Effects Of Sample’s Unconventional Morphology On Terahertz Transmission Spectra

Terahertz spectroscopy technology shows great potential and value in public security inspection and non-destructive testing based on coherence,penetrability,and low-energy photon characteristics.However,in the current terahertz spectroscopy test,the solid-state material is required to be made into a standard shape with a parallel and smooth surface.On the contrary,samples with unconventional morphology are inevitable,and sample preparation methods cannot be performed,especially in practical situations such as security inspection.Therefore.the sample’s morphology will hinder the application process of terahertz spectroscopy.The samples with common unconventional morphology are divided into regular shape and irregular shape samples in this paper.The optical parameters extraction and terahertz spectral transfer function of irregular shape samples with a rough surface and regular shape samples with concave features on the surface is studied.The main contents and innovations of this paper are as follows:(1)In the study of the Gaussian random rough surface influence on terahertz transmission spectra,a model based on the Fresnel transmission process is established.In this model,the rough surface is divided into microelements,and the model value is obtained by rough surface with known morphological parameters.The model’s transmission coefficient and phase probability distribution are analyzed,and the mechanism for terahertz wave amplitude attenuation caused by frequency,roughness,and correlation length are summarized.Above all,the quantitative relationship between the ratio of roughness and correlation length and the distortion degree of the spectral transfer function is obtained.Polytetrafluoroethylene without absorption characteristics within the terahertz band is selected as the experimental material,and the experimental results confirm the accuracy of the model and related conclusions.(2)In the study of extracting the optical parameters of Gaussian random rough surface samples in terahertz transmission spectra,surface roughness is introduced,and we establish a stepped terahertz transmission model which makes terahertz wave path associated with surface roughness.The model includes the average thickness,roughness,refractive index,and extinction coefficient.In the experiment,polytetrafluoroethylene without absorption characteristics and α-Lactose and Lhistidine with absorption characteristics within the terahertz band are selected as samples.The experiment results show that the model extraction algorithm can effectively reduce the influence of roughness.It can extract the sample’s refractive index and absorption coefficient accurately compared with the traditional algorithm under the condition of known roughness and average thickness.This research provides a new method to extract optical parameters of samples with a rough surface.(3)In the study of the influence of regular concave surface on terahertz transmission spectra,we establish a model according to the transmission process of terahertz wave on the concave surface based on Gaussian beam optics.The terahertz Gaussian beam parameters are measured by the small-aperture fitting method.In the experiment,polytetrafluoroethylene is selected as the experimental material.The theoretical model value of terahertz transfer function is compared with the experiment value to verify the model’s applicability.The necessity of treating terahertz wave as Gaussian beam in the case of defects on the surface is confirmed by experiments,and it is inferred that the quantitative influence of concave depth and concave radius on terahertz transmission spectra in the parallel and perpendicular propagation direction of the terahertz wave.On the one hand,this research is meaningful in non-destructive testing of surface defects of plastic products by terahertz spectra;On the other hand,it is valuable in designing the sample’s surface morphology to have a specific spectral transfer function.