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National Science Review

Femtosecond coherence dynamics of exciton–polaritons

Haoyuan Jia, Junhui Cao, Fei Chen, Fangying Peng, Yihui Li, Yihan Xu, Leizhu Chen, Ziyu Ye, Xianyan Zhao, Shian Zhang, Jietai Jing, Hongxing Xu, Zhanghai Chen, Tim Byrnes, Hui Li, Alexey Vitalievich Kavokin, Jian Wu

Published Nov 7, 2025• 2 citations
Publisher

Abstract

Exciton-polaritons perform as ideal carriers of macroscopic quantum coherence, which can be potentially manipulated through precisely shaping the driving laser field. However, the connection of the coherence properties between the pumping laser and the strongly coupled light-and-matter system is studied to a lesser extent. In this paper, we visualize the femtosecond dynamics of coherence transfer from the driving laser field to the resonantly excited exciton-polariton by an interferometric measurement. The resonant polaritons can effectively preserve the coherence of the pumping laser field in femtosecond timescales. At a high excitation strength, non-resonant polaritons appear at higher energies delayed by several picoseconds, without the phase coherence from the pump, which is understood by a coupled oscillator model. Our results offer the possibility of regulating the polariton coherence by finely shaping the external pump laser fields.

DOI
10.1093/nsr/nwaf493

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Exciton-polaritons perform as ideal carriers of macroscopic quantum coherence, which can be potentially manipulated through precisely shaping the driving laser field. However, the connection of the coherence properties between the pumping laser and the strongly coupled light-and-matter system is studied to a lesser extent. In this paper, we visualize the femtosecond dynamics of coherence transfer from the driving laser field to the resonantly excited exciton-polariton by an interferometric measurement. The resonant polaritons can effectively preserve the coherence of the pumping laser field in femtosecond timescales. At a high excitation strength, non-resonant polaritons appear at higher energies delayed by several picoseconds, without the phase coherence from the pump, which is understood by a coupled oscillator model. Our results offer the possibility of regulating the polariton coherence by finely shaping the external pump laser fields.
We demonstrate macroscopic coherence between quantum condensates generated at different times, separated by more than the particle dephasing time. This is possible due to the dressed light-matter nature of exciton-polaritons, which can be injected resonantly by optical excitation at well-defined momenta. We show that the build-up of coherence between condensates depends on the interaction between the particles, particle density, as well as temperature despite the non-equilibrium nature of the condensate, whereas the mass of the particles plays no role in the condensation of resonantly injected polaritons. This experiment also makes it possible for us to measure directly the large nonlinear phase shift resulting from the polariton-polariton interaction energy. Our results provide direct evidence for coherence between different condensates and demonstrate a new approach for probing their ultrafast dynamics, opening new directions in the study of matter coherence as well as in practical applications such as quantum information and ultrafast logic.
Coherence transfer is a multi-disciplinary topic of interest, including chemistry, biology, and physics. In quantum technologies, achieving non-local coherent coupling between solid-state qubits is of the utmost importance. Here, we demonstrate that excitons—i.e., electron–hole pairs bound by the Coulomb force within a quantum well—can act as a medium for mesoscopic optical coherence transfer in semiconductors. To this end, we use a femtosecond laser pulse to resonantly generate excitons within the light cone. These excitons can then either recombine radiatively or scatter out of the light cone, gaining an in-plane momentum in the process. In samples without disorder, such as the CdTe quantum wells used here, the resulting fast excitons can diffuse over mesoscopic distances before recombining radiatively. Using coherent nonlinear micro-spectroscopy, we carry out exciton time-of-flight measurements. Specifically, we monitor the spatio-temporal propagation of launched exciton wave packets, selectively observing their coherence or density on a scale of up to 10 µm. Our proof-of-principle experiment demonstrates that free excitons inherit a phase modulation from the optical pulsed excitation and can generate coherent links within excitonic circuits, offering a higher level of miniaturisation and compactness than photonic or polaritonic architectures.
Coherence transfer is a multi-disciplinary topic of interest, including chemistry, biology and physics. In quantum technologies, achieving non-local coherent coupling between solid-state qubits is of the utmost importance. Here, we demonstrate that excitons - i.e. electron-hole pairs bound by the Coulomb force within a quantum well - can act as a medium for mesoscopic optical coherence transfer in semiconductors. To this end, we use a femtosecond laser pulse to resonantly generate excitons within the light cone. These excitons can then either recombine radiatively or scatter out of the light cone, gaining an in-plane momentum in the process. In samples without disorder, such as the CdTe quantum wells used here, the resulting fast excitons can diffuse over mesoscopic distances before recombining radiatively. Using coherent nonlinear micro-spectroscopy, we carry out exciton time-of-flight measurements. Specifically, we monitor the spatio-temporal propagation of launched exciton wave packets, selectively observing their coherence or density on a scale of up to 10 μ\,μμm. Our proof-of-principle experiment demonstrates that free excitons inherit a phase modulation from the optical pulsed excitation and can generate coherent links within excitonic circuits, offerring a higher level of miniaturisation and compactness than photonic or polaritonic architectures.
Exciton polaritons are coupled states of matter and light, originated by the strong interaction between an optical mode and semiconductor excitons. This interaction can be obtained also at a single-particle level, in which case it has been shown that a quantum treatment is mandatory. In this work we study the light-matter entanglement of polaritons from a fully quantum formalism including pumping and dissipation. We find that the entanglement is completely destroyed if the exciton and photon are tuned at the resonance condition, even under very low pumping rates. Instead, the best condition for maximizing entanglement and purity of the steady state is when the exciton and photon are out of resonance and when incoherent pumping exactly compensates the dissipation rate. In the presence of multiple quantum dots coupled to the light mode, matter-light entanglement survives only at larger detuning for a higher number of quantum dots considered.
Time-dependent spatial modulation of a quantum condensate plays a key role in the investigation of the dynamics of collective coherent systems. Cold-atom condensates were already manipulated into forming solitons, vortices and optical lattices via the dynamic Stark effect. In the solid-state framework, semiconductor microcavity exciton-polaritons present an excellent platform for observing nonequilibrium condensate dynamics. Furthermore, the dynamic manipulation of exciton-polariton condensates offers numerous potential implementations in all-optical logic devices. We report the first observation of the energy modulation of an exciton-polariton condensate via the dynamic Stark effect. A Stark pulse is employed to induce a transient, coherence-preserving blueshift in the energy of the condensate of femtosecond-scale duration. A novel approach is presented for discerning the Stark effect in a condensate from the uncondensed case according to distinctive features in differential reflectance spectra, including spectro-temporal oscillations enabling to detect the buildup of coherence. The ultrafast and non-invasive modulation of the exciton-polariton condensate demonstrated here opens new research avenues of nonequilibrium physics of solid-state condensates as well as opportunities for polariton-based elements in both classical and quantum information technologies.
Abstract Investigations of femtosecond dynamics of excitons in two different regimes are reported: 1. perturbative or weak coupling regime where the exciton decay is irreversible (i.e. excitons in quantum wells), and 2. non‐perturbative or strong coupling regime (i.e. quantum well excitons in microcavities) where the coherent exchange of energy between the coupled cavity and exciton modes dominates the femtosecond dynamics. In the perturbative regime, the first femtosecond luminescence measurements are reported, which show (i) a finite rise time for the emission from resonantly‐excited excitons and (ii) femtosecond quantum beats of heavy‐hole and light‐hole excitons. In microcavities, the first nonlinear measurements are reported, which show quantum beats from the coupled cavity‐exciton modes, and provide information about the system not available from linear measurements.