Nonlinear Optical Materials Research Articles

Blue Na2O - CoO - P2O5 glasses: Structural, physico-chemical, thermal and optical characterizations

Two blue phosphate glasses series, (50-x/2)Na2O - xCoO - (50-x/2)P2O5 (0 ≤ x ≤ 35) and (50-x)Na2O - xCoO - 50P2O5 (0 ≤ x ≤ 50) labelled NaCoPO(I) and NaCoPO(II), were synthesized by the melt quenching route and characterized by diverse techniques. Introduction of cobalt oxide CoO in vitreous NaPO3 metaphosphate induces an increase in the glass transition temperature, a decrease in the molar volume and the dissolution rate of the glasses in distilled water, and therefore a strengthening of the glass network. Raman spectra of NaCoPO(I) series, where O/P ratio varies from 3 to 3.54, show that Co2+ ions breakdown the infinite metaphosphate chains and provoke remarkable structural modifications. In contrast, no significant change was observed in the glassy network of NaCoPO(II) series, where O/P ratio is constant and equal to 3. Optical study results show that Co2+ ions have both octahedral and tetrahedral environments with predominance of tetrahedral coordination. The reduction in band gap energy when CoO content rises is due to the increase of non-bridging oxygen number in the vitreous network. The values of CIE L∗a∗b∗ parameters are consistent with the blue color, sought for commercial pigments. The physical parameters deduced from optical spectra suggest that the studied glasses are promising materials for several applications such as pigments and nonlinear optical materials.

Structure and optical properties of alkali rare-earth double phosphates of M3RE(PO4)2 (M = K, Rb; RE = Y, La, and Lu)

Designing and synthesizing phosphate nonlinear optical materials with novel structures and excellent properties remains challenging. In this work, the electronic structure and optical properties (second harmonic generation (SHG) response or birefringence) of alkali rare-earth double phosphates M3RE(PO4)2 (M = K, Rb; RE = Y, La, and Lu) are systematically investigated. It is worth mentioning that P31m-Rb3Lu(PO4)2 was successfully synthesized by a flux-method, and the UV–vis–NIR diffuse reflectance spectroscopy showed that Rb3Lu(PO4)2 exhibits a short absorption edge at 207 nm and a high transmittance of 77.9 %. Based on first-principles calculations, the results show that the birefringence of the M3RE(PO4)2 series of crystals exhibits significant differences (0.009-0.028@1064 nm). Subsequently, analysis of the electronic structure and the real-space atom cutting method reveals that rare-earth polyhedra play a major role in enhanced birefringence. Overall, this work enriches the study of nonlinear optical crystals of rare earth phosphates and provides a case for further exploration.

Exploration of the structure, electronic and optical properties of AB 2(PO3)5 polyphosphates: a first-principle approach

The density functional theory method was applied using the gradient PBE, incorporating the hybrid PBE0 functional along with dispersion corrections D3(BJ). These objectives were accomplished through the utilization of the CRYSTAL package, employing a localized atomic orbital basis. Calculations encompassing crystal properties, including electronic, structural, and linear/nonlinear opto-electronic characteristics, were conducted for monoclinic AB 2(PO3)5. Specifically, the polyphosphates investigated in this study are RbPb2(PO3)5, CsPb2(PO3)5, KBa2(PO3)5, and RbBa2(PO3)5. It was demonstrated that in monoclinic phosphates, phosphorus and oxygen atoms form infinite chains [PO4]∞ based on the· ⋯ O1-P-O1 ⋯ ·structure. Alkali metal and lead (barium) atoms are surrounded by oxygen atoms O2, forming polyhedra. Infrared absorption spectra calculations confirmed the presence of chain backbone structural units. Additionally, band structure and partial density of electronic states were determined. The nature of valence and unoccupied states was also investigated. In lead-doped compounds, lead atoms contribute to the formation of upper-valence and lower-unoccupied states. Conversely, in barium-doped compounds, the upper valence states are primarily formed by O2 p-states, while the lower unoccupied states are formed by phosphorus. Also, the coefficients of second harmonic generation and birefringence were calculated, assessing the potential use of phosphates as nonlinear optical materials.

