Preparation of graphene on SiC by laser-accelerated pulsed ion beams

Laser-accelerated ion beams (LIBs) have been increasingly applied in the field of material irradiation in recent years due to the unique properties of ultra-short beam duration, extremely high beam current, etc. Here we explore an application of using laser-accelerated ion beams to prepare graphene. The pulsed LIBs produced a great instantaneous beam current and thermal effect on the SiC samples with a shooting frequency of 1 Hz. In the experiment, we controlled the deposition dose by adjusting the number of shootings and the irradiating current by adjusting the distance between the sample and the ion source. During annealing at 1100 ℃, we found that the 190 shots ion beams allowed more carbon atoms to self-assemble into graphene than the 10 shots case. By comparing with the controlled experiment based on ion beams from a traditional ion accelerator, we found that the laser-accelerated ion beams could cause greater damage in a very short time. Significant thermal effect was induced when the irradiation distance was reduced to less than 1 cm, which could make partial SiC self-annealing to prepare graphene dots directly. The special effects of LIBs indicate their vital role to change the structure of the irradiation sample.     

Generating Proton Beams Exceeding 10 MeV Using High Contrast 60TW Laser

A prototype of a laser driven proton accelerator is built at Peking University. Protons exceeding 10 MeV are accelerated from micrometer-thick aluminum targets irradiated by tightly focused laser pulse with 1.8 J energy and 30 fs duration. The beam energy spectrum and charge distribution are measured by a Thomson parabola spectrometer and radiochromic film stacks. The sensitivity of proton cut-off energy to the focusing of the laser beam, the pulse duration, and the foil thickness are systematically investigated in the experiments. Stable proton beams have been produced with an optimized parameter set, providing a cornerstone for the future applications of laser accelerated protons.     

相位光栅分离双波长宽角度光线的理论分析

使用衍射光栅的分色分光方式一般以平行光入射情况为讨论基础,对于非平行光入射的情况讨论较少.本文基于菲涅尔衍射理论和角谱理论,将球面波与平面波的衍射波场联系起来,从而在传播函数中引入角度参量,结合分数泰伯效应的理论基础,推导出双波长宽角度入射光线经相位光栅衍射后的波场分布函数,并对推导出的函数进行数值模拟,得到像面不同位置衍射波场分布.与平行光入射时的标准波场分布相比较,得到宽角度入射时的衍射波场的横向展宽量和偏移量.通过调节光栅台阶的宽度,改变衍射场的展宽和偏移,使各个单一波长的衍射波场宽度小于光栅周期的一半,从而减少双波长光衍射波场的混叠.同时本文给出波场宽度与光栅台阶宽度的变化关系,选取光栅面上多个位置作为台阶宽度的计算点,并对整个光栅的台阶宽度进行曲线拟合,得到可以使双波长宽角度入射光实现良好空间分离的光栅参量.该结果可用于各类宽角度入射光线的光谱分离场合,如双波长成像、液晶显示和液晶投影等.     

一款小型化可见光两档7.5×变焦系统的设计

许多应用都期望以大视场搜索定位目标，以小视场瞄准跟踪目标。提出并设计一款小型化两档7.5×变焦距成像系统，其工作于486 nm～656 nm的可见光波段，短焦视场为±15°，长焦视场为±2°，两视场下F数均为2.8，系统总长仅为60 mm。系统由玻璃镜片及注塑镜片混合组成，包含一个前固定组和一个可动组，通过可动组在2个变焦位置间的切换完成广角搜索功能和长焦凝视功能的转换，具有小型化、轻量化、低成本的优势。     

Analysis of electromagnetic pulses generated from ultrashort laser irradiation of solid targets at CLAPA

Electromagnetic pulses(EMPs)produced by the interaction of a TW femtosecond laser with solid targets at the Compact Laser Plasma Accelerator(CLAPA)are measured and interpreted.The statistical results confirm that the intensities of the EMPs are closely related to both target material and thickness.The signal of the titanium target is more abundant than that of the copper target with the same thickness,and the intensity of EMP is positively correlated with the target thickness for aluminium foil.With the boosted EMP radiations,the energy of accelerated protons is also simultaneously enhanced.In addition,EMPs emitted from the front of the target exceed those from the rear,which are also pertinent to the specific target position.The resonant waveforms in the target chamber are analyzed using the fast Fourier transform,and the local resonance and the attenuation lead to changes of the frequency spectra of EMPs with variation of detecting positions,which is well supported by the modeling results.The findings are beneficial to gaining insight into the mechanism of EMP propagation in a typical target chamber and providing more information for EMP shielding design.     

长波红外与激光共孔径双模导引光学系统研究

为提高导引结构的特征分辨能力和全天候工作能力,提出一种长波红外与激光共孔径的双模导引光学系统设计方案,利用被动红外模块搜索目标,通过主动激光雷达模块锁定目标并精确制导。为解决导引头内光学系统尺寸受限的问题,以Ritchey-Chretien结构为共用部分,通过次镜镀分光膜实现长波红外（8～12μm）反射光路与激光（1.064μm）透射光路的组合,并分析了不同光学遮拦情况对非相干成像系统调制传递函数衍射极限的影响。展示了F数为0.98、光学遮拦比为1/3的共孔径双模导引系统的实例,使用多片折射镜片实现对主、次镜残余像差的补偿,利用光学被动式消热差方法完成-40～60℃范围的长波红外无热化,具有良好的热稳定性和可加工性,可为双模导引光学系统的分析与设计提供参考。     

微小球面显微光学系统设计与照明分析

机器视觉检测技术具有非接触、高精度的优点,正在逐渐取代人工检测,成为工业生产检测的重要一环,作为前端的光学系统是机器视觉检测技术的核心部分。文中针对半径10 mm以内的光滑球体表面检测设计了一款双远心光学成像系统。系统由7片镜片和1片滤光片组成,采用了2个非球面校正像差,调制传递函数曲线在40 lp·mm^-1处大于0.3,具有良好的成像性能。由于该系统光圈较小且待测物为类镜面反射材料,设计完成后进行了照明模拟,选择采用多个面光源对待测物进行照明。模拟结果表明,文中设计的双远心光学系统实现了对较大曲率表面成像,并给出了一种配套的照明方式。     

LED背光源中侧发光导光管长度与出光性能的关系

微细侧发光导光管可以将LED点光源转化成线光源.为了同时发挥LED光源和CCFL光源的优势,本文对导光管模型进行了改进,使不同长度导光管均能满足良好的出光性能.在网点公式基础上引入非线性修正参数α,并找到导光管长度与α的对应关系,得到不同长度导光管的优化网点排布形式.针对较短导光管光能利用率偏低的问题,采用导光管一端入...     

用于激光周向探测的发射光学系统

为了提高激光周向探测中出射光场的均匀度，增加其有效作用区域，本文提出了一种采用圆锥型视场的前倾探测方案。基于六象限分区的布局方式，该系统利用非球面透镜对激光二极管的输出光束在子午和弧矢两个方向同时进行准直，通过鲍威尔棱镜实现弧矢方向扩束匀光，采用偏转棱镜保证光束前倾角为60°，并借助弧矢方向上的柱面镜进行完整的圆锥型视场拼接。在不同距离处的模拟仿真结果表明，系统在子午方向上出射视场角为±0.75°，在弧矢方向上可以覆盖360°视场完成周向探测。系统出射光场在四个目标平面上的照度均匀度均大于90%，能量利用率可达98%以上。该系统结构紧凑，整体仅由三片透镜构成，实现了一体化设计，为激光周向探测系统中的光学设计提供了参考。