Uniaxial negative birefringence

Interference color, birefringence straight or wavy extinction biaxial or uniaxial negative or positive 2 V dispersion in transmitted light. 

Fig. 5.102. Index matching for SHG in a uniaxial negatively birefringent crystal...

According to the definition at the end of Sect. 6.2, type-I phase matching is achieved in uniaxial negatively birefringent crystals when e(2a>, 0) = no(a>). The polarizations of the fundamental wave and the SH wave are then orthogonal. From (6.13) and the condition n ilco, 9) = no co), we obtain the phase-matching angle 9 as... 

In thin sections natural graphite is translucent, strongly pleochroic, and uniaxial. It has a negative sign of birefringence and two extinctions per revolution under crossed Nicol prisms. The atomic number of carbon accounts for its low absorption coefficient for x-rays and electrons. 

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

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. 

A peculiarity of polystyrene in bulk is the fact that it has a negative optical anisotropy. The optical anisotropy is revealed, for instance, by birefringence when the material is strained. Let us consider a uniaxial strain produced by a mechanical tension 3. For small deformations, the difference Sn of the indices for directions parallel and perpendicular to the axis vary linearly with respect to F... 

Besides, both s tructures posses an optical anisotropy in the spectral range of X a where a i s the "lattice" constant. This is the so called anisotropy of shape described in [4]. Studies of birefringence in 2D PC from macroporous silicon [5] shows it is a positive uniaxial crystal with its optical axis oriented along the pores. The present work is focused on the studies of ID PC from grooved Si. It is expected to be a negative uniaxial crystal whose axis is perpendicular to the Si ribs. 

In systems with uniaxial orientation, the optical axis is coincident with the direction of principal orientation (Xin Fig. 54b). The system will exhibit two polarizabilities / , and and two principal refractive indices n, and T2 /, parallel and perpendicular to X respectively. The birefringence A is designated as being positive if n, > ni and negative if n, <( ni and its m nitude is given by... 

---