Polarization-coupled polariton pairs in a birefringent microcavity

We demonstrate a clear anticrossing between two polarization-orthogonal exciton-polariton modes in ZnO microwires. The strong coupling between exciton polaritons with orthogonal polarization is evidenced by using angle-resolved photoluminescence spectroscopy. The evolution of the coupling strength with the detuning between the two coupled polariton modes is studied by scanning the excitation spot on a tapered ZnO microwire. Steady oscillations are established between the two differently polarized polariton modes with resonant wave vectors and frequencies, which is well described by the plane wave coupling model.

Figure 1

(a) The scanning electron microscope image of a single ZnO microwire. c is the crystallographic axis. (b) Magnification image of the partial polarization detail. The dark points reflect the polarization direction of ordinary light ($o$), and the blue lines the polarization direction of extraordinary light ($e$). The green and black dashed lines are the components of the polarization direction of extraordinary light ($e$).

Figure 2

The low-temperature angle-resolved photoluminescence spectral ($\mathbf{k}$-space) mapping under continuous He-Cd laser (325 nm) excitation. (a) TE- and (b) TM-polarized polaritons. Dashed curves: Theoretical fitting [with the coupling term in Eq. (4)] for the lower polariton branches. Blue dashed curves represent TM-polarized polaritons and the green dashed curves represent TE-polarized polaritons. Traditional theoretical fitting [without the coupling term in Eq. (4)] for the lower polariton branches. White dashed curves represent TE-polarized polaritons and the black dashed curves represent TM-polarized polaritons. The field distribution simulated by the finite element analysis (FEA) method for (c) TE- and (d) TM-polarized polaritons.

Figure 3

(a) TE component distribution of the upper and lower coupled polariton modes depending on ${\mathbf{k}}_{\parallel }$. The black squares and red dots reflect the experimental TE component of the upper and lower coupled polariton modes relative to the total TE component, respectively. The black and red curves denote the theoretical TE component of the upper and lower coupled polariton modes, respectively. (b) TM component distribution of the upper and lower coupled polariton modes depending on ${\mathbf{k}}_{\parallel }$, correspondingly.

Figure 4

(a) TE-polarized polariton detected from different positions of the ZnO microwire. (b) TM-polarized polariton detected from different positions of the ZnO microwire. The mode number of the TE- and TM-polarized polaritons is $N=59$. The black dashed curves denote TM-polarized polaritons obtained by theoretical fitting, and the white dashed curves denote TE-polarized polaritons. $\Delta E=E_{\text{TE}}({\mathbf{k}}_{\parallel }=0)-E_{\text{TM}}({\mathbf{k}}_{\parallel }=0)$, where ${\mathbf{k}}_{\parallel }$ is the component of the polariton wave vector parallel to the c axis, $E_{\text{TE}}$ (${\mathbf{k}}_{\parallel }=0$) is the energy of the TE-polarized polariton with ${\mathbf{k}}_{\parallel }=0$, and $E_{\text{TM}}$ (${\mathbf{k}}_{\parallel }=0$) is the energy of the TM-polarized polariton with ${\mathbf{k}}_{\parallel }=0$.