Spin Wave Frequency Comb At Terahertz

A frequency comb is a spectrum composed of a series of discrete and evenly distributed lines.It was first proposed in optical systems,it is very important for high precision frequency measurement.Magnons(quanta of spin waves)and photons are both bosons and have many similar properties.Therefore,in recent years,researchers have found that it is possible to induce the generation of magnonic frequency comb(MFC)by using the topological spin structure of skyrmion,which mainly relies on the nonlinear three-magnon process between magnons and skyrmion.Since its discovery,magnonic frequency comb has attracted wide attention in magnonics because of its potential application prospect.Terahertz magnonic frequency comb combines the advantages of terahertz and magnonic frequency comb technologies,which greatly promotes the application of magnonic frequency comb in magnon measurement,sensing and communication.Here,we add two different microwave fields to excite the spin wave mode and the skyrmion breathing mode respectively,and generate the sum frequency mode and the difference frequency mode through the three-magnon process,and these modes can be further coupled with the breathing mode to produce higher order frequency mode.This chain reaction will cause a cascade of magneton excitation in the system.The result is a magnonic frequency comb with frequency intervals of skyrmion breathing mode.The conditions for generating the spin frequency comb in terahertz band are discussed by changing the amplitude and frequency of microwave field.In this work,it is found that the intensity of the nonlinear coupling between the magnonic and skyrmion breathing presents a linear relationship with the driving frequency,and the signal of terahertz spin frequency comb can be observed in a wide driving frequency range.Moreover,it is found that there is no threshold for the driving field amplitude,but the propagation of the frequency comb will be limited by damping.The results of this study extend the operating frequency of MFC from gigahertz to terahertz,have potential applications in terahertz spintronic devices,and promote the further development of terahertz magnonics...