Mg?Si?As: An Unexplored System with Promising Nonlinear Optical Properties

Two new non‐centrosymmetric ternary compounds, MgSiAs₂ and Mg₃Si₆As₈, are discovered via metal flux and solid‐state synthetic methods. MgSiAs₂ belongs to the well‐known II‐IV‐V₂ family, which is extensively studied experimentally and computationally for their optical properties. MgSiAs₂ is computationally predicted but not experimentally known prior to this work. Mg₃Si₆As₈ crystallizes in a new non‐centrosymmetric cubic chiral structure type with the Pearson symbol cP68. The syntheses, crystal structure, thermal and chemical stabilities, electronic structures, and optical properties of these two new compounds are investigated in this work. Optical absorption measurements and electronic structure calculations reveal the two compounds to be direct or pseudo‐direct bandgap semiconductors (1.8–2 eV). The crystal structures of both compounds are non‐centrosymmetric, though Mg₃Si₆As₈ belongs to the 432 chiral crystal class, which is optically active but does not exhibit second harmonic generation (SHG) behavior. The SHG response of MgSiAs₂ is 60% of that for AgGaS₂, but MgSiAs₂ exhibits a higher laser damage threshold than AgGaS₂ at 33.2 MW cm^?2.     

Noncentrosymmetric Tetrel Pnictides RuSi4P4 and IrSi3P3: Nonlinear Optical Materials with Outstanding Laser Damage Threshold

Noncentrosymmetric (NCS) tetrel pnictides have recently generated interest as nonlinear optical (NLO) materials due to their second harmonic generation (SHG) activity and large laser damage threshold (LDT). Herein nonmetal-rich silicon phosphides RuSi₄P₄ and IrSi₃P₃ are synthesized and characterized. Their crystal structures are reinvestigated using single crystal X-ray diffraction and ²⁹Si and ³¹P magic angle spinning NMR. In agreement with previous report RuSi₄P₄ crystallizes in NCS space group P1, while IrSi₃P₃ is found to crystallize in NCS space group Cm, in contrast with the previously reported space group C2. A combination of DFT calculations and diffuse reflectance measurements reveals RuSi₄P₄ and IrSi₃P₃ to be wide bandgap (E_g) semiconductors, E_g = 1.9 and 1.8 eV, respectively. RuSi₄P₄ and IrSi₃P₃ outperform the current state-of-the-art infrared SHG material, AgGaS₂, both in SHG activity and laser inducer damage threshold. Due to the combination of high thermal stabilities (up to 1373 K), wide bandgaps (≈2 eV), NCS crystal structures, strong SHG responses, and large LDT values, RuSi₄P₄ and IrSi₃P₃ are promising candidates for longer wavelength NLO materials.     

Designing Non‐Centrosymmetric Molecular Crystals: Optimal Packing May Be Just One Carbon Away

Molecular nonlinear optical (NLO) crystals feature important advantages compared to inorganic counterparts, such as low dielectric constants, ultrafast response times, and large electro‐optic coefficients. Conjugated push–pull chromophores connecting electron‐donating with accepting groups are often employed in the design of these crystals. However, associated large molecular dipole moments induce antiparallel or centrosymmetric conformations in the solid‐state, which leads to NLO inactivity. The cation–anion hydrogen bond interactions of a hydroxy‐piperidino electron donor group are combined with increased van der Waals volume effects induced by an ethyl modification of the electron‐accepting moiety. This produces non‐centrosymmetric packing in the organic salt EHPSI‐4NBS ((E)‐1‐ethyl‐2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐3,3‐dimethyl‐3H‐indol‐1‐ium 4‐nitrobenzenesulfonate). Converting a methyl group into ethyl changes the packing symmetry in the molecular crystal to switch on NLO activity. This behavior is attributed to the increased size of the ethyl group, which pushes apart the van der Waals contacts of the cation that lead to centrosymmetric packing in the methyl derivative. To test the NLO properties of EHPSI‐4NBS, THz generation experiments are performed at 1200 nm pump wavelength. Spectral amplitude similar to DAST ((E)‐4‐(4‐(dimethylamino)styryl)‐1‐methylpyridin‐1‐ium tosylate) crystal is observed with generation profile from 0 to 3.8 THz.     