Carbonatoperoxovanadates with Strong Second-Harmonic Generation: Insights from First-Principles Calculations on Anisotropic Structures and Optical Parameters.

Carbonatoperoxovanadates are considered as promising functional materials in optoelectronic devices due to their excellent optical properties, particularly strong second-harmonic generation (SHG) response. However, the relationship between their geometric structures and optical properties remains unclear. Herein, the structural, electronic, and optical properties of carbonatoperoxovanadates A3VO(O2)2CO3 (A=K, Rb, and Cs) were investigated using first-principles calculation. Results suggest that high density and parallel arrangement of nonlinear optical active [VO(O2)2CO3] units are conducive to generating large SHG response in A3[V(O2)2O]CO3. Optical anisotropy was observed. Birefringence values for A3[V(O2)2O]CO3 were comparable to those of commonly used infrared nonlinear optical materials. Specifically, results of tiny optical characteristics (local dipole moments, HUMO-LUMO gap, polarizability anisotropy, and hyperpolarizability) indicate that asymmetry [VO(O2)2CO3] is an excellent nonlinear optical active functional unit, owing to the synergistic effect between its non-centrosymmetric nonlinear optical elements. This study elucidates the structure-property relationship of carbonatoperoxovanadates, offering valuable insights for designing novel high-performance SHG materials.

RE(HSeO3)(SeO3)(H2O)·(H2O) (RE = Y, Gd): Two Rare-Earth Selenite Nonlinear Optical Crystals with Wide Band Gaps.

The band gap is one of the significant parameters for nonlinear optical (NLO) materials, which is the key factor in their application band in laser technology. This study reports the synthesis of two rare-earth selenite NLO crystals, namely, RE(HSeO3)(SeO3)(H2O)·(H2O) (RE = Y, Gd), utilizing the hydrothermal technique. The two compounds are isostructural, and both belong to the orthorhombic chiral space group of P212121 and possess three-dimensional structures consisting of two-dimensional rare-earth selenite layers connected by hydrogen bonds (O-H··O). These two compounds exhibit a moderate second harmonic generation intensity of 0.5/0.8 × KDP and wide energy band gaps reaching 4.8 eV. Comprehensive density functional theory analyses based on first-principles were carried out to unravel the relationship between the structural and corresponding properties. This work provides an approach for band gap enlargement in rare-earth selenite systems and is beneficial to the design of future NLO materials.

Growth, characterization and cognition of 2-cyanopyridinium perchlorate single crystals for nonlinear optical applications.

The fascinating electronic applications attracted researchers to explore the field of nonlinear optical (NLO) materials. The slow evaporation of solvent technique (SEST) was employed to grow the 2-cyanopyridinium perchlorate (2-CPPC) NLO single crystals. The cell parameters of the grown 2-CPPC crystal are confirmed by the single crystal X-ray diffraction (SCXRD) study. The powder X-ray diffraction studies confirm the crystallinity of 2-CPPC crystals, and the peaks were indexed. The computation for the geometry optimization, HOMO-LUMO energy gap, global reactivity parameters, natural bond orbital (NBO) analysis, polarizability, and hyperpolarizability of the 2-CPPC molecule was done using B3LYP (6-311G basis set) functional of DFT method. The experimental FTIR and UV-Vis results of the 2-CPPC compound were compared with the simulated results. The second harmonic generation (SHG) study for the 2-CPPC crystal was employed using Kurtz-Perry powder technique. Single beam Z-scan technique using He-Ne laser is used to study the third-order NLO properties.

