The newly developed two-dimensional layered materials provide a perfect platform for valley-spintronics exploration. To determine the prospect of utilizing the valley degree of freedom, it is of great importance to directly detect and understand the valley dynamics in these materials. Here, the exciton valley dynamics in monolayer WSe2 is investigated by the two-color pump–probe magneto-optical Kerr technique. By tuning the probe photon energy in resonance with the free excitons and trions, the valley relaxation time of different excitonic states in monolayer WSe2 is determined. Valley relaxation time of the free exciton in monolayer WSe2 is confirmed to be several picoseconds. A slow valley polarization relaxation process is observed to be associated with the trions, showing that the valley lifetime for trions is one order of magnitude longer than that of free excitons. This finding suggests that trion can be a good candidate for valleytronics applications.

In this work, we systematically investigate the linear absorption associated with the excitons’ interband transition of a semiconductor quantum dot (SQD) in proximity to a metal nanoparticle (MNP), where the competition between local field enhancement and nonradiative resonant energy transfer (NRET) plays a critical role. It is shown that the linear absorption coefficient of the SQD depends strongly on the geometrical parameters of the hybrid nanostructure. In particular, a continuous transition from absorption enhancement to quenching with varying MNP size and SQD location is clearly observed. Three regimes are identified unambiguously where NRET or local field enhancement governs the absorption response of the SQD in the hybrid nanostructure. In the first regime, where the separation distance between the SQD and the MNP is relatively short and the SQD–MNP coupling is strong, the dominant contribution to the absorption response of the SQD is the NRET effect, and the multipole effect must be considered in addition to the dipole effect. As the separation distance between the SQD and the MNP increases, the coupling is slightly weakened, and the local field enhancement becomes the dominant contribution to the optical response of the SQD in the second regime. When the coupling is further weakened by increasing the distance between the SQD and the MNP, the strong coupling interaction is diminished, and the optical response approaches the case of the bare SQD. Controlling the geometrical parameters of the nanostructure not only provides a further engineering degree of freedom to elucidate the underlying physics of these structures, but also offers a general guide for the optimal design of SQD–MNP hybrid nanostructures toward novel optoelectronics applications.

●Laser-triggered coherent magnetization dynamics study in (Ga,Mn)As by TR-MOKE.●Dynamic phase reversal of magnetization precession is observed by giant MLD effect.●Dynamic phase reversal is sensitive to modulation of the effective magnetic fields.●Demonstrates an efficient method to visualize the magnetization reorientation.Ultrafast laser-triggered coherent magnetization dynamics in ferromagnetic (Ga,Mn)As films have been investigated by time-resolved magneto-optical spectroscopy. Dynamic phase reversal in the magnetic precession process is observed when the ambient temperature or the external magnetic field is varied. This phenomenon is found to be sensitive to the spontaneous magnetization orientation, and is attributed to the giant magnetic linear dichroism (MLD) effect in (Ga,Mn)As. Our findings suggest that this effect will enable the sensitive measurement of the dynamic phase of in-plane magnetization precession on picosecond time scale in the collective spin excitation in (Ga,Mn)As, thus enabling efficient and ultrafast magneto-optical detection for magnetization dynamics in ferromagnetic semiconductor-based spintronic devices.

Semiconductor quantum dots (SQDs) have good nonlinear figures of merit (FOMs) but relatively poor nonlinear refraction. The linear and nonlinear optical processes of SQDs in the proximity of metallic nanostructures can be enhanced by the surface plasmon resonance, but the nonlinear FOMs are limited by the enhanced linear absorption. Here, we investigate optical third-order nonlinearity and the corresponding FOMs of the CdTe quantum dots heavily doped with Ag. The excitonic resonant absorption is enhanced and the 1S peak is red-shifted and broadened after doping silver. Intriguingly, the nonlinear refraction near the band edge is enhanced more than 35 times, while the nonlinear absorption keeps very small at the crossover of the one-photon saturation absorption and two-photon excitation near the band edge, leading to the desired one- and two-photon FOMs (W and T) for the demands of all-optical waveguide switching. Our observations offer a strategy to prepare doped semiconductor quantum dots with large third-order susceptibility and good nonlinear FOMs and thus show prospective applications in optical information processing, switching, and modulating.

