Focus Displacement Correction in Focused Beam Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) is becoming the standard method for the characterization of novel materials e.g. for photovoltaics or in sensing and security. Since THz-TDS is capable of measuring an electromagnetic field in amplitude and phase—contrary to measuring a mere intensity—both the refractive index and the absorption coefficient of materials can be quantitatively determined at the same time. A complex mathematical treatment of transmission spectra is not necessary.

A typical THz-TDS set-up consists of a THz generator, a beam path with a focus in which the sample is placed, and a gated THz detector. However, the insertion of the sample into the focused THz beam will induce a focus displacement on the THz detector and affect the accuracy of the quantitative determination of the dielectric function of the sample.

The group led by Prof. Thomas Dekorsy, from the University of Konstanz (Germany), investigated the defocusing influence on the quantitative determination of the refractive index and the absorption coefficient in focused THz-TDS setup based on asynchronous optical sampling (ASOPS) system. They demonstrated theoretical and experimental procedures to reduce the errors induced by the focus displacement. The results are reported in Chinese Optics Letters, Vol. 13, No. 9, 2015 (Q. Liang et al., Origin of potential errors in the quantitative determination of terahertz optical properties in time-domain terahertz spectroscopy).

The propagation of the THz beam in the focused beam THz-TDS setup is described using the Gaussian beam and the ABCD method. In this way, a theoretical correction of the quantitative determination is obtained. A precise extraction of the refractive index and the absorption coefficient from the experimental data is achieved using a correction procedure.

"Since the accuracy and metrology in the THz frequency range becomes more and more important, as time-domain THz analysis is becoming a standard tool in advanced material characterization, all effects influencing quantitative data analysis are becoming more relevant." says Prof. Thomas Dekorsy.

Future work will concentrate on generating accurate data base values of THz properties of different materials of technological relevance.

太赫兹时域光谱技术中光束聚焦时的焦点偏移修正

太赫兹时域光谱技术（THz-TDS）在新型材料表征（如光伏材料）、物质探测以及安全检测方面变得愈发重要。与一般只探测光强的测量方法不同，这种方法可以同时测量振幅和相位，进而能够计算出物质的折射率和吸收系数，而不再需要复杂的数学处理方法。

典型的THz-TDS装置由太赫兹产生器，在样品位置聚焦的太赫兹光路以及选通太赫兹探测器组成。然而在聚焦的太赫兹光束中放入具有一定厚度的样品会引起光束焦点的偏移，进而影响样品介电函数的计算准确性。

德国康斯坦茨大学的Thomas Dekorsy教授课题组利用基于异步光脉冲取样系统（ASOPS）的聚焦THz-TDS装置研究了焦点偏移对定量计算样品折射率和吸收系数的影响，并提出了修正焦点偏移所引起的定量计算误差的方法。相关研究成果发表在 Chinese Optics Letters 2015年第13卷第9期上（Q. Liang et al., Origin of potential errors in the quantitative determination of terahertz optical properties in time-domain terahertz spectroscopy）。

他们利用高斯光束和ABCD矩阵传输理论描述太赫兹光束在聚焦THz-TDS装置中的传播，对计算误差进行了理论修正。另外，他们也在实验方面对测量方法进行了修正，从而获得了比一般实验方法更为精确的样品折射率和吸收系数。

Thomas Dekorsy教授说道：“随着THz-TDS逐渐成为表征先进材料的标准测量手段，太赫兹波段的测量和准确度变得越来越重要，因此需要更全面地探究所有能对实验数据的定量分析产生影响的因素。”

后续工作将集中在建立与太赫兹技术相关材料的精确太赫兹特性数据库。