一个新的简单的磁光模型揭示了一个意想不到的自旋开关
A new and simple model for magneto-optics uncovers an unexpected spin switching
摘要：
在磁光学中，样品的自旋角动量$S_z$是通过旋转角和椭圆率间接探测的，而旋转角和椭圆率主要由非对角极化率$χ^{(1)}_{xy}$决定。$S_z$和$χ^{(1)}_{xy}$之间的直接和解析关系是必要的，并且对于磁光的成功至关重要，但是通常难以获得，因为量子力学的关系隐藏在求和状态中。在这里，我们提出了一个新的和简单的模型来建立这样一个急需的关系。我们的模型是基于Hookean模型，但包括自旋轨道耦合。在连续波激发下，我们证明了对于固定的光子频率$ω$，$χ^{(1)}_{xy}(ω)$确实与$S_z$成正比。这种优雅的关系令人鼓舞，我们想知道我们的模型是否也能描述自旋动力学。通过允许自旋动态变化，令我们惊讶的是，我们的模型预测超快激光脉冲可以引起自旋进动;通过适当的参数，激光甚至可以将自旋从一个方向反转到另一个方向。这对圆偏振光和线偏振光都有效。自旋反转窗口很窄。这些意想不到的结果非常类似于在更复杂的亚铁磁性稀土化合物中发现的全光学螺旋度依赖的磁开关。因此，我们相信我们的自旋轨道耦合模型可能会发现一些重要的应用自旋开关过程中，一个热门的话题，飞秒磁性。
英文摘要：
In magneto-optics the spin angular momentum $S_z$ of a sample is indirectly probed by the rotation angle and ellipticity, which are mainly determined by the off-diagonal susceptibility $χ^{(1)}_{xy}$. A direct and analytic relation between $S_z$ and $χ^{(1)}_{xy}$ is necessary and of paramount importance to the success of magneto-optics, but is often difficult to acquire since quantum mechanically the relation is hidden in the sum-over-states. Here we propose a new and simple model to establish such a much needed relation. Our model is based on the Hookean model, but includes spin-orbit coupling. Under cw excitation, we show that $χ^{(1)}_{xy}(ω)$ is indeed directly proportional to $S_z$ for a fixed photon frequency $ω$. Such an elegant relation is encouraging, and we wonder whether our model can describe spin dynamics as well. By allowing the spin to change dynamically, to our surprise, our model predicts that an ultrafast laser pulse can induce a spin precession; with appropriate parameters, the laser can even reverse spin from one direction to another. This works for both the circularly and linearly polarized light. The spin reversal window is narrow. These unexpected results closely resemble all-optical helicity-dependent magnetic switching found in much more complicated ferrimagnetic rare earth compounds. Therefore, we believe that our spin-orbit coupled model may find some important applications in spin switching processes, a hot topic in femtomagnetism.
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