Polarization Control of the Nonlinear Emission of Semiconductor Microcavities

The degree of circular polarization ($\wp$) of the nonlinear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards $\mathbf{K}\sim 0$ cavitylike states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the $\pm 1$ spin levels at $\mathbf{K}\sim 0$. At short times after a pulsed excitation $\wp$ reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy ($+1/-1$ spin for positive/negative detuning).

Figure 1

Contour plot of the PL at 10 ps measured in different points of the sample. Black indicates high intensity; light gray indicates low intensity. The dashed lines are guides to the eye. The arrows indicate the detunings discussed in the text. Inset: integrated intensity (log scale) of the lower polariton branch (LPB) emission at 20 ps as a function of the excitation power density for $\delta =-10\mathrm{m}\mathrm{e}\mathrm{V}$. The arrow points to the threshold for observing nonlinear effects in the emission.

Figure 2

(a) Time evolution of the circularly polarized PL of the UPB at $\delta =10\mathrm{m}\mathrm{e}\mathrm{V}$. The filled circles/solid line (open circles/dashed line) denote the ${\sigma }^{+}$ (${\sigma }^{-}$) emission. (b) Same as in (a) for the LPB at $\delta =-10\mathrm{m}\mathrm{e}\mathrm{V}$. Inset: maximum value of the polarization degree at 20 ps as a function of excitation density for $\delta =-10\mathrm{m}\mathrm{e}\mathrm{V}$. The line is a guide to the eye. (c) Time evolution of the circular polarization degree of the PL emission of the UPB at $\delta =10\mathrm{m}\mathrm{e}\mathrm{V}$. (d) Same as in (c) for the LPB at $\delta =-10\mathrm{m}\mathrm{e}\mathrm{V}$. The data are taken with an excitation density of $18\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$

Figure 3

(a) Polarization-resolved PL spectra at 20 ps for an excitation density of $18\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ at $\delta =10\mathrm{m}\mathrm{e}\mathrm{V}$. The filled circles/solid line (open circles/dashed line) denote the ${\sigma }^{+}$ (${\sigma }^{-}$) emission. The lines are Gaussian fits of the experimental data. Inset: Spin splitting ($\Delta$) at 20 ps as a function of excitation density for $\delta =10\mathrm{m}\mathrm{e}\mathrm{V}$. The line is a guide to the eye. (b) Same as in (a) for $\delta =-10\mathrm{m}\mathrm{e}\mathrm{V}$.