在飞秒激光与固体靶相互作用中,利用光学CCD相机和光学多道分析仪(OMA),分别在固体薄膜靶背表面法线方向测量了渡越辐射(TR)积分成像图案和渡越辐射光谱。测量结果显示,渡越辐射光斑呈现圆盘状结构,在圆盘中明亮而强的光信号呈局部化分布,并且有分离的光斑出现。该现象表明,超热电子在输运过程中存在成丝效应,引起严重的不稳定性; 渡越辐射光谱在800 nm附近出现了尖峰,是基频波(ω0),并且光谱向红光方向移动; 基频波产生的原因归结于超热电子束在传输过程中产生的微束团而引起的相干渡越辐射(CTR); 光谱红移的原因是由于等离子体临界面的迅速膨胀所致; 渡越辐射光强随靶厚度的增加而减小。

For studying transport of hot electrons in solid targets by transition radiation (TR), the image pattern of spatial distribution and spectrum of optical emission were measured at the normal direction from the rear side of targets employing optical charge coupled device (CCD) camera and optical multi-channel spectrometer apparatus (OMA) on the 100 TW femtosecond laser facility. The image pattern of spatial distribution presents a tray-shape and in the tray-shape there is a bright localized signal, which shows filament effect during hot electron transport. The spectrum of optical emission presents a sharp peaks in the wavelength near 800 nm, which is attributed to the one-order harmonic (ω0). The production of one-order harmonic is ascribed to the coherent transition radiation (CTR) generated by microbunches produced in the transporting of hot electron beams. The red shift is caused by the expansion of the critical density surface. With the increase of the laser energy, the peak of the CTR moves further to the long wavelength side. Intensity of transition radiation decreases with the increase of the target thickness.