Crystal optical studies of lithium tetraborate

Abstract

Using the HAUP-type universal polarimeter and the Senarmont technique, detailed crystal optical studies of Li2B4O7, lithium tetraborate, are carried out. It is shown that the optical indicatrix rotation and the optical activity are absent from the crystal, in accordance with symmetry considerations. Measurements of optical birefringence reveal the existence of a regular staircase-like temperature behaviour in the whole range under investigation (290-480 K), a hysteresis character of the birefringence under cycling temperature and a pronounced thermooptical memory effect. The origins of the above phenomena are analysed, in particular the possible influence of the pyroelectric effect and systematic errors of the optical equipment. A conclusion is drawn that the main features of the birefringence are well explained by an incommensurately modulated super-structure which is at present a matter of debate. The peculiarities of the optical properties of lithium tetraborate are compared with those of incommensurate crystals known from the literature.

Simultaneous measurements of crystal optical characteristics of deuterated triglycine sulphate

“…Otherwise, higherorder corection terms appear in formulae (1), which include the parameters of the latter effects. It is most convenient to detect these corrections with just the s parameter [18,21,24]:…”

“…Especial care was taken of a plane parallelism of the sample surfaces, which was not worse than 10 À4 rad. This was done in order to diminish "smearing" out the phase retardation of the sample D (D ¼ 2pd Dn=l, with d the thickness of the crystal plate along the light propagation direction, l the wavelength of light in vacuum, and Dn the birefringence) through the cross section of a laser beam, due to the relevant thickness variations (see [18]). Using the beam diaphragms, the residual "smearing" effect was reduced to the value of dD 7 Â 10 À3 rad.…”

“…Its working principles and construction of the optical apparatus are in detail described elsewhere (see, e.g., [18,[20][21][22]). According to the method, one measures the linear dependences of the azimuth c and the ellipticity e of the light emergent from a crystal, upon the azimuth q of the incident light, under some additional conditions.…”

Temperature dependence of optical activity and circular dichroism in α‐ZnP2 crystals

“…In view of extremely large linear birefringence (Dn ¼ n e À n o ' À0:15 [2], where n o and n e denote respectively the ordinary and extraordinary refractive indices), we paid especial care of a plane-parallelity of surfaces of the latter sample, which was not worse than 2 Â 10 À4 rad. This allowed us to minimize the noises associated with "smearing out" the optical phase retardation D ¼ 2pd Dn=l (with d the sample thickness and l the light wavelength in vacuum) through the cross section of the laser beam [11].…”

“…Our optical thermostat enabled us to control the temperature with the accuracy of about 0:03 K. Its peculiarity was the absence of any optical windows, in order to prevent the systematic errors originated from the temperature-dependent stress-induced birefringence in the windows [11,12]. The optical activity along the z direction was measured so that the temperature was being scanned continuously in a heating run (the scan rates…”