Terahertz Radiation Based On Ultrafast Spin Dynamics In Ferromagnetic Heterostructures

In the 1980s,THz technology based on ultrafast photonics was born,which aroused wide interest among scientists.Especially,after THz spectroscopy and imaging technologies were developed,THz science and technology show great application prospects.But the THz wave has only received widespread attention in the last decade or so,this is because of the lack of a variety of photonic devices in the THz band.In particular,the low-cost,miniaturized,chip-level THz emitter.It is urgent to consider new material system,new physical origin and new structure of micro-nano devices to develop the THz broadband coherent source.In this paper,based on THz radiation spectrum system,we researched THz radiation characteristics in one-layer film?ferromagnetic?FM??,two-layer film?FM/non-ferromagnetic?NM??and three-layer film?NM/FM/NM?,respectively.And we utilized simple stripe and block structures to modulate THz waves radiated by heterostructures effectively.The specific content can be summarized as the following three aspects:?1?Based on the ultrafast demagnetization effect,we investigated THz radiation in ferromagnetic half-metallic Heusler alloy Co₂MnSn thin film.We first observed the broadband emission of THz pulse in Co₂MnSn,by the irradiation of a femtosecond laser pulse at 800 nm.The magnetic-,sample symmetry-,and pump fluence-dependent THz emission reveal that the THz radiation is originated from the magnetic-dipole radiation,the light-induced sub-picosecond demagnetization.Our experiments demonstrate the possibility of ultrafast manipulation of the carrier transport and magnetic order of the half-metallic Co₂MnSn with femtosecond laser excitation.?2?Based on the inverse spin Hall effect?ISHE?,We studied the broadband THz radiation of two-and three-layer heterostructures upon the irradiation of a femtosecond laser pulse.The THz emission is dominated by the photo-excited transient charge current,which is converted from the spin current with ISHE.Experiment of two-layer film:we tailored the metallic heterostructures with different NM layer?Pd or Ru?and FM layer?CoFeB or CoFe?,to shape the THz transients.Moreover,we find that a saturation effect of THz radiation for CoFeB/Pd is less compared to CoFeB/Ru.Experiment of three-layer film:according to the THz radiation results of the two-layer heterostructures,we designed three-layer films W?4 nm?/CoFeB?4 nm?/Pt?4 nm?and Ta?4 nm?/CoFeB?4 nm?/Pt?4 nm?.Our results indicate that the peak-values of the THz radiation from W/CoFeB/Pt and Ta/CoFeB/Pt are stronger than that from 0.5 mm-thick ZnTe?110?crystal,under very similar excitation conditions.?3?Based on the charge confinement effect,we exploit the combined spintronic and photonic ultrathin metal heterostructures to realize actively modulated THz radiation.We demonstrate that the THz radiation can be mediated coherently through the charge current induced ISHE and the build-in transient current quasi-simultaneously created within the stripe and block heterostructures.The waveforms of THz radiation can be shaped by the charge confinement effect,including the amplitude modulation,the full width at half maximum change and the center-frequency shift.Our findings can be utilized for device-oriented opto-spintronics,and also extend the established THz emitter to THz modulator.