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Urea-doped barium tartrate (UBT) crystals were developed using the method of an aqueous solution. The crystal structure was determined through XRD analysis. Optical transmittance studies were conducted to analyze multiple linear optical parameters such as transmittance, band gap, refractive index, absorption coefficient and more. Vickers microhardness testing was performed to assess mechanical properties like hardness and yield strength. The efficiency of Second harmonic generation (SHG) was calculated for the harvested crystal. Laser damage threshold (LDT) testing was completed as well. Nonlinear optical characteristics were analyzed by applying the Z-scan method. Electronic polarizability was calculated for the UBT crystal.

Dipole and cavity field transient spectra are investigated for a modified Jaynes-Cummings (JC) model due to the presence of a second harmonic generation (SHG) cavity field. For initially de-excited atom and field in a coherent state, detuning effects due to SHG affects the symmetry and splitting structure of both spectra.

Mentioned communication explores the modification in properties of Zinc Thiourea Sulphate (ZTS) crystal due to L-Threonine (LT) addition. Superior quality LT-doped ZTS crystal with 0.5 M% concentration of LT was grown by the slow evaporation solution growth technique. Powder X-ray diffraction technique was applied to study the cell parameters which confirmed orthorhombic crystal structure of both pure and LT-ZTS crystal with slight variation in cell parameters. Shimatzu make spectrophotometer confirmed the UV-Visible spectral analysis in the range of 200–900 nm which affirmed the 94% transmittance, enhanced bandgap value (4.72 eV), lower cut-off value (246 nm) and lower optical constants viz. extinction coefficient, polarizability, refractive index, and reflectance of LT-ZTS crystal. The higher second harmonic generation (SHG) efficiency of LT-doped ZTS was pointed by Kurtz Perry powder method (3.06 times of pure ZTS crystal and 3.62 times of KDP crystal) using Nd: YAG laser. The colour centered emission and electronic purity of parent and doped ZTS crystals were examined which resulted in the violet emission in visible region for both pure and LT-ZTS crystals. Z-scan technique is used to identify the Kerr lensing nonlinearity in pure and LT-doped ZTS crystal. Close aperture Z-scan curve demonstrated negative refraction nonlinearity (self-defocusing nature) for pure and positive refraction nonlinearity (self-focusing nature) for LT-ZTS crystal. Calculated value of refraction nonlinearity n2 is −2.2 × 10${}^{-11}$ cm²/W for pure ZTS and $+$4.99 × 10${}^{-12}$ cm²/W for LT-ZTS crystal. Open aperture Z-scan showed reverse saturable absorption effect (RSA) in pure ZTS and saturable absorption effect (SA) in LT-ZTS crystal. The $\beta$ value is 2.85 × 10${}^{-5}$ cm/W for pure ZTS and 3.92 × 10${}^{-5}$ cm/W for LT-ZTS crystal. The $\chi$3 of ZTS crystal is 6.133 × 10${}^{-5}$ cm/W and 1.655 × 10${}^{-4}$ cm²/W for LT-ZTS crystal. The transition in TONLO parameters is observed due to doping of LT in ZTS. The obtained parameters indicated the superiority of LT-ZTS for application in distinct laser stabilization systems, UV shelter and NLO applications.

Single crystals of pure and various amount of L-lysine doped KDP crystals were grown from aqueous solution. The doping of L-lysine was confirmed by CHN analysis and FT-IR spectroscopy. Powder XRD was carried out to assess the single phase nature of the samples. The effect of doping on thermal stability of the crystals was carried out by TGA and the kinetic and thermodynamic parameters of dehydration were evaluated. It was found that as the amount of doping of amino acid, L-lysine, increased the thermal stability of the grown crystals decreased. However, the second-harmonic generation (SHG) efficiency of Nd:YAG laser and UV-vis spectroscopy studies indicated that as the L-lysine doping increased in KDP crystals the SHG efficiency and optical transmission percentage increased. The dielectric constant and the dielectric loss of L-lysine doped KDP crystals are lower than the pure KDP crystals. Hence L-lysine doped KDP crystals are found to be more beneficial from an application point of view as compared to pure KDP crystals. The results are discussed.

