Direct Observation of Optically Injected Spin-Polarized Currents in Semiconductors

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.

Figure 1

Schematic drawing of the electron motion and their corresponding spins (from 4). The light propagation direction is parallel to $z$ and the sample surface lies in the $xy$ plane. Thin arrows denote the current direction and thick arrows the spin orientation. Case (a) corresponds to cocircular and case (b) to cross-linear polarization.

Figure 2

Schematic drawing of the PL-spot movement. The solid line marks the excitation spot, which also marks the position of the resulting photoluminescence (PL) without spin currents [e.g., for $(2{\varphi }_{\omega }-{\varphi }_{2\omega })=0,2\pi ,4\pi ,\dots$]. The dashed lines mark the position of the PL spots for spin-up and spin-down electrons for maximum spin current [$(2{\varphi }_{\omega }-{\varphi }_{2\omega })=\frac{\pi }{2},\frac{5\pi }{2},\frac{9\pi }{2},\dots$]. For $(2{\varphi }_{\omega }-{\varphi }_{2\omega })=-\frac{\pi }{2},\frac{3\pi }{2},\frac{7\pi }{2},\dots$ the spin-up current flows to the right and the spin-down current to the left; i.e, the positions of the ${\sigma }^{+}$ and ${\sigma }^{-}$ PL are interchanged.

Figure 3

Experimental setup. The $\omega$-light beam is delayed relative to the $2\omega$-light beam by a delay stage with coarse and fine adjustment.

Figure 4

Results of the measured displacement in ZnSe at 100 K with excitation with 400 and 800 nm light, respectively. The lower curve shows the relative phase relation $(2{\varphi }_{\omega }-{\varphi }_{2\omega })$ between the $\omega$ and $2\omega$ light beams as a function of the delay. The upper curve displays the displacement of spot due to the spin currents.