Optically induced abnormal terahertz absorption in black silicon

The absorption responses of blank silicon and black silicon(silicon with micro/nano-conical surface structures) wafers to an 808-nm continuous-wave(CW) laser are investigated at room temperature by terahertz time-domain spectroscopy. The transmission of the blank silicon shows an appreciable change, from ground state to the pump state, with amplitude varying up to 50%, while that of the black silicon(BS) with different cone sizes is observed to be more stable. Furthermore,the terahertz transmission through BS is observed to be strongly dependent on the size of the conical structure geometry.The conductivities of blank silicon and BS are extracted from the experimental data with and without pumping. The non-photo-excited conductivities increase with increasing frequency and agree well with the Lorentz model, whereas the photo-excited conductivities decrease with increasing frequency and fit well with the Drude–Smith model. Indeed, for BS, the conductivity, electron density and mobility are found to correlate closely with the size of the conical structure.This is attributed to the influence of space confinement on the carrier excitation, that is, the carriers excited at the BS conical structure surface have a stronger localization effect with a backscattering behavior in small-sized microstructures and a higher recombination rate due to increased electron interaction and collision with electrons, interfaces and grain boundaries.     

基于THz光谱成像的纸币真伪鉴别研究

根据纸币防伪特征区域载体材料与制作工艺的不同,利用透射式太赫兹脉冲成像技术对2005版真伪百元人民币的白水印和安全线两处防伪特征区域进行逐点扫描获取相应的太赫兹时域信号,然后对太赫兹波形进行数据处理获得时域显示模式成像及频域显示模式成像。将真钞和假钞的成像结果进行对比,发现白水印、安全线部位的真假钞成像结果区别明显,证实太赫兹脉冲成像技术能够真实、有效地对真假人民币进行多区域、多维度的准确鉴定。