A new optically nonlinear material, poly[N-(4-benzoylphenyl)-2-methylacrylamide], for conversion of wavelength 1064 nm to 532 nm has been synthesized. The material possesses strong electrical and optical nonlinearity required for second harmonic generation (SHG). The thin films of poly[N-(4-benzoylphenyl)-2-methylacrylamide] were prepared by the vacuum-deposition technique. The optical UV-visible, IR, NMR studies, mass spectrometry, surface topography and nonlinear optical properties of the thin films were studied. The vacuum-deposited thin films of the polymer were subjected to optimized high potential multi-point corona poling at temperatures just below the glass transition temperature (T_g) to create directional orientation of the side chain groups. The optical UV-visible absorption indicates a complete transparency in the visible and IR regions with an absorption peak at ~350 nm. The energy band gap of the polymer was estimated to be ~2.9 eV, from the optical absorption spectra of the thin films. The electrical current–voltage (IV) and optical nonlinear characteristic indicate the strong potential of these polymeric thin films for nonlinear optical (NLO) applications.

The cyano and nitro groups were chosen as acceptor groups, and the substituent amino or ether groups as donor groups to the matrix. Polyurethane (PU) was modified by glycerol to increase the content of chromophore and to improve the stability of the nonlinearity. T_g and T_m were raised and solubility, film-forming ability and other physical properties were improved. The average functional groups of the reactant can be adjusted to ≥ 2 with polygroups and monogroups mixture. These poled polymers show high second-order optical nonlinearity and would have potential application in frequency-doubling or electro-optical controlling devices.

A nonlinear optical (NLO) microscope has been developed based on recording the second harmonic generation (SHG) signals of the B₂- and B₄-phases of bent shaped liquid crystalline (LC) materials. We observed that the microstructures of the polar order can be accurately characterized by 2D scanned images implementing this device. Specifically, we have investigated two bent-core compounds, which have either B₄ or B₂ phases at room temperature and exhibit SHG activity. The NLO microscope was calibrated according to the Maker-fringes method allowing the in situ evaluation of the relative SHG efficiency. The developed NLO microscopy device may serve as an important tool to characterize liquid crystalline features at a microscopic scale for both fundamental and applied research.

A nonlinear optical (NLO) microscope has been developed based on recording the second harmonic generation (SHG) signals of the B₂- and B₄-phases of bent-shaped liquid crystalline (LC) materials. We observed that the microstructures of the polar order can be accurately characterized by 2D scanned images implementing this device. Specifically, we have investigated two bent-core compounds, which have either B₄ or B₂ phases at room temperature and exhibit SHG activity. The NLO microscope was calibrated according to the Maker fringe method allowing the in situ evaluation of the relative SHG efficiency. The developed NLO-microscopy device may serve as an important tool to characterize liquid crystalline features at a microscopic scale for both, fundamental and applied research.

A theoretical analysis of the condition of scalar phase-matched second harmonic generation and optical parametric oscillation (OPO) in single-crystalline samples Zn_0.52Mg_0.48Se is presented. The calculated range of pumping for second harmonic generation with phase matching is 5.1-13 μm. The tuning range of the optical parametric oscillator based on Zn_0.52Mg_0.48Se is determined. The combination of active media and nonlinear converter for OPO in the same optical element are proposed.

Here, we propose a novel plasmonic structure, called asymmetric plasmonic nanocavity grating (APNCG), which is shown to dramatically enhance nonlinear optical process of second harmonic generation (SHG). The proposed structure consists of two different metals on both sides of lithium niobate and a thin layer of graphene. By using two different metals the nonlinear susceptibility of the waveguide would be increased noticeably causing to increase SHG. On the other hand, it consists of two identical gratings on one side. By two identical gratings, the pump beam is coupled to two opposing SPP waves, which interfere with each other and result in SPP standing wave in the region between the two gratings. The distance between two gratings will be optimized to reach the highest SHG. It will be shown that by optimizing the geometry of proposed structure and using different metals, field enhancement in APNCG waveguides can result in large enhancement of SHG.