一种新型材料的LB膜及非线性光学特性研究

研究了一种包含西佛氏碱和萘酰亚胺的新型材料的LB膜的制备 ,并采用π A等温曲线和紫外 可见吸收谱对其LB膜的制备特性进行了表征。单分子膜的崩溃压力在 30mN/m左右 ,对应的单分子的面积约为 1.8nm2 。π A等温线的固相区较为陡峭 ,在空气 水界面上 ,能形成了较好的单分子膜 ,并可以较好的转移到固体基板上 ,转移比可保持在 1± 0 .1的范围内。采用二次谐波产生的方法研究了LB单层膜的二阶非线性光特性。测量了二次谐波强度随基频光入射角的变化关系 ,其二次谐波信号的最大值在约 6 0°的入射角处。这种化合物具有较大的二阶非线性极化率。它的二阶非线性光特性起源于由苯乙烯形成的共轭π 电子体系     

1D Van der Waals Polymers with Nonlinear Optical Performance Approaching Theoretical Upper Limit

Nonlinear optical (NLO) materials are of great importance for applications in lasers, atomic clocks, free-space communication, etc. Herein, inspired by the recent prediction of excellent second harmonic generation (SHG) performance in van der Waals (vdW) materials with 1D building blocks, 14 new NLO materials are found from 244 bulk crystals constructed with 1D polymers using high-throughput first-principles calculations. Nearly half of the new NLO materials exhibit superior NLO performance with SHG susceptibilities approaching the theoretical upper limit. The 2D form of 11 candidates inherits the NLO property covering UV, visible, and infrared regions. Bader charge analysis reveals that the SHG susceptibility is determined by the charge difference of ions on the chains. Finally, it is proposed that superior NLO materials can be found in materials with proper bandgaps and large charge differences on the chains. This work not only screens out candidates with outstanding NLO performance in vdW materials with 1D building blocks but also provides a guideline for the search and design of NLO vdW 1D polymer patterns with excellent NLO properties.     

NMOB分子LB膜的光谱及其非线性光学特性研究

通过稳态和时间分辨荧光及二次谐波产生(SHG)的方法研究了2-硝基-5-(N-甲基-N-十八烷基)氨基苯甲酸(NMOB)分子朗缪尔-布罗基特(Langmuir-Blodgett,LB)多层膜的光谱及非线性光学特性。LB膜的稳态荧光谱较溶液红移大。而纯的NMOBY型LB多层膜与NMOB/花生酸交替的LB多层膜之间差别较小;由于分子间的相互作用,使得NMOB分子在溶液中比在LB膜中的荧光衰减时间大。在LB膜中,纯的NMOB和NMOB与花生酸交替的LB多层膜的荧光衰减时间相差不大,说明NMOB分子层内的相互作用较大,而层间的相互作用较小;由于苯环两侧的不对称取代,分子内部形成固有的偶极矩,使得NMOB/花生酸交替的LB多层膜具有较大的宏观偶极矩,因而具有大的二阶非线性极化率,其超分子极化率约为β=4×10-29esu。     

Octupolar Films with Large Second Harmonic Generation and Electro‐Optical Effects

Thin films that benefit from efficient octupolar molecular packing are prepared for second harmonic generation (SHG) and electro‐optic (EO) applications. The films are composed of 1,3,5‐tricyano‐2, 4,6‐tris(p‐diethylaminostyryl)benzene (TTB) in a ploymethylmetacrylate (PMMA) matrix on aluminum/BK7 glass (Al/BK7) and polyimide/indium tin oxide (PI/ITO) substrates. Octupolar films prepared on both substrates display polycrystalline and cylindrical domains. The molecular orientation, SHG efficiencies, and EO coefficients of the crystalline domains are measured. In the cylinders, the molecular crystal planes are oriented perpendicularly to the major cylinder axis, whereas in the polycrystals, the planes are randomly oriented. While both structures exhibit high and stable SHG and EO efficiencies, the cylinders, in particular, exhibited a very large SHG, a large EO coefficient, and high thermal stability; these characteristics will be useful in second order nonlinear optical applications.     