Unveiling the NLO Potential of New Zn (II) Complex of 6‐Methylpyridine‐2‐Carboxaldehyde: Experimental/DFT Study on Spectral, Static, and Frequency‐Dependent Linear/Nonlinear Optical Parameters

ABSTRACTOngoing exploration focuses on synthesizing and characterizing coordination compounds to improve the design of nonlinear optical (NLO)‐based materials. In this regard, to examine spectral and static/frequency–dependent linear/NLO parameters, the new Zn (II) complex {[Zn(6‐MePyAld)2(Cl)]; 6‐MePyAld: 6‐methylpyridine‐2‐carboxaldehyde} was synthesized and characterized by using 1H and 13C NMR, mass (LC‐HRMS), powder XRD, and FTIR spectra. The electronic features of synthesized complex were investigated by considering the TD‐CAM‐B3LYP/ and TD‐M06L/6‐311G(d,p)//LanL2DZ levels of time‐dependent density functional theory (TD‐DFT). Moreover, the theoretical linear optical (LO), second‐, and third‐order NLO susceptibility tensors/polarization (χ(1)/P(1), χ(2)/P(2), χ(3)/P(3)) parameters for the Zn (II) complex were computed using the DFT/M06L and DFT/CAM‐B3LYP levels. The external electric field (E), polarization (P), and electric displacement (D) values of the Zn (II) complex were also calculated using the same DFT levels. To investigate microscopic LO (isotropic polarizability <α(0;0)>/<α(−ω;ω)>, and anisotropic polarizability (∆α (0;0)/∆α (−ω;ω)) and second‐/third‐order NLO (<β(0;0,0)>/<β(−ω;ω,0), <β(−2ω;ω,ω)>/<γ(0;0,0,0)>/<γ(−ω;ω,0,0)> and <γ(−2ω;ω,ω,0)>) parameters for the Zn (II) complex, the DFT/M06L and DFT/CAM‐B3LYP levels in the gas phase were used. The ∆α (0;0), ∆α (−ω;ω), <β(0;0,0)>, <β(−ω;ω,0)>, and <β(−2ω;ω,ω)> for Zn (II) complex were computed at 17.288 × 10−24, 21.782 × 10−24, 14.692 × 10−30, 466.80 × 10−30, and 210.79 × 10−30 esu, respectively, by using the DFT/CAM‐B3LYP level. Moreover, the <γ(0;0,0,0)>/<γ(−ω;ω,0,0)> and <γ(−2ω;ω,ω,0)> in the gas phase computed at the DFT/CAM‐B3LYP level for Zn (II) complex were obtained at 129.74 × 10−36, 3997.6 × 10−36, and −886.60 × 10−36 esu, in turn. According to the CAM‐B3LYP level, the <γ(0;0,0,0)> value is 8.65 and 18.53 times higher than the values of para‐nitroaniline (pNA) and urea, respectively. The obtained static/dynamic β and γ values of Zn (II) complex are greater than those of urea and pNA. Zn (II) complex exhibited remarkably microscopic second‐order and particularly third‐order NLO features. It is predicted that our study will shed light on NLO materials that might be used in telecommunication and optoelectronics.

Toward Strong UV-Vis-NIR Second-Harmonic Generation by Dimensionality Engineering of Zinc Thiocyanates.

The precise modulation of the spatial orientations and connection modes of primitives is vital for certain critically important optical functions for nonlinear optical (NLO) materials (specifically, second-harmonic generation (SHG) and optical bandgap); however, we are yet to achieve a sufficient level of control for the designed construction of efficient broadband NLO materials. Exploiting the changes in microscopic polarization that may result from dimensional increase, we propose herein a zero-dimensional (0D)-to-three-dimensional (3D) dimensionality-increase strategy to realize strong broadband SHG responses for the first time. The novel 3D pseudo diamond-like Zn(SCN)2 has been synthesized by removing SHG-inactive [NH4]+ counter cations and H2O molecules that are located between the adjacent discrete [Zn(SCN)4] building blocks within the 0D (NH4)2Zn(SCN)4·3H2O. The 0D-to-3D dimensionality engineering, proceeding from (NH4)2Zn(SCN)4·3H2O to Zn(SCN)2, results in significantly enhanced SHG responses and efficient broadband activity (8 × KH2PO4 @ 1064 nm, 4.18 eV bandgap for the former c.f. 2 × β-BaB2O4 @ 380 nm, 30 × KH2PO4 @ 1064 nm, 2 × KTiOPO4 @ 2100 nm, 4.78 eV bandgap for the latter) from the UV to the NIR regions (SHG@300-1050 nm). Theoretical calculations and crystal structure analyses reveal that the coordination-bond-connected [Zn(SCN)4] building blocks within the diamond-like structure of Zn(SCN)2 are responsible for its giant broadband SHG responses.

