First-Principles Design and Simulations Promote the Development of Nonlinear Optical Crystals.

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.

Direct evidence of interaction-induced Dirac cones in a monolayer silicene/Ag(111) system.

Silicene, analogous to graphene, is a one-atom-thick 2D crystal of silicon, which is expected to share many of the remarkable properties of graphene. The buckled honeycomb structure of silicene, along with enhanced spin-orbit coupling, endows silicene with considerable advantages over graphene in that the spin-split states in silicene are tunable with external fields. Although the low-energy Dirac cone states lie at the heart of all novel quantum phenomena in a pristine sheet of silicene, a hotly debated question is whether these key states can survive when silicene is grown or supported on a substrate. Here we report our direct observation of Dirac cones in monolayer silicene grown on a Ag(111) substrate. By performing angle-resolved photoemission measurements on silicene(3 × 3)/Ag(111), we reveal the presence of six pairs of Dirac cones located on the edges of the first Brillouin zone of Ag(111), which is in sharp contrast to the expected six Dirac cones centered at the K points of the primary silicene(1 × 1) Brillouin zone. Our analysis shows clearly that the unusual Dirac cone structure we have observed is not tied to pristine silicene alone but originates from the combined effects of silicene(3 × 3) and the Ag(111) substrate. Our study thus identifies the case of a unique type of Dirac cone generated through the interaction of two different constituents. The observation of Dirac cones in silicene/Ag(111) opens a unique materials platform for investigating unusual quantum phenomena and for applications based on 2D silicon systems.

High-power 266 nm laser generation with a NaSr3Be3B3O9F4 crystal.

We demonstrate a 266 nm ultraviolet (UV) picosecond laser by fourth-harmonic generation of a Nd:YAG laser with a 5.4 mm thick NaSr3Be3B3O9F4 (NSBBF) crystal. A maximum output power exceeding 1 W at 266 nm was obtained (the highest output power being 1.6 W), corresponding to a conversion efficiency of 10.3%. The stability measurements on the NSBBF crystal with a fluctuation of 3.34% at 200 mW within 1 h indicate that it is a promising UV nonlinear optical material for practical applications. In addition, for the first time, to the best of our knowledge, we measured the effective nonlinear coefficient of NSBBF crystal at 266 nm and compared it with that of ß-BaB2O4 crystal.

High-energy single-frequency 167 nm deep-ultraviolet laser.

We report a high-energy single-frequency deep-ultraviolet (DUV) solid-state laser at 167.079 nm by the eighth-harmonic generation of a diode-pumped Nd:LGGG laser. A maximum DUV laser output energy of 1.5 µJ at a 5 Hz repetition rate with a 200 µs pulse duration is achieved. The central wavelength of the DUV laser is located at 167.079 nm and can be finely tuned from 167.075 to 167.083 nm. The linewidth is estimated to be 0.025 pm. To the best of our knowledge, this is the first Letter reporting a high-energy single-frequency solid-state DUV laser below 170 nm. The successful demonstration of the high-energy single-frequency DUV laser source with the unique wavelength is useful for direct detection of a Al+27 ion via resonance fluorescence in a multi-ion optical clock.

Deep-Ultraviolet Nonlinear Optical Crystal Cs2 Al2 (B3 O6 )2 O: A Benign Member of the Sr2 Be2 (BO3 )2 O Family with [Al2 (B3 O6 )2 O]2- Double Layers.

For the explorations of deep ultraviolet (DUV) nonlinear optical (NLO) borates, a type of important optoelectronic material, the (BO3 )3- group has been long regarded as the sole microscopic optically-active unit, and toxic Be-containing raw materials are frequently-adopted. Herein, a new DUV NLO crystal, Cs2 Al2 (B3 O6 )2 O (CABO), was designed and synthesized by simultaneously replacing the (BO3 )3- groups and Be2+ cations for (B3 O6 )3- units and Al3+ cations in Sr2 Be2 (BO3 )2 O, which possesses a favorable structure, through a chemical co-substitution approach. CABO exhibits a considerable DUV NLO capability because of the wide band gap and large birefringence originating from the [Al2 (B3 O6 )2 O]2- double layers. Remarkably, CABO melts congruently and does not contain the toxic beryllium, which is favorable for bulk-size crystal growth and practical applications.

