基于探针光调制的皮秒分辨X-ray探测方法与实验

Picosecond resolving detection method and experiment for ultrafast X-ray by modulation of an optical probe

高能密度物理研究中涉及许多单次皮秒现象的诊断测量,然而对单次X-ray脉冲形状、X-ray与激光脉冲的皮秒精度同步依然是极具挑战的课题.传统行波选通分幅相机受电子渡越时间限制,难以突破40 ps时间分辨极限.本文围绕半导体中光学探针光的全光调制效应,提出一种以低温GaAs材料为基础,实现皮秒时间分辨X-ray探测的新方法,详细阐述了该探测器的工作机理、器件参数设计和时间分辨能力.通过飞秒激光打靶实验,验证了其概念设计的正确性.结果表明该探测器具有约1.5 ps时间响应和10 ps时间分辨能力,通过材料优化可将时间分辨提升至1 ps以内.

Diagnostic measurement of single picosecond event in high energy density physics,laser fusion,plasma radiation,and combustion,is of great importance.However,the measuring of the shape of the single X-ray pulse and the synchronization of X-ray and the laser pulse in picosecond resolution is still a great challenge.Restricted by the transit time of electrons,the time-resolution limit of a conventional framing camera based on the microchannel plate is 40 ps.Centered on the full-optical modulation effect of the light-probe,a novel method for X-ray detection of picoseconds temporal resolution based on low temperature GaAs is proposed in this work.The basic physical mechanism of the detector can be explained in both macroscopical and microcosmic ways.In the macroscopical way,the X-ray radiation absorption in the sensor material produces a transient,non-equilibrium electron-hole pair distribution that results in a transient differential change of the local refractive index,which is then sensed by the reflectivity changes of the optical probe beam.In the microcosmic way,X-ray absorption creates photoelectrons and the core level holes are subsequently filled through Auger or fluorescence processes.These excitations ultimately increase conduction and valence band carriers that perturb optical reflectivity.To verify the proposed X-ray detection method,a Fabry-Perot detector is designed,which consists of a 5（im thick GaAs layer surrounded by a GaAs/AlAs distributed Bragg reflector.The test is carried out on a femtosecond laser facility,where the X-ray source is produced by focusing the 56 fs Ti：Sapphire facility laser,with a central wavelength of 800 nm,onto an aluminum foil.Then the X-ray pulse induces a transient optical reflectivity change in GaAs,which is a powerful tool for establishing the high-speed X-ray detection.The experimental results indicate that this technology can be used to provide X-ray detectors with a temporal resolution of tens of picoseconds.By optimizing the material,the temporal resolution can be enhanced to be less than 1ps.Through further development,this X-ray detector could provide an insight into previously unmeasurable phenomena in many fields.Future work will focus on developing much faster devices characterizing both the rise and fall time and imaging array technology.