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Van der Waals (vdW) deep‐UV (DUV) nonlinear optical (NLO) crystal is an important material system recently developed. Herein, we review its concept and original intention, and then summarized the discovery process of related materials, including the role of A‐site cations and the resulting two‐/one‐dimensional vdW DUV NLO systems. Finally, we evaluate the practical DUV NLO performance and prospected the opportunities and challenges. [ABSTRACT FROM AUTHOR]

Van‐der‐Waals (vdW) deep‐ultraviolet (DUV) nonlinear‐optical (NLO) materials hold great potential to extend DUV NLO applications to two dimensions, but they are rare in nature. In this study, we propose a design principle to realize vdW DUV NLO materials via structural evolution from the non‐vdW (BO3)‐(BeO3F) layers in KBe2BO3F2 (KBBF) to the vdW (BO3)‐(BeO4H) layers in berborite Be2BO5H3 (BBH) and the vdW (BO4)‐(BeO4) layers in beryllium metaborate BeB2O4 (BEBO). Based on first‐principles calculations, the fundamental NLO properties of BBH and BEBO demonstrate that a balanced DUV NLO performance can be achieved in these two systems. Importantly, BBH, a layered material existing in nature, can achieve an available DUV phase‐matched output with strong second harmonic generation (SHG) for 177.3/193.7 nm DUV lasers, which is almost identical to that of KBBF. Remarkably, BEBO shows an excellent DUV SHG capacity and an even shorter phase‐matching wavelength than KBBF. Therefore, the newly discovered vdW BBH and BEBO, once verified by experiments, could provide an ideal platform to study DUV NLO effects in three dimensions and two dimensions. [ABSTRACT FROM AUTHOR]

A novel concept to obtain the deep‐ultraviolet (DUV) nonlinear optical (NLO) materials is proposed based on the assembling of one‐dimensional (1D) polar motifs into quasi‐1D polymer patterns. Based on the first‐principles calculations, we have successfully discovered an excellent DUV NLO polymer, i.e. poly(difluorophosphazene), with the chemical formula of (PNF2)n. Calculations reveal that PNF2 has a larger band gap, a stronger second harmonic generation effect, a larger birefringence, and a shorter phase‐matching cutoff than KBe2BO3F2. These findings not only demonstrate that the PNF2 is the first reported DUV NLO polymer, but also could open a new direction to discover novel DUV NLO materials in polymer systems. [ABSTRACT FROM AUTHOR]

In this work, we propose a strategy to design new deep‐ultraviolet (deep‐UV) nonlinear optical (NLO) materials through the combination of borate and phosphate NLO material genome. Accordingly, a series of borophosphates are predicted with very large energy bandgaps, strong second harmonic generation (SHG) effects, and moderate birefringence in the deep‐UV region. In particular, two novel fluoride borophosphates, PBO3F2 and PB3O6F2, are able to achieve the deep‐UV SHG laser output with shorter phase‐matching wavelengths (≈155 and 137 nm) than that of KBe2BO3F2 (KBBF, ≈161 nm). Further calculations show that these two structures are dynamically and thermodynamically stable, thus triggering possible synthesis, growth, and applications of them. The results attest that the proposed strategy is instructive and pave a comprehensive road map for us to discover new NLO crystals, especially in the borophosphate catalog with excellent deep‐UV phase‐matching capabilities beyond KBBF. [ABSTRACT FROM AUTHOR]

Abstract: KBe2BO3F2 (KBBF) is still the only practically usable crystal that can generate deep‐ultraviolet (DUV) coherent light by direct second harmonic generation (SHG). However, applications are hindered by layering, leading to difficulty in the growth of thick crystals and compromised mechanical integrity. Despite efforts, it is still a great challenge to discover new nonlinear optical (NLO) materials that overcome the layering while keeping the DUV SHG available. Now, two new DUV NLO beryllium borates have been successfully designed and synthesized, NH4Be2BO3F2 (ABBF) and γ‐Be2BO3F (γ‐BBF), which not only overcome the layering but also can be used as next‐generation DUV NLO materials with the shortest type I phase‐matching second‐harmonic wavelength down to 173.9 nm and 146 nm, respectively. Significantly, γ‐BBF is superior to KBBF in all metrics and would be the most outstanding DUV NLO crystal. [ABSTRACT FROM AUTHOR]

