A theoretical model of a driven-dissipative polariton condensate reports a preferred direction for stripe-like density patterns in an anisotropic case with unequal effective masses. In a selected simulation, stripes oriented along y appeared at about 25 ps; the modeled condensate number fluctuated, and the stripes repeatedly appeared and disappeared as the run continued. The work reports modeled behavior from equations and numerical trajectories.
A directional instability, not a new instability rule
The study asks how mass anisotropy affects modulational instability — a modeled tendency for small disturbances to grow into larger patterns — along with pattern formation and collective motion. It represents the condensate and its reservoir as mean fields governed by an open-dissipative Gross-Pitaevskii equation and a reservoir rate equation. For homogeneous pumping, the authors used Bogoliubov-de Gennes linear stability analysis and integrated the coupled equations numerically with a split-step Fourier method. Together, those calculations identify unstable modes and follow their evolution.
In the homogeneous-pump calculation, the modulational-instability criterion was unchanged by mass anisotropy. What changed was the shape of the unstable-mode dispersion: as the mass ratio increased, the most-unstable wavelength became shorter along the direction with the larger mass. In everyday terms, the calculation kept the same basic condition for instability but shifted the preferred pattern scale in a direction-specific way.
The directional shift was visible in the reported wavevectors. For mass ratios of 1, 2, 3 and 4, the most-unstable pairs were respectively (1.21, 1.21), (1.21, 1.71), (1.21, 2.09) and (1.21, 2.42) µm⁻¹. The x component stayed at 1.21 µm⁻¹, while the y component increased with the ratio.
The pump width changed the story
With a Gaussian pump, the isotropic-mass baseline showed several modeled regimes as the beam width changed. At σ = 2.95 µm, the state remained stable. Near the characteristic wavelength, σ = 3.53 µm corresponded to self-breathing and σ = 3.71 µm to self-rotation. Further widening was accompanied by pattern formation.
For larger beam widths, the anisotropic profile changed from a symmetric Gaussian to an ellipse whose major axis lay along x while the model remained modulationally stable. That profile transformation persisted only up to a critical mass ratio, called mc in the analysis; above it, the model entered a modulationally unstable, pattern-forming regime. The text identifies the threshold as a function of pump width but does not state its numerical values.
Stripes that move in one direction
In the reported anisotropic run with my/mx = 4 and σ = 2.36 µm, stripes appeared around t = 25 ps. The condensate number showed rapid fluctuations, and the stripes repeatedly appeared and disappeared during the simulated evolution.
The motion was directional in the model. The condensate center of mass stayed at x = 0, while its y coordinate began oscillating after about 25 ps. The calculated x current remained zero throughout, whereas the y current oscillated after about 25 ps.
Momentum space supplied a matching signature. At t = 40 ps, the modeled distribution showed symmetric side peaks at ky = ±2.60 µm⁻¹, consistent with a y-direction density modulation whose reported wavelength was 2.42 µm. This gave the simulated stripe state a corresponding momentum-space signature.
At the larger width of σ = 4.71 µm with my/mx = 4, stripes initially developed along y but later became dynamically distorted. Even after that distortion, the anisotropic model retained a preferred direction relative to the isotropic case.
Evidence bounded by the model
The evidence is bounded by the setup: a mean-field open-dissipative model, a linear stability calculation and numerical simulations under selected parameter choices. The reported wavevector values cover selected mass ratios of 1, 2, 3 and 4. The supplied analysis reports no experimental validation, replicate analysis, error bars or statistical tests.
That means the results describe what the specified equations and simulations do; they do not establish that directional stripes or oscillations occur in a physical condensate. The document is a preprint, arXiv:2608.25825v1, dated 26 Aug 2026. Whether the modeled predictions are reproduced experimentally remains an open question.
Paper data and sources
Original title: Mass-anisotropy driven stripe pattern formation and directional modulational instability in polariton condensate
Authors: Hari Sadhan Ghosh, Soumyadeep Halder, Subrata Das, Sonjoy Majumder
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text