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Single crystals of para Nitroaniline (pNA), an organic nonlinear optical (NLO) material, have been grown by slow solvent evaporation technique at room temperature. Good optical quality single crystals with dimensions of up to 10 × 11 × 3 mm³ have been grown by adopting the above technique, and good quality single crystals were obtained. The grown single crystals were subjected to different characterization analyses. The lattice dimensions have been determined from the powder X-ray diffraction analysis. Its functional groups and optical behavior have been identified from the FTIR and UV-Vis. analyses respectively. The thermal stability of the crystal has been analyzed by thermogravimetric (TG) and differential thermal (DT) analyses respectively.

Single crystals of 2-Naphthalenol (2N), new organic nonlinear optical (NLO) material, have been grown by slow evaporation solution growth technique at room temperature. The crystal system has been confirmed from the single crystal X-ray diffraction analysis. The functional groups were identified using FTIR spectroscopy. UV-vis-NIR spectrum showed absence of absorption in the wavelength region 400–1400 nm. The second harmonic generation efficiency is two times higher than that of standard potassium dihydrogen phosphate (KDP).

Growth hillocks on the {100} faces of L-arginine phosphate monohydrate (LAP) single crystals grown at 25°C and at a supersaturation of 0.32 have been discussed. The typical dislocation growth hillocks are lopsided and elongate along the b direction. The dislocation sources are probably caused by the extra stress field which is introduced by the hollow cavities distributing on the steps and hillocks generated by the two-dimensional nucleus. The elongated shape is due to the characteristic structure of the LAP crystal. Apart from that, the formation of the lopsided growth hillocks is explained by the liquid flow theory.

Morphology of the {100} cleavage faces of L-arginine phosphate monohydrate single crystal before and after its growth at 40°C at a supersaturation of 0.031 has been studied. Monolayer and multilayer steps are found on the original cleaved surfaces. The regrowth morphology varies in nature with different growth time. With the increase of the growth time more growth layers and two-dimensional nucleus come into being, and they become more stable.

Surface morphology of the {001} faces of MMTWD crystals grown from by the temperature-lowering method has been studied. Monolayer and multilayer steps elongate along the a direction, which is determined by the crystal structure. Apart from that, the elementary steps have narrower terraces than the bunched ones, which may be resulting from the faster growth rates of the former than the latter. The formation of the protuberances at the step fronts is primarily associated with the uneven growth rates. The hollow cavities also elongate along the a direction, which demonstrates that the formation of them is also restricted by the crystal structure. Cracks are supposed to occur during harvesting, handling, or temperature stress afterwards. Growth of the 3D hillocks in high density can probably cause large stress and induce structure mismatch and serious cracks at the later stage.

Growth mechanisms and defects formation of the manganese mercury thiocyanate (MMTC) crystal have been investigated by atomic force microscopy (AFM). Both screw dislocation controlled growth and 2D nucleation growth occur on the {110} faces. Stacking faults are observed among dislocation hillocks and the formation of them probably results from the different crystallization orientations of different spirals. Hollow channels are found around the nucleation islands and the formation of them is due to the instability of the interface generated by the rapid nucleation and growth speeds.

We synthesized 4,4'-dinitrophenyl 1,1'-adipamide (DNPA) and studied optical second harmonic generation (SHG) in this organic material using the Kurtz method. The experimental analysis of SHG was made for a range of particle size of DNPA. It was observed that DNPA is nearly three times more SHG active than urea. It was also found that the compound is non-phasematchable and did not show any optical degradation for about an year when kept in ambient atmosphere.

Amino acids (L-Glutamic acid, L-Histidine, L-Valine) doped potassium dihydrogen phosphate crystals were grown by the solution growth technique. Slow cooling as well as slow evaporation methods were employed to grow these crystals. The concentration of dopants in the mother solution was varied from 0.1 mole % to 10 mole %. The solubility data for all dopant concentrations were determined. The variation in pH and the corresponding habit modification of the grown crystals were characterized with UV - VIS, FT-IR and SHG trace elements, and dielectric studies reveal slight distortion of lattice parameter for the heavily doped KDP crystals. TGA-DTA studies reveal good thermal stability. The dopants increase the hardness value of the material, which also depends on the concentration of the dopants. Amino acids doping improved the NLO properties. The detailed results on the spectral parameters, habit modifications and constant values will be presented.

Para-nitroaniline(p-NA)–dimethyl formamide (DMF) adduct has been synthesized chemically. The formation of adduct is confirmed via UV, FTIR, CHN analyses and other techniques. The adduct formation takes place due to dipolar interaction and hydrogen bonding. The parent polar molecules, viz. p-NA and DMF, crystallize in symmetric forms and do not exhibit optical second harmonic generation (SHG). The adduct has been found to exhibit a considerable SHG efficiency and hence this confirms that the adduct crystallizes in a noncentrosymmetric form. These observations suggest a Λ-shaped structure of the molecule, which is further established theoretically via geometry optimization.