Metal sulfides with diamond-like (DL) structures are systematically investigated in this study for nonlinear optical (NLO) applications in the mid-infrared (mid-IR) spectral region. All the selected materials are characterized by a 3D framework connected by various sulfide tetrahedral units, such as [PS 4 ], [SiS 4 ], [GeS 4 ], [GaS 4 ], [InS 4 ], [CdS 4 ], [ZnS 4 ], [LiS 4 ], and [AgS 4 ]. The linear and nonlinear optical properties of these materials are analyzed on the basis of first-principles calculations. It is concluded that it is relatively easy to achieve good balance between the bandgap and the NLO effect. Additionally, moderate birefringence Δn (∼0.03–0.10) is required for practical mid-IR NLO applications. In particular, several metal sulfides with normal DL and defect DL structures, exhibiting superior mid-IR NLO properties, are highlighted. These provide an instructive guide for the design and discovery of new materials with good mid-IR NLO performance. [ABSTRACT FROM AUTHOR]

Metamaterials have enabled the realization of unconventional electromagnetic properties not found in nature, which provokes us to rethink the established rules of optics in both the linear and nonlinear regimes. One of the most intriguing phenomena in nonlinear metamaterials is 'backward phase-matching', which describes counter-propagating fundamental and harmonic waves in a negative-index medium. Predicted nearly a decade ago, this process is still awaiting a definitive experimental confirmation at optical frequencies. Here, we report optical measurements showing backward phase-matching by exploiting two distinct modes in a nonlinear plasmonic waveguide, where the real parts of the mode refractive indices are 3.4 and −3.4 for the fundamental and the harmonic waves respectively. The observed peak conversion efficiency at the excitation wavelength of ∼780 nm indicates the fulfilment of the phase-matching condition of k2ω = 2kω and n2ω = −nω, where the coherent harmonic wave emerges along a direction opposite to that of the incoming fundamental light. [ABSTRACT FROM AUTHOR]

Nonlinear optical (NLO) crystals are very important optoelectronic functional materials and their developments have significantly contributed to the progress of laser science and technology for decades. In order to explore new NLO crystals with superior performances, it is greatly desirable to understand the intrinsic relationship between the macroscopic optical properties and microscopic structural features in crystals. In this paper, the applications of density functional theory (DFT) method to the elucidation of the structure-property relationship and to the exploration on novel NLO materials in the ultraviolet and infrared spectrum regions are reviewed. The great success in the linear and NLO property predictions has been achieved using the first-principles computational simulations, and the mechanism understanding obtained by various analysis tools can give substantial guidance to the search and design of new NLO crystals. [ABSTRACT FROM AUTHOR]

• Single crystal structure of a novel ultraviolet NLO tourmaline was solved. • Sellmeier equations were obtained by fitting the measured refractive indices. • Phase matching range of the title crystal is obtained from the Sellmeier equations. • The linear and NLO properties were investigated by the experimental and theory calculation. Transparent tourmaline NaAl 9 (BO 3) 3 Si 6 O 21 OH is first reported by combining experimental and theory calculation methods, which exhibits potential ultraviolet nonlinear optical properties. [Display omitted] Tourmaline, as an important mineral, has received intensive interest, since it is not only a valuable gemstone, but also is a piezoelectric or pyroelectric material, due to the polar structural configuration. However, its nonlinear optical (NLO) properties were not explored so far, because a transparent crystal with high transmittance is scarce, being essential to optical functional materials. Here, we report a novel ultraviolet (UV) NLO tourmaline crystal with chemical composition NaAl 9 (BO 3) 3 Si 6 O 21 OH for the first time by combining first-principles analysis and experimental measurements. It exhibits a short UV absorption edge, considerable second harmonic generation effect and moderate birefringence, thereby facilitating its phase-matching coherent beam output in the range from visible to UV regions. [ABSTRACT FROM AUTHOR]

