Collaborative Research: Novel Terahertz Generators Based on Magnetic Materials

频率接近1太赫兹的电磁波发生器是几种实用应用所需的，例如新的生物医学成像技术、高灵敏度的化学传感器和节能的无线计算机芯片。现有的太赫兹辐射发生器存在严重缺陷，严重限制了它们的用途。这些发电机要么在低于室温的温度下工作，要么基于昂贵而笨重的激光系统。该项目的目标是创造一种新型的太赫兹发生器，这种发生器结构紧凑，价格低廉，可以在室温下工作。这些发电机基于容易获得的磁性材料，如氧化铁和氧化镍，并将通过将这些材料中的磁振荡转换为太赫兹电磁波来运行。拟议研究计划的目标将通过磁性设备制造专家(加州大学欧文分校)和磁性设备领域的领先理论家(奥克兰大学)的协同努力来实现。拟议的研究计划的结果将以多种方式影响社会。太赫兹信号产生的新方法将有助于保持美国在太赫兹技术方面的领先地位。一些本科生和研究生将接受现代设备制造技术的培训，这将增强美国纳米技术劳动力。外展活动，包括磁性和超导演示，将以代表人数较少的群体的中学生为目标，并将有助于吸引少数族裔进入科学和工程领域。拟议的研究计划基于提出者大量的初步实验和理论工作，他们通过实验证明了铂/赤铁矿双层中的自旋泵浦，并从理论上预测，当直流电施加到铂层时，具有强易平面和弱易轴磁各向异性的重金属(铂)和反铁磁材料的双层膜可以作为相干THz辐射的源。在这种基于反铁磁体的自激振荡器中，铂层中的电流向反铁磁体注入纯的自旋霍尔电流，并驱动其序参数进入持久进动状态。这种由垂直于反铁磁体易平面各向异性的自旋电流分量激发的进动在时间上是不均匀的，这是由于易平面各向异性内存在的弱易轴磁各向异性。反铁磁有序参数振荡的频率与注入的自旋电流成正比，并且随着铂层电流密度的增加，从大约0.1THz增加到2.0THz。通过自旋泵浦和铂层中的逆自旋霍尔效应，将有序参数振荡转换为电场幅度超过1V/cm的THz电磁信号。太赫兹频室温反铁磁自谐振子的动力学在数学上等同于约瑟夫森结自振子的动力学，反铁磁体的弱单轴磁各向异性的能量起约瑟夫森能量的作用。这些紧凑、可调谐和结构简单的基于反铁磁体的太赫兹辐射源的展示将使用于成像、化学检测、无线芯片到芯片通信和太赫兹光谱/显微镜的紧凑和廉价的固态太赫兹设备的开发成为可能。

Generators of electromagnetic waves with frequencies near one terahertz are needed for several types of practically useful applications such as new bio-medical imaging techniques, highly sensitive chemical sensors and energy-efficient wireless computer chips. Existing generators of terahertz radiation have significant deficiencies that severely limit their usefulness. These generators either work at temperatures below room temperature or are based on expensive and bulky laser systems. The goal of this project is to create a new type of terahertz generator that is compact, inexpensive and works at room temperature. These generators are based on readily available magnetic materials such as iron oxide and nickel oxide and will operate via conversion of magnetic oscillations in these materials into terahertz electromagnetic waves. The goal of the proposed research program will be achieved via a collaborative effort of a synergistic team of experts in magnetic device fabrication (University of California, Irvine) and leading theorists in the field of magnetic devices (Oakland University). The results of the proposed research program will impact society in multiple ways. The new method of terahertz signal generation will help maintain the US leadership in terahertz technology. A number of undergraduate and graduate students will be trained in modern device fabrication techniques, which will enhance the US nanotechnology workforce. The outreach activities, including demonstrations on magnetism and superconductivity, will target middle school students from underrepresented groups, and will help attract minorities to science and engineering.The proposed research program is based on a substantial preliminary experimental and theoretical work of the proposers, who experimentally demonstrated spin pumping in Pt/hematite bi-layers, and theoretically predicted that a bi-layer of a heavy metal (Pt) and an antiferromagnetic material with strong easy-plane and weak easy-axis magnetic anisotropies can function as a source of coherent THz radiation when direct current is applied to the Pt layer. In such antiferromagnet-based auto-oscillators, an electric current in the Pt layer injects pure spin Hall current into the antiferromagnet and drives its order parameter into a state of persistent precession. This precession excited by the component of spin current perpendicular to the easy plane anisotropy of the antiferromagnet is non-uniform in time due to the weak easy-axis magnetic anisotropy present within the easy plane anisotropy. The frequency of the antiferromagnetic order parameter oscillations is proportional to the injected spin current, and increases from approximately 0.1 THz to 2.0 THz with increasing current density in the Pt layer. The order parameter oscillations are converted into a THz electromagnetic signal with electric field amplitude exceeding 1 V/cm via spin pumping and the inverse spin-Hall effect in the Pt layer. The dynamics of the THz-frequency room-temperature antiferromagnetic auto-oscillator is mathematically equivalent to that of a Josephson junction auto-oscillator, with the energy of the weak uniaxial magnetic anisotropy of the antiferromagnet playing the role of the Josephson energy. The demonstration of these compact, tunable and structurally simple antiferromagnet-based sources of THz radiation will enable the development of compact and inexpensive solid state THz devices for imaging, chemical detection, wireless chip-to-chip communication and THz spectroscopy/microscopy.