Nonbonding Electrons Driven Strong SHG Effect in Hg2GeSe4: Experimental and Theoretical Investigations.

A Hg-based ternary infrared nonlinear optical (NLO) material, Hg2GeSe4, with the defect diamond-like (DL) structure was systematically investigated for the first time. The experimental results show that Hg2GeSe4 exhibits an enhanced second harmonic generation (SHG) response about 2.1 times that of the normal DL selenide AgGaSe2 ( d36 = 33 pm/V) at the particle size of 150-200 µm, as well as good phase-matchable ability. Moreover, theoretical analysis reveals that the nonbonding electrons around Se atoms in the defect DL structure make a dominant contribution to the improvement of the NLO property: d36 = 78.83 pm/V and Δ n = 0.11. This study highlights the promise of electronic engineering strategies and opens new avenues toward the design of new infrared NLO crystals with high performance.

High-efficiency UV generation at 266 nm in a new nonlinear optical crystal NaSr3Be3B3O9F4.

266 nm laser output in NaSr3Be3B3O9F4 crystal by the fourth harmonic generation process with a picosecond mode-locked Nd-based YAG laser has been done for the first time. When the input pumping energy was 870 µJ at 532 nm, a 280 µJ 266 nm UV laser was obtained and the corresponding conversion efficiency was 35.9%. Further investigations identified that NaSr3Be3B3O9F4 has a large acceptance angle width of 0.47 (mrad • cm), a small walk-off angle of 35.43 mrad and a large deff as 0.62 pm/V for the fourth harmonic generation. These results indicate that NSBBF is applicable for high-power 266 nm laser generation.

Unconventional Superconductivity in the BiS_{2}-Based Layered Superconductor NdO_{0.71}F_{0.29}BiS_{2}.

We investigate the superconducting-gap anisotropy in one of the recently discovered BiS_{2}-based superconductors, NdO_{0.71}F_{0.29}BiS_{2} (T_{c}∼5 K), using laser-based angle-resolved photoemission spectroscopy. Whereas the previously discovered high-T_{c} superconductors such as copper oxides and iron-based superconductors, which are believed to have unconventional superconducting mechanisms, have 3d electrons in their conduction bands, the conduction band of BiS_{2}-based superconductors mainly consists of Bi 6p electrons, and, hence, the conventional superconducting mechanism might be expected. Contrary to this expectation, we observe a strongly anisotropic superconducting gap. This result strongly suggests that the pairing mechanism for NdO_{0.71}F_{0.29}BiS_{2} is an unconventional one and we attribute the observed anisotropy to competitive or cooperative multiple paring interactions.

Structure and Characterization of a Zero-Dimensional Alkali Tin Dihalides Compound Cs3Sn3F2Cl7 with the [Sn2F2Cl4]2- Clusters.

A new perovskite stoichiometric alkali tin dihalides compound, Cs3Sn3F2Cl7, is synthesized by a hydrothermal method. This compound belongs to the monoclinic space group of P21/c with cell parameters of a = 9.5645(4) Å, b = 14.2057(7) Å, c = 13.5828(6) Å, and ß = 93.2450(10)°. Unlike the common perovskites in which octahedra are interconnected to be a three-dimensional network, Cs3Sn3F2Cl7 possesses a zero-dimensional structure consisting of Cs+ cations, isolated [SnCl3]- trigonal pyramids, and dimer structural units [Sn2F2Cl4]2-; the latter microscopic unit is found for the first time. The thermal stability and UV-vis-NIR diffuse reflectance spectroscopy in Cs3Sn3F2Cl7 are measured, and the electronic structure is calculated. Interestingly, the 5s2 lone-pair electrons on Sn2+ cations are stereochemically active, which results in a pretty good photocatalytic activity of the title compound.

Two KBBF-Type Beryllium Borates MBe2B2O6 (M = Sr, Ba) with a Three-Dimensional (Be2B2O6)∞ Network.