We identify the microscopic structural origins determining the bandgaps in the deep-ultraviolet borates, and propose an efficient method for the prediction of their bandgaps. This method considers only the chemical bond lengths around oxygen atoms and achieves the very high precision with the relative error <5% typically. Its validity is verified by the first-principles studies, which reveal the strong dependence of bandgaps on the coordination environment around oxygen atoms. Our studies have great implications on the search and design of optoelectronic functional materials with large bandgap. [ABSTRACT FROM AUTHOR]

Highlights • This is the first systematic review for NLO metal halides. • The relationships between microscopic structures and NLO performances in NLO metal halides are summarized and analyzed. • The development prospects for NLO metal halides are discussed. Abstract Infrared (IR) nonlinear optical (NLO) materials are of current interest owing to their technological applications in many military and civil fields. Metal halides are among the most favorable candidates for IR laser frequency conversion owing to their relatively large NLO effects, wide IR transparency and high laser damage thresholds (LDT). However, the comprehensive review for metal halides NLO materials is absent. Herein, we focus on the relationship between the structural features and NLO performances in the reported metal halide NLO materials. In particular, metal halides are classified according to the NLO-active [ MX k ] anionic units and NLO-active cations, i.e. , they are sorted into traditional metal halides containing linear units, triangle pyramidal units, tetrahedral units, rectangular pyramidal units, five-coordinated pseudo-octahedra, six- and more-coordinated polyhedral units, organic-inorganic and supramolecular metal halides. The determination rules of these microscopic structures on NLO properties in metal halides are summarized and analyzed on the basis of the combination of available experimental data and first-principles results. From the deduced structure-property relationship, the development prospects for NLO metal halides are discussed. [ABSTRACT FROM AUTHOR]

A computer-assisted material design system based on first-principles DFT calculations is employed to predict that NH4Be2BO3F2 possesses nonlinear optical properties, such as deep-UV second harmonic generation capability, very close to those of KBe2BO3F2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Recent developments in mid-IR NLO materials utilizing f 0 rare earth chalcogenides (RE = Sc, Y, La) are reviewed. • The advantages of utilizing f 0 rare earth chalcogenides as mid-IR NLO materials are thoroughly analyzed. • Several useful conclusions and considerations for future research are presented. Mid-infrared (mid-IR) nonlinear optical (NLO) materials are extensively researched as a vital optoelectronic material family with widespread applications ranging from civil to military fields. Metal chalcogenides possess a wide range of IR transparency and vast structural diversity, making them ideal system for exploration of novel mid-IR NLO materials. Incorporating rare earth elements with 4 f 0 and 5 f 0 electrons, including Sc, Y, and La, into chalcogenide structures in mid-IR NLO materials has been actively studied in the recent years. In this review, we categorize all typical f 0 rare earth (RE = Sc, Y, La) chalcogenides in the inorganic crystal structure database based on their NLO functional groups. We then assess their energy band gaps, NLO coefficients, birefringence, IR cut-offs, and mechanical properties using available experimental data and first-principles results and investigate the correlation between structural features and mid-IR NLO performance. It is showed that the f 0 rare earth chalcogenides can possess suitable birefringence and NLO response for mid-IR NLO applications, while not compromising energy band gap, thus may have a well-balanced mid-IR NLO performance. The prospects of novel f 0 rare earth chalcogenides with favorable mid-IR NLO performance are also discussed. [ABSTRACT FROM AUTHOR]

The non-linear optical (NLO) material BiB2O4F is prepared by hydrothermal synthesis employing a 1:20:2 molar mixture of Bi(NO3)3, H3BO3, and NaF (autoclave, 200 °C, 3 d, ≈90% yield). [ABSTRACT FROM AUTHOR]