Ultrafast Oscillations of a Ballistically Propagating Polariton Condensate Driven by Inter-mode Coherent Energy Transfer

The interplay of dynamics and transport leads to intriguing spatiotemporal behaviors of nonequilibrium macroscopic quantum systems. By means of time-resolved spectroscopy, we here provide microscopic insights into the interplay of ballistic transport and many-particle interactions for an exciton-polariton condensate. We observed anomalous condensation of a coherent polariton flow propagating away from the hot reservoir, accompanied by ultrafast oscillations of its population in the time domain with a period of a few picoseconds. On the basis of time- and spatially-resolved photo luminescence imaging characterization, we reveal that the inter-mode coherent energy transfer, controllable via an incoherent excitonic reservoir, gives rise to the observed ultrafast oscillations. Theoretically, modeling was conducted using an open-dissipative Gross-Pitaevskii equation, with the simulation results fully reproducing our experimental observations. These results advance the fundamental understanding of light-matter interactions in nonequilibrium systems associated with the interplay of dynamics and transport.

💡 Research Summary

In this work the authors investigate the spatiotemporal dynamics of a ballistically propagating exciton‑polariton condensate in a one‑dimensional ZnO microrod, focusing on a striking ultrafast oscillation of the condensate population that appears only above a certain non‑resonant pump threshold. The experimental platform consists of a 370 nm femtosecond laser (300 fs pulse width, 200 kHz repetition) that creates a hot exciton reservoir in the microrod. Angle‑resolved photoluminescence (AR‑PL) maps reveal a series of lower‑polariton branches (labelled 79th, 80th, 81st, etc.) that follow the coupled‑oscillator dispersion. At low pump powers (≈0.1 P_th) the system is in the linear regime, showing well‑defined parabolic emission belts. When the pump power reaches the condensation threshold (≈P_th) the condensate is expelled from the pump spot by the repulsive exciton reservoir, acquiring a finite in‑plane momentum and forming bright spots at large angles (≈±34.5°) on the 81st branch.

Increasing the pump further (≈3.4 P_th) leads to an unexpected accumulation of polaritons at the bottom of the 81st branch (k≈0) together with a clear oscillatory modulation of the emission intensity in the time domain, as recorded by a streak camera with ~2 ps resolution. A similar phenomenon occurs on the 80th branch at even higher pump powers (≈7.5 P_th). The oscillations have a period of a few picoseconds and become more pronounced with increasing pump density.

The authors attribute these observations to an inter‑mode coherent energy transfer mediated by the decaying exciton reservoir. As the reservoir density drops, the blue‑shifted condensate on the N‑th branch experiences a red‑shift. When the instantaneous energy of the N‑th branch matches the energy gap ΔE_N between the N‑th and (N+1)‑th branches, a resonant condition is fulfilled. Under this condition, stimulated scattering (bosonic stimulation) from the N‑th branch into the dispersion minimum of the (N+1)‑th branch is strongly enhanced. The scattered polaritons accumulate at k≈0, where their group velocity vanishes, allowing efficient collisions with the still‑propagating condensate. This feedback loop produces a rapid rise and subsequent decay of the (N+1)‑th branch population, which repeats as the reservoir continues to evolve, giving rise to the observed picosecond oscillations.

To substantiate the picture, the authors formulate a set of coupled open‑dissipative Gross‑Pitaevskii equations for the wavefunctions Ψ_N, Ψ_{N+1}, Ψ_0 (the condensate at the (N+1)‑th branch minimum) and the reservoir density R. The equations include polariton‑polariton interaction (g), polariton‑reservoir interaction (g_R), stimulated scattering rates (β for reservoir‑to‑condensate, Γ for inter‑branch scattering), and decay rates (γ). Numerical integration reproduces the experimental intensity traces for the 79th, 80th, and 81st branches at both 3.4 P_th and 9.1 P_th, capturing the emergence, amplitude, and period of the oscillations.

Additional evidence comes from (i) the temporal evolution of the condensate blueshift: the blueshift of the 81st branch crosses the energy gap ΔE_81 at ≈14.5 ps, precisely when the second intensity peak appears (≈18.2 ps); (ii) a power‑dependent analysis of the intensity ratio between two spatial points (A: k≈0 on the 81st branch, B: same energy on the 80th branch) shows a clear threshold behavior, consistent with the nonlinear nature of the stimulated scattering; (iii) spatially resolved streak images confirm that the oscillation‑generating scattering occurs predominantly at the edge of the pump spot, where the energy difference between centre and edge polaritons is maximal and the scattered polaritons remain stationary.

The study provides several key insights. First, the dynamic decay of the exciton reservoir can act as a temporal “tuning knob” that brings different polariton modes into resonance, enabling ultrafast, coherent energy transfer. Second, bosonic stimulation can amplify this transfer to the point where the population of a higher‑energy mode is periodically replenished, leading to picosecond population oscillations. Third, the phenomenon is intrinsically linked to the spatial profile of the pump: the edge of the excitation spot is the preferred region for resonant scattering because of the large kinetic‑potential energy conversion and the zero‑group‑velocity condition of the scattered polaritons.

From an application perspective, the ability to trigger and control inter‑mode coherent scattering on a picosecond timescale opens avenues for ultrafast polaritonic switches, modulators, and logic elements that exploit the non‑equilibrium nature of polariton condensates. Moreover, the demonstrated agreement between experiment and an open‑dissipative Gross‑Pitaevskii framework validates this model as a versatile tool for describing multi‑mode, non‑linear dynamics in other low‑dimensional semiconductor microcavities, such as 2D transition‑metal dichalcogenide monolayers or perovskite nanowires.

In summary, the paper uncovers a novel mechanism—inter‑mode coherent energy transfer driven by the decaying exciton reservoir—that generates ultrafast oscillations in a ballistically propagating polariton condensate. The combined experimental and theoretical approach convincingly demonstrates that resonant stimulated scattering between adjacent polariton branches can dominate the dynamics once the condensate blueshift exceeds the inter‑branch energy gap, leading to a rich, controllable, picosecond‑scale population dynamics in a nonequilibrium quantum fluid.