS facility at Peking University. Low energy spread RFQ beam dynamics design was approached by the method of internal discrete bunching. 14C+ will be accelerated from 40 keV to 500 keV with the length of about 1.1 m. The designed transmission efficiency is better than 95% and the energy spread is as low as 0.6%. Combining the beam dynamics design, a trapezoidal IH-RFQ structure was proposed, which can be cooled more easily and has better mechanical performance than traditional RFQ. Electromagnetic field distribution was simulated by using CST Microwave Studio (MWS). The specific shunt impedance and the quality factor were optimized primarily.

Beam dynamics and rf designs of a 104 MHz ladder type IH-RFQ (L-IH-RFQ) accelerator are finished at Peking University for the acceleration of 14C+ from 40keV to 500keV. As a specific feature, the output beam energy spread is as low as 0.6% achieved with the internal discrete bunching method, which makes potential applications of RFQ feasible, such as accelerator mass spectrometry and ion implantation. Tolerances of the beam dynamics design are studied by means of changing the input beam parameters, and the results are quite satisfying. On the other hand, the L-IH-RFQ structure is employed, taking advantage of its mechanical stability and the absence of inter-electrode voltage asymmetry. Radio-frequency properties are studied and optimized for reducing power loss with Microwave Studio (MWS). Tuning of the field flatness and frequency is investigated in principle.