Multimodal Optically Nonlinear Nanoparticles Exhibiting Simultaneous Higher Harmonics Generation and Upconversion Luminescence for Anticounterfeiting and 8-bit Optical Coding

Nonlinear optical materials are essential in areas such as nanophotonics, optical information processing, and biomedical imaging. However, nanomaterials employed for these diverse applications to date are efficient only for one type of nonlinear optical activity. Herein, the first multimodal nonlinear optically active class of nanomaterials based on lanthanide-doped lithium niobate nanoparticles, which simultaneously exhibit unprecedentedly efficient second and third harmonic generation, as well as up-conversion photoluminescence, is reported. These dielectric nanoparticles retain their high nonlinear optical conversion efficiency both as powder and as aqueous colloidal solution. The high stability also allows for the fabrication of optically active biocompatible micron-sized fibers and polymer-based 3D-printable objects, as well as for fingerprint detection. Finally, the first 8-bit coding platform purely based on multimodal nonlinear optical activity originating from different parametric and nonparametric processes is demonstrated, showcasing the technological potential of these materials for both anti-counterfeiting and advanced optical information processing.     

The Directional Peeling Effect of Nanostructured Rigiflex Molds on Liquid‐Crystal Devices: Liquid‐Crystal Alignment and Optical Properties

We report a new strategy, the directional peeling of a rigiflex mold with a nanostructure, to overcome several problems with general patterning techniques for liquid‐crystal (LC) alignment. These include difficulty in generating the pretilt angle and in controlling the LC rising‐up direction, formation of local domains, and weak optical properties. The directional peeling of the rigiflex mold results in pretilt‐angle formation and controls the LC rising‐up direction. In addition, a nanostructure with small spacing aligns the LC with a high order parameter because of a strong confinement effect and suppresses diffraction due to its small spacing. Eventually, the nanostructure achieves improvements in the optical properties. In summary, while recent patterning techniques for LC alignment only solve one problem, the directional peeling of the rigiflex mold with a nanostructure simultaneously overcomes several problems with LC alignment and optical properties.     

Doxorubicin Nanocarriers Based on Magnetic Colloids with a Bio‐polyelectrolyte Corona and High Non‐linear Optical Response: Synthesis,Characterization, and Properties

Magnetic drug nanocarriers are synthesized following an arrested mineralization of magnetic spinel iron oxides in the presence of the biopolymer of sodium carboxymethylcellulose. Based on the experimental results, the polyelectrolyte corona probably attains a brushlike configuration around the magnetic particles. The inner core of these colloids may be constituted of polymer‐associated nanocrystallites, forming nanogel colloids. The hybrid colloids are endowed with a high loading capacity for the anticancer agent doxorubicin and pronounced pH responsiveness. They also display a dramatic increase in non‐linear optical response as compared to previous studies of similar materials. Furthermore, as cell studies indicate, the blank nanocarriers are cytocompatible and the drug retains its activity after loading in the nanocarriers.     

Polymorphism in Crystalline Microfibers of Achiral Octithiophene: The Effect on Charge Transport,Supramolecular Chirality and Optical Properties

Polymorphic crystalline microfibers from an achiral octithiophene with one S‐hexyl substituent per ring are separately and reproducibly grown with the same characteristics on various solid surfaces, including the interdigitated electrodes/SiO₂ surface of a bottomcontact field‐effect transistor. The arrangement of the same molecule in two diverse supramolecular structures leads to markedly different electronic, optical, and charge mobility properties. The microfibers—straight and yellow emitting or helical and red emitting—exhibit p‐type charge transport characteristics, with the yellow ones showing a charge mobility and on/off current ratio of one and three orders of magnitude, respectively, greater than the red ones. Both forms show circular dichroism signals with significant differences from one form to the other. DFT calculations show that the octithiophene exists in two different quasi‐equienergetic conformations aggregating with diverse orientations of the substituents. This result suggests that the observed polymorphism is conformational in nature. The self‐assembly, driven by sulfur–sulfur non‐bonding interactions, amplifies the small property differences between conformers, leading to quite different bulk properties.     

Synthesis and Multiscale Evaluation of LiNbO3‐Containing Silicate Glass‐Ceramics with Efficient Isotropic SHG Response

A study of bulk second harmonic generation (SHG) response of lithium niobium silicate glass‐ceramics is presented. The observed macroscopic SHG signals have an isotropic 3D nature. To interpret this particular nonlinear optical response, a multiscale approach is used in which clear correlations between structure and optical response are characterized from the sub‐micrometer to the millimeter scale. In particular, it is inferred that the