Martin Alexander T, Nichols Shane M, Li Sichao, Tan Melissa, Kahr Bart
Department of Chemistry, New York University, New York, NY 10003, USA.
Department of Advanced Science and Engineering, Waseda University, Tokyo 162-8480, Japan.
J Appl Crystallogr. 2017 Jul 20;50(Pt 4):1117-1124. doi: 10.1107/S1600576717007865. eCollection 2017 Aug 1.
Circular birefringence (CB) is generally responsible for only a small perturbation to the state of light polarization in crystals that also exhibit linear birefringence (LB). As such, the magnetoelectric tensor of gyration, which gives rise to CB and optical activity, is less well determined than the electric permittivity tensor in optical models of the Mueller matrix. To visualize the effect of the magnetoelectric tensor on polarimetric measurements, reported here are experimental mappings of the Mueller matrix and of the CB in a new chiral crystal with accidental null LB at an accessible optical frequency. Single crystals of ethylenediammonium selenate (EDSeO) were synthesized and characterized by X-ray diffraction and Mueller matrix measurements in transmission and reflection. The crystals are isomorphous with the corresponding sulfate salt. They are tetragonal, space group 422. The constitutive relations of EDSeO were recovered using a partial wave summation of incoherent reflections. The extraordinary and ordinary refractive indices cross at 364 nm (3.41 eV), a scenario commonly called the 'isotropic point' or 'iso-index point'. At this wavelength, the magnetoelectric tensor fully describes the polarization transformation, giving rise to a double cone of eigendirections.
圆双折射（CB）通常仅对同时表现出线性双折射（LB）的晶体中的光偏振态产生微小扰动。因此，在穆勒矩阵的光学模型中，产生CB和旋光性的回转磁电张量比电容率张量更难确定。为了可视化磁电张量对偏振测量的影响，本文报道了一种新型手性晶体在可及光频率下的穆勒矩阵和CB的实验映射，该晶体意外地具有零LB。合成了乙二胺硒酸盐（EDSeO）单晶，并通过X射线衍射以及透射和反射中的穆勒矩阵测量对其进行了表征。这些晶体与相应的硫酸盐同构。它们是四方晶系，空间群为422。利用非相干反射的部分波求和恢复了EDSeO的本构关系。非常折射率和寻常折射率在364 nm（3.41 eV）处相交，这种情况通常称为“各向同性点”或“等折射率点”。在该波长下，磁电张量完全描述了偏振变换，产生了一个本征方向的双锥。
