Resonant light interacting with matter supports different phases of a polarisable medium, and optical bistability where two phases coexist. Such phases have previously been actively studied in cavities. Here, we identify signatures of optical phase transitions and optical bistability mapped onto scattered light in free-space planar arrays of cold atoms. Methods on how to explore such systems in superradiant and extreme subradiant states are proposed. The cooperativity threshold and intensity regimes for the intrinsic optical bistability, supported by resonant dipole-dipole interactions alone, are derived in several cases of interest analytically. Subradiant states require lower intensities, but stronger cooperativity for the existence of non-trivial phases than superradiant states. The transmitted light reveals phase transitions and bistability that are predicted by mean-field theory as large jumps in coherent and incoherent signals and hysteresis. In the quantum solution, traces of phase transitions are identified in enhanced quantum fluctuations of excited level populations.

与物质相互作用的共振光支持可极化介质的不同相，并且在两个相共存的地方具有光学双稳性。以前已经在腔体内积极研究了此类相。在这里，我们确定了冷相自由空间平面阵列中映射到散射光上的光学相变和光学双稳性的特征。提出了在超辐射和极端亚辐射状态下探索这类系统的方法。在某些感兴趣的分析案例中，仅由共振偶极-偶极相互作用支持的固有光学双稳性的合作阈值和强度机制。与超辐射态相比，亚辐射态要求的强度较低，但对于存在非平凡相的合作性要求更高。透射光揭示了相干和双稳态，这是由均场理论预测的，因为相干和非相干信号以及磁滞的大跃变。在量子解决方案中，在激发能级群的增强量子涨落中识别出了相变的痕迹。