•The magnetization dynamics of MBE-grown Fe/GaAs film is studied by TR-MOKE.•The magnetization damping of Fe/GaAs is studied at different temperature and fields.•The intrinsic Gilbert damping constant is extracted at higher external magnetic fields.•The spin-flip scattering and the two-magnon scattering dominated the damping process.In this paper, we studied the ultrafast magnetization dynamics and damping process in a hybrid epitaxial Fe/GaAs structure by using time-resolved magneto-optical measurements over various temperatures. The phenomenological Gilbert damping constant depended strongly on both the direction and strength of the applied magnetic field. The damping process was revealed to be dominated by incoherent spin-flip scattering, with involvement of the Fe/GaAs interfacial defects mediated two-magnon scattering. The intrinsic damping parameter was extracted at sufficiently high magnetic fields (or magnetization precession frequencies). Our results allow for better understanding of magnetization relaxation of Fe/GaAs heterostructure for its potential application in novel spintronic devices.

•The magnetization switching process in MBE-grown Co2MnAl film is studied by MOKE.•The complex single, double, and triple jumps are observed experimentally.•The complex jumping processes are explained based on magnetic domain energetics.•The hard axis direction is precisely determined experimentally and theoretically.Single-crystalline Co2MnAl Heusler alloy film has been successfully grown on GaAs (001) substrate by molecular-beam epitaxy (MBE). The complex multistep magnetic switchings with single, double, and triple loops, deriving from the in-plane uniaxial magnetic anisotropy superimposed with a cubic anisotropy, have been observed experimentally. All switching processes are revealed to be mediated by the sweeping of 90° and 180° domain walls, and can be explained successfully based on the domain energetics. Theoretical calculation of hard axis orientation using free energy density shows excellent agreement with the experimental result.

The electron density-dependent in-plane spin dephasing anisotropy in GaAs/AlGaAs single quantum wells has been investigated by time-resolved magneto-Kerr rotation (TR-MOKE) measurements. Large anisotropy of in-plane spin dephasing time has been observed and compared for samples with different degrees of structural asymmetry. The breakdown of spin dephasing anisotropy with increase in electron density is ascribed to the combined effect of relative strength change in the Rashba and Dresselhaus fields and contribution of cubic Dresselhaus term at higher excitation electron density. It is revealed that Dresselhaus spin–orbit coupling strength is stronger than Rashba for all samples studied in our experiment. The measured anisotropy dependence on electron density and sample asymmetry agrees with previous theoretical studies.Research highlights► Large in-plane spin dephasing anisotropy has been observed in highly asymmetric GaAs quantum wells. ► In-plane spin dephasing anisotropy reduced with increase in photo-excited electron density. ► Measured degree of in-plane spin dephasing anisotropy also qualitatively agrees with the designed degree of structural inversion asymmetry. ► It is discussed that the Dresselhaus spin–orbit coupling dominates Rashba one for all samples.

The third-order optical nonlinear refractive properties of InAs/GaAs quantum dots grown by molecular beam epitaxy have been measured using the reflection Z-scan technique at above-bandgap energy. The nonlinear refractive index and nonlinear absorption index of the InAs/GaAs quantum dots were determined for wavelengths from 740 to 777 nm. The measured results are compared with the nonlinear refractive response of several typical III–V group semiconductor materials. The corresponding mechanisms responsible for the large nonlinear response are discussed.

The newly developed two-dimensional layered materials provide a perfect platform for valley-spintronics exploration. To determine the prospect of utilizing the valley degree of freedom, it is of great importance to directly detect and understand the valley dynamics in these materials. Here, the exciton valley dynamics in monolayer WSe2 is investigated by the two-color pump–probe magneto-optical Kerr technique. By tuning the probe photon energy in resonance with the free excitons and trions, the valley relaxation time of different excitonic states in monolayer WSe2 is determined. Valley relaxation time of the free exciton in monolayer WSe2 is confirmed to be several picoseconds. A slow valley polarization relaxation process is observed to be associated with the trions, showing that the valley lifetime for trions is one order of magnitude longer than that of free excitons. This finding suggests that trion can be a good candidate for valleytronics applications.