Search

Light induced refractive index modulations in photorefractive crystals can be used to advantage to obtain optical phase conjugation (OFT) through Degenerate Four Wave Mixing (DFWM). The applications of OPC in various adaptive optical systems are well known. Among the of photorefractive crystals, $LiNBO_3$ has been chosen for the present studies on account of its greater spontaneous polarization and higher refractive index modulation constant.In this paper, we report the studies in pure $LiNb0_3$ and Fe (0.02%) doped sample, using 488 nm, which is towards the absorption edge, whereas the earlier reports were with 514.5 nm. The formation and erasure times of the phase grating and the phase conjugate signal are measured using 488 nm wavelength with the pump beams of power 8 mW and probe beam of power 1.2 mW. In the Fe doped crystal, the formation and decay of the phase grating follow a near exponential pattern. The phase conjugate signal also behaves similarly with a formation time of 840sec and a decay time of 1200 sec. These results are in agreement with the previously predicted values by other workers. In pure $LiNbo_3$ crystal, though the decay of the phase grating follows the exponential form, with a decay time of 2000 sec, the formation of the phase grating is observed to follow a rather different pattern. After a gradual increase in the diffraction efficiency in the first 600 sec, there is a fall during the next 200 sec and an increase thereafter and subsequent fall. The reasons for this type of behaviour is explained on the basis of a dynamic approach to the volume grating that is formed.

One-hundred and sixteen patients were surgically treated for constrictive pericarditis over a period of 18 years. Twenty-eight patients were less than 15 years old. All patients had exertional dyspnoea and elevated jugular venous pressure. Eighty-eight patients had NYHA class III or IV functional disability. Only 2 cases had atrial fibrillation. Seventeen patients had roentgenographic evidence of pericardial calcification. Fluoroscopy showed diminished cardiac movements in 110 cases. Cardiac catheterisation in 77 patients demonstrated classical haemodynamic pattern of constrictive pericarditis. All our patients underwent subtotal pericardiectomy through a left anterolateral approach. Seventy-one patients had histological evidence of tuberculous pericarditis. Nearly 88 per cent of the followed up cases reported good to excellent relief. The hospital mortality was 6.9 per cent. Our observations and inferences are compared with those of other published reports.

A continuous flow cryostat was designed (80-300 K) for the study of dielec. and thermal expansion behavior of solid samples at high vacuum (.apprx.10-5 torr). The temp. up to 150 K was reached by increasing the opening or a control valve near the suction pump. Further opening of the valve for obtaining lower temps. revealed a slow oscillation of temp. between 150 and 125 K. And several methods tried out for avoiding the temp. oscillations are discussed

The Pippard-Janovec relations are derived for correlating the anomalous elastic coefficient and the anomalous specific heat near the phase transitions of ferroelectric crystals. These relations are verified in the case of ferroelectric triglycine selenate crystal.

Introduction Dicalcium strontium propionate (DCSP) undergoes a ferroelectric phase transition at about 28 1.5 K, with the spontaneous polarization occurring along the tetragonal C-axis.1 Takashige et al.2,3 have recently reported ferroelectricity in annealed samples of dicalcium lead propionate (DCLP) in the range 191 K to 331 K. The removal of the inner biasing field by annealing has been known in the case of DCLP3 and DCSP.4 Because of the possible dependence of the inner biasing field on the particle size, a study of the temperature dependence of the dielectric behaviour of the powdered samples of these compounds was undertaken.

The dielectric measurements on diglycine sulphate in the temperature range 5-400 K, show that it is a normal dielectric, unlike triglycine sulphate which is a ferroelectric. The difference in the dielectric behavior of these two glycine sulphates has been explained on the basis of certain structural features derived from a study of their vibration spectra.

