Two-photon spin injection in semiconductors

Abstract

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.

Second-harmonic generation spectroscopy of excitons in ZnO

“…The dipole-allowed Γ 5 state can occur for a one-photon process and forms two polariton branches in the given geometry. The Γ 1 state can become excited by a two-photon process if one of the involved photons is due to the quadrupole perturbation or due to the involvement of intermediate virtual states in remote bands 49 . As a result, the 1s exciton states of the A and C excitons might be observed in the SHG spectrum due to the Zeeman spin mixing of the Γ 5y and Γ 1 states.…”

“…The relevant two-photon matrix element is M 2ph 2p,G ∝ D ω,a D ω,f . On the other hand, the direct two-photon absorption by the s exciton states in noncentrosymmetric semiconductors may occur within the ED approximation via the intermediate virtual states in remote bands 49 . In this case the two-photon matrix element is M 2ph s,G ∝ D ω,a D ω,a .…”

Fast Optical Control of Spin in Semiconductor Interfacial Structures

“…This leads to a decrease of the degree of spin polarization P e which is practically achievable in the subsystem. Ideally, each absorbed laser pulse should generate P e ≈ 50% at the instant of absorption [25]. If a material with τ s ≈ 16 ns is excited by a train of circularly polarized laser pulses with a repetition rate of 80 MHz, it should lead to time average P e ≈ 36% when this unideal renewal is not considered.…”

Many-body contributions to spin relaxation in semiconductors

“…The number of spin-up electrons is three times larger than the number of spin-down electrons, owing to selection rules for two-photon absorption of circularly polarized light. [32] Then, if the spin relaxation time is neglected we see immediately, by direct substitution, that the solution of Eq. (2.11) is…”

“…The pump pulse is tightly focused to excite carriers with a Gaussian spatial profile of w 0 =1.5 µm (full width at half maximum) with an excitation excess energy of about 40 meV. The density of spin-up electrons excited n ↑ is approximately three times higher than the density of spin-down electrons n ↓ owing to the selection rulesfor two-photon absorption of circularly polarized light [32].…”