We experimentally demonstrate a dissipative phase transition in a mirrorless optical parametric oscillator (MOPO) using a double-$\mathrm{\Lambda }$ system in thermal atomic vapor. Bistable behavior is observed in the output power of the beams generated via the MOPO with the variation of the input power of the driving fields. We show that the long-lived hyperfine ground-state coherence induced by the Gaussian pump and the generated fields leads to the observation of optical bistability in the MOPO threshold. We further study the critical slowing-down of nonequilibrium dynamics in the vicinity of the forward threshold where the switching time of the generated fields diverges with the critical exponent, $\alpha =-0.86\pm 0.02$. Our observations pave the way to understanding and characterizing the phase transitions in driven-dissipative systems using a simple nonlinear optical system like the MOPO.

中文翻译：

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无反射镜光参量振荡器中的耗散相变

我们通过实验证明了使用双倍镜的无镜光学参量振荡器（MOPO）中的耗散相变$\mathrm{\Lambda }$热原子蒸气中的体系。随着驱动场输入功率的变化，在通过MOPO生成的光束的输出功率中观察到了双稳态的行为。我们表明，由高斯泵和产生的场引起的长寿命超精细基态相干导致在MOPO阈值中观察到光学双稳态。我们进一步研究了正向阈值附近非平衡动力学的临界减慢，其中产生的场的切换时间与临界指数不同，$\alpha =-0.86\pm 0.02$。我们的观察为使用简单的非线性光学系统（如MOPO）了解驱动和耗散系统中的相变铺平了道路。