Two new polyborates, BaBe2B2O6 and SrBe2B2O6, in a three-dimensional (Be2B2O6)∞ network featuring KBBF-type two-dimensional planes are synthesized. Compared with KBBF, both compounds possess comparable optical birefringence and deep-ultraviolet (deep-UV) cutoff edges and exhibit better bulk growth habits owing to their three-dimensional networks, which make them applicable deep-UV optical materials.

Structural Design of Two Fluorine-Beryllium Borates BaMBe2(BO3)2F2 (M = Mg, Ca) Containing Flexible Two-Dimensional [Be3B3O6F3]∞ Single Layers without Structural Instability Problems.

Molecular structural design is a compelling strategy to develop new compounds and optimize the crystal structure by atomic-scale manipulation. Herein, two fluorine-beryllium borates, BaMgBe2(BO3)2F2 and BaCaBe2(BO3)2F2, have been rationally designed to overcome the structural instability problems of Sr2Be2B2O7 (SBBO). When relatively large Ba atoms were introduced, the [Be6B6O15]∞ double layers of SBBO were successfully broken, generating flexible [Be3B3O6F3]∞ single layers. Also, the strategy adopted in this work has many implications in understanding the structural chemistry and designing novel optical functional materials in a beryllium borate system.

Electronic evidence of an insulator-superconductor crossover in single-layer FeSe/SrTiO3 films.

In high-temperature cuprate superconductors, it is now generally agreed that superconductivity is realized by doping an antiferromagnetic Mott (charge transfer) insulator. The doping-induced insulator-to-superconductor transition has been widely observed in cuprates, which provides important information for understanding the superconductivity mechanism. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic bad metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. No evidence of doping-induced insulator-superconductor transition (or crossover) has been reported in the iron-based compounds so far. Here, we report an electronic evidence of an insulator-superconductor crossover observed in the single-layer FeSe film grown on a SrTiO3 substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with increasing carrier concentration. In particular, the insulator-superconductor crossover in FeSe/SrTiO3 film exhibits similar behaviors to that observed in the cuprate superconductors. Our results suggest that the observed insulator-superconductor crossover may be associated with the two-dimensionality that enhances electron localization or correlation. The reduced dimensionality and the interfacial effect provide a new pathway in searching for new phenomena and novel superconductors with a high transition temperature.

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.

High average power third harmonic generation at 355 nm with K3B6O10Br crystal.

We demonstrate a nanosecond 355 nm laser by third harmonic generation of an Nd: YAG laser with a 4 × 4 × 13.3 mm3 type I cut K3B6O10Br crystal for the first time. The maximum average power of 19.3 W is achieved with the corresponding conversion efficiency of 16.3% and the maximum conversion efficiency of 18.3% is recorded when the 355 nm average power is 15.8W. In addition, several nonlinear optical parameters including the angle and temperature bandwidths at third harmonic generation are also investigated.

Phase-matched frequency conversion below 150 nm in KBe2BO3F2.

Sum frequency mixing has been demonstrated below 150 nm in KBeBO3F2 by using the fundamental with its fourth harmonic of a 6 kHz Ti: sapphire laser system. The wavelength of 149.8 nm is the shortest ever obtained to our knowledge by phase matching in nonlinear crystals. The output powers were 3.6 µW at 149.8 nm and 110 µW at 154.0 nm, respectively. The phase matching angles measured from 149.8 to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The measured transmission spectra of KBeBO3F2 crystals support the generation of coherent radiation below 150 nm.

Beryllium-Free KBBF Family of Nonlinear-Optical Crystals: AZn2BO3X2 (A = Na, K, Rb; X = Cl, Br).

A series of a novel beryllium-free KBBF family of nonlinear-optical materials AZn2BO3X2 (A = K, Rb and X = Cl; A = Na, K, Rb and X = Br) were successfully synthesized through molecular engineering design, and single crystals of AZn2BO3Cl2 (A = K, Rb) were grown by a spontaneous nucleation technique from self-flux systems. As a representative for the halogen KBBF family of crystals, KZn2BO3Cl2 features the infinite lattice layer [Zn2BO3Cl2]∞ made up of BO3 and ZnO3Cl anionic groups, and the in-layer BO3 groups are completely coplanar and well-aligned. Besides, KZn2BO3Cl2 exhibits high transmittance in the range of 300-2000 nm with a UV-transmission cutoff of around 200 nm according to transmission spectra. The compounds of AZn2BO3Cl2 (A = K, Rb) are both phase-matchable with powder second-harmonic-generation efficiencies of 1.3 and 1.17 times that of KH2PO4 for KZn2BO3Cl2 and RbZn2BO3Cl2, respectively, which are similar to that of KBBF.

