Room-Temperature Spin Polariton Diode Laser

A spin-polarized laser offers inherent control of the output circular polarization. We have investigated the output polarization characteristics of a bulk GaN-based microcavity polariton diode laser at room temperature with electrical injection of spin-polarized electrons via a $\mathrm{FeCo}/\mathrm{MgO}$ spin injector. Polariton laser operation with a spin-polarized current is characterized by a threshold of $\sim 69\mathrm{A}/{\mathrm{cm}}^2$ in the light-current characteristics, a significant reduction of the electroluminescence linewidth and blueshift of the emission peak. A degree of output circular polarization of $\sim 25\%$ is recorded under remanent magnetization. A second threshold, due to conventional photon lasing, is observed at an injection of $\sim 7.2\mathrm{kA}/{\mathrm{cm}}^2$. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed with the carrier and exciton rate equations and the Gross-Pitaevskii equations for the condensate and there is good agreement between measured and calculated data.

Figure 1

(a) Schematic representation of the spin-polarized double heterostructure GaN-based microcavity diode (not drawn to scale) in the quasi-Voigt measurement geometry. (b) Subthreshold angle-resolved electroluminescence spectra recorded after in-plane magnetization of the ferromagnetic contacts with $H\sim +1.6\mathrm{kOe}$. The spectra have been vertically offset for clarity. The dashed lines are guides to the eye representing the lower-polariton and excitonic transitions. (c) Corresponding polariton dispersion curves calculated using a coupled harmonic oscillator model. The red (wine) circles represent the computed cavity photon (upper-polariton) dispersion and the blue circles correspond to the “data points” in (b). (d) Normal incidence ($k_{\parallel }\sim 0$) LP electroluminescence intensities recorded as a function of injected current density after in-plane magnetization of the contacts with $H\sim \pm 1.6\mathrm{kOe}$. The vertical arrow indicates the onset of nonlinearity.

Figure 2

(a) Normal incidence ($k_{\parallel }\sim 0$) LP electroluminescence intensities recorded as a function of injected current density. (b) Two threshold lasing behavior with the nonlinearities due to polariton and photon lasing. The inset shows an enlargement highlighting the threshold at higher injection. (c) LP emission linewidth and blueshift of the LP electroluminescence peak emission energy as a function of injected current density. The resolution of the measurement ($\sim 4.6\mathrm{meV}$) is indicated as a horizontal dashed line. (d) LP emission intensities for different $k_{\parallel }$ states as a function of the energy difference $E(k_{\parallel })-E(k_{\parallel }\sim 0)$, determined from angle-resolved electroluminescence measurements at different injection densities. The measurements were made after in-plane magnetization of ferromagnetic contacts with $H\sim +1.6\mathrm{kOe}$. The vertical arrows in (a) and (b) indicate the onset of the nonlinearity due to polariton lasing. The solid lines are guides to the eye.

Figure 3

(a) Measured normal incidence ($k_{\parallel }\sim 0$) circular polarization-resolved LP electroluminescence intensities as a function of injected current density recorded after in-plane magnetization of ferromagnetic contacts with $H\sim +1.6\mathrm{kOe}$. The total light intensity $S$ is the summation of the right-hand ($S^{+}$) and left-hand circularly polarized ($S^{-}$) LP emission intensities. The arrow indicates the onset of nonlinearity and the solid lines are guides to the eye. (b) Measured steady state degree of circular polarization as a function of injected current density recorded after in-plane magnetization of ferromagnetic contacts with $H\sim +1.6\mathrm{kOe}$. The red (green) solid lines in (b) represent the calculated values assuming strong (weak) coupling in the microcavity.