We studied theoretically optical spectra when circularly polarized light obliquely impinges on a slab of a chiral photonic structure with linearly and uniformly spatial varying pitch. The material that we considered locally has a $\overline{4}2m$ point-group symmetry and presents Pockels effect, hence, we controlled optical spectra by a low frequency (DC) electric field applied along the nonhomogeneity axis. The spectra display a Bragg-type broadband, where the edge wavelengths of the photonic band are expressed in terms of the medium parameters and the external electric-field magnitude. This allows us to select the region of the electromagnetic spectrum, where the band could become as broad as we choose. We studied three samples with iridescent, silver, and golden colors due to their reflection properties. We observed an enhancement as well as a broadening of the optical band by the application of the electric field. We found that, under certain conditions, when the optical band is practically absent, a very broad band could be created (comparable to the whole visible spectrum) when a DC electric field is applied. This is the result of electro-optic contributions in the permittivity tensor elements, which give rise to a tremendous increase in both the rotatory power and dichroism. Therefore, these media could be used as electrically controlled broadband frequency and polarization filters. Moreover, we observed the usual blueshift of the band as the incidence angle of light increases and an asymmetry in the reflection band amplitude as the incidence angle of the light increases, depending on whether the pitch increases or decreases and the pitch gradient; which endows the sample with different reflection colors for different gradients and incidence angles of the light. The asymmetry in the reflection band vanishes as the magnitude of the electric field increases.

中文翻译：

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倾斜照明下渐变间距手性光子结构的电光效应

我们从理论上研究了当圆偏振光斜射在具有线性且均匀空间变化间距的手性光子结构板上时的光谱。我们在当地考虑的材料有一个$\overline{4}2米$点群对称并呈现普克尔斯效应，因此，我们通过沿非均匀性轴施加的低频 (DC) 电场来控制光谱。光谱显示布拉格型宽带，其中光子带的边缘波长用介质参数和外部电场大小表示。这使我们能够选择电磁频谱的区域，在该区域中频带可以变得与我们选择的一样宽。由于反射特性，我们研究了三个具有彩虹色、银色和金色的样品。我们观察到通过施加电场增强以及加宽了光带。我们发现，在某些条件下，当光带实际上不存在时，当施加直流电场时，可以产生一个非常宽的波段（与整个可见光谱相当）。这是介电常数张量元件中电光贡献的结果，这导致旋转能力和二色性的巨大增加。因此，这些介质可用作电控宽带频率和偏振滤波器。此外，我们观察到随着光的入射角增加，通常的带蓝移和随着光入射角增加反射带幅度的不对称，这取决于节距是增加还是减小以及节距梯度；它赋予样品针对不同梯度和光入射角的不同反射颜色。