Collaborative Research: Novel Terahertz Generators Based on Magnetic Materials

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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.