Thermo-mechanical properties of the Ba 2 Ga 8 GeS 16 nonlinear optical crystal

Nonlinear optical (NLO) crystal is one of the important opt-electrical functional materials that can convert laser frequency and obtain wide band tunable coherent sources, thus it possesses crucial strategic and application value in military and civil fields. On the basis of more than 30 years' efforts, the NLO crystals in visible and near infrared region, including -BaB2O4 LiB3O5 and KTiOPO4, have been basically mature. However, there are still many shortcomings for those NLO crystals used in deep ultraviolet (DUV) and mid/far-infrared (IR) regions, thus putting forward more requirements for high performance crystals. For DUV KBe2BO3F2 (KBBF) crystals, the main shortcomings are the use of toxic BeO raw materials and strong layer growth tendency. Wide transparent region and high second harmonic generation (SHG) effect are also expected in new developed DUV NLO crystals. More importantly, a large enough birefringence is highlighted to satisfy the phase-matchable condition and DUV harmonic generation capacity below 200 nm. On the other hand, the main requirement for mid/far-infrared NLO crystals is to maintain the balance between high laser damage threshold and strong SHG response. Indeed, it is a very difficult task to search for good NLO crystals through the traditional trial and error experimental methods. Theoretical studies, especially first principles calculations, can provide an efficient way to investigate and design new NLO materials with superior properties. In this paper, the recent progress of deep-UV and mid-IR NLO crystals is summarized. In addition, the crucial role of first principles calculations in new material exploration and design is highlighted by introducing several typical new NLO crystals, including defect diamond-like compound AgZnPS4, trigonal alkaline metal fluorooxoborate KB4O6F and alkaline earth fluorooxoborate SrB5O7F3. Moreover, some advanced analysis tools are introduced, such as real space atomic cutting method, SHG-weighted mapping, flexible dipole moment model, and non-bonding atomic orbitals analysis, and used to investigate the structure-property relationship in langasite La3SnGa5O14, metal cyanurate Ca3(C3N3O3)2, vanadium-carbonate K3[V(O2)2O]CO3, etc. Further, the flow chart of high-throughput first principles calculations of NLO crystal is proposed. According to the known or predicted crystal structure, we can obtain the chemical stability, band gap, NLO coefficient, birefringence and phase-matchable capacity quickly, thus easily judging the research potential of a new NLO material. On the basis of these ideas, a great blueprint for NLO crystal material genome engineering is highly put forward. Finally, the difficulties in research and challenges in NLO material investigations are discussed, and the direction of future research priorities based on first principles calculations are pointed out.

Crystalline materials have a variety of applications ranging from catalysis to semiconductors. One interesting and emerging application is nonlinear optics. When nonlinear optical (NLO) crystals are irradiated, the nonlinear responses of the crystals to the oscillating electric field produces light with a different frequency than the incident radiation. Using different NLO crystals, any frequency of light can be generated. For this reason, NLO crystals are in increasing demand for many photonic applications, such as the generation of terahertz radiation (THz) to investigate and utilize the vibrational frequencies of collective atom motions. To accelerate the discovery of new NLO crystals, we combine data mining and DFT calculations to find potential NLO crystals and predict their optical nonlinearity. To be a potential NLO crystal, a structure must be noncentrosymmetric as the sum of the molecular NLO responses in a centrosymmetric structure is zero due to the inversion center. Thus, X-ray structure data is the excellent starting point to identify potential NLO crystals. For this reason, our data mining efforts have focused first on searching for noncentrosymmetric materials from the Cambridge Structural Database (CSD) to be used as THz generators. Using a custom-built code, we filtered the structures for desirable properties such as a conjugated -system and then extracted the candidates’ molecular and structural information. From over 1.2 million materials in the CSD, we isolated 77,842 NLO candidates based on our selection criteria. Using DFT, we calculated the molecular hyperpolarizability of these candidates and used those values in oriented gas models to estimate the polarization values for each candidate’s crystal structure. Because crystal polarization is directly related to the strength of the nonlinear response, crystal polarization estimates should provide a means of accounting for optical nonlinearity. To verify our approach, we selected the four candidates (PNPA, ZPAN, NMBA and TMOAT), synthesized them on a large scale, and observed their THz generating ability. The structures of all crystals were characterized using single-crystal X-ray diffraction, confirming their molecular identities and noncentrosymmetric packing. Large crystals of each material (Figure 1) were grown, and all generated THz radiation during a standard electro-optic measurement (Figure 2), which is a clear indication that our method successfully discovered new NLO crystals. In addition to discovering new NLO crystals, we are endeavoring to predict the THz spectra that they generate. The frequencies and magnitudes in the THz spectum directly impacts a crystal’s potential applications, and as Figure 2 illustrates, the spectra generated from different NLO crystals vary widely because they are influenced by many factors such as molecular vibrations, phonons, crystal thickness, density, refractive index, absorption, crystal face, etc. Combining our crystal polarization values with refractive indices and absorption coefficients obtained through solid-state DFT calculation, we were able compute theoretical THz generation spectra for NMBA and PNPA. As Figure 3 shows, our calculated spectra agree with the experimental spectra reasonably well, confirming that our method can viably predict THz generation spectra.

