The 266-nm ultraviolet-beam generation of all-fiberized super-large-mode-area narrow-linewidth nanosecond amplifier with tunable pulse width and repetition rate

We report on a compact, stable, all-fiberized narrow-linewidth(0.045 nm) pulsed laser source emitting laser beam with a wavelength of 266 nm, and tunable pulse width and repetition rate. The system is based on all-fiberized nanosecond amplifier architecture, which consists of Yb-doped fiber preamplifiers and a super-large-mode-area Yb-doped fiber power amplifier. The fiber amplifier with a core of 50 μm is used to raise the threshold of the stimulated Brillouin scattering(SBS) effect and to obtain high output power and single pulse energy. Using lithium triborate(LBO) crystal and betabarium borate(BBO) crystal for realizing the second-harmonic generation(SHG) and fourth-harmonic generation(FHG),we achieve 17 μJ(1.73 W) and 0.66 μJ(66 mW), respectively, at wavelengths of 532 nm and 266 nm and a repetition rate of 100 kHz with pulse width of 4 ns. This source has great potential applications in fluorescence research and solar-blind ultraviolet optical communication.

Broadband and high efficiency terahertz metasurfaces for anomalous refraction and vortex beam generation

The applications of metasurfaces are currently a highly active research field due to their extraordinary ability to manipulate electromagnetic waves. The ultra-thin characteristics of metasurfaces allow the miniaturization and integration of metasurface devices. However, these devices work typically under a low efficiency and narrow bandwidth condition. In this work, we design eight multilayered unit cells with similar amplitudes and a phase interval of π/4, which convert the polarization states of the terahertz(THz) waves between two orthogonal directions. The average cross-polarized transmission amplitudes of these cells are all around 0.9 in an ultra-broad frequency range from 0.5 THz to 1.4 THz. Furthermore,unit cells are used to construct both an ultra-thin anomalous refraction metalens and a vortex phase plate. Our simulation results show that the anomalous refraction for the transmitted linear polarization component is comparable to the theoretical prediction, and the maximum error is determined to be below 4.8%. The vortex phase plate can also generate an ideal terahertz vortex beam with a mode purity of 90% and more. The distributions of longitudinal electric field, intensity, and phase illustrate that the generated vortex beam has excellent propagation characteristics and a weak divergence. Simulations of the two types of metasurface devices, based on the eight unit cells, exhibit very high efficiencies in a wide bandwidth. Our research will assist in the improvement in the practical applications of metasurfaces. It also provides a reference for the design of high efficiency and broadband devices that are applied to other frequency ranges.

Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere

We propose theoretically and verify experimentally a method of combining a q-plate and a spiral phase plate to generate arbitrary vector vortex beams on a hybrid-order Poincaré sphere. We demonstrate that a vector vortex beam can be decomposed into a vector beam and a vortex, whereby the generation can be realized by sequentially using a q-plate and a spiral phase plate. The generated vector beam, vortex, and vector vortex beam are verified and show good agreement with the prediction. Another advantage that should be pointed out is that the spiral phase plate and q-plate are both fabricated on silica substrates, suggesting the potential possibility to integrate the two structures on a single plate. Based on a compact method of transmissive-type transformation, our scheme may have potential applications in future integrated optical devices.

Generation of Laguerre-Gaussian beam from end-pumped and c-cut Nd:YVO_4 laser

We demonstrate an end-pumped, c-cut Nd：YVO4 laser that emitted first-order Laguerre-Gaussian （LGol） beam by adjusting the position of focused pump beam relative to laser crystal. The pumping light reached the laser crystal has circular and solid intensity profile. The laser is compact and stable, and the obtained LG01 beam power reaches 202 mW with -25% slope efficiency.

Generation of cylindrical vector beams in a mode-locked fiber laser using a mode-selective coupler

We experimentally obtain cylindrical vector beams（CVBs） in a passively mode-locked fiber laser based on nonlinear polarization rotation. A mode-selective coupler composed of both a single-mode fiber（SMF） and a twomode fiber（TMF） is incorporated into the cavity to act as a mode converter from LP01 mode to LP11 mode with broad spectral bandwidth. CVBs in different mode-locked states including single-pulse, multi-pulse, and bound pulse are obtained, for the first time to our best knowledge. The ultrafast CVBs with different operation states have potential applications in many fields such as laser beam machining, nanoparticle manipulation, and so on.

