Pifu Gong

Beryllium-free Li4Sr(BO3)(2) for deep-ultraviolet nonlinear optical applications

Nonlinear optical (NLO) materials are of great importance in laser science and technology, as they can expand the wavelength range provided by common laser sources. Few NLO materials, except KBe2BO3F2 (KBBF), can practically generate deep-ultraviolet coherent light by direct second-harmonic generation process, limited by the fundamental requirements on the structure-directing optical properties. However, KBBF suffers a strong layering tendency and high toxicity of the containing beryllium, which hinder the commercial availability of KBBF. Here we report a new beryllium-free borate, Li4Sr(BO3)2, which preserves the structural merits of KBBF, resulting in the desirable optical properties. Furthermore, Li4Sr(BO3)2 mitigates the layering tendency greatly and enhances the efficiency of second-harmonic generation by more than half that of KBBF. These results suggest that Li4Sr(BO3)2 is an attractive candidate for the next generation of deep-ultraviolet NLO materials. This beryllium-free borate represents a new research direction in the development of deep-ultraviolet NLO materials.

Beryllium-Free Rb3Al3B3O10F with Reinforced Interlayer Bonding as a Deep-Ultraviolet Nonlinear Optical Crystal

A new beryllium-free borate Rb3Al3B3O10F has been synthesized and characterized by single-crystal X-ray diffraction. It features a framework structure consisting of alveolate [Al3(BO3)3OF]∞ layers tightly bound via Al-O and Al-F bridged bonds, with the in-layer [BO3](3-) groups in nearly coplanar and aligned arrangement. This compound is transparent down to 200 nm and is phase-matchable with a powder second-harmonic generation efficiency of 1.2 times that of KH2PO4. Remarkably, it exhibits a strong interlayer bonding which is about one order larger than that of the benchmark KBe2BO3F2, thus no layering tendency was observed during the crystal growth. In addition, it is nonhygroscopic and thermally stable up to ∼1462 K. These attributes make Rb3Al3B3O10F a promising nonlinear optical crystal in the deep-ultraviolet region. First-principles calculations, combined with the anionic group theory, were adopted to rationalize the optical properties.

Deep-ultraviolet transparent phosphates RbBa2(PO3)5 and Rb2Ba3(P2O7)2 show nonlinear optical activity from condensation of [PO4](3-) units.

It is challenging to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance between deep-UV transparency and high NLO activity. Known deep-UV NLO materials are almost exclusively limited to borates, except few newly discovered phosphates despite their small NLO activities. Here we report two asymmetric phosphates, RbBa2(PO3)5 (I) and Rb2Ba3(P2O7)2 (II), which feature [PO3]∞ chains and [P2O7](4-) dimers formed by condensation of [PO4](3-) units, respectively. Remarkably, I achieves the desired balance, with the shortest deep-UV absorption edge at 163 nm and the largest NLO activity of 1.4 × KDP (KH2PO4) in deep-UV NLO phosphates. According to first-principles calculations, the enhanced macroscopic SHG response of I can be attributed to the [PO3]∞ chains which exhibit significantly larger microscopic SHG coefficients as compared with the [P2O7](4-) dimers.

First-principles materials applications and design of nonlinear optical crystals

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.

Tailored Synthesis of a Nonlinear Optical Phosphate with a Short Absorption Edge

A nonlinear optical phosphate Ba5P6O20 was rationally developed by a tailored synthetic approach based on the use of flexible [P3O10](5-) units. The phosphate exhibits a very short absorption edge of λ=167 nm, which is among the shortest known in phase-matchable phosphates. First-principles electronic structure analysis elucidated the origin of the changes in the optical properties, and specifically in the absorption edge, of the material. Such a tailored synthetic approach provides a new opportunity to design nonlinear optical materials with short absorption edges.

Trigonal Planar [HgSe3]4– Unit: A New Kind of Basic Functional Group in IR Nonlinear Optical Materials with Large Susceptibility and Physicochemical Stability

A new mercury selenide BaHgSe2 was synthesized. This air-stable compound displays a large nonlinear optical (NLO) response and melts congruently. The structure contains chains of corner-sharing [HgSe3](4-) anions in the form of trigonal planar units, which may serve as a new kind of basic functional group in IR NLO materials to confer large NLO susceptibilities and physicochemical stability. Such trigonal planar units may inspire a path to finding new classes of IR NLO materials of practical utility that are totally different from traditional chalcopyrite materials.

Near-Zero Thermal Expansion and High Ultraviolet Transparency in a Borate Crystal of Zn4B6O13

Intrinsic isotropic near-zero thermal expansion is discovered in borate crystal Zn4 B6 O13 with high transparency in the ultraviolet region. First-principles calculations demonstrate that the very low thermal expansion originates mainly from the invariability of the solid [B24 O48 ] truncated octahedra that are fixed by the [Zn4 O13 ] clusters in the ZBO structure.

BaGa2SnSe6: a new phase-matchable IR nonlinear optical material with strong second harmonic generation response

By introducing heavy Sn ions to a Ba–Ga–Q system, a new IR nonlinear optical material, BaGa2SnSe6, has been obtained. It crystallizes in space group R3 of the trigonal system with a = 10.1449(14) A, c = 9.2490(18) A and Z = 3. In the structure, three (Ga/Sn)Se4 tetrahedra are connected via corner-sharing to generate (Ga/Sn)3Se9 building groups, which are further joined to produce a three-dimensional network with Ba2+ lying in the cavities. Due to the contribution of the large and easily-polarizable Sn atom and the polar arrangements of the (Ga/Sn)Se4 tetrahedra, the compound exhibits a very strong NLO response, i.e., ∼5.2 times that of the benchmark AgGaS2 at a fundamental laser wavelength of 2.09 μm and shows type-I phase-matchable behavior. Furthermore, the calculated birefringence index is Δn = 0.1649 at 1.064 μm and the major SHG tensors are d11 = 62.91 pm V−1 and d33 = 50.26 pm V−1.

Isotropic Negative Area Compressibility over Large Pressure Range in Potassium Beryllium Fluoroborate and its Potential Applications in Deep Ultraviolet Region

Isotropic negative area compressibility, which is very rare, is observed in KBBF and the related mechanism is investigated by combined high-pressure X-ray diffraction (XRD) experiments and first-principles calculations. The strong mechanical anisotropy leads to a large Poissons ratio and high figure of merit for the acoustic-optics effect, giving KBBF potential applications as smart strain converters and deep-ultraviolet (DUV) acoustic-optic devices.

Ag3Ga3SiSe8: a new infrared nonlinear optical material with a chalcopyrite structure

An infrared (IR) nonlinear optical (NLO) material, Ag3Ga3SiSe8, has been synthesized for the first time. It crystallizes in the tetragonal chalcopyrite AgGaSe2 structure type and the substitution of Ga3+ with Si4+ generates an equal amount of vacancies at the Ag+ position. Ag3Ga3SiSe8 is a congruent melting compound with a melting temperature of 784 °C. The second-harmonic generation (SHG) effect of Ag3Ga3SiSe8 is similar to that of AgGaSe2, as confirmed by the first-principles studies. Moreover, the diffuse reflectance spectra reveal that the energy band gap of Ag3Ga3SiSe is 2.30 eV, 0.44 eV larger compared with AgGaSe2 (1.86 eV). The birefringence of Ag3Ga3SiSe8 is predicted to be above 0.1 at the wavelength of 1 μm, significantly larger than that of AgGaSe2 (Δn ~ 0.02). Our results indicate that Ag3Ga3SiSe8 may possess improved optical properties compared with AgGaSe2 and has a promising application as a good IR NLO material.