Temperature dependent terahertz giant anisotropy and cycloidal spin wave modes in BiFeO3 single crystal

THz time-domain spectroscopy(THz-TDS)is used to study the THz-optical properties of a single crystal bismuth ferrite BiFeO3(BFO).It can be found that the anisotropy of BiFeO3 is strongly dependent on the temperature.A giant birefringence up to around 3.6 is observed at 1 THz.The presence of a spatially modulated cycloidal antiferromagnetic structure leads to spin cycloid resonances(SCR)ψandΦ,corresponding to the out-of-plane and in-plane modes of the spin cycloid,respectively.We distinguish the SCR with respect to their response to orthogonal polarizations of the electric fields of the incident THz beam.In addition,we observe a resonance appearing below 140 K,which might be interpreted as an electromagnon mode and related to a spin reorientation transition.Our present observations present that the temperature and polarization,as the external control parameters,can be used to modulate the THz optical properties of BFO single crystal.

Photoinduced large polaron transport and dynamics in organic-inorganic hybrid lead halide perovskite with terahertz probes

Organic-inorganic hybrid metal halide perovskites(MHPs)have attracted tremendous attention for optoelectronic applications.The long photocarrier lifetime and moderate carrier mobility have been proposed as results of the large polaron formation in MHPs.However,it is challenging to measure the effective mass and carrier scattering parameters of the photogenerated large polarons in the ultrafast carrier recombination dynamics.Here,we show,in a one-step spectroscopic method,that the optical-pump and terahertz-electromagnetic probe(OPTP)technique allows us to access the nature of interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH_(3)NH_(3)PbI_(3)(MAPbI_(3))of about 10μm grain size.Firstly,we demonstrate a direct spectral evidence of the large polarons in polycrystalline MAPbI_(3).Using the Drude-Smith-Lorentz model along with the Frӧhlich-type electron-phonon(e-ph)coupling,we determine the effective mass and scattering parameters of photogenerated polaronic carriers.We discover that the resulting moderate polaronic carrier mobility is mainly influenced by the enhanced carrier scattering,rather than the polaron mass enhancement.While,the formation of large polarons in MAPbI_(3)polycrystalline grains results in a long charge carrier lifetime at room temperature.Our results provide crucial information about the photo-physics of MAPbI3 and are indispensable for optoelectronic device development with better performance.