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Register a new userMultiple transfer of angular momentum quanta from a spin-polarized hole to magnetic ions in ZnMnSe/ZnBeSe quantum wells
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The magnetization kinetics in (Zn,Mn)Se/(Zn,Be)Se quantum wells has been studied on a ps-time scale after pulsed laser excitation. The magnetization induced by an external magnetic field is reduced by up to 30% during ~100 ps due to spin and energy transfer from photocarriers to Mn spin system. The giant Zeeman splitting leads to a complete spin polarization of the carriers, resulting in a strong suppression of flip-flop processes between carriers and magnetic ions. Therefore a multiple angular momentum transfer from each spin-polarized hole to the Mn ions becomes the dominant mechanism in the magnetization dynamics. A model based on spin-momentum coupling in the valence band is suggested for explaining this transfer.
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The dynamics of spin-lattice relaxation of the Mn-ions in (Zn,Mn)Se-based diluted-magnetic-semiconductor quantum wells is studied by time-resolved photoluminescence. The spin-lattice relaxation time varies by five orders of magnitude from 10-3 down to 10-8 s, when the Mn content increases from 0.4 up to 11%. Free carriers play an important role in this dynamics. Hot carriers with excess kinetic energy contribute to heating of the Mn system, while cooling of the Mn system occurs in the presence of cold background carriers provided by modulation doping. In a Zn0.89Mn0.11Se quantum well structure, where the spin-lattice relaxation process is considerably shorter than the characteristic lifetime of nonequilibrium phonons, also the phonon dynamics and its contribution to heating of the Mn system are investigated.
We introduce the continuity equation for the electromagnetic spin angular momentum (SAM) in matter and discuss the torque associated with the SAM transfer in terms of effective spin forces acting in a material. In plasmonic metal, these spin forces result in plasmogalvanic phenomenon which is pinning the plasmon-induced electromotive force to atomically-thin layer at the metal interface.
The optical spin-orbit coupling occurring in a suitably patterned nonuniform birefringent plate known as `q-plate allows entangling the polarization of a single photon with its orbital angular momentum (OAM). This process, in turn, can be exploited for building a bidirectional spin-OAM interface, capable of transposing the quantum information from the spin to the OAM degree of freedom of photons and textit{vice versa}. Here, we experimentally demonstrate this process by single-photon quantum tomographic analysis. Moreover, we show that two-photon quantum correlations such as those resulting from coalescence interference can be successfully transferred into the OAM degree of freedom.
Magnetic phenomena are ubiquitous in our surroundings and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times. This phenomenon, in the meantime also observed in many other materials, has connected magnetism with femtosecond optics in an efficient, ultrafast and complex way, offering opportunities for rapid information processing or ultrafast spintronics at frequencies approaching those of light. Consequently, the physics of ultrafast demagnetization is central to modern material research, but a crucial question has remained elusive: If a material loses its magnetization within only femtoseconds, where is the missing angular momentum in such short time? Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons that appear as quickly as the magnetic order is lost. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. We explain these observations by means of circularly polarized phonons that quickly absorb the missing angular momentum of the spin system before the slower onset of a macroscopic sample rotation. The time that is needed for demagnetization is related to the time it takes to accelerate the atoms. These results provide an atomistic picture of ultrafast demagnetization under adherence to all conservation laws but also demonstrate the general importance of polarized phonons for non-equilibrium dynamics and provide innovative ways for controlling materials on atomic dimensions.
Electron spins in GaAs quantum dots have been used to make qubits with high-fidelity gating and long coherence time, necessary ingredients in solid-state quantum computing. The quantum dots can also host photon qubits with energy applicable for optical communication, and can show a promising photon-to-spin conversion. The coherent interface is established through photo-excitation of a single pair of an electron and a Zeeman-resolved light-hole, not heavy-hole. However, no experiments on the single photon to spin conversion have been performed yet. Here we report on single shot readout of a single electron spin generated in a GaAs quantum dot by spin-selective excitation with linearly polarized light. A photo-electron spin generated from a Zeeman-resolved light-hole exciton is detected using an optical spin blockade method in a single quantum dot and a Pauli spin blockade method in a double quantum dot. We found that the blockade probability strongly depends on the photon polarization and the hole state, heavy- or light-hole, indicating a transfer of the angular momentum from single photons to single electron spins. Our demonstration will open a pathway to further investigation on fundamental quantum physics such as quantum entanglement between a wide variety of quantum systems and applications of quantum networking technology.
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