Nonresonant spin selection methods and polarization control in exciton-polariton condensates

Bosonic condensates of exciton-polaritons are characterized by a well-defined pseudospin, which makes them attractive for quantum information schemes and spintronic applications, as well as the exploration of synthetic spin-orbit coupling. However, precise polarization control of coherent polariton condensates under nonresonant injection, the most important ingredient for such advanced studies, still remains a core challenge. Here, we address this problem and demonstrate unprecedented control of the pseudospin of an exciton-polariton condensate. The ultrafast stimulated scattering process allows the observation of completely spin-polarized condensates under highly nonresonant, circularly polarized excitation. This conservation of spin population translates, in the case of linearly polarized excitation, into an elliptically polarized emission. The degree of ellipticity can be controlled by varying the exciton-photon detuning and condensate density. Additionally, cavity engineering allows us to generate completely linearly polarized condensates with a deterministically chosen orientation. Our findings are of fundamental importance for the engineering and design of polaritonic devices that harness the spinor degree of freedom, such as chiral lasers, spin switches, and polaritonic topological insulator circuits.

Figure 1

(a) Dispersion below and above the condensation threshold at 1 pJ/pulse and 110 pJ/pulse input power. (b), (c) Input-output characteristics of the planar microcavity under nonresonant linearly (black) and circularly (${\sigma }^{+}$ in red) polarized excitation. Intensity, linewidth, and energy were extracted from a Lorentzian fit of the ground state integrated around $k=0\pm 0.1\mu {\mathrm{m}}^{-1}$. (d) Circular polarization of the emission as a function of the input power (circularly polarized excitation). (e) Calculated energy shift as a function of the pump power, and (f) degree of circular polarization, calculated by utilizing the corresponding Boltzmann rate equation model (see text). The parameters for the modelling are $W={10}^{-3}{\text{ps}}^{-1}$, ${\gamma }_c={\left(20\right)}^{-1}{\text{ps}}^{-1}$, ${\gamma }_r=5^{-1}{\text{ps}}^{-1}$, ${\gamma }_{rs}=1{\text{ps}}^{-1}$.

Figure 2

(a) Degree of circular polarization characterized by the Stokes parameter $S_3$ of the condensate as a function of normalized input power for different photon-exciton detunings of the planar sample. (b) Modelled polarization $S_3$ of the condensate under the nearly linear polarization of the pump ($S_{3P}=0.05$) calculated for different values of the spin-scattering parameter ${\gamma }_{rs}$. The parameter ${\gamma }_{rs}$ depends on the photon-exciton detuning and can therefore explain the behavior of the condensate observed in panel (a).

Figure 3

(a) Stokes parameter resolved measurement of a $6\mu \mathrm{m}$ micropillar microcavity below threshold under nonresonant ${\sigma }^{-}$ excitation (red) and ${\sigma }^{+}$ excitation (orange) and the same measurement technique above threshold (${\sigma }^{-}$ blue, ${\sigma }^{+}$ green), where a high degree of polarization conservation can be observed. (b) Linear polarization study of randomly chosen $2\mu \mathrm{m}$ micropillars showing linearly polarized emission oriented along different axes, driven by a linearly polarized pump laser. (d) Scanning electron microscopy (SEM) images of an elliptical micropillar with (c) $3:2$ aspect ratio and (d) $2:3$ aspect ratio. (e) Stokes parameter resolved measurement for polarized excitations of an elliptical micropillar in the condensate regime: linear (black), ${\sigma }^{+}$ (green) and ${\sigma }^{-}$ (red). Similar Stokes parameters, indicating linear polarization, are observed for all pump polarizations in contrast to the system with rotational symmetry. The exciton-photon detuning is $-5$ meV. (f) Linear polarization resolved intensity is measured with a $\lambda /2$ wave plate for two elliptical systems with the long axis orthogonal to each other.