Time-and polarization-resolved differential transmission measurements employing same and oppositely circularly polarized 150 fs optical pulses are used to investigate spin characteristics of conduction band electrons in bulk GaAs at 295 K. Electrons and holes with densities in the 2 ϫ 10 16 cm −3-10 18 cm −3 range are generated and probed with pulses whose center wavelength is between 865 and 775 nm. The transmissivity results can be explained in terms of the spin sensitivity of both phase-space filling and many-body effects ͑band-gap renormalization and screening of the Coulomb enhancement factor͒. For excitation and probing at 865 nm, just above the band-gap edge, the transmissivity changes mainly reflect spin-dependent phase-space filling which is dominated by the electron Fermi factors. However, for 775 nm probing, the influence of many-body effects on the induced transmission change are comparable with those from reduced phase space filling, exposing the spin dependence of the many-body effects. If one does not take account of these spindependent effects one can misinterpret both the magnitude and time evolution of the electron spin polarization. For suitable measurements we find that the electron spin relaxation time is 130 ps.

A comparison is made between the degree of spin polarization of electrons excited by one- and two-photon absorption of circularly polarized light in bulk zincblende semiconductors. Time- and polarization-resolved experiments in (001)-oriented GaAs reveal an initial degree of spin polarization of 49% for both one- and two-photon spin injection at wavelengths of 775 and 1550 nm, in agreement with theory. The macroscopic symmetry and microscopic theory for two-photon spin injection are reviewed, and the latter is generalized to account for spin-splitting of the bands. The degree of spin polarization of one- and two-photon optical orientation need not be equal, as shown by calculations of spectra for GaAs, InP, GaSb, InSb, and ZnSe using a 14x14 k.p Hamiltonian including remote band effects. By including the higher conduction bands in the calculation, cubic anisotropy and the role of allowed-allowed transitions can be investigated. The allowed-allowed transitions do not conserve angular momentum and can cause a high degree of spin polarization close to the band edge; a value of 78% is calculated in GaSb, but by varying the material parameters it could be as high as 100%. The selection rules for spin injection from allowed-allowed transitions are presented, and interband spin-orbit coupling is found to play an important role.Comment: 12 pages including 7 figure

Ballistic charge current gratings are induced in GaAs at 300 K by quantum interference of single-and two-photon absorption using noncollinearly incident 775 and 1550 nm, 150 fs pulses. First-order diffraction of time-delayed 830 nm, 150 fs probe pulses is used to observe carrier evolution for injected densities near 10 17 cm −3. The current grating forms electron and hole charge-density gratings during pumping, and because the pumping is uniform while the carrier density and hence electronic specific heat is not, a carrier temperature grating also forms. The peak diffraction efficiency from both grating types is only ϳ10 −9. The temperature grating, with modulation amplitude ϳ1 K, decays through cooling in ϳ500 fs. Space-charge fields neutralize the electron and hole density gratings by the end of pumping, but nonetheless leave a neutral, electron-hole pair density grating with amplitude of ϳ10 −3 of the injected carrier density. At the highest injected carrier densities, the pair grating amplitude builds on a few picosecond time scale before decaying by recombination and ambipolar diffusion with an ϳ15 ps time constant. A model based on continuity equations for carrier density, momentum, and energy during ballistic and drift motion is used to help interpret the experimental data. Besides qualitatively confirming the above dynamics, the model suggests that the pair grating amplitude and evolution is determined by two factors: ͑1͒ the warping or nonparabolicity of the hole bands and ͑2͒ the transfer of some electrons from the ⌫-valley electron-density grating to the L, X conduction band valleys during excitation, and their subsequent return to the ⌫ valley on a few picosecond time scale.

Articles you may be interested inFast electron spin resonance controlled manipulation of spin injection into quantum dots Appl. Phys. Lett.We demonstrate that the quantum mechanical interference between the probability amplitudes for the two-photon absorption of a fundamental (1.55 m)ϳ150 fs pulse and for the one-photon absorption of a noncollinearly propagating second-harmonic ͑775 nm͒ pulse can create transient, ballistic, purely spin-polarized current gratings in bulk GaAs at room temperature. For fundamental and second-harmonic pulses having orthogonal linear polarizations, two periodically modulated ballistic spin-polarized current gratings are injected that have opposite spins and opposite propagation directions at each point along the grating. Consequently, there is no initial modulation of the charge current, carrier population, or net spin. Before the carrier momentum relaxes, the transport associated with these spin currents forms two oppositely spin-polarized population gratings that are exactly out of phase spatially and that decay by electronic spin diffusion in a time of 3.2 ps. In addition, charge density gratings are directly produced by the quantum interference process, and they decay by ambipolar diffusion and recombination ͑ϳ17.6 ps͒. The polarization selection rules and sample orientation are used to separate the contributions of the current and density gratings.

A comparison is made between the degree of spin polarization of electrons excited by one-and two-photon absorption of circularly polarized light in bulk zincblende semiconductors. Time-and polarization-resolved experiments in (001)-oriented GaAs reveal an initial degree of spin polarization of 49% for both one-and two-photon spin injection at wavelengths of 775 and 1550 nm, in agreement with theory. The macroscopic symmetry and microscopic theory for two-photon spin injection are reviewed, and the latter is generalized to account for spin-splitting of the bands. The degree of spin polarization of one-and two-photon optical orientation need not be equal, as shown by calculations of spectra for GaAs, InP, GaSb, InSb, and ZnSe using a 14 × 14 k · p Hamiltonian including remote band effects. By including the higher conduction bands in the calculation, cubic anisotropy and the role of allowed-allowed transitions can be investigated. The allowed-allowed transitions do not conserve angular momentum and can cause a high degree of spin polarization close to the band edge; a value of 78% is calculated in GaSb, but by varying the material parameters it could be as high as 100%. The selection rules for spin injection from allowed-allowed transitions are presented, and interband spin-orbit coupling is found to play an important role.