基于太赫兹时域光谱系统的双折射率晶体非接触式表征方法

Non-Contact Characterization Method of Birefringence Crystal Based on Terahertz Time-Domain Spectroscopy

晶体的各向异性探测技术正逐渐向非接触和非破坏性方法发展。太赫兹辐射由于其对许多介电材料的大穿透深度和非电离特性,在各向异性材料的双折射研究中具有广阔的前景。石英、蓝宝石、液晶以及含有亚波长结构的超材料等都表现出太赫兹双折射,作为偏振功能器件中的常用材料,其参数测量对于太赫兹器件研发具有重要意义。材料双折射率的提取往往依赖光轴方向和晶体厚度等先验参数。材料的光轴方向表征其各向异性的优先方向,对于已知光轴取向的晶体,可以根据经验选择合适的琼斯向量,而在实际应用中,只要选择探测太赫兹波的线偏振方向、光轴、探测轴。可以很容易地测量出寻常光和非常光,并直接从时域信号中计算出各自的折射率。对于光轴方向未知的材料,需要旋转样品以不同的方位角进行测量。双折射率的提取依赖于材料厚度的确定,通过游标卡尺或千分尺所得测量值与真实值存在较大差异,容易在样品表面造成划痕。无论是旋转样品还是厚度测量,人为操作都会为双折射率表征引入不确定性。基于透射式太赫兹时域光谱系统(THz-TDS)开发了一种双折射晶体光轴方向和晶片厚度的非接触式测量方法,不依赖于晶体先验参数,即可获取晶体完整的折射率。通过控制样品旋转配合光学延迟线动作实现光轴自动定位,利用传递函数迭代逼近算法提取晶体厚度和完整的折射率。为验证该方法,选择了在太赫兹波段具有双折射特性的(10-10)取向蓝宝石晶体进行测量,准确地提取了晶体在0.3~1.5 THz频率范围的平均寻常光折射率3.39±0.02,非常光折射率3.08±0.02,双折射率-0.31±0.02,并绘制了吸收谱。结果证明,所建立的测量方法避免了人工操作引起的样品损坏和定位误差,提高了双折射晶体太赫兹频段材料参数的测量速度、稳定性和精确度,对偏振敏感太赫兹测量技术及其应用具有重要意义。

The anisotropic detection technology of crystals is gradually developing towards non-contact and non-destructive methods.Terahertz radiation has broad prospects in studying birefringence of anisotropic materials due to its large penetration depth and non-ionization characteristics for many dielectric materials.Quartz,sapphire,liquid crystal,and metamaterials containing sub-wavelength structures exhibit terahertz birefringence.As a common material in polarization functional devices,the parameter measurement is of great significance for developing terahertz devices.The extraction of material birefringence often depends on previous knowledge,such as optical axis direction and crystal thickness.The material's optical axis direction characterizes its anisotropy's preferred direction.The appropriate Jones vector can be selected according to experience for crystals with known optical axis orientation.In practical applications,only the terahertz wave's linear polarization direction,optical axis,and detection axis can be selected.It is easy to measure the ordinary and extraordinary light and calculate their refractive index directly from the time domain signal.For materials with unknown optical axis direction,it is necessary to rotate the sample to measure in different orientations.In addition,the extraction of birefringence depends on the thickness of the materials.The measured value obtained by vernier caliper or micrometer is quite different from the true value,and it is easy to cause scratches on the sample's surface.At the same time,whether it is sample rotating or thickness measurement,human operation will introduce uncertainty for birefringence characterization.Based on the Terahertz time-domain spectroscopy(THz-TDS),a non-contact measurement method for the optical axis direction and the thickness of the birefringent crystal is developed in this paper.The complete refractive index properties of the crystal can be obtained without relying on the crystal's prior parameters.The automatic positioning of the optical axis is realized by controlling the rotation of the sample and the action of the optical delay line.The iterative approximation algorithm of the transfer function is used to extract the crystal thickness and complete refractive index information.To validate our method,the(10-10)oriented sapphire,which exhibits birefringence at terahertz frequencies,was selected.We extracted the average extraordinary and ordinary refractive indices of sapphire in the frequency range of 0.3~1.5 THz,which is 3.08±0.02 and 3.39±0.02,respectively.The birefringence is-0.31±0.02,and the absorption spectrum was plotted.The results show that our method avoids sample damage and positioning errorscaused by manual operation and improves the efficiency,stability,and accuracy of terahertz birefringence extraction.It is significant to the polarization-sensitive terahertz measurement technology and its application.