Injection of ballistic pure spin currents in semiconductors by a single-color linearly polarized beam

We demonstrate the injection and control of pure spin currents in [110]-oriented GaAs quantum wells at room temperature by one-photon absorption of a single linearly polarized optical pulse. These currents result from the interference of absorption processes associated with the right and left circularly polarized components of the pulse, with the current direction determined by their relative phase. The current generation process differs from the circular photogalvanic effect, which relies only on the intensity of circularly polarized beams. By using spatially resolved pump-probe techniques, we obtain signatures for the currents by measuring the resulting spin separations of 1–4 nm. The separation decreases with increasing excitation fluence, consistent with a reduction in the momentum relaxation time with increasing carrier density.

Figure 1

(a) Sample orientation and definition of a laboratory coordinate system schematically depicting the injection of a PSC along the $y$ direction by one-photon absorption of an $x$ polarized field propagating along $z$. Small arrows inside spheres show the direction of net electron spin polarization, while the larger arrows intersecting the spheres show the direction of ballistic electron propagation. The spin-up (spin-down) electrons are shown moving in the $+y$ $(-y)$ direction, yielding no net charge current. (b) Schematic representation of the spin-up and spin-down electron spatial distributions (solid lines) produced by the currents in (a), immediately following momentum relaxation. The dashed line shows the corresponding difference in the spin-up and spin-down electron densities resulting from this motion. (c) Experimental geometry for producing and measuring pure spin currents: $\lambda ∕4$ represent a quarter-wave plate.

Figure 2

Measurement of $\Delta T_{+}∕T_{+}\propto n_{\downarrow }\left(y\right)$ (open squares) and $\Delta T_{-}∕T_{+}-\Delta T_{+}∕T_{+}\propto n_{\uparrow }\left(y\right)-n_{\downarrow }\left(y\right)$ for an $x$ polarized (solid circles) and a $y$ polarized (solid triangles) pump pulse at room temperature for a pump fluence of $10\mu \mathrm{J}∕{\mathrm{cm}}^2$. The lines are the fits to the data for spin separation $d=2.8\mathrm{nm}$.

Figure 3

Separation of the spin-up and spin-down profiles associated with the injection of PSC by single photon absorption as a function of pump fluence. The estimated momentum relaxation times $\tau$ extracted from these separations (right axis) and the approximate carrier density (top axis).