基于连续波太赫兹频域光谱系统的样品折射率测定方法

Refractive Index Measurement Using Continuous Wave Terahertz Frequency-Domain Spectroscopy

经过二十年的发展,太赫兹光谱技术已经在生物医疗、工业质检、国防安全、新材料研发等多个领域证明了其强大的应用潜力,然而传统太赫兹时域光谱技术(THz-TDS)对飞秒激光器的依赖,严重限制了太赫兹技术在实际工业领域中的应用,近年来,以商业成熟的中红外激光混频作为产生与探测机制的太赫兹频域光谱技术(THz-FDS),凭借其高集成度、低成本以及高分辨率等适用于实际应用的特点得到了广泛的研究关注。但是,作为新型光谱技术,THz-FDS的数据处理算法与传统THz-TDS相比仍处于起步阶段,此前所开展的物质表征研究大多局限于信号幅值的利用,而其相干探测机制所蕴含的太赫兹波相位信息并未得到充分挖掘,与相位信息紧密相关的样品折射率、介电常数、极化率等关键参数因此无法准确获取。从THz-FDS的相干探测原理出发,结合实测数据,详细阐述了太赫兹波相位信息与THz-FDS原始光电流数据周期振荡之间的联系,建立了由原始光电流数据振荡周期求取样品折射率的理论模型。在此基础上,指出了传统零频基点求取折射率方法在低频段表现欠佳的原因,提出了所改进的双基点折射率求取算法。为了验证所提出的折射率测定方法的可靠性,选取了在太赫兹波段应用广泛的聚合物聚四氟乙烯作为表征对象,制备了厚度不同的多个样片,对所有样品测定折射率的结果为1.456±0.006,不同样片间折射率的测量不确定度仅为0.5%,且平均值与前期报道值吻合,充分验证了该方法复现性。此外,选取其中一枚样片考察了实验参数设置变化对折射率测定结果的影响,使用THz-FDS多种积分时间和采样步长的设置组合采集原始光电流数据,结果显示实验参数设置对折射率测定结果没有显著影响,验证了算法的鲁棒性。提出的折射率测定方法丰富了太赫兹频域光谱系统的表征参数,结合频域光谱系统相比传统时域光谱系统在工业应用方面的优势,该方法对太赫兹光谱技术的实际应用发展具有重要意义。

In the past two decades,terahertz spectroscopy has been proven to be a powerful tool potentially applied in biomedical research,industrial quality inspection,national defence,and public security.However,the dependence of traditional Terahertz time-domain spectroscopy(THz-TDS)on femtosecond lasers seriously limits the application of THz technology in practical industrial fields.Recently,terahertz frequency-domain spectroscopy(THz-FDS)based on the photo-mixing mechanism of two commercially available mid-infrared lasers has been widely studied as an alternative due to its better compactivity,lower cost,and outperformed frequency resolution.However,the data processing algorithm for THz-FDS is not comparable to the traditional THz-TDS.Previous spectroscopic studies using THz-FDS generally focus on the absorption spectra of target material extracted from the signal amplitude.Nevertheless,the phase information of the THz wave provided by the coherent detection mechanism has not been well explored.Hence,various valuable parameters such as refractive index,dielectric constant,and polarizability closely related to the phase information cannot be accurately obtained.In this paper,the relationship between the phase information of the THz wave and the periodic oscillation of the FDS original photocurrent data is well explained,accompanied by presenting actual measurement data.A theoretical model is then constructed to calculate the refractive index spectra from the original photocurrent data oscillation period.The primary reason the previous DC-basepoint algorithm could not output the correct refractive index over the low-frequency band is then pointed out.We propose an improved dual-basepoints index algorithm capable of extracting accurate refractive index spectra over the whole spectrum.In order to verify the reliability of our proposed method,polytetra fluoroethylene,a polymer widely used in the THz band,was selected as the characterization object.Multiple samples with different thicknesses were prepared.The refractive index of all those samples was 1.456±0.006.The measurement uncertainty of the refractive index between different samples is only 0.5%,and the average value is consistent with the previously reported value,which thoroughly verifies the reproducibility of this method.In addition,one sample was selected to investigate the influence of the experimental parameter setting on the refractive index measurement results.The original photocurrent data were collected on the same sample with THZ-FDS using various integration times and frequency step-size combinations.The results show that the experimental parameter setting does not significantly affect the refractive index measurement results,which verifies the algorithm’s robustness.The refractive index measurement method proposed in this paper explored the characterization parameters of the terahertz frequency domain spectral system.Therefore,this method is of great significance to the practical application and development of terahertz spectral technology.