Terahertz emission from multiple-microcavity exciton-polariton lasers

Terahertz emission between exciton-polariton branches in semiconductor microcavities is expected to be strongly stimulated in the polariton laser regime due to the high density of particles in the lower state (final-state stimulation effect). However, nonradiative scattering processes depopulate the upper state and greatly hinder the efficiency of such terahertz sources. In this work, we suggest a scheme using multiple microcavities and exploiting the transition between two interband polariton branches located below the exciton level. We compare the nonradiative processes loss rates in single and double cavity devices and we show that a dramatic reduction can be achieved in the latter, enhancing the efficiency of the terahertz emission.

Figure 1

(a) Band structure of the AQW along the growth axis $z$, together with the electron (red) and heavy hole (blue) wave functions of the states 1 (full line) and 2 (dashed line). (b) Computed dispersion spectrum for nine identical AQWs placed in a single microcavity. The color scale shows the photonic part of the polariton states. The white dashed lines indicate the energy of the bare $e1hh1$ and $e1hh2$ excitons and the red arrow points to the optically pumped state. (c) Schematic representation of the microcavity with the AQWs layer (red line), also showing the pump and THz beams. (d) Schematic polariton band structure for the single microcavity scheme. The THz transition is represented by a green vertical arrow, scattering by interface roughness and acoustic phonons are indicated with full black and dotted red arrows, respectively. Horizontal polariton-polariton collisions are represented by the two dashed blue arrows.

Figure 2

(a) Schematic representation of the double microcavity with the AQWs layers (red lines). In order to generate THz radiation, the AQWs layer of the top cavity should be removed (dashed red line). (b) and (c) Computed dispersion spectrum for nine identical AQWs placed in the bottom microcavity. The color scale shows the photonic part of the polariton states inside the bottom (b) and top cavity (c). The white dashed lines indicate the energy of the bare $e1hh1$ and $e1hh2$ excitons and the red arrow points to the optically pumped state. The energy splitting between the two bare cavity modes (without AQW) is 12 meV.

Figure 3

(a) THz emission (green), low-temperature acoustic phonon scattering (red) interface roughness scattering (black) and polariton-polariton collision rates (blue) are plotted as a function of the detuning $\delta$ between the polariton state $|U\rangle$ and the excitonic reservoir $e1hh1.$ (b) Schematic representation of the polariton dispersion for the double cavity of Fig. 2 at negative $\delta$. The considered scattering processes are represented as in Fig. 1. The density of final states after scattering is strongly reduced compared to the single cavity scheme.