A hot topic in materials science is to search for nonlinear optical (NLO) crystals, which are indispensable in current laser technology, future optical information, and precision measurements. In the period of the 1980s and 1990s, the anionic group theory proposed by Prof. Chuangtian Chen has greatly promoted the inventions of BaB2O4 (BBO), LiB3O5 (LBO), and KBe2BO3F2 (KBBF) which are widely applied in the ultraviolet (UV) spectral region today. From the beginning of this century, the rapid development of laser science and technology urgently demands new NLO crystals for wider application ranges. However, commercial NLO crystals in deep-UV and mid-infrared (mid-IR) regions are scarce. The challenge arises from the stringent criteria at various wavelengths and inefficient exploration strategy. As such, more comprehensive and quantitative theoretical guidance is necessary to improve and supplement the NLO structure-property understandings. Benefiting from high-performance computing resources, first-principles design and simulations came into being, which is more applicable to the understanding of mid-IR NLO mechanism and suitable for the efficient design of new NLO structures for current needs. In the past decade, a complete set of computational research programs based on first-principles simulations have been developed, which have promoted the development of NLO crystals in the deep-UV and mid-IR regions, and guided the subsequent and further experimental explorations. Based on our developed first-principles materials design system, the discoveries of NLO materials have ranged from basic theoretical design to rapid-prototyping and final experimental synthesis. In this Account, we will concisely summarize our ab initio guided and forward-looking studies on NLO crystals, which are our original contributions to this field and can be consulted by other material fields. First, we will review the development of NLO crystals and the important features of NLO materials. Second, we will summarize the important role of computer-aided design in advancing the NLO material field and our developed NLO material design system based on the first-principles simulations. Third, we will introduce the first-principles design for new deep-UV NLO crystals using two novel design proposals, i.e., interlayer cationic replacement and intralayer anionic substitution. Meanwhile, we will illustrate the hierarchical molecular engineering optimizations for mid-IR NLO crystals by illustrating an extended mid-IR NLO family pedigree, from which many promising mid-IR NLO systems were predicted theoretically and confirmed experimentally. Finally, we will give an outlook to explore new functional NLO crystals guided by our first-principles design and simulations. We believe that the computer-assisted exploration for new functional NLO materials is useful for understanding structure-property relationships and can provide researchers with a new approach to cost-effective and data-driven materials design.

A new inorganic nonlinear optical single crystal of sodium manganese tetra chloride (SMTC) has been successfully grown form aqueous solution by the slow evaporation technique at room temperature. The crystals obtained by the above technique were subjected to different characterization analysis. The crystalline nature of the grown crystal of SMTC was analyzed by powder X-ray diffraction. Single crystal X-ray diffraction study reveals that the crystal belongs to orthorhombic system with non-centrosymmetric space group Pbam. Optical transmission study on SMTC crystal shows high transmittance in the entire UV–Vis region and the lower cutoff wavelength is found to be 240 nm. The mechanical strength of the grown crystal was estimated by Vicker’smicrohardness test. The second harmonic generation (SHG) efficiency of the crystal was measured by Kurtz’s powder technique infers that the crystal has nonlinear optical (NLO) efficiency 1.32 times that of KDP.The dielectric constant and dielectric loss of the compound were measured at different temperature with varying frequencies. Photoconductivity study confirms that the title compound possesses a negative photoconducting nature. Growth mechanism and surface features of the as grown crystals were analyzed by chemical etching analyzing.

