中文摘要：

实验研究了双色超快强激光场作用于氮气分子束所产生的宽带太赫兹（THz）辐射光谱随等离子体介质的密度和长度的依赖关系,发现THz辐射的中心频率随等离子体密度提高和长度减小而增大（0.8—1.4 THz）,且谱宽也随之增加（0.78—1.53 THz）.分析和计算表明,太赫兹光谱的变化由等离子体振荡频率和谱宽决定.该发现为等离子体宽带太赫兹辐射源的光谱操控提供了新思路.

英文摘要：

The terahertz（THz） radiation becomes an attractive light source utilized in molecular dynamic spectroscopy,remote sensing,medicine,communication and fundamental research.The controlling of the THz spectrum is necessary for the applications.In this paper,a method is proposed for controlling the terahertz spectra generated from the laser induced plasma by two-color pluses based on the contribution of plasma oscillation.The plasma current oscillation can shift the THz spectrum when the length of medium is less than plasma skin depth.Experimentally we use a short length of molecules by means of the molecular beam method.We investigate the changing spectrum of broadband ultrashort terahertz THz generated from a jet of nitrogen（N_2） molecules pumped with the two-color laser pulses following the varying plasma density and plasma length.With the increase of plasma density and the decrease of the plasma length,we observe the increase of THz central frequency（0.8–1.4 THz） and the broadening of the THz spectral width（0.78–1.53 THz）.The analysis and the calculation show that the THz spectrum changes due to the frequency and the width of the plasma resonance.This scheme of controlling the THz spectrum by changing the plasma density and length is easier to implement and do not need to use complex shaped optical pulses.The discovery provides a new way of controlling the low-frequency broadband THz spectrum.