Terahertz Studies Of Correlated Electron Materials Under Different External Stimuli

Terahertz(THz)waves,for the unique advantages of their bandwidth and photon energy,are of significant importance in many applications such as ultrafast wireless communications,radar detection,imaging,spectral analysis,and biomedicine,have been increasingly studied in recent years.Over the last two decades,the THz community is growing fast.In the fundamental research area,the interaction between all kinds of materials and THz waves has been continuously revealed.Particularly,the studies on various low-energy excitations of strongly correlated electron materials by THz terahertz spectroscopy are extremely impressive.In the field of functional devices,a lot of THz emitters,detectors,and modulators have been fabricated for the generation,detection,and manipulation of THz waves.What’s more,in the application areas,the THz imaging system has already been commercialized,THz communication has been officially designated for the 6G network,and THz spectroscopy has been demonstrated for biomedicine detection.However,the development of THz technology is still limited by high-power sources,sensitive detectors,and efficient modulators.Compared to laser or microwave technology,many essential devices are still lacking in THz range,and even the principles of terahertz waves are inadequate.The reason lies in the unexplored interaction between materials and THz waves.On this basis,it is necessary to explore the characteristics of THz spectra in different kinds of materials for the designing of high-performance THz devices.In this paper,the THz modulation properties of correlated electron materials were studied and different active THz modulators based on correlated electron materials with high modulation performance were designed.These main contents are outlined as follows:(1)Photoinduced broadband tunable terahertz absorber based on VO2 thin film.By utilizing an unpatterned strongly correlated electron oxide VO2 thin film,a photoinduced broadband tunable THz absorber is realized.The absorption of THz waves can be actively controlled by an external pump laser.Furthermore,broadband(0.2～1 THz)and～74.7%strong absorption modulation depth can be realized in this device.Apart from other modulators,the photoexcitation-assisted dual-phase competition is identified as the origin of this active THz modulation.(2)Active and smart terahertz electro-optic modulator based on VO2 structure.Modulating THz waves actively and smartly through an external field is highly desired in the development of THz spectroscopic devices.Here.we demonstrate an active and smart electro-optic THz modulator based on a strongly correlated electron oxide VO2.With milliampere current excitation on the VO2 thin film,the transmission,reflection,absorption,and phase of THz waves can be modulated efficiently.In particular.the antireflection condition can be actively achieved and the modulation depth reaches～99.9%,accompanied by a～180° phase switching.Repeated and current scanning experiments confirm the high stability and multi-bit modulation of this electro-optic modulation.Most strikingly,by utilizing a feedback loop of "THz-electroTHz" geometry,a smart electro-optic THz control is realized.For instance,the antireflection condition can be stabilized precisely no matter what the initial condition is and how the external environment changes.(3)Terahertz spectral study of doping and magnetic-field induced effects on spin reorientation in Ho1-xYxFeO3 single crystals.Ho1-xYxFeO3 single crystals were chosen to study the magnetic-field and Y3+doping dependence of spin states and spin reorientation by the terahertz time-domain spectroscopy(THz-TDS).The magnetic field can not only tune the resonant peaks of quasi-ferromagnetic mode(q-FM)spin waves but also induce the spin reorientation in Ho1-xYxFeO3 single crystals.These magnetic-field induced spin reorientation phenomena can be achieved more easily if the temperature will get closer to the intrinsic spin reorientation temperature range of the single crystals and the critical magnetic field inducing spin reorientation will increase with the doping of Y3+.(4)Terahertz magnetic modulation in spin textured band EuTe2.In order to develop efficient magnetically controlled THz modulators,appropriate materials with tunable optical properties under magnetic fields are important.Spin textured band EuTe2 is one of the ideal materials for this purpose because of its spin-dependent band structure governed by the magnetic field.The THz responses under different temperatures,magnetic directions,and magnetic fields were systematically studied by using THz-TDS.The results show that the magnetic THz modulation depth of EuTe2 can reach as high as 100%and the phase shift is～300°.In addition,this magnetic modulation is frequency-dependent.In the critical magnetic field,an absorption peak will appear in the terahertz range and the THz transmission phase will jump at the frequency of the absorption peak.Furthermore,the peak will redshift with the increase of the magnetic field,the absorption peak will shift more than 2 meV with the magnetic variation of 0.02 T.In addition,this magnetic modulation has strong anisotropy and the maximum anisotropy modulation depth can reach near 100%.