In epoxy based polymeric nonlinear optical (NLO) materials, the nonlinear optically active moiety forms crosslink between polymer chains. Some of the important moieties include 4, 4'diamino diphenyl sulfone (DADS). We have examined second harmonic generation (SHG) in DADS and have compared it with 4, 4'diamino diphenyl methane (DADM). The geometry optimization suggests that both molecules have Λ-shaped structure. The theoretical calculations as well as experimental observations of SHG in both the molecules suggest the suitability of DADS over DADM for making polymeric NLO materials.

In this paper, we dress bare quantum graphs with finite delta function potentials and calculate optical nonlinearities that are found to match the fundamental limits set by potential optimization. We show that structures whose first hyperpolarizability is near the maximum are well described by only three states, the so-called three-level Ansatz, while structures with the largest second hyperpolarizability require four states. We analyze a very large set of configurations for graphs with quasi-quadratic energy spectra and show how they exhibit better response than bare graphs through exquisite optimization of the shape of the eigenfunctions enabled by the existence of the finite potentials. We also discover an exception to the universal scaling properties of the three-level model parameters and trace it to the observation that a greater number of levels are required to satisfy the sum rules even when the three-level Ansatz is satisfied and the first hyperpolarizability is at its maximum value, as specified by potential optimization. This exception in the universal scaling properties of nonlinear optical structures at the limit is traced to the discontinuity in the gradient of the eigenfunctions at the location of the delta potential. This is the first time that dressed quantum graphs have been devised and solved for their nonlinear response, and it is the first analytical model of a confined dynamic system with a simple potential energy that achieves the fundamental limits.

Nanoparticles of Titanium dioxide $({\mathrm{\text{TiO}}}_2)$ with its unique optical and electronic characteristics is an important material for photochemical catalysis. The efficiency of catalytic activity of ${\mathrm{\text{TiO}}}_2$ anatase nanostructures is greatly influenced by the photo-generated bound excitons. It is found that the interaction of bound excitons generated in ${\mathrm{\text{TiO}}}_2$ enhances the cubic nonlinearity of the system due to strong oscillation of photo-generated bound excitons. The trapped electron hole pair concentration is directly proportional to the photocatalytic efficiency of the ${\mathrm{\text{TiO}}}_2$ anatase nanostructures. In our report, we show how these photo-generated bound excitons play a significant role in origin of third-order optical nonlinearities. In particular, we have measured large phase shift and seen two photon absorption process through closed and open aperture Z-scan, respectively, using femtosecond pulses at 532nm.

In this paper, 7,8-dihydroxyaminoacenaphthalene was synthesized by a convenient method. Its crystal structure was determined by X-ray diffraction. The crystal structure shows that the molecules aggregated to monoclinic crystal with space group $P{2}_1/n$. The semi-empirical method ZINDO was employed to study the static first hyperpolarizabilities of this compound. The static first hyperpolarizability value is $8.524\times 10^{-30}$esu, which is 6.00 times as much as that of KH₂PO₄ (KDP) molecule. The results indicate that the title molecule would be potential nonlinear optical materials.

By performing the femtosecond-pulsed $Z$-scan experiments at a wavelength of 780nm, we report the observation of simultaneous third- and fifth-order refractive nonlinearities in the film of a side-chain azobenzene copolymer synthesized by free-radical polymerization method. Theoretically, we extend two continuous wave $Z$-scan theories suitable for the excitation of ultrafast laser pulses. First one is valid only for the situation of the sample with the pure third-order refractive nonlinearity. Second one is applicable when the sample exhibits the simultaneous third- and fifth-order refractive nonlinearities. Both $Z$-scan theories allow the precise analysis of femtosecond-pulsed $Z$-scan experimental data. Using the femtosecond time-resolved optical Kerr effect measurements, we estimate the nonlinear response time of the sample to be in the sub-picosecond regime. The physical mechanisms of the observed nonlinearity are also discussed. The high nonlinear figure of merit with ultrafast response in this polymer is desirable in nonlinear photonic devices.

In this work, MoSe₂ film is synthesized by the chemical vapor deposition method. Saturable absorber (SA) is assembled with the tapered fiber and MoSe₂ films. Q-switched operation is achieved with the MoSe₂-based SA. The experimental result reveals that the MoSe₂-based SA has excellent saturable absorption characteristic with 23.41% modulation depth. When the pump power is greater than 289.2mW, Q-switched all-fiber lasing action is demonstrated. The repetition rate of the Q-switched pulses varies from 185.6 to 252.8kHz. The shortest pulse duration is 801.9ns, and the largest output energy is 69.3nJ. Results indicate that MoSe₂ films can provide new opportunities for large-modulation-depth optical modulators in ultrafast photonics.

With the advancement of near-infrared laser technology, there is a significantly increased possibility of exposure of the human eye and optical devices to intense laser conditions. Consequently, research on protection against near-infrared lasers is becoming increasingly prevalent. This review has provided an overview of the recent advancements in the field of nonlinear optical materials, focusing on materials such as graphene, carbon nanotubes, inorganic metal clusters, transition metal sulfides, perovskites and copper niobate. These materials exhibit promising performance in near-infrared optical limiting applications, showcasing good broadband capabilities, but still face urgent issues in terms of structural control, performance regulation, high-purity preparation, and device applications.