Flexible and high-efficiency generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere

We propose theoretically and verify experimentally a compact optical configuration to directly generate arbitrary vector vortex beams on a hybrid-order Poincare sphere with good flexibility and high efficiency based on a reflective phase-only liquid crystal spatial light modulator （LC-SLM）. The conversion system, consisting of an LC-SLM and a quarter-wave plate, can be considered a flexible dielectric metasurface to simultaneously modulate inhomogeneous polarization and helical phase-front. This approach has some advantages, including a simple experimental setup, good flexibility, and high efficiency. superposition existing in the conventional method are potential applications in many advanced domains. Orthogonally polarized modes alignment and an explicit not necessary in the proposed method, which exhibits

Generation and propagation of partially coherent vortex beams

The generation and propagation of partially coherent vortex beams have been investigated experimentally.It is found that a vortex beam with more uniform intensity distribution can be obtained by decreasing the coherence of the beam.We find that the beam shape of a completely coherent beam will keep hollow profile invariant during its propagation in free space.However,the center of the beam turns to be blurred for a partially coherent vortex beam on propagation.It is also found that the beam spreads more rapidly for the vortex beam with lower coherence,and this influence can be decreased by increasing the topological charge of the vortex beam.

High power generation of adaptive laser beams in a Nd：YVO4 MOPA system

We report efficient power scaling of the laser output with an adaptive bemn profile from an Nd：YAG dual-cavity master oscillator using a three-stage end-pumped Nd：YVO4 amplifier. We succeed ill the last switching of an excited laser mode by modulating an acousto-optic modulator loss in a dual-cavity master oscillator, thereby achieving temporal modulation of the output beam profile. The outputs from the master oscillator are amplified via a three-stage power amplifier yielding 36.6, 40.5, and 45.4 W of the maximum output at 116.8 W of incident pump power for the transverse electromagnetic, Laguerre-Gaussian, and quasi-top-hat beam, respectively. The prospects for further power scaling and applications via the dual-cavity master-oscillator power-amplifier （MOPA） system are considered.

Forward acceleration and generation of femtosecond megaelectronvolt electron beams by an ultrafast intense laser pulse

We present a new mechanism of energy gain of electrons accelerated by a laser pulse. It is shown that when the intensity of an Tiltrafast intense laser pulse decreases rapidly along the direction of propagation, electrons leaving the pulse experience an action of ponderomotive deceleration at the descending part of a lower-intensity laser field than acceleration at the ascending part of a high-intensity field, thus gain net energy from the pulse and move directly forward. By means of such a mechanism, a megaelectronvolt electron beam with a bunch length shorter than 100 fs could be realized with an ultrafast (≤30 fs), intense (≥1019 W/cm2) laser pulse.

Polarization evolution of vector beams generated by q-plates

The polarization evolution of vector beams(VBs) generated by q-plates is investigated theoretically and experimentally.An analytical model is developed for the VB created by a general quarter-wave q-plate based on vector diffraction theory.It is found that the polarization distribution of VBs varies with position and the value q.In particular,for the incidence of circular polarization,the exit vector vortex beam has polarization states that cover the whole surface of the Poincarésphere,thereby constituting a full Poincarébeam.For the incidence of linear polarization,the VB is not cylindrical but specularly symmetric,and exhibits an azimuthal spin splitting.These results are in sharp contrast with those derived by the commonly used model,i.e.,regarding the incident light as a plane wave.By implementing q-plates with dielectric metasurfaces,further experiments validate the theoretical results.

High Azimuthal Index Doughnut Beam Generated by Liquid Crystal Spiral Phase Plates

A novel technique of producing azimuthal index 1>1 doughnut beam of good quality by use of multi-liquid crystal cells is presented, it has the advantages of high conversion efficiency and flexibility.