The advancement of crystal growth and characterization methods allows us to investigate new substances with excellent nonlinear optical characteristics. To synthesize nonlinear optical material L-histidine hydrochloride hydrate (L-HHCLH), the gradual evaporation process was used. The produced samples were characterized by single-crystal X-ray crystallography, Fourier transform infrared (FTIR) and Raman spectroscopy, UV–visible (UV–Vis) spectroscopy, second harmonic generation (SHG) test, dielectric, and mechanical investigations. The L-HHCLH sample was crystallized in an orthorhombic structure with the P2₁2₁2₁ space group, as verified by the crystallographic data. FTIR and Raman spectroscopy were applied to examine the molecular vibrations and availability of the functional groups of the compound. The L-HHCLH is significantly transparent across the UV and visible ranges, as shown by the UV–Vis spectra measurements. The bandgap of L-HHCLH is 5.45 eV. The SHG test showed that the L-HHCLH crystals produced a significant amount of SHG output thrice that of the potassium dihydrogen phosphate (KDP) sample. The frequency dependences of the dielectric parameters were investigated in the dielectric tests. With increasing frequency, both the dielectric constant and loss dropped exponentially. The crystal hardness was determined using a microhardness test.

The development of crystal growth and measurement methods opens new possibilities for studying materials with remarkable nonlinear optical features. The development of bulk crystals is a constant concern to be beneficial for device applications. This study focuses on the development of nonlinear optical crystals of L-histidine complexes, characterization approaches, and prospective applications. The solution growth approach, a straightforward, inexpensive, and low-temperature growth technique, was adopted broadly. Recent studies show that L-histidine derivatives have potential optical, SHG, thermal, dielectric, and mechanical properties, making them an excellent choice for nonlinear optical devices. In this work, structural, spectroscopic, chemical composition, linear and nonlinear optical analyses, thermal, and dielectric characterizations of L-histidine derivatives were conducted. Most of the L-histidine complexes were found to be crystallized in orthorhombic and monoclinic crystal systems with P2₁2₁2₁ and P2₁ space groups. The crystalline perfections of the samples were analyzed by the HR-XRD method. The energy-dispersive X-ray (EDX) spectroscopic technique was applied to evaluate the chemical content of the compounds. FT-IR and FT-Raman techniques were used to recognize the molecular vibrations and functional groups of the L-histidine derivatives. The UV-visible and photoconductivity studies were done to examine the optical behaviors of the developed crystals. Most of the crystals were found to have excellent optical transparency and wide bandgaps. It has been observed that L-histidine hydro-fluoride dehydrate and L-histidine tetrafluoroborate are found to have the highest second harmonic generation (SHG) efficiencies. The dielectric analyses were utilized to study the dielectric constant and dielectric loss variations with the applied frequency. According to the thermal analyses, most of the crystals were found to have substantial thermal stabilities suitable for device applications.

2-methylimidazole (2MI) single crystal has been successfully grown by slow evaporation solution technique (SEST) by using water as a solvent. The synthesized 2MI crystal was purified by repetitive recrystallization. Various other characterizations were carried out for the grown 2MI single crystal, such as powder X-ray diffraction (PXRD), FTIR, UV–Visible NIR, Microhardness, Photoluminescence, SEM, Piezoelectric, and SHG. The 2MI crystal was subjected to PXRD to endorse the crystalline quality of the material. The vibrational assignments and their functional groups were identified from the FTIR analysis. The UV–Visible NIR study exposes the transparency and cut-off wavelength of the 2MI crystal. The mechanical strength was investigated by Vickers microhardness tester. The surface investigation of the 2MI crystal was analyzed by a scanning electron microscope technique to obtain the presence of defects and surface texture in the developed 2MI crystal. The photoluminescence spectrum confirmed the green light emission of the crystal. The value of the piezoelectric d${}_{33}$ coefficient is measured using piezoelectric analysis. Kurtz powder’s second harmonic generation test reveals that the frequency conversion efficiency of 2MI is 3.7 times higher than that of potassium dihydrogen phosphate (KDP) crystal.

