Walk-off compensated nonlinear crystal stacks for efficient harmonic wavelength conversions

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.

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.

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.

Second-order nonlinear optical crystals with mixed anions

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.

Nonlinear optical (NLO) crystals are the key materials in modern laser technology and science because of their intrinsic capability to convert the wavelength of the light source. The search for new NLO materials is still very active in both scientific and industrial communities. Machine learning (ML) becomes a powerful tool to explore new candidates of NLO materials and to reveal the underlying relationship between structures and properties. In this work, we have proposed multilevel features that are relevant to the atomic properties, the characters of fundamental structural groups, and the crystal structures to describe inorganic NLO crystals for machine learning. The first-level and second-level descriptors can be obtained based on chemical compositions of crystals without prior knowledge about crystal structures. Several ML classifiers have been optimized using a database that consists of hundreds of NLO crystals to identify the samples with desired birefringence (Δn) and second-order nonlinear coefficients (dij). In particular, almost all of the ML models that only involve the first-level and second-level features, called as the crystal-structure-free model, exhibit good classification performance. It is still far from perfect but suitable to act as a filter in the first step of high-throughput materials discovery. Using the optimized ML models, feature importance analyses and virtual screening processes have been performed to understand the relationship between the features and targeted properties and to extract the statistical pictures on elements and fundamental structural groups. Several unexplored crystals are also picked out as ML-proposed candidates, and three of them are suggested as new potential NLO materials based on further first-principle calculations. The present ML models are expected to accelerate the inverse design for new NLO crystals with desired properties.

High-power tunable mid-infrared (MIR) and far-infrared (FIR) lasers in a range of 3-20 μm, especially in the atmospheric windows of 3-5 μm and 8-12 μm are essential for the applications, such as in remote sensing, minimally invasive surgery, telecommunication, national security, etc. At present, the technology of MIR and FIR laser have become a research hotspot. As the core component of all-solid-state laser frequency conversion system, nonlinear optical (NLO) crystals for coherent MIR and FIR laser are urgently needed by continuously optimizing and developing. However, compared with several outstanding near infrared, visible, and ultraviolet NLO crystals, such as <i>β</i>-BaB<sub>2</sub>O<sub>4</sub>, LiB<sub>3</sub>O<sub>5</sub>, LiNbO<sub>3</sub>, KTiOPO<sub>4</sub>, and KBe<sub>2</sub>BO<sub>3</sub>F<sub>2</sub>, the generation of currently available NLO crystals for 3-20 μm laser is still underdeveloped. Traditional NLO oxide crystals are limited to output wavelengths ≤ 4 μm due to the multi-phonon absorption. In the past decades, the chalcopyrite-type AgGaS<sub>2</sub>, AgGaSe<sub>2</sub> and ZnGeP<sub>2</sub> have become three main commercial crystals in the MIR region due to their high second-harmonic generation coefficients and wide IR transparency ranges. Up to now, ZnGeP<sub>2</sub> is still the state-of-the-art crystal for high energy and high average power output in a range of 3-8 μm. Unfortunately, there are still some intrinsic drawbacks that hinder their applications, such as in poor thermal conductivity and low laser damage threshold for AgGaS<sub>2</sub>, non-phase-matching at 1.06 μm pumping for AgGaSe<sub>2</sub>, and harmful two-photon absorption at 1.06 μm for ZnGeP<sub>2</sub>. In addition, ZnGeP<sub>2</sub> has significant multi-phonon absorption in an 8-12 μm band, which restricts its applications in long wavelength MIR. With the development of research, several novel birefringent crystals, as well as all-epitaxial processing of orientation-patterned semiconductors GaAs (OP-GaAs) and GaP (OP-GaP), have been explored together with attractive properties, such as large NLO effect, wide transparency ranges, and high resistance to laser damage.<br/>In this paper, from the angle of the compositions of NLO crystal materials, several kinds of phosphide crystals (ZnGeP<sub>2</sub> CdSiP<sub>2</sub>) and chalcogenide crystals (CdSe, GaSe, LiInS<sub>2</sub> series, and BaGa<sub>4</sub>S<sub>7</sub> series) are summarized. In addition, the latest achievements of the orientation-patterned materials such as OP-GaAs and OP-GaP are also reviewed systematically. In summary, we review the above-mentioned attractive properties of these materials such as in the unique capabilities, the crystal growth, and the output power in the MIR and FIR region.

