Determining the sign of g factor via time-resolved Kerr rotation spectroscopy with a rotatable magnetic field

Time-resolved Kerr rotation spectroscopy is used to determine the sign of the g factor of carriers in a semiconductor material, with the help of a rotatable magnetic field in the plane of the sample. The spin precession signal of carriers at a fixed time delay is measured as a function of the orientation of the magnetic field with a fixed strength B. The signal has a sine-like form and its phase determines the sign of the g factor of carriers. As a natural extension of previous methods to measure the （time-resolved） photoluminescence or time-resolved Kerr rotation signal as a function of the magnetic field strength with a fixed orientation, such a method gives the correct sign of the g factor of electrons in GaAs. Furthermore, the sign of carriers in a （Ga, Mn）As magnetic semiconductor is also found to be negative.

Dependence of femtosecond time-resolved magneto-optical Kerr rotation on the direction of polarization of the probe beam

We demonstrate that the femtosecond time-resolved magneto-optical Kerr rotation oscillates with the direction of polarization of the probe beam when a sample of Al0.25Ga0.75As/GaAs multi-quantum wells is excited by a circularly polarized pump and detected by a linearly polarized probe at wavelengths from 800 to 830 nm. Analytical expressions are derived to explain the mechanism, which is in good agreement with the numerical computation and the experimental data. The results suggest that the Kerr signal can be enhanced by choosing an optimal direction of polarization, which is of benefit to the measurement of the weak Kerr rotation.

Effects of Nd Substitution on Magnetic and Magneto-optical Properties of TbCo/Cr Films

NdTbCo/Cr amorphous films with high perpendicular magnetic anisotropy were prepared onto glass substrates by rf magnetron sputtering. The effects of Nd substitution on the magnetic and magneto-optical properties of TbCo/Cr films were investigated. It was found that partial Tb substitution by Nd would increase the saturation magnetization and the Kerr rotation angle, change the temperature dependence of magneto-optical characteristics. These results can be explained by the ferrimagnetic structure of the rare earth-transition metal alloy. When the magnetic layer composition was （Nd0.265Tb0.735）31Co69, a saturation magnetization of 247 emu/cm^3 and a coercivity of 3.8 kOe at room temperature could be obtained.

Effects of Nd Doping on Magnetic-Optical Properties of GdFeCo Thin Films

The GdFeCo and NdGdFeCo thin films were prepared by sputtering, and their hysteresis loops, the temperature dependence of the saturation magnetization Ms and the magneto-optical Kerr spectrum in the visible light range were measured. By studying the effects of light rare earth element Nd doping on the magneto-optical Kerr rotation angle of GdFeCo thin films, it is found that proper Nd additives in GdFeCo films could enhance Kerr rotation at short wavelengths. So it could be better medium used as the readout layer of center aperture detection magnetically induced super resolution （CAD-MSR）.

Valley dynamics of different excitonic states in monolayer WSe_(2)grown by molecular beam epitaxy

Monolayer transition-metal dichalcogenides possess rich excitonic physics and unique valley-contrasting optical selection rule,and offer a great platform for long spin/valley lifetime engineering and the associated spin/valleytronics exploration.Using two-color time-resolved Kerr rotation and time-resolved reflectivity spectroscopy,we investigate the spin/valley dynamics of different excitonic states in monolayer WSe_(2)grown by molecular beam epitaxy.With fine tuning of the photon energy of both pump and probe beams,the valley relaxation process for the neutral excitons and trions is found to be remarkably different-their characteristic spin/valley lifetimes vary from picoseconds to nanoseconds,respectively.The observed long trion spin lifetime of>2.0 ns is discussed to be associated with the dark trion states,which is evidenced by the photon-energy dependent valley polarization relaxation.Our results also reveal that valley depolarization for these different excitonic states is intimately connected with the strong Coulomb interaction when the optical excitation energy is above the exciton resonance.

Spin depolarization dynamics of WSe_(2) bilayer

In this work,the spin dynamics of a centrosymmetric WSe2 bilayer has been investigated by the two-color timeresolved Kerr rotation together with helicity-resolved transient reflectance techniques.Two depolarization processes associated with the direct transition are discovered at a low temperature of 10 K,with the characteristic decaying time of~3.8 ps and~20 ps,respectively.The short decay time of~3.8 ps is suggested to be the exciton spin lifetime of the WSe2 bilayer,which is limited by the short exciton lifetime of the WSe_(2) bilayer and the rapid intervalley electron–hole exchange interaction between K^(+)and K^(-)valley in the same layer as that of monolayer.The long decay time of~20 ps is suggested to be the spin lifetime of photo-excited electrons,whose spin relaxation is governed by the rapid intervalley scattering from the K valley to the global minimumΣvalley and the subsequent interlayer charge transfer in WSe_(2) bilayer.Our experimental results prove the existence of the spin-polarized excitons and carriers even in centrosymmetric transition metal dichalcogenides(TMDCs)bilayers,suggesting their potential valleytronic and spintronic device applications.

Transmission characteristics of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals

The propagation properties of linearly polarized light in reflection・type one-dimensional magnetoph tonic crystals are studied by using the 4×4 transmission matrix method.The structure models of reflectiotype one-dimensional magnetophotonic crystals are designed,the magnetic field direction control characteristics of reflection spectrum and Kerr rotation angle are discussed,and the effect of applied magnetic field direction and strength on reflection spectrum and Kerr rotation angle are analyzed.The results show that the non-diagonal elements in the dielectric constant of magneto optical materials change when the angle φ between applied magnetic field and optical path changes,the reflectivity and Kerr rotation angle decrease when the angle φ increases;when the applied magnetic field strength changes,the reflectivity and Kerr rotation angle increase when the applied magnetic field strength increases;by adjusting the angleφ and strength of the applied magnetic field,the rotation angle of Kerr can be adjusted to 45°,and a more flat reflection spectrum can be obtained by designing the appropriate structure.