Midinfrared Nonlinear Optical Thiophosphates from LiZnPS4 to AgZnPS4: A Combined Experimental and Theoretical Study.

Our earlier theoretical calculation and preliminary experiment highlighted LiZnPS4 as a good mid-infrared (mid-IR) nonlinear optical (NLO) material. However, this compound suffers from problems including corrosion of the silica tubes, a pungent smell, deliquescence, and incongruent-melting behavior in the further single crystal growth and applications. In order to overcome these problems, herein, we investigate the analogues of LiZnPS4 and propose that AgZnPS4 would be a good candidate. The combination of experimental and theoretical study demonstrates that AgZnPS4 exhibits a much stronger NLO effect than that of LiZnPS4 despite the relatively smaller energy band gap. More importantly, AgZnPS4 melts congruently with a melting point as low as 534 °C, much lower than those of traditional IR NLO crystals, and is nondeliquescent with enough stability in the air. Such a good crystal growth habit and chemical stability enable AgZnPS4 to possess much better overall performance for the practical mid-IR NLO applications.

Be2BO3F: A Phase of Beryllium Fluoride Borate Derived from KBe2BO3F2 with Short UV Absorption Edge.

A phase of beryllium fluoride borate Be2BO3F (BBF) was successfully developed and grown by spontaneous nucleation from high temperature solution. The crystal belongs to the trigonal space group of R3̅c (No. 167), with lattice parameters a = 4.442(1) Å, c = 24.956(5) Å, and Z = 2. It is constructed by the infinite planar [Be2BO3F2]∞ layers, in which the planar triangle [BO3](3-) and the tetrahedral [BeO3F](5-) anionic groups are arranged in parallel via corner-sharing O atoms in each ab plane. BBF is an incongruent compound and decomposes at about 650 °C. The deep-ultraviolet (DUV) transmittance spectrum reveals that its UV cutoff wavelength is down to ∼150 nm. Theoretical calculations show that BBF has a large birefringence (Δn = 0.13 at 200 nm), which mainly originates from the infinite planar [Be2BO3F2]∞ layers. In conclusion, BBF may be served as a potential DUV birefringent material.

Cs3W3PO13: A Tungsten Phosphate with One-Dimensional Zigzag Tunnels Exhibiting Strongly Anisotropic Thermal Expansion.

A new tungsten phosphate, Cs3W3PO13, is synthesized using the high-temperature flux method. Cs3W3PO13 crystallizes in the space group Pnma and contains one-dimensional zigzag tunnels, which are found for the first time in tungsten phosphate. This highly anisotropic structural feature results in a very strong anisotropic thermal expansion, with thermal expansion coefficients of 14.15 ± 1.11 and 0.72 ± 0.22 M K(-1) along the a and b axes, respectively, over the temperature range from 13 to 270 K. In addition, thermal analysis, UV-vis-near-IR diffuse reflectance, and first-principles electronic structure calculations on Cs3W3PO13 are performed.

Dispersion relations of refractive indices suitable for KBe2BO3F2 crystal deep-ultraviolet applications.

KBe2BO3F2 (KBBF) is the only nonlinear optical crystal available to generate deep-ultraviolet (DUV) laser output by direct harmonic generation. High-precision refractive indices, including in the DUV region, were measured, and starting from a double resonance model of polarizability, new dispersion relations of the refractive indices were deduced from the measured data. The predicted phase matching angles for second-harmonic generation down to 165 nm from the new relations agree well with the previous reported values. Moreover, the new dispersion relations show superior results in an even shorter wavelength range, giving perfectly calculated phase matching angles for fifth-harmonic generation down to as short as 149.8 nm.