二氧化钒薄膜在光的照射下, 会从绝缘体相变为金属, 伴随有电导率的剧变, 该现象称之为光致绝缘体-金属相变。 二氧化钒薄膜的这种相变对于太赫兹波段的调制器或者其他功能器件有重要应用。 利用太赫兹时域光谱技术研究了二氧化钒薄膜光致绝缘体-金属相变前后的透射光谱变化, 分析了该薄膜的这种光致相变在太赫兹波段的特性。 实验中二氧化钒薄膜相变成金属的过程分别通过连续激光照射和飞秒激光照射实现。 两种照射方式下, 均观察到了明显的太赫兹波形变化, 并且随着照射光功率的增大, 信号的幅度衰减以及时域波形畸变逐渐加剧。 进而通过对透射太赫兹时域信号的傅里叶变换光谱分析发现, 在照射光功率增加时, 不但该薄膜的透射光谱幅值在下降, 而且其谱线形状也在随之改变, 其原因为二氧化钒薄膜的色散特性在光照条件下逐渐趋向金属性所致。 为清晰的描述光致相变的色散特点, 用二氧化钒薄膜光照前后透射光谱的幅度差定义了透射率调制函数来描述上述现象。 在透射率调制函数曲线上, 能够明显的看出二氧化钒的这种光致相变在太赫兹波段具有强烈的频率相关性质, 并且随照射光功率变化呈规律性演化。 进一步对比发现, 虽然连续光和飞秒激光照射方式都能引发光致相变, 但在同样透射光谱情况下, 对应的激发光功率存在明显不同, 对这两种照射方式下的相变效率差别进行了分析和讨论。

Vanadium dioxide(VO2) film will be phase-transitioned from insulator into metal, accompanied with dramatic change on conductivity, which is named as photo-induced insulator-metal phase transition. Such phase transition of VO2 film has important application potentials in modulators or other functional devices for terahertz waves. In this paper, the transmission spectrum variations before and after the photo-induced insulator-metal phase transition of vanadium dioxide film are investigated, and the phase transition properties in terahertz(THz) region are analyzed. In the experiment, the phase transition of the VO2 film was induced by a continuous wave(CW) laser source and a femtosecond(fs) laser source, respectively. Obvious changes on the THz waveforms were observed for the both mentioned means of excitation, and the amplitude attenuation, as well as the signal distortion, was intensified with the increase of the impinging optical power. The fast Fourier transform(FFT) spectra of the transmitted THz time-domain signals were analyzed and it was found that the amplitude of the transmitted spectrum decreased synchronously with the increase of the optical power, accompanied with deformation of the spectrum line shape at the same time. The reason was that the macroscopic dielectric properties of the VO2 film approached gradually to that of a metal as laser power was increased. A parameter, transmission modulation function, was defined in the paper as the amplitude difference between the transmission spectra of the VO2 film before and after the laser excitation, to describe the dispersivity of the photo-induced phase transition more clearly. From the curve of the transmission modulation function, strong frequency-dependent properties at THz frequencies were found to vary regularly with the incident light power. After furthermore comparison, it was found that, though the insulator-metal phase transition could be trigged by both CW laser source and fs laser source, the corresponding impinging optical power values were obviously alternative for the equivalent transmission modulation function. At the end of the paper, the difference of the phase transition efficiency between the two excitation methods was analyzed and discussed.