Zuanming Jin

Terahertz magnetic field induced coherent spin precession in YFeO3

We present the magnetic dipole transition at 0.299 THz excited by magnetic component of terahertz electromagnetic pulse in an antiferromagnetic YFeO3 crystal. The impulsive magnetic field of the terahertz pulse tilts the macroscopic magnetization, causing deviation from the equilibrium position, which is manifested by a sharp absorption at the frequency of the quasiferromagnetic mode of the crystal. The rotating coherent macroscopic magnetization radiates elliptically polarized emission at the frequency of the quasiferromagnetic resonance due to the dichroic absorption in the crystal.

Dynamical spin reorientation transition in NdFeO3 single crystal observed with polarized terahertz time domain spectroscopy

Temperature-induced spin reorientation transition in NdFeO3 single crystal is studied by terahertz time-domain spectroscopy in the temperature range from 50 K to 290 K. Following the resonant excitation of quasi-antiferromagnetic (AF) mode, the nature of temperature dependence of emission from AF-mode is investigated systematically in the spin reorientation temperature interval. The emission frequency is observed at 0.485 THz for both Γ4 and Γ24 phases, and it shifts abruptly to 0.456 THz (around 110 K) corresponding to Γ2 phase. The evolution of vector G is obtained from the temperature-dependent polarization changes of the AF-mode excitation. Our results demonstrate that the polarized terahertz time-domain spectroscopy is a sensitive tool to explore the dynamical spin reorientation transition in RFeO3 crystals, and the terahertz magnetic pulse shows potential application for non-thermally manipulating ultrafast spin reorientation.

Ultrafast Terahertz Photoconductivity of Photovoltaic Polymer-Fullerene Blends: A Comparative Study Correlated with Photovoltaic Device Performance.

Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.

Electron spin relaxation in intrinsic bulk InP semiconductor

Electron spin dynamics in intrinsic bulk indium phosphide (InP) semiconductor is studied by time resolved pump probe reflectivity technique using the cocircularly and countercircularly polarized femtosecond pulses at room temperature and 70 K. The reflectivity change from bleaching into absorption is observed with increasing pump photon energy, which can be explained in terms of the spin sensitive band filling and band gap renormalization effects. Density dependence of electron spin relaxation time shows similar tendency at room temperature and 70 K. With increasing carrier density, the electron spin relaxation time increases and then decreases after reaching a maximum value. Our experimental results agree well with the recent theoretical prediction [Jiang and Wu, Phys. Rev. B 79, 125206 (2009)] and D’yakonov–Perel’ mechanism is considered as a dominating contribution to the electron spin relaxation in intrinsic bulk InP semiconductor.

Ultrafast dynamics of the Mn3+ d-d transition and spin-lattice interaction in YMnO3 film

We investigate the photo-induced carrier dynamics and spin-lattice interaction in hexagonal YMnO3 film by the temperature-dependent femtosecond pump-probe spectroscopy. The spin-lattice interaction is identified from the slow component of the transient transmittance change with the excitation energies tuned to 1.7 eV and 2.0 eV, which are close to Mn3+ ions d(xz),(yz)→d(z2) and d(x2-y2),(xy)→d(z2) transition, respectively. Temperature dependences of the spin-lattice relaxation parameters demonstrate that the spin-lattice interaction is strongly connected with the d-d transition within Mn3+ ions and enhanced by spin ordering.

Structural dependent ultrafast electron-phonon coupling in multiferroic BiFeO3 films

The electronic energy relaxation of polycrystalline BiFeO3 films is studied using ultrafast pump-probe spectroscopy. After photo-excitation with femtosecond laser pulses, the relaxation of hot electrons is identified to decay with two different characteristic times. The fast process is attributed to scattering of electrons with lattice-vibration modes, and the slow one is corresponding to the spin-lattice thermalization. The electron-phonon coupling is characterized by the second moment of the Eliashberg function, λ〈ω2〉. Due to the structural strain and symmetry breaking, the electron-phonon interaction strength of tetragonal BiFeO3 films is larger than that of rhombohedral counterparts.

Ultrafast all-optical magnetic switching in NaTb(WO4)2

The operation of an all-optical magnetic switching based on the paramagnetic NaTb(WO4)2 crystals is carried out by the time-resolved magneto-optical Faraday effect. Our results demonstrate that the switching time can be as fast as ∼500 fs at room temperature. The switching amplitude shows a linear dependence on the excitation intensity, which is proportional to the magnetization induced by the circularly polarized light. Based on the inverse Faraday effect in magneto-optical crystal, the switching mechanisms arising from circular dichroism and birefringence are discussed. By tailoring the magneto-optical properties of NaTb(WO4)2 crystal, the switching magnitude can be modulated.

Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature

Excitation photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal was studied by time resolved pump-probe reflectivity technique at room temperature. The results show that spin relaxation time decreases monotonously. While with increasing excitation carrier density, the time constants increases initially then decreases after reaching a maximum value. Our experimental results reveal that both D’yakonov–Perel’ [M. I. D’yakonov and V. I. Perel’, Sov. Phys. JETP 38, 177 (1974)] and Elliot–Yafet [R. J. Elliott, Phys. Rev. 96, 266 (1954); Y. Yafet, Solid State Phys. 14, 1 (1963)] mechanisms dominate the spin relaxation process in CdTe crystal.

Resolving the spin reorientation and crystal-field transitions in TmFeO3 with terahertz transient

Rare earth orthoferrites (RFeO3) exhibit abundant physical properties such as, weak macroscopic magnetization, spin reorientation transition, and magneto-optical effect, especially the terahertz magnetic response, have received lots of attention in recent years. In this work, quasi-ferromagnetic (FM) and quasi-antiferromagnetic (AFM) modes arising from Fe sublattice of TmFeO3 single crystal are characterized in a temperature range from 40 to 300 K, by using terahertz time-domain spectroscopy (THz-TDS). The magnetic anisotropy constants in ac-plane are estimated according to the temperature-dependent resonant frequencies of both FM and AFM modes. Here, we further observe the broad-band absorptions centered ~0.52, ~0.61, and ~1.15 THz below 110 K, which are reasonably assigned to a series of crystal-field transitions (R modes) of ground multiplets (6H3) of Tm3+ ions. Specially, our finding reveals that the spin reorientation transition at a temperature interval from 93 to 85 K is driven by magnetic anisotropy, however, which plays negligible role on the electronic transitions of Tm ions in the absence of applied magnetic fields.

Selective excitation of spin resonance in orthoferrite PrFeO3 with impulsive polarized terahertz radiation

Single cycle terahertz (THz) pulses were employed to excite coherent spin waves in (110)-oriented PrFeO3 single crystal. The free induction decay radiations at frequency of 0.34 THz (quasi-ferromagnetic mode, FM mode) and 0.41 THz (quasi-antiferromagnetic mode, AFM mode) were observed arising from the coupling of magnetic moment with the impulsive magnetic field of polarized terahertz radiation. These two spin modes in PrFeO3 can be excited and modulated by the magnetic field of THz pulse with a specific polarization with respect to the crystal axis. The extracted complex dielectric permittivity and magnetic permeability dispersion in THz range suggest a higher efficiency of energy transfer from the impulsive THz pulse into the AFM than the FM spin system.