We detail the design and characterization of a terahertz range achromatic quarter-wave plate based on a stack of aligned variable thickness birefringent sapphire discs. The disc thicknesses and relative rotations of the discs are determined through a basin-hopping Monte Carlo thermal annealing routine. The basin-hopping scheme allows an improved refinement of the required thicknesses and rotations to give a predicted phase error from the ideal π/2 of only 0.5%, which is a factor of approximately 6 better than previous efforts. Moreover, the large contrast between extraordinary and ordinary axes of sapphire allows us to greatly decrease the overall optical path length of our wave plate design by approximately a factor of 10 over similar designs based on quartz discs. However, this very same contrast requires very precise tolerances in the milled thicknesses of the discs and their assembly. We detail a method to compensate for differences in the thickness from their calculated ideal values. We have constructed one of our designs and found it similar in performance to other configurations, but with our much more compact geometry.

中文翻译：

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紧凑型宽带太赫兹范围四分之一波板

我们详细介绍了基于对准的可变厚度双折射蓝宝石光盘堆栈的太赫兹范围消色差四分之一波片的设计和特性。盘片的厚度和盘片的相对旋转是通过盆地跳跃式蒙特卡洛热退火程序确定的。盆跳方案可以改进所需厚度和旋转的精度，从而从理想的π / 2预测的相位误差仅为0.5 ％，比以前的努力要好大约6倍。此外，蓝宝石的普通轴和普通轴之间的较大对比度使我们可以大大减小波片设计的总光程长度，比基于石英光盘的类似设计大约减少10倍。然而，这种非常相同的对比要求在磨盘的厚度及其组装上有非常精确的公差。我们详细介绍了一种方法，可以根据计算出的理想值来补偿厚度差异。我们已经构造了一种设计，并且发现它的性能与其他配置相似，但是几何形状更加紧凑。