表面粗糙样品的太赫兹光谱参数提取方法研究

Study on Extraction Method of Terahertz Spectral Parameters of Rough Surface Samples

很多物质在太赫兹波段内的光谱参数具有指纹特征,这是太赫兹技术在安检等众多领域有所应用的基础。但是,目前常用的太赫兹时域光谱(THz-TDS)技术提取物质光学参数的Duvillaret算法,要求样品上下表面平行且充分光滑。然而在很多有潜力的实际应用场合中,尤其是对于固体样品,表面粗糙度不可避免,并且不能使用模具压片等实验室制样手段来保证其表面的平行光滑特性,由此需要一种不受样品表面粗糙度影响且更具有普适性的光学参数提取算法来增加太赫兹时域光谱技术在实际场合下的应用能力。分析了太赫兹波透射表面粗糙样品的光程,建立了包含有样品粗糙度、折射率和消光系数的表面粗糙样品太赫兹透射模型,在已知粗糙度及平均厚度的条件下,提取出在太赫兹波段有特征吸收的表面粗糙样品的折射率和吸收系数,验证了此透射模型的适用性。首先,选择在太赫兹波段有特征吸收的α-乳糖和L-组氨酸作为实验材料,制作了质量分数和密度相同的表面粗糙样品和光滑样品,并在太赫兹时域光谱系统中获取它们的太赫兹透射实验数据;然后,利用建立的透射模型从实验数据中提取出了表面粗糙α-乳糖和L-组氨酸样品的折射率和吸收系数;最后,为衡量透射模型提取结果的优劣性,把Duvillaret算法作为另一种光学参数提取的参照方法,将本模型算法、Duvillaret算法对表面粗糙样品的提取结果分别与从平行光滑样品中提取的标准结果进行比较,计算两种提取方式与标准值之间的均方根误差(RMSE)。比较结果表明,相比于Duvillaret算法,本透射模型方法所提取出的表面粗糙样品的折射率和吸收系数与标准值之间偏差更小,在一定程度上减少了粗糙度对提取结果的影响,具有更高的精确度。因此,在已知表面粗糙样品的粗糙度以及平均厚度的条件下,利用此模型可较为精确地在太赫兹时域光谱系统中提取出物质的光学参数。此研究结果将有力推进太赫兹波光学参数提取技术的实际应用。

The spectral parameters of many substances within the terahertz band have fingerprint characteristics,which is the basis of the application of the terahertz technology in many fields such as security inspection.However,it is required that the upper and lower surfaces of samples should be parallel and smooth for the Duvillaret algorithm,which is commonly used to extract the optical parameters of materials in terahertz time-domain spectroscopy(THz-TDS).Moreover,surface roughness is inevitable,and laboratory sample preparation methods such as mold pressing can not be utilized to ensure the parallelism and smoothness of the surface in many potential practical applications,especially for solid samples.Therefore,a universal optical parameter extraction algorithm that is not easily affected by the surface roughness of samples is needed to increase the application ability of THz-TDS in different situations.In this paper,the optical path of the transmission of the terahertz wave through rough samples is analyzed,and a terahertz transmission model of rough samples,including roughness,refractive index and extinction coefficient parameters,is established.Under the condition of known roughness and average thickness,the applicability of the transmission model is verified by extracting the refractive index and absorption coefficient of rough samples with characteristic absorption peaks in the terahertz band.-----Theα-Lactose and L-Histidine which have typical absorption features at terahertz frequencies are selected as experimental material.Pellets at the same concentration are prepared and further separated into the rough surface group and smooth surface group.The refractive index,as well as absorption coefficient,are calculated with both our proposed model and conventional Duvillaret algorithm.The comparison between the results is further analyzed for evaluating our method,the root-mean-square error(RMSE)between the two extraction methods and the standard value is calculated,and the extraction results of the transmission model,the Duvillaret algorithm and the standard value extracted from the parallel and smooth sample are compared at the same time.The comparison results show that,compared with the Duvilaret algorithm,the refractive index and absorption coefficient of the rough sample extracted by the transmission model method has smaller deviations from the standard value,which reduces the influence of roughness on the extraction result to a certain extent and has higher accuracy.Therefore,under the condition of known roughness and average thickness of the surface rough sample,this model can be used to more accurately extract the optical parameters of the substance in the terahertz time-domain spectroscopy system.The research results will greatly promote the practical application of terahertz optical parameter extraction technology.