Nonlinear optical (NLO) crystals are a critical, enabling technology in the development of solid state laser sources, allowing the output from the most mature laser source operating a few discrete wavelengths to be shifted almost anywhere in the electromagnetic spectrum spanning from the ultraviolet to the far‐infrared. For efficient frequency conversion, a nonlinear crystal must simultaneously satisfy a long list of material requirements, some of which are intrinsic, while others are extrinsic and must be controlled through careful processing. A wide range of materials and growth techniques are required to in order to produce crystals which operate in the various wavelength regimes of interest. The basic principles of NLO frequency conversion are introduced and used to derive the material property requirements. The crystal growth, properties, and performance of state‐of‐the‐art nonlinear optical crystals are surveyed, and future directions in the development of new and improved NLO materials are identified.

With the development of laser technology and related scientific fields, understanding of the structure–property relationships in nonlinear optical (NLO) crystals is becoming more and more important. In this article, first-principles studies based on density functional theory, and their applications to elucidate the microscopic origins of the linear and NLO properties in NLO crystals, are reviewed. The ab initio approaches have the ability to accurately predict the optical properties in NLO crystals, and the developed analysis tools are vital to investigating their intrinsic mechanism. This microscopic understanding has further guided molecular engineering design for NLO crystals with novel structures and properties. It is anticipated that first-principle material approaches will greatly improve the search efficiency and greatly help experiments to save resources in the exploration of new NLO crystals with good performance.

Second-order nonlinear optical crystals with mixed anions

Nonlinear optical (NLO) crystals are the vital components of laser science and technology, as they can convert lasers in common wavelengths into new wavelength bands for ultraviolet (UV), IR, and even terahertz laser output. Known UV NLO crystals mainly focus on crystals containing cations, but covalent crystals have rarely been reported. Here we report two covalent NLO crystals, B2 O3 I and B2 O3 II. According to the first-principles calculations, B2 O3 I and II have extremely short absorption edges of about 134 nm and 141 nm, large NLO coefficients of d22 =1.38 pm/V and d24 =0.702 pm/V, as well as sufficient birefringences of 0.037 and 0.031, respectively. Notably, the absorption edges are almost the shortest among NLO crystals. Meanwhile, the NLO coefficients are evidently larger than that of another well-known covalent NLO crystal α-SiO2 and are comparable to those of the commercial UV NLO crystal LiBO3 with Li+ cation. Furthermore, the birefringences are significantly larger than that of α-SiO2 , which are favorable to the phase matching for both crystals. These results reveal that B2 O3 I and B2 O3 II are excellent candidates for UV NLO applications. In-depth calculations are carried out to reveal the origin of excellent NLO properties. These covalent crystals provide a new direction for the research of UV NLO crystals.

Linear and nonlinear optical, mechanical, electrical and surface studies of a novel nonlinear optical crystal – Manganese mercury thiocyanate (MMTC)

The developments of nonlinear optical (NLO) crystals in the last decade are reviewed from two aspects. In the theoretical part, on the ab initio plane-wave pseudopotential energy method, calculation of the optical responses, including the second harmonic generation coefficients, a computer-program package, and a real-space atom-cutting method are described and employed to evaluate the degree of approximation of the anionic group theory for nonlinear optical crystals. The ab initio method combined with the anionic group theory gives much more insight into the understanding of the relationship between the optical responses and microscopic structures of NLO crystals, borate-based NLO crystals in particular. In the experimental part, we describe how to use the anionic group theory method and an experimental evaluation system to develop a series of borate-based UV-NLO crystals such as KBe2BO3F2 (KBBF) and K2Al2B2O7 (KABO). The capability of these new UV-NLO crystals in producing deep-UV harmonic output is evaluated and determined. Finally, recent advances in the generation of deep- and vacuum-UV harmonic output with these new NLO crystals are reported.

We describe experiments characterizing a new nonlinear optical crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/ (YCOB). This crystal has a number of advantages over other commonly available nonlinear optical crystals. It has a higher nonlinear coefficient than KDP, can be fabricated to large sizes (/spl sim/3-in diameter, 8-in length), and has a high damage threshold. Moreover, this new nonlinear optical crystal is nonhygroscopic, has good optical quality and mechanical properties, allowing easy optical polishing. This crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/, commonly termed YCOB, is one of a family of new nonlinear crystals, the oxyborates, that include RECa/sub 4/O(BO/sub 3/)/sub 3/ (RE=La/sup 3+/, La/sup 3+/, Y/sup 3+/, Sm/sup 3+/, Gd/sup 3+/, Er/sup 3+/, and Nd/sup 3+/). In this paper, we also successfully demonstrate a technique for improving the nonlinear optical properties of this crystal. This technique, ion substitution, has previously had limited success with other crystal hosts. However, the inclusion of yttrium in YCOB provides the opportunity to exploit this technique. Yb/sup 3+/, which has larger mass, but approximately the same atomic size as Y/sup 3+/ can be substituted into the crystal structure without introducing stress and nonuniformities. A systematic investigation of the linear and nonlinear characteristics of several crystals doped with various levels of Yb demonstrate that selective substitution of Yb in YCa/sub 4/O(BO/sub 3/)/sub 3/ improves the second-harmonic conversion efficiency by increasing the optical nonlinearity.

