We demonstrate the existence of a cross Feshbach resonance by strongly driving a lower polariton mode and by monitoring in time the transmission of a short optical pulse at the energy of the upper polariton mode in a semiconductor microcavity. From the signatures of the optical resonance, strength, and sign of the energy shift, we attribute the origin of the scattering process between polariton modes with opposite circular polarization to a biexciton bound state. From this study, we infer the conditions required for a strong enhancement of the generation of entangled photon pairs.
We demonstrate the existence of a cross Feshbach resonance by strongly driving a lower polariton mode and by monitoring in time the transmission of a short optical pulse at the energy of the upper polariton mode in a semiconductor microcavity. From the signatures of the optical resonance, strength, and sign of the energy shift, we attribute the origin of the scattering process between polariton modes with opposite circular polarization to a biexciton bound state. From this study, we infer the conditions required for a strong enhancement of the generation of entangled photon pairs.
We develop a many-body theory for the properties of exciton-polaritons interacting strongly with a Bose-Einstein condensate (BEC) of exciton-polaritons in another spin state. Interactions lead to the presence of a two-body bound state, the biexciton, giving rise to a Feshbach resonance in the polariton spectrum when its energy is equal to that of two free polaritons. Using the minimal set of terms to describe this resonance, our theory recovers the main findings of two experiments probing interaction effects for upper and lower polaritons in a BEC. This strongly supports that Feshbach physics has, indeed, been realized, and we furthermore extract the energy and decay of the biexciton from the experimental data. The decay rate is predicted to be much larger than that coming from its dissociation into two free polaritons, indicating that other decay channels are important.
We investigate polariton quantum correlations in a coherently driven box cavity in the low driving regime, with a particular focus on accounting for the polarization degree of freedom. The possibility of having different interaction strengths between co- and cross-circularly polarized polaritons as well as a realistic linear-polarization splitting allows one to model the system as two coupled nonlinear resonators with both self- and cross-Kerr-like nonlinearities, thus making our results potentially relevant for other experimental platforms. Within an effective wave-function approach, we obtain analytical expressions for the steady-state polarization-resolved polariton populations and second-order correlation functions, which agree very well with our numerical results obtained from a Lindblad master equation. Notably, we highlight that depending on the excitation polarization (circular or linear), both the unconventional (interference-mediated) and conventional (mediated by nonlinearities) antibunchings can be investigated in a single cavity. Moreover, using our results, we argue that recent experiments on confined fiber-cavity polaritons are likely to have probed a regime where the dominant interaction is between cross-polarized polaritons, which is characteristic of the polariton Feshbach resonance. We furthermore investigate the regime close to resonance using a two-channel model and we show that systems with large biexciton binding energies, such as atomically thin semiconductors, are promising platforms for realizing strong polariton antibunching.
Summary form only given. Semiconductor microcavities can be engineered to produce strong coupling between photons and quantum well excitons leading to the formation of new eigenstates called polaritons. Due to their peculiar dispersion, enabling non-equilibrium Bose-Einstein condensation [1], and their straightforward experimental accessibility, microcavity polaritons have gained considerable interest in recent years. Polariton states can be selectively populated by choosing the excitation angle and the wavelength of the excitation laser. After a lifetime of a few picoseconds the polariton decays and a photon is emitted from the cavity, carrying the complete information on the underlying polariton state. Several theoretical works [2-3] suggest that polariton-polariton interactions can be a source of non-classical photon correlations, producing photon pairs that are entangled in either the energy, the polarization, or the momentum degree of freedom. One appeal of such a system is that many parameters such as the polariton-polariton interaction strength are tunable [4] and multipartite entangled states can be created in a single device [5]. However, due to the lack of high-quality samples, experimental realisations of these schemes have been elusive so far. In this contribution, we will discuss some of our experimental work on the statistics of polariton-polariton parametric scattering. Excited polaritons can scatter in a four-wave mixing process into signal and idler states lying on a figure-eight shaped curve in k-space, where phase-matching is fulfilled. We make use of a sample with ultra narrow exciton resonances, which allows for the generation and detection of correlated light. We present our measurements of the second order correlation functions using time-correlated photon counting. We discuss in detail, how polariton quantum correlations depend on a number of experimental parameters, such as phase space location and excitation density and find that the results can be understood in the framework of the well known Bogoliubov theory for 2D Bose gases utilised for microcavity polaritons [6]. We find bunching below the parametric threshold and a slow buildup of coherence above the threshold similar to the quantum statistics of a non-degenerate optical parametric oscillator, which is well known for its nonclassical properties [7]. We discuss in particular the influence of strong background processes on the observed photon statistics.
Nonlinear optical response of cavity polaritons formed in high-g semiconductor microcavities has attracted much attention because of their potential applications to entangled-photon sources and other quantum information technologies. Theoretical ideas of entangled-photon generation, e.g., biexcitonic cavity quantum electrodynamics and polariton-polariton scattering, have been suggested, however these theories are limited in scope of target because they restrict two-exciton states to either biexciton or unbound two-exciton states. As for exciton system with small binding energy of biexciton, it is likely that the interplay between the biexciton and unbound two-exciton states have much impact on generation efficiency of entangled photons. The authors introduces a general model including both biexciton and unbound two-exciton states and discuss generation efficiency of entangled photons.
We theoretically investigate polarization-entangled-photon generation by using a quantum dot embedded in a microcavity. In the absence of a biexciton, two of the four Bell states can be generated by applying cross-circularly polarized laser fields because of a cavity quantum electrodynamics effect [Ajiki et al., New J. Phys. 11, 033033 (2009)]. In the presence of the biexciton state, the generation of these Bell states can be forbidden under specific conditions. Furthermore, another type of Bell state can be generated by applying cross-circularly polarized fields. Our results indicate that one can generate three different types of Bell-state photon pairs just by changing the frequencies of the applied fields.
Microcavity polariton spin dynamics is investigated in high-finesse microcavities under resonant excitation using polarization- and time-resolved secondary emission spectroscopy. Using an appropriate calibration procedure, the instantaneous polariton occupation numbers of the modes with $\mathbf{k}\ensuremath{\approx}\mathbf{0}$ wave vectors can be determined. When the phase-matching conditions are achieved, we show that Coulomb scattering dominates the other processes, and that stimulation occurs when the occupation of the $\mathbf{k}\ensuremath{\approx}\mathbf{0}$ final state with a given mode polarization approaches unity. Under elliptically polarized light, we show that the stimulated polariton scattering leads to an increase with time of the circular polarization of the $\mathbf{k}\ensuremath{\approx}\mathbf{0}$ states, while the linear polarization dynamics is characterized first by a fast drop to negative or zero values immediately after the excitation pulse, followed by weak-amplitude nonperiodic oscillations. The latter are interpreted as the manifestation of the pseudospin precession of the pumped polariton modes around the self-induced exchange effective field.