Crystal biaxial, positive

Further turning to 8 =0 leads to the same picture (dark cross) as shown for 8 =7t/2. The second optical axis of the stretched LC-elastomer is parallel to the axis of deformation. The use of a quarter wave plate finally indicates that the deformed LC-elasto-mer corresponds to an optically biaxial positive crystal. 

Orthorhombic, monoclinic and triclinic crystals are biaxial, while tetragonal, hexagonal and rhombohedral crystals are uniaxial. For uniaxial crystals, the principal axis that exhibits the anisotropy is called the optic axis. The crystal is positive uniaxial if > and negative uniaxial if < e. 

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

Interference figures These will indicate whether the crystal is optically positive or negative and, if biaxial, the angle between the optic axes. 

Birefringence The difference between the refractive index for the extraordinary and ordinary rays (he — no) in a uniaxial or biaxial crystal. If a crystal, such as quartz, is positively birefringent, no is less than ns, and the velocity of the ordinary-ray is greater than that of the extraordinary ray. The reverse is true for a negatively birefringent crystal. 

Optical investigations of smetic liquids indicated a behaviour of uniaxial or biaxial crystals depending on the special type of material. They are usually positive birefringent, which means that with transmitted light the ordinary beam has a lower refractive index. The nematic liquid is optically positive. 

In the first case (-S =tt/2) polarizer and axis of deformation are in a perpendicular position. The conoscopic picture is schematically shown in Fig. 7 ( 3 =tt/2). The dark cross is typical for monaxial crystals between crossed polarizers if the optical axis is parallel to the incident light. This also holds for biaxial crystals, if the plane of the polarized light (plane of oscillating electric field vector) is perpendicular to the plane formed by the two optical axis of a biaxial crystal. 

For monaxial crystals the picture of the dark cross has to be invariant of changing when the microscopic stage is turned. This is not the case for the LC-elastomer as to be seen in the second position =tt/4). One finds a picture which is comparable with those received for biaxial crystals in the diagonal position. 

SmE D2 X T(0) Biaxial crystal with upright molecules having true three- dimensional positional order. Rectangular in-plane lattice and herringbone packing of molecules (orthorhombic syngony). Soft crystal with small shear elastic modulus... 

Sign of double refraction n. An empirical classification of crystals it is positive for uniaxial crystals when s > o), for biaxial crystals when y — > J — a. It is negative... 

In addition, an anisotropic substrate can change the in-plane symmetry of the interfacial layer. For example, it may induce a weak biaxiality of the nematic phase or a positional order in the plane of the interface (a type of epitaxy). Hence, in general, the symmetry group of the interfacial layer (Synun 1-2) is a subgroup of the synunetry groups of a liquid crystal (Symm 2) and a substrate (Synun 1), Fig. 3.1. In other cases, a substrate can induce a multistable orientation of a nematic when the director can choose between different equivalent directions in the plane of the substrate (2, 3],... 

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