A new inorganic nonlinear optical single crystal of sodium cadmium tetra chloride (SCTC) is successfully grown from aqueous solution by the slow evaporation technique at room temperature. The crystalline nature of the grown crystal is analyzed by powder X‐ray diffraction. The lattice parameters and crystal system is confirmed by single crystal X‐ray diffraction. Optical transmission study on SCTC crystal shows high transmittance in the entire UV–Vis region. The mechanical strength of the grown crystal is estimated by Vicker's microhardness test. The second harmonic generation (SHG) efficiency of the crystal is measured by Kurtz's powder technique infers that the crystal has nonlinear optical (NLO) efficiency 1.75 times that of KDP. The variation of dielectric constant and dielectric loss as a function of log frequency at various temperature reveals that the crystal shows normal dielectric behaviour. The growth process of SCTC crystal is analysed by etching study. The features of surface of grown crystal is analysed by SEM/EDAX spectrum. The third order nonlinear optical property is investigated for the grown crystal by z – scan technique and the results indicates that SCTC crystal should be a promising material for nonlinear optical device applications.

Synthesis growth and characterization of l-Valinium Picrate a new nonlinear optical crystal

Growing large, high-quality nonlinear optical (NLO) crystals remains a central challenge for advancing ultraviolet (UV) laser technologies. KBe2BO3F2 (KBBF)-type crystals exhibit outstanding NLO properties but suffer from pronounced layer separation that impedes the growth of application-scale single crystals. Here, we introduce a three-dimensional (3D) charge-assisted hydrogen-bonded framework strategy by incorporating sulfanilamide and nitrate anions as hydrogen-bond donors and acceptors into a KBBF-like structure. This approach led to the successful synthesis of a novel ultraviolet (UV) NLO crystal, C6H9N2SO2·NO3 (SAN), which preserves a KBBF-like architecture while being stabilized by an extensive hydrogen-bonding network. SAN demonstrates a stronger second-harmonic generation (SHG) response (2.5 × KH2PO4, KDP) and higher birefringence (0.112 at 546 nm) compared with KBBF. Crucially, the 3D hydrogen-bonded network suppresses layer exfoliation and enables direct growth of centimeter-scale crystals (4.5 × 1 × 0.5 cm3) from aqueous solution by simple evaporation at room temperature. This work pioneers the integration of hydrogen-bonded framework design into the structural modification of NLO crystals. Through rational hydrogen-bond engineering within the KBBF-type framework, the crystal growth behavior has been significantly improved, establishing a generalizable strategy for modifying other layered functional crystalline materials.

The e-ray spatial walk-off effect is intrinsic with all anisotropic crystals but is of special concern for nonlinear optical (NLO) crystals in critical phase matching harmonic conversions. It presents one of the fundamental limitations for obtaining optimum conversion efficiency with NLO crystals. The principle of walk-off compensation has long been recognized and various schemes for overcoming walk-off has been discussed in the literature. A key prerequisite in this process for compensation is the ability to determine the deviation from the theoretical cut angle in starting NLO crystals and its subsequent correction. Our approach is to combine phase-angle-corrected NLO crystals by Adhesive-Free Bond (AFB®) into a stack of components for WOC by periodically inverting stack components to minimize walk-off and allow the generated power to grow proportionally with the length of the stack. We have implemented a measuring and correction system that consists of a precision six axes hexapod of a 0.0003° angular scanning resolution and an in-situ detection system with feedback of the generated power level as a function of angular position. As-received nonlinear crystals are oriented within ≤±.05° accuracy. We report on critical parameters of stack formation and improvement of a BBO (BaB₂O₄) quadruplet for conversion efficiency of 532 nm to 266 nm over a single BBO crystal and considerations for bonding LBO (LiB₃O₅) stacks.