$Me_4N)_2ZnCl_4 (TmAZnCl_4)$ shows 6 successive phase transitions at 80-300 K. Polarized Raman spectra of oriented single crystals of $TMAZnCl_4$ and the IR spectra by the $KBr$ pellet technique were recorded at 130-298 K, covering all the known phase transitions,. The frequency of the $Zn-Cl$ stretching vibration increases in Raman and IR spectra with decreasing temp. Three bands are found in the Raman spectrum in the region where Me frequencies usually occur and, in a similar way, 3 bands appear in the IR spectrum at low temp. due to the deformation vibrations of the Me group. The triggering mechanism of the phase transition at low temp. can be due to a coupling of the distortion of $ZnCl_{42}$- groups and the Me deformation, where the motion of relatively free Me rotations is slowed down, favoring 1 equilibrium position below 161 K. The phase transition appears to be an order-disorder one with Me rotational disorder in the high temperature paraelectric phase.

Introduction The alum family of double salts with the general formula M1+M3+(RO4)2.12H2O where M1+ is a monovalent ion (M1+ = K, Rb, Cs, Tl, NH4, CH3NH3, NH3OH3 NH3NH2, etc.) and M3+ is a trivalent metal such as Al, Fe, Cr, V, In, Ga, etc. and R is S or Se, form an isomorphous series and their general features indicate a common cubic space group Pa3. Lipson1 showed subsequently that there exist three different structure types agr, β and γ and the structure of a particular alum is dependent on the radius of the monovalent atom. The agr structure is typical of medium sized ions, the β of the larger ones and the γ of the small Na atom.2 Ferroelectricity has been reported only in alums containing NH4, CH3NH3, NH3NH3 and NH3OH. Their hindered rotations as well as the influence of sulphate group disorder on the dielectric behaviour of alums is still not clear.3 No study of the temperature dependence of the low frequency dielectric constant of some of the alums, particularly those of Cs, Rb and Tl, have been made so The present investigation was undertaken to correlate their dielectric behaviour with their composition and structural differences. Under the same experimental conditions, methyl ammonium and ammonium alums also were studied and compared with the known data.

The dielec. behavior of $KClO_4$ was studied at low temp. to liq. He temp. and confirmed by DSC. The dielec. and thermal studies of $KClO_4$ at low-temp. indicates the presence of a low-temp. structure transition with considerable thermal hysteresis (.apprx.$20^0$). The intensity of the dielec. peak reduces considerably during successive thermal cycling. The dielec. anomaly might be due to a disorder in the orientation of the $ClO_4$ groups in the as-grown crystals. This disorder can contribute to the dielec. anomaly in the crystal. The disorder in the $ClO_4$ orientations in addn. to making a contribution directly, can also through the ionic polarization play a role in the anomaly. By thermal cycling this disorder possibly gets minimized and the structure acquires the spatial ordering corresponding to the space group Pnma and the dielec. anomaly slowly decreases

The emission spectrum of bismuth monobromide has been investigated and a vibrational analysis of the A→X system has been made. About 286 bands were recorded in the region λλ 4595–6063 and the isotope effect due to Br79 and Br81 was observed in about 87 bands. A value of 2·74 ev. for the dissociation energy of the excited state has been obtained and arguments have been given to show that the dissociation products in the excited state are Bi(4S3/2) and Br(2P3/2) and that those of the ground state are most probably Bi (4S3/2) and Br (2P1/2) atoms.

The observation of (A-X) system of BiF has been extended up to λ 5316 and twenty new bands belonging to this system have been recorded. The band heads could be represented by the following equation: {Mathematical expression} Seven other faint bands in the region λ 5316-5492 have also been reported, which, however, could not be classified. By our analysis of the present data and from known thermochemical data it has been deduced that the ground state dissociation energy is, in all probability, around 20000 cm.-1 (∼2·5 ev.) and that the dissociation products are the normal Bi and F atoms. The dissociation energy of the upper state and the correlation rules have been used to show that the dissociation products in the upper state are very likely to be Bi atom in the excited state2D3/2 and F atom in its ground state (2P3/2).

Teen the ehe dielec. consts. of thiourea powders of different particle sizes were detd. at 70-300 K, and the differences betwxptl.results and the theory of M. Marutake (1955) are discussed.