The development of crystal fabrication and characterization approaches makes it possible to study novel materials with enhanced linear and nonlinear optical properties. The solution growth method was adopted in the present investigation to create Barium thiourea chloride (BTCL), a metal–organic single crystal. Structural, optical, and thermal characteristics of the BTCL crystals were evaluated by XRD analysis, Fourier transform infrared spectroscopy, UV–Vis spectrometry, photoluminescence, SHG, and thermal analyses. The XRD method was applied to investigate the structural characteristics of the BTCL, which is related to the monoclinic structure. The FT-IR spectral investigation was conducted to observe the vibrational modes in BTCL. The optical measurements showed the transmittance in the near-UV and visible spectrums. Optical characteristics include bandgap, refractive index, extinction coefficient, optical, and electrical conductivities, and dielectric constants were obtained. The PL curve showed two notable emission peaks at 482.5nm and 491nm. The ability to generate second harmonics was investigated using the Kurtz and Perry method, and the SHG was more than 1.79 times the KDP sample. The thermal behavior of the BTCL was evaluated by TGA/DTA measurements. The Coats–Redfern relation was used to estimate the activation energy of the thermal system and the value was determined to be 9.48KJ/mol. The findings indicate that the BTCL crystals possess excellent optical transparency, higher SHG potency, and good thermal stability, which are suitable for use in photonic structures.

2-Amino-5-nitropyridinium halides (2A5NPBr)/(2A5NPCl) were synthesized by dissolving 2-amino-5-nitropyridine in hydrobromic/hydrochloric acid. Nanoparticles of 2A5NPBr and 2A5NPCl were attained by hydrothermal method. For comparison and estimation of mechanical properties, single crystals of the samples were grown by slow evaporation process. SEM analysis shows the formation of nanoparticles of 2A5NPBr and 2A5NPCl with average grain size of 50 and 100nm, respectively. UV-Vis analysis reveals that the materials possess low cut-off wavelength and a wide optical transmission window. By Vickers test, the Meyer index (n) was estimated to be 1.33 for 2A5NPBr and 2.49 for 2A5NPCl crystals. The SHG efficiency of 2A5NPBr and 2A5NPCl nanoparticles was calculated to be 1.5 and 4.3 times that of well known NLO material KDP. The role of cations in demonstrating the desired NLO properties were discussed in detail.

L-Glycine Glycinium Hydrochloride Monohydrate (L-GGHCM), a new second-order nonlinear optical crystal, was produced using the slow evaporation process with double distilled water as a solvent. Powder XRD analysis was completed in order to identify the crystallinity sample. In the orthorhombic system with space group P${}_{212121}$, L-GGHCM expands in accordance with structural improvement. A single crystal XRD analysis discovered the extended framework architecture of L-GGHCM, a self-assembly process involving short-range and directed bonds, among many other interactions. The optical quality and transmission of the synthesized crystal were determined using UV–visible and fluorescence spectrum analysis. The effectiveness of L-GGHCM’s second harmonic generation is roughly 1.7 times compared to a standard potassium dihydrogen orthophosphate sample. Thermal analysis is used to study a material’s thermal stability and internal decomposition. L-glycine glycinium hydrochloride monohydrate crystal is shown to have good thermal stability.

The following sections are included:

• Introduction
• Linear vis-à-vis Nonlinear
• Nonlinear Equation of Ultrafast Optics: Mathematical Formulation of Propagation of Ultrafast Pulse in a Nonlinear Medium
• Solution of Nonlinear Optical Equation
• Further Discussion on Phase Matching Taking Example of SHG
• Further Discussion on Third-Order Processes
• Degenerate 4WM
• High Harmonic Generation (HHG)
• References
• Further Reading