Two-Dimensional Optical Bistability

我们提出并研究了一种新型的光学双稳，它与普通的具有滞后性的光学双稳有很大的不同。光学系统非常简单，它由一个含有基态自旋原子的盒、一个Lambda/8板和一个反射镜组成。该系统有一个关于自旋偏振度的正反馈回路，因此即使入射光线性偏振没有角动量，当入射光强超过一个临界值时，原子也开始有自旋极化。在本工作中，我们从实验和理论上研究了特殊耦合双稳系统的行为，这种耦合是通过(1)自身发光和(2)自旋软化原子的扩散来实现的。首先对两个系统的耦合进行了研究，发现耦合系统在两个不同的入射光强下表现为干叉分叉(对称性破缺)，并且在强光强区域有四个稳定态。其次，研究了连续分布双稳系统的行为。通过探测法拉第旋转的特殊贡献，将一束大截面的激光照射到该池上，观察到该池的每个位置上自发产生的自旋极化。我们发现了磁畴的存在，在磁畴中所有的自旋都指向相同的方向。研究还发现，自旋极化的稳态分布具有空间模，特别是在(1)的情况下。所有的实验都是在钠蒸汽中进行的。用简化模型进行的计算机模拟很好地解释了上述实验结果。

We Proposed and studied on a new type optical bistability, which is largely different from the ordinary one behaving with hysteresis. The optical system is very simple, which consists of a cell containing atoms with spin in the ground state, a lambda/8 plate and a mirror. This system has a positive feedback loop with respect to the spin polarizatim, so even if the incident light is linearly polarized with no angular momentum, atoms begin to have spin polarization when the incident light intensity exceeds a exceeds a critical value. In the present work, we have studied experimentally and theoretically on the behavior of specially coupled bistable systems, the coupling being through(1) light it self and (2) diffusion of spin-soarized atoms. At first we studied about the coupling of two systems, and found that the coupled system behaves with pitchfork bifurcation (symmetry breaking) at two different values of incident light intensity and it has four stable states in the strong intensity region. Next, the behavior of the system with continuously distributed bistable system has been studied. A laser beam with a large cross section is applied to the cell, and observed the spontaneously produced spin-polarization at each position of the cell, by detecting the special tribution of the Faraday rotation. We have found the existence of magnetic domain, in which all of spins are oriented to the same direction. It is also found that the steady state distribution of spin-polarization has spatial modes particularly in case of (1). All of experiments have been carried out with sodium vapor. Computer simulation with a simplified model was found to explain well the experimental results mentioned above.