基于GaP:N晶体体相位光栅的动态可调太赫兹滤波研究

Terahertz(THz) filters have widespread applications in terahertz radar and communication fields, etc. At present, the study on THz filter concentrates on the photonic crystal and superlattice, but its artificial microstructure did not adapt to normal dynamic adjustable properties. In this project, we intend to adopt the acousto-optical resonance effect of the GaP:N crystal. First of all, the characteristics of GaP:N crystal (physical, optical, elastic, lattice dynamics) will be analyzed by using the first principles method. Terahertz absorption and acousto-optic effect and phonon frequency of GaP:N will be studied via experimental detection method. Second, the physical process for the formation of TO-phonon polaritons will be studied by coupling between coherent ultrasonic and solid-state excitation under the condition of THz wave and ultrasonic collinear incident to GaP:N crystal. We will establish the dispersion equation and the mathematical relationship between the frequency and width of the gap with the frequency and power of ultrasonic. The characteristic of the dispersion curve and the physical mechanism of band gap formation will be clarified. Finally, we will establish the mathematical model for Bragg reflectivity of total and other orders(0、±1，±2) by without second-peaks of double-phonons reflection. The influence of intensity of acousto-optic coupling on the reflection spectrum will be analyzed, and the terahertz transmission spectrum will be simulated and verified by experimental. We try to propose a new technique to build a dynamic tunable terahertz filtering based on the volume phase grating by combining theory with experiment.

太赫兹雷达及通信等众多应用领域对太赫兹滤波器提出了迫切的需求。目前针对太赫兹滤波器研究主要集中于光子晶体和超晶格，但其人工微结构不满足于常态可调滤波技术。本项目拟采用以掺氮磷化镓晶体声光共振效应为切入点，首先通过第一性原理方法分析GaP:N晶体的物理、光学、弹性及晶格动力学特性，以实验探测方法研究其太赫兹吸收率、声光效应及声子频率；其次以太赫兹电磁波和超声波同向共线入射GaP:N晶体，研究相干超声波与固态激发耦合形成改性TO声子耦合场量子的物理过程，建立色散方程并分析其性质，阐明带隙形成的物理机制并建立带隙频率及宽度与超声波频率和功率的数学关系；最后建立不含双声子反射次峰的各（0、±1，±2）级及总Bragg反射率数学模型，分析声光耦合强度对反射谱影响，模拟并通过实验验证太赫兹透射谱。力图通过理论与实验相结合的方法，建立一种基于体相位光栅进行动态可调太赫兹滤波的新技术。

本项目利用超声波在GaP1-xNx晶体内产生的体相位光栅对THz波反射滤波进行了研究。（1）获得了具有宽滤波频谱的GaP0.75N0.25声光晶体。利用第一性原理计算了GaP1-xNx晶体光学性质、弹性系数、声光系数等参数，根据GaP1-xNx晶体声子振动关系，TO声子频率跟x成正比例关系，本项目选择GaP0.75N0.25晶体为声光晶体；根据声子共振关系获得低频带滤波范围为0.2THz-9THz，滤波频谱带宽比纯GaP晶体提高1.5THz，对应超声波频率为0MHz-200MHz；（2）建立GaP:N晶体TO声子耦合场量子色散关系。结合改性布拉格条件获得TO声子耦合场量子色散关系，随着超声波频率的增加，带隙频率出现蓝移现象，带隙宽度与超声波频率以及声光耦合强度均为正比例关系；（3）获得GaP:N晶体体相位光栅作用下太赫兹反射谱。获得了0级、-1级和-2级布拉格反射谱，在第一带隙位置，各级反射谱都会出现反射峰，谱峰随着带隙级别的增加而减小，获得滤波输出的太赫兹波为总布拉格反射波，反射峰随超声波频率增加而蓝移，反射谱峰宽度随带隙宽度增加而增加，跟超声波频率和耦合强度成正比例关系，声光耦合强度不影响反射谱频率，强度跟声光耦合强度成正比例关系，可调节超声波频率和强度实现可调太滤波。本研究为探讨可调太赫兹滤波器奠定了坚实的工作基础。项目资助发表SCI论文8篇，培养6名在读硕士研究生。