Magnetic field-engineered optical nonlinearity in germanene nanotubes

Abstract The linear and nonlinear optical properties of germanene nanotubes (GeNTs) under the influence of magnetic fields are investigated through theoretical analysis. By employing a tight-binding model beyond the Dirac cone approximation, the electronic band structure, density of states, dipole matrix elements, and third-order nonlinear optical susceptibilities are calculated. The application of a magnetic field induces a Zeeman splitting of the energy levels, breaking the band degeneracy. Consequently, the dipole matrix elements exhibit a magnetic field dependence, altering the intensity and number of allowed transitions. In the infrared and ultraviolet energy regions, the linear optical absorption spectrum displays the splitting of peaks into dual neighboring peaks, with the splitting distance linearly dependent on the magnetic field strength. The third-order nonlinear optical susceptibilities, χTHG(3)3ω and χIDIR(3)ω, reveal the emergence of multiple resonance peaks in different energy regions due to two-photon and three-photon absorption processes. The magnetic field significantly influences the number, position, and height of these optical peaks, enabling effective control over the nonlinear optical response. By investigating the scaled energy (E/E_g) dependence, a universal behavior is observed for the three- and two photon resonance peaks, where their positions remain constant, but their peak intensities are significantly enhanced under the influence of the magnetic field. These findings demonstrate the potential of GeNTs as tunable nonlinear optical materials, with the magnetic field serving as an effective parameter for engineering their optical properties for various applications in optoelectronics, photonics, and related fields.

Substrate Interference and Strain in the Second-Harmonic Generation from MoSe2 Monolayers.

Nonlinear optical materials of atomic thickness, such as non-centrosymmetric 2H transition metal dichalcogenide monolayers, have a second-order nonlinear susceptibility (χ(2)) whose intensity can be tuned by strain. However, whether χ(2) is enhanced or reduced by tensile strain is a subject of conflicting reports. Here, we grow high-quality MoSe2 monolayers under controlled biaxial strain created by two different substrates and study their linear and nonlinear optical responses with a combination of experimental and theoretical approaches. Up to a 15-fold overall enhancement in second-harmonic generation (SHG) intensity is observed from MoSe2 monolayers grown on SiO2 when compared to its value on a Si3N4 substrate. By considering an interference contribution from different dielectrics and their thicknesses, a factor of 2 enhancement of χ(2) was attributed to the biaxial strain: substrate interference and strain are independent handles to engineer the SHG strength of non-centrosymmetric 2D materials.

Bismorpholin-4-ium tetrabromo-cadmium metal-organic single crystal delivering Crystal structure, physicochemical properties and Characterizations for Optoelectronic Applications

For the development of nonlinear optical (NLO) materials, compositions of cadmium chloride (CC) and Morpholinium bromide (MB) molecules were utilized to form metal-organic materials. Morpholinium bromide was incorporated with cadmium metal to have electron charge transfer by constructing acentric structures that take advantage of strong third-order nonlinear optical properties. Bis Morpholinium cadmium bromide (BMCB) crystals were synthesized by a slow evaporation method (SEST). The BMCB crystal belongs to monoclinic P21/c space group with four molecules in the unit cell (a=6.7607(4) Å, b=16.5811(2) Å, c=14.9744(2) Å, β=94.136(3) ° and Z=4). The grown BMCB crystal phase purity and crystallinity were further investigated using powder X-ray diffraction (PXRD) analysis. Using Fourier transform infrared (FTIR) spectroscopy, the functional groups and their vibrational states were evaluated. From the results obtained from UV-vis-NIR spectral analysis, it is clear that the title compound is transparent in the visible range with the lower cut-off wavelength of 254 nm. The TG-DTA measurement has been used to evaluate thermal stability. Microhardness measurements were carried out to evaluate the mechanical stability of the grown crystal. The third-order nonlinear optical (NLO) characteristics were studied using Z-scan technique. The grown crystal exhibits outstanding third-order NLO response and wide laser damage threshold (LDT), which are crucial characteristics for developing practical NLO devices. The findings consequently open a novel path for the logical design of enormous, thermally stable and NLO-active metal-organic complexes for NLO applications.

