实时多次取样的超快太赫兹传感/成像技术研究

Up to date, terahertz time-domain spectroscopy (THz-TDS) has been widely applied in a variety of fields,e.g.biomedical imaging, detection- security,quality control of food and so on, except for real-time sequential multi- samplings for imaging/sensing due to its utilizing pumping-probing technology. This project focuses on real-time sequential multi-sampling ultrafast terahertz imaging/sensing, which is of the extension and development of our about 20-year research work both in ultrafast (nonlinear) optics and in pulse terahertz imaging and sensing fields. This work will begin from developing a novel design of electro-optic(EO) sampling with high terahertz optical modulation, large dynamical range and strong ability to conceal background noises, which shall be very helpful for the successive experiments of this project with high signal-noise ratio. Then the project will realize ultrafast sequential multi-samplings for terahertz imaging/sensing in a creative way, called parallel EO sampling by multi-color pulses (PEOS-MCP). In PEOS-MCP, some multi-color pulses probe simultaneously the terahertz pulses by use of the same one electro-optic (EO) sensor. After passing through the EO sensor, the multi-color pulses propagate seperatedly by a spatial dispersion (imaging) system and detected by corresponding receivers. As a result, it can avoid effectively both the restraint of the response time of the receivers upon the sampling separations and the need of the temporal synchronization among the receivers thereby simplify the configuration but keep high temporal resolution and high sampling frequency. The terahertz pulses propagate co-axially, and their tempoarl separations from one to next can be tunned from serveral tens of picoseconds to nanoseconds which match those of the corresponding multi-color probing pulses, so our setup can operate effectively for quite wide temporal span. Another important creative work in the project is to probe the co-axial terahertz pulses by a chirped pulse which, correspondingly, is received by spectrometers instead of the detectors, e.g. photodiodes. According to this design, one can finish the ultrafast real-time sequential multi-sampling for terahertz sensing in a sigle shot mode, which is necessary in some applications, e.g.the diagnoses in ICF experiments. Because THz-TDS can not only capture both the amplitude and phase information of terahertz fields, to record the evolution of an ultrafast dynamical event by sequential multi-samplings basing on THz-TDS will not only provide accurate characterizations with high sensivitive phase-imaging/sensing but also reveal the dynamical spectroscopy of the ultrafast events, which is of cutting-edge work, very significant not only from a physics viewpoint but also from engineering perspective.

太赫兹时间域光谱(THz-TDS)传感/成像技术因其泵浦-探针特性迄今还未涉足实时连续多次成像/传感领域。本项目首次提出发展实时多次取样的超快太赫兹成像/传感技术。基于约二十年在超快（非线性）光学和太赫兹电光取样技术方面的研究积累和技术优势，本项目以发展高调制度、强消背底噪声能力和宽动态范围的新型太赫兹电光取样技术为起点，创造性地利用多色激光脉冲串作为探针对脉冲间隔相应的太赫兹脉冲串实行平行电光取样，从而实现对超快事件的实时多次超高时间分辨的太赫兹传感/成像。另外，还将首次利用啁啾脉冲激光探针实现只需单个脉冲就能完成对超快事件的实时多次超高时间分辨的太赫兹光谱传感。利用THz-TDS能同时检测光场振幅和相位的特性对超快事件进行实时多次传感/成像，不但可满足超快相位事物动态演变过程的瞬时成像所需的高灵敏度需求,还可精确获得事件的太赫兹动态光谱特性，非常前沿且具有重要的科学研究和工程应用价值。

首先成功研制了新型的电光取样装置。实验证实该装置具有基于 45度光学偏置的电光取样系统相当的消背底噪声能力和动态测量范围，同时与近 0度光学偏置电光取样系统相当的光学调制度，从而大大提高了测量信噪比。进一步地，还发展出一种太赫兹脉冲的正交平衡单次电光测量技术。该技术无论从光学调制度、可测量的线性范围以及测量信噪比方面均比同类型传统测量技术有明显的提高（2倍以上）。在此基础上，还发展了二种适合于强太赫兹脉冲信号单次相干测量的新型电光取样技术：基于啁啾脉冲光谱干涉技术的强太赫兹时域光谱单次探测技术。这两种测量技术不再受制于传统的要求太赫兹信号引入相位调制需显著小于1的限制，而且实验证明其测量噪声也显著小于同类光谱全息干涉测量技术。其次，还利用多色脉冲串为探针实现对多个太赫兹传感脉冲进行平行电光取样并应用到对超快事件进行多时间点太赫兹成像研究。研究结果发现利用太赫兹波对激光激发产生等离子体成像的不可行性，其原因是受到太赫兹波衍射极限分辨的限制。于是，对激光激发产生等离子体超快事件进行分幅成像方案进行了修正调整，从实际出发，成功对该事件实现了高空间分辨的4幅实时分幅飞秒成像。在现有激光系统的条件下，实现了超快4幅摄影频率达5 Tfps的非共线光参量闲频光成像，成像的时间分辨约35飞秒，空间分辨约29微米。最后，我们还引进二步相移技术实现高稳定大范围超短光脉冲的时间光谱特性的检测技术。这为下一步结合光学全息干涉技术，发展出新型适合测量形状复杂的强太赫兹脉冲信号的相干检测技术奠定基础。