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.

ConspectusThe invention of the laser is a pivotal milestone in the evolution of modern science and technology. Second-order nonlinear optical (NLO) crystals, which possess the ability to convert frequencies, have found widespread applications in laser science, information transmission, industrial Internet, and other cutting-edge fields within materials and optics. As modern science and technology continue to advance at a rapid pace, existing ultraviolet (UV) and deep ultraviolet (DUV) NLO crystals struggle to meet the ever-growing demands of various applications. Consequently, the development of novel UV and DUV NLO crystals has become an urgent necessity. For a UV NLO crystal to be considered outstanding in the UV/DUV range, it must exhibit three fundamental yet crucial properties: large second-order NLO coefficients, suitable birefringence, and short UV cutoff edge corresponding to a wide band gap. However, these key factors often conflict with one another, making it challenging to achieve a harmonious balance within a single crystal. It is widely believed that these mutually constrained optical properties are codetermined by microscopic NLO-active units and macroscopic structure features. Therefore, how to design high performance UV NLO-active groups to balance these three key properties is an essential scientifically question and serious challenge. In this Account, we present three strategies for designing high-performance UV NLO-active groups: (1) The "tetrahedron partial substitution" strategy by employing various substituents to replace one or more atoms in the traditional nonpolar tetrahedral groups, might achieve the aim of increasing the polarizability anisotropy and hyperpolarizability of the newly formed polar tetrahedral functional groups, such as from SO4 to SO3NH2 or SO3CH3 groups. (2) The "structure-analogue" strategy to develop a range of organic functional groups exhibiting more strong polarizability anisotropy and hyperpolarizability by using inorganic π-conjugated groups, such as BO3 and B3O6 groups, as templates. (3) The "two in one" strategy for integrating groups featuring planar triangle configurations and tetrahedrons to create NLO-active functional groups possessing large band gaps, strong hyperpolarizability, and moderate polarizability anisotropy. These three strategies successfully guide us to design and explore various kinds of organic-inorganic composite NLO crystal materials with excellent performances, like Ba(SO3CH3)2, M(SO3NH2)2 (M = Sr, Ba), C(NH2)3SO3F, KLi(HC3N3O3)·2H2O, KLi(C3H2O4)·H2O, and so on. Finally, we briefly conclude these strategies and propose some prospects for exploring new excellent UV/DUV NLO materials with practical applications. These findings could inspire novel thoughts for researchers designing new UV/DUV NLO materials and providing abundant materials used in UV/DUV regions.

Nonlinear optical (NLO) crystals have been playing an increasingly important role in laser science and technology. The NLO crystals used in the middle infrared (mid-IR) region, compared with the NLO crystals in the other wavelength regions, are still not good enough for the application of high-energy laser. The main defect is that their laser damage thresholds (LDT) are low. Chinese scientists have made a lot of important contributions to the UV and visible NLO crystals. In the last decade, they also did a lot of work on the mid-IR NLO materials. The main purpose of these researches is to increase the LDT and simultaneously balance the other properties. This paper presents a brief summary of their research progress in this topic on three types of materials: chalcogenides, oxides, and halides. The emphasis is put on the design strategy and quality control of the crystals.

Nonlinear optical (NLO) crystals play a pivotal role in solid‐state lasers and have broad applications in both civilian and military technologies. Ideal NLO crystal materials should possess a wide transmission range, moderate birefringence, a high second‐harmonic generation (SHG) coefficient, and excellent physical and chemical stability. Recent research has highlighted that incorporating metals with stereochemically active lone pairs (SCALPs) can significantly improve the NLO performance of these materials. This review presents a comprehensive overview of the latest 20 years’ advancements in NLO crystals featuring SCALPs, with a focus on their crystal structures and optical properties, particularly their SHG responses. The article also explores how metal‐based functional units containing SCALPs influence the polarizability of these materials. It is anticipated that this review will provide valuable insights into the functional design of high‐performance NLO crystals, contributing to the discovery of new SHG materials with the desired properties to meet the growing demands of modern technologies.

Recent development of nonlinear optical borate crystals: key materials for generation of visible and UV light

ASn(C10H12N2O8)Cl·H2O (A = K, Rb): Nonlinear optical crystals with new distorted Sn(IV)-centered nitrogen-oxygen-chloride polyhedron and wide band gaps