Blue phases at work!
Blue phases at work!
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DOI：
10.1002/cphc.200500506
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发表时间：
2006-01
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影响因子：
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通讯作者：
H. Kitzerow
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作者：
H. Kitzerow
蓝色相是液晶领域中最有趣的自组织结构之一。然而，尽管液晶对平板显示器具有普遍的重要性，但到目前为止，这些特殊的中间相仅引起了学术上的兴趣。对于可能的应用来说，主要的缺点是温度范围通常很小，在这个温度范围内bp是稳定的。在最近一期的《自然》杂志上，Harry Coles和Mikhail Pivnenko报道了在40 - 508摄氏度的温度区间内表现出bp稳定的新化合物。考虑到人们对光子晶体的兴趣，这一突破可能会推动BP研究，甚至最终导致液晶的新应用。液晶是具有各向异性物理性质的液体。通常，这些材料由棒状有机分子组成，它们可能表现出优先的平行取向和非常有限的位置顺序，从而在固态和各向同性液态之间的温度范围内形成一个或几个中间相（中间相）。由于取向顺序的关系，折射率和静态介电常数取决于定向器的方向（表示分子优选方向的伪矢量）和电场，从而分别导致双折射和介电各向异性。bp是个例外。这些中间相只出现在手性体系中，温度范围刚好低于各向同性液相，它们在光学上是各向同性的，也就是说，它们没有双折射。然而，由于圆偏振光的选择性反射，它们显示出鲜艳的颜色。事实上，可能是BP的选择性反射引起了植物学家Friedrich Reinitzer在1888年的注意，并导致了液晶的发现。与其他中间相相比，在Reinitzer发现液晶后的前80年里，bp仍然是一个相对未知的现象。由于bp只出现在高手性体系中，而且大多出现在非常窄的温度范围内（< 1k），因此bp经常被忽视或被认为是过渡前现象，通常被认为只具有学术意义。它们的结构在接下来的十年里仍然是个谜，即使阿尔弗雷德·索普（Alfred Saupe, 1969）提出了一个具有周期性上部结构的局部扭曲定向场。然而，在1980年，选择性反射的特性和双折射的缺失确实可以解释为纯粹定向有序的周期结构的布拉格散射。
Blue phases (BPs) are among the most interesting self-organized structures in the field of liquid crystals. However, in spite of the general importance of liquid crystals for flat panel displays, these particular mesophases have been only of academic interest, so far. The main drawback for possible applications is the usually small temperature range, in which BPs are stable. In a recent issue of the journal Nature, Harry Coles and Mikhail Pivnenko [1] reported on new compounds that exhibit BPs stable in a temperature interval of 40–508C. Taking into account the topical interest in photonic crystals, this breakthrough may cause a boost of BP research and finally even lead to new applications of liquid crystals. Liquid crystals are liquids with anisotropic physical properties. Typically, these materials consist of rodlike organic molecules which may show a preferred parallel orientation and very limited positional order, thereby forming one or several mesophases (intermediate phases) in the temperature range between the solid state and the isotropic liquid state. Due to the orientational order, the refractive index and the static dielectric constant depend on the orientation of the director (a pseudo-vector indicating the preferred direction of the molecules) and the electric field, thereby causing birefringence and dielectric anisotropy, respectively. BPs are an exception. These mesophases, which appear exclusively in chiral systems in the temperature range just below the isotropic liquid phase, are optically isotropic, that is, they show no birefringence. Nevertheless, they show brilliant colors due to the selective reflection of circularly polarized light. As a matter of fact, it was probably the selective reflection from a BP which attracted the attention of the botanist Friedrich Reinitzer in 1888 and lead to the discovery of liquid crystals.[2]In contrast to other mesophases, BPs remained a relatively unknown phenomenon during the first 80 years after Reinitzer’s discovery of liquid crystals. Due to the fact that they appear only in highly chiral systems and—if so—mostly in a very narrow temperature range (< 1 K), BPs were often overlooked or considered as pretransitional phenomena and generally thought to be of only academic interest. Their structure remained a mystery for another decade, even after Alfred Saupe (in 1969)[3] had proposed a locally twisted director field with a periodic superstructure. In 1980, however, the characteristics of the selective reflection and the lack of birefringence could indeed be explained as Bragg scattering from a periodic structure which is characterized by pure orientational order,