Polariton lasing by exciton-electron scattering in semiconductor microcavities

Abstract

The relaxation bottleneck present in the dispersion relation of exciton polaritons in semiconductor microcavities has prevented the realization of low threshold lasing based on exciton-polariton condensation. Here we show theoretically that the introduction of a cold electron gas into such structures induces efficient electronpolariton scattering. This process allows the condensation of the polaritons accumulated at the bottleneck to the final emitting state with a transition time of a few picoseconds, opening the way to a new generation of low-threshold light-emitting devices. DOI: 10.1103/PhysRevB.65.153310 PACS number͑s͒: 78.47.ϩp, 42.50.Ϫp, 42.65.Ϫk, 71.36.ϩc The observation of the strong coupling of light with excitons in semiconductor microcavities 1 has generated much speculation regarding the possibility for low-threshold optical devices.2,3 Realization of such devices based on the Bosonic character of the optical eigenmodes ͑exciton polaritons 4 ͒ of these structures would be a revolutionary step in semiconductor optics. However, Bose condensation of exciton polaritons has not yet been observed. One of the main obstacles is set by a bottleneck in the polariton relaxation rate.5-7 As a result, the emission of the microcavity under nonresonant excitation remains weak and nondirectional.Here we propose an original relaxation mechanism based on the scattering of polaritons with free electrons. This allows polaritons to relax efficiently from the bottleneck region to the lowest energy state once free electrons are introduced into the active region either via doping or by photoexcitation. This opens the way to realization of lowthreshold laserlike devices based on cavity polaritons.The basic principle of a polariton laser is illustrated in Fig. 1͑a͒ using the dispersion curve of the lower branch exciton polaritons in a typical microcavity. The strong-coupling regime creates a trap containing a small number of polariton states at energies below all other states in the semiconductor. This polariton trap is sharp with a depth equal to nearly half the splitting ⍀ between the two polariton modes. Polaritons in the trap are half photon and half exciton, and have properties suitable for the Bose condensation of exciton polaritons once sufficiently populated. Recombination from this state in the Bose condensation regime is coherent, monochromatic, and sharply directed, characteristic of laser emission. The relaxation of polaritons into the kϭ0 state is found to be stimulated if the population of the final state is larger than 1. The amplification process proposed here is physically very different from the classical lasing process.9 In particular, the threshold to lasing which in conventional lasers is conditional on the inversion of population is only dependent on the lifetime of the ground state in the polariton laser. As soon as relaxation to the ground state of the trap becomes faster than the radiative recombination from this state, optical amplification is achieved. Note that the absorption and re-absorption...

Generic picture of the emission properties of III-nitride polariton laser diodes: Steady state and current modulation response

“…This is attributed to the difference in the efficiency of the free electron scattering mechanism as a function of temperature. 55 For the direct electrical pumping geometry, the electron distribution does not depend on T latt since electrons are not thermalized, thus leading only to slight changes in the free electron scattering rate with temperature and thereby explaining the weak δ opt (T ) variation dis-played in Figs. 4(a) and 4(c).…”

Spontaneous Rotating Vortex Lattices in a Pumped Decaying Condensate

“…One particular difference shown by these systems is that the quasiparticles have finite lifetimes, and as a result, they can be made to form condensates out of equilibrium, which are best understood as a steady state balance between pumping and decay, rather than true thermal equilibrium. The effects of pumping and decay in these condensates have been the subject of several recent works [5,11,12,13,14,15,16,17,18,19,20] which have shown that even when collisions can rapidly thermalise the energy distribution of a system, there may yet be noticeable effects associated with the energy scale introduced by the pumping and decay.The Gross-Pitaevskii equation (GPE) has been applied to successfully describe many features of equilibrium condensates when far in the condensed regime, including density profiles, the dynamics of vortices, hydrodynamic modes -see e.g. [21] and Refs.…”

Disorder Effects on Exciton–Polariton Condensates

“…Despite their short life time, they can thermalize to a quasi-thermal (Bose) distribution [9,10,11,12,13] which can in principle allow the polariton gas to undergo a Berezinskii-Kosterlitz-Thouless phase transition towards a superfluid state [14,15,16]. In CdTe or GaN cavities, this superfluid behavior of the condensed phase was not observed because of the presence of a strong in plane disorder which tends to localize the condensate, leading to the formation of a glassy phase [17].…”