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Polycrystalline samples of oxides of the general formula LiM(V)M(VI)O(6) (M(V) = Nb, Ta; M(VI) = Mo, W), crystallizing in a non-centrosymmetric (space group P (4) over bar 2(1)m) trirutile structure, exhibit second harmonic generation (SHG) of 1064 nm radiation with efficiencies 15-45 times that of alpha-quartz; interestingly, the SHG response is retained by the protonated derivatives HM(V)M(VI)O(6) . xH(2)O, and their n-alkylamine intercalates as well.

Several substituted anilines were converted to binary salts with L-tartaric acid. Second harmonic generation (SHG) activities of these salts were determined. The crystal packing in two structures, (i) m-anisidinium-L-tartrate monohydrate (i) and (ii) p-toluidinium-L-tartrate (2), studied using X-ray diffraction demonstrates that extensive hydrogen bonding steers the components into a framework which has a direct bearing on the SHG activity

Several transition metal phosphates of the general formula $KM_{0.5}M'_{0.5}OPO_4$, where $M = Nb^v$ or $Ta^v$ and $M' = Ti^{III}, V^{III}, Cr^{III}, Fe^{III}$, possessing the $KTiOPO_4$ (KTP) structure have been synthesized. Unlike KTP, none of these phosphates exhibit second harmonic generation (SHG) of the 1064-nm radiation. Soft-chemical oxidation of the $Ti^{III}$ and $V^{III}$ phosphates using chlorine in $CHCI3$, however, restores the SHG property to the oxidized materials, $K_{0.5}M_{0.5}M'_{0.5}OP0_4 (M = Nb^v, Ta^v; M' = Ti^{Iv}, V^{IV})$. $K_{1-x} Ti_{1-x}V_x OP0_4 (x \leq 0.25)$ prepared similarly from $KTi_{1-x}V_x OPO_4$ also exhibit SHG. The results reveal that exclusive presence of d° cations at the $Ti^{iv}$ site preserves the SHG response of KTP derivatives.

Soft-chemical oxidation of KTiOPO4-like KM(0.5)(V)Ti(0.5)(III)OPO(4) (M = Nb, Ta) using chlorine in CHCl3 is accompanied by partial deintercalation of potassium, yielding K(0.5)MV(0.5)Ti(0.5)(IV)OPO(4) compounds which are new non-linear optical materials that exhibit efficient second-harmonic generation of 1064 nm radiation, as does KTiOPO4.

We report some new results of optical phase conjugation(OPC) studies on three halogenated methanes by stimulated Brillouin scattering (SBS) technique using the second harmonic of pulsed Nd:YAG laser which have not been examined so far.The calculated and experimentally determined SBS gains have been compared. Phase conjugate reflectivities of the order of 70% have been obtained at a pressure as low as 1 bar.

A number of salts of substituted pyridines with $L$-tartaric acid were prepared. Their nonlinear optical behaviour e.g. second harmonic generation (SHG) activity was measured. The crystal structure of $L$-tartaric acid-4-imethylaminopyridine (1:1) dihydrate salt 1 was determined in order to understand the relationship between crystal packing and SHG activity within salt 1.

The development of optical phase conjugation using thermally formed gratings in absorbing media, especially the dye solutions, has besides the high efficiencies, many other characteristics which are worthy of study. Here the wavelength depcndence of phase conjugate reflectivity in relation to absorption spectrum of rhodamine-6G and rhodamine-B in water, methanol and ethanol is reported. Further the Bragg condition has been verified for the dominant diffracted beam which shows that the thermal'grating behaves like an optically thick grating.

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.

$Me_4N)_2ZnCl_4 (TmAZnCl_4)$ 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.