DFT Study on the Second-Order NLO Responses of 2-Phenyl Benzoquinoline Ir(III) Complexes by Substituents and Redox Effects.

Metal complexes have received extensive attention in nonlinear optical (NLO) materials because of their advantages, such as shorter response times and more flexible structural properties. Density functional theory is used in investigating the geometric structures, electronic structures, charge centroid, and first hyperpolarizability (βtot) of a series of selected 15 coordinated Ir(III) complexes. The substitute effect and one-electron redox process effects on the structures and properties of 15 coordinated Ir(III) complexes are considered. When the electron-withdrawing group is introduced into the ligand, the HOMO-LUMO energy gap decreases and the βtot value increases, positively correlating with the electron-withdrawing ability. The single electron redox process can also improve the NLO responses of complexes, especially the reduction process. The βtot value of complex 4- is the largest, 2078 times higher than that of complex 4. The analysis shows that the variation of NLO responses of complexes is ascribed to the change of electronic structures and the charge transfer modes induced by the ligand modification and redox process, which are considered to be two effective methods in enhancing the NLO responses of 2-phenyl benzoquinoline Ir(III) complexes. This study aims to offer design insights into high-performance nonlinear optical materials.

K4Cd3Ge4O13: A Congruent-Melting Germanate Nonlinear Optical Crystal with a Moderate Second-Harmonic Generation Intensity and Broad Transparent Window.

Mid-infrared (IR) nonlinear optical (NLO) materials have generated extensive research interest because of their crucial role in laser technology applications. Here, we report the synthesis of a novel cadmium germanate NLO crystal, K4Cd3Ge4O13, using spontaneous crystallization. K4Cd3Ge4O13 demonstrates a distinct three-dimensional structural framework characterized by twisted [Ge4O13] and [Cd3O10] clusters, composed of [GeO4], [CdO4], [CdO5], and [CdO6] basic building units, respectively, which represents an unprecedented structural feature. The title compound undergoes a desirable congruent melting behavior at about 727 °C. Notably, K4Cd3Ge4O13 demonstrates a short UV cutoff edge at 261 nm, coupled with a wide energy gap of 4.4 eV, and maintains an extended IR transparency window at around 6.0 μm. More importantly, it demonstrates a strong second-harmonic generation activity comparable to that of KH2PO4 (KDP) at 1064 nm. Theoretical analyses further elucidate that the remarkable optical performances of K4Cd3Ge4O13 are predominantly attributed to the cooperative effects of Ge-O and Cd-O bond-based motifs. These desired characteristics underscore the potential of K4Cd3Ge4O13 as a good candidate material for mid-IR NLO applications.

Rational analysis of hydrogen bonding interaction in phenazine, 2-hydroxynaphthalene (1:1) cocrystal: from molecular modeling to photophysical properties.

Organic cocrystals have a wide range of applications in the field of optics due to their photo responsive property. We present here a newly synthesized phenazine 2-hydroxynaphthalene (1:1) cocrystal, its structural and theoretical calculations which tend to the nonlinear optical property. In the crystal structure of the title cocrystal, the phenazine and 2-hydroxynaphthalene molecules from one- and two-dimensional supramolecular frameworks via O‒H…N hydrogen bonds and C‒H…N, C‒H…π interaction, respectively. The phenazine molecules from an infinite off-set stacking through π…π interactionin the three-dimensional molecular packing of the title cocrystal. The contribution of intermolecular interaction in the three-dimensional molecular packing and the interaction energy calculation is studied by the Hirshfeld surface analysis. The molecular geometry retrieved from the experimental X-ray diffraction analysis is in good agreement with the theoretically calculated parameters. Further, the molecular electrostatic potential (MEP) and frontier molecular orbital (FMO) analysis have been carried out to study the charge distribution and molecular reactive mechanism. Third-order nonlinear optical property of the cocrystals has been analyzed by Z-scan measurements. The determined nonlinear optical absorption coefficient value 6.442 × 10-05 (m/W) and the nonlinear refractive index value - 5.535 × 10-2 (m/W) suggest that the crystalline solid can be a good choice of potential nonlinear optical material. The crystal structures of phenazine 2-hydroxynaphthalene cocrystal was solved by direct methods procedure using SHELXS program and refined by full-matrix least square procedure on F2 using SHELXL-2018 program on Olex2 software. The computational calculation has been carried out using DFT/B3LYP quantum chemical function with triple zeta 6-311 + + basis set in the ground state molecular stability using Gaussian 09W program suite. The Hirshfeld surface analysis mapping, associated 2D fingerprint plot, and intermolecular molecular interaction energy calculations were carried out using CrystalExplorer (version 21.5) software.

Covalent Organic Frameworks in Computational Design of Second-Harmonic Generation Materials: Role of Tetrel Atoms and Their Interactions.

Modern approaches to the design of nonlinear optical materials often rely on computational techniques. Here, we discuss the effects of the variation in the center tetrel atoms, Tt = C, Si, or Ge, in a series of covalent organic frameworks of the COF-102 family. The effects of halogen substitution, Hal = Cl, Br, or I on intramolecular tetrel bonding are also discussed. The characteristics of the calculated electron density have been implemented to describe the features of the electron distribution around the central fragment involving a tetrahedral tetrel atom. The effect of the central Tt atom leads to a dramatic change in the character of electron delocalization on the Tt-Car bond with aromatic rings. The location of the halogen atom at the ortho-position of the aromatic ring leads to the formation of tetrel bonds, halogen bonds, or other noncovalent interactions. The changes in the second-order electric susceptibility χ(2) have been studied in order to describe the strength of nonlinear optical properties within the periodic couple-perturbed Kohn-Sham approach. A counterintuitive trend for the χ(2) decrease is observed upon substitution of H > Cl > Br > I at the ortho-position of the phenyl ring. This is due to the corresponding elongation of the Tt-Car bond.

An Investigation on Optical, Larvacidal and Cytotoxicity Analysis of Sulfanilic Acid Single Crystal for Optical and Biomedical Applications.

Sulfanilic acid (SFA) crystal is well known as an effective material for photonic, electro-optical, harmonic generating and biomedical applications. A well-known nonlinear optical material, a high-quality SFA single crystal made utilizing the slow evaporation solution method (SEST) is the subject of this article. A 75 days development period yielded a transparent SFA single crystal measuring 5 × 5 × 2 mm3. The grown crystal used for different characterizations like Single crystal XRD used to find out the cell parameters. Fourier transforms infrared utilized to identify the band assignments. UV-Visible analysis used to detect the absorbance of the crystal and it is utilized for optical application. Photoluminescence studies utilized to recognize the excitation and emission of the grown crystal. Fluorescence used for determining the crystallinity and purity of the sample. The quantitative analysis is verified by using Elemental Dispersive Analysis by X-Rays. Scanning Electron Microscopy utilized to identify the structural and morphological characteristics. To the best of our knowledge, this paper is the first to provide the generated crystal that was used to analyze cytotoxicity and larvacidal activity. Assessment of larvicidal activity was used to ascertain the anti-malarial efficacy. We tested the items on MCF7-Human Breast cancer cell line and MCF7 Vero cells using the MTT Assay to identify the molecular basis of their cytotoxicity in vitro. Biological and optical are two areas that could benefit from the created crystal.

Constructing Ultraviolet Nonlinear Optical Crystals with Superior Thermal Stability Based on Organic π-Conjugated [HCOO] Groups.

In recent years, more and more organic π-conjugated moieties have been screened out to create excellent ultraviolet (UV) and deep-UV nonlinear optical (NLO) crystal materials. However, NLO crystals with organic groups usually exhibit poor thermal stability compared with those of traditional inorganic NLO materials. Herein, the organic group [HCOO], which is similar to traditional planar trigonal π-conjugated anionic groups, was employed to successfully build three UV NLO crystals RE(HCOO)3 (RE = Eu, Gd, and Dy) and two centrosymmetric compounds RE(HCOO)2(OH) (RE = Eu, and Gd) with superior thermal stability. Their structures and properties were further studied and characterized, especially the structure evolutions between noncentrosymmetric RE(HCOO)3 (RE = Eu, Gd, and Dy) and centrosymmetric RE(HCOO)2(OH) (RE = Eu, and Gd), as well as the NLO properties of the three crystals RE(HCOO)3 (RE = Eu, Gd, and Dy).

Efficient second-harmonic generation in a lithium niobate metasurface governed by high-Q magnetic toroidal dipole resonances.

Lithium niobate (LN) is an excellent nonlinear optical material due to its large nonlinear coefficient, low loss, and broad optical transparency window. So, it is widely used in the generation of nonlinear harmonics. Magnetic toroidal dipole (MTD) resonance is a special optical resonance mode, which can effectively localize the light field inside the device, thus enhancing the nonlinear effects of the materials. In this work, we numerically study the second-harmonic generation (SHG) effect of the LN metasurface based on the MTD mode with a high quality factor (Q-factor). The designed LN nanorod dimer metasurface supports high Q-factor MTD guided mode resonances (GMRs), which are excited by varying the center spacing of the two nanorods, and the Q-factor can be controlled by the offset distance. The excited MTD can effectively confine the electric field within the device, which enables the LN metasurface SHG conversion efficiency to reach 1.15 × 10-2. In addition, by adjusting the structural parameters, it is possible to effectively modulate the wavelength and conversion efficiency of the SHG. Our results provide a new route for high-quality nonlinear light sources.

The electronic structure and optical properties of BaBiBO4, a promising compound with high birefringence

Birefringence is a crucial parameter for various advanced optical devices like birefringent crystals, and nonlinear optical materials. In this paper, the electronic structures and optical properties of centrosymmetric BaBiBO4 are investigated using the first-principles method. The results show that BaBiBO4 could be a good birefringent compound whose birefringence is 0.195 @ 1064 nm and 0.266 @ 532 nm. The spin–orbit-coupling effect plays an important role in electronic structures, leading to downshift and band splitting of the conduction band, resulting in a reduced bandgap of 3.32 eV and enhanced birefringence. The projected density of states and real-space atom-cutting results reveal that BiO5 groups with asymmetric distribution of stereochemical active lone pairs and the staggered parallel arrangement of the BO3 groups give the main contribution to birefringence.

K0.9Rb2.1B8PO16: Design and Synthesis of a Deep-Ultraviolet Nonlinear Optical Crystal with Balanced Performance Derived from π-Conjugated and Non-π-Conjugated Groups.

A new deep-ultraviolet (DUV) nonlinear optical (NLO) material K0.9Rb2.1B8PO16 (KRBPO) has been designed and synthesized by adjusting the B/P molar ratio to 8:1. The KRBPO crystals were synthesized by a flux method and crystallized in noncentrosymmetric (NCS) and polar space group Cc. This compound exhibits a double-layer structure in which the A layer is composed of [B2O5] and [BO4] units and the B layer is formed by interconnected [B3O7] and [BO3] groups, and the two layers are connected by [PO4] tetrahedron. The theoretical calculations and experiments show that the synergistic interaction of π-conjugated and non-π-conjugated units leads to relatively well-balanced NLO properties of the title compound, including moderated SHG (0.7 × KDP), short UV cutoff edge (λcutoff < 190 nm), and a large band gap of 6.16 eV. Specifically, the coplanar arrangement of B-O groups in double-layer makes the KRBPO display a large birefringence (0.075@532 nm) and enables the shortest phase-matched wavelength to reach an important laser output wavelength of 266 nm.