Ultrafast Dynamic Control of Spin and Charge Density Oscillations in a GaAs Quantum Well

We use subpicosecond laser pulses to generate and monitor in real time collective oscillations of electrons in a modulation-doped GaAs quantum well. The observed frequencies match those of intersubband spin- and charge-density excitations. Light couples to coherent density fluctuations through resonant stimulated Raman scattering. Because the spin- and charge-related modes obey different selection rules and resonant behavior, the amplitudes of the corresponding oscillations can be independently controlled by using shaped pulses of the proper polarization.

Figure 1

(a) Sample structure showing Si donors (+) introduced by $\delta$ doping. The black and gray areas denote, respectively, GaAs and ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$. A smoothing superlattice consisting of 100 periods of $30\AA$ GaAs and $100\AA$ ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ was grown on top of the substrate, which is on the left-hand side of the diagram. The two doping layers close to the sample surface help pull the conduction band edge to very near the 2DEG Fermi level at the position of the Si donors close to the QW. (b) Energy level diagram. (c) Photoluminescence spectrum obtained at 7 K with $0.01\mathrm{W}/{\mathrm{cm}}^2$ of the $4880\AA$ Ar line. The dashed line denotes the QW gap. The weaker peak at $\sim 1.516\mathrm{eV}$ is not associated with the 2DEG. (d) Wave vector dependence of intersubband excitations for $q\ll k_F$. The shaded area denotes single-particle excitations ${\mathrm{SP}}_{01}$. (e) Raman spectra recorded in the backscattering configuration [19]. The (continuous wave) laser energy is 1.556 eV.

Figure 2

(a) Time-resolved differential reflectivity (top) and magnetic Kerr (bottom) data. Curves are linear prediction fits [28]. The associated insets are Fourier transform intensity spectra showing peaks due to charge- and spin-density oscillations. The top and bottom traces were obtained, respectively, with linearly and circularly polarized pump pulses. For the top (bottom) trace, the scattered field, $\delta {\mathbf{E}}_R$, is parallel (perpendicular) to the polarization of the incident probe beam. Inset: Dependence of the spontaneous ${\mathrm{CD}}_{01}$ Raman cross section on laser energy. Results obtained with a continuous wave Ti:sapphire laser at $\sim 1\mathrm{W}/{\mathrm{cm}}^2$. Also shown is the energy spectrum of the light pulses centered at 1.545 eV. (b) Same as the bottom trace of (a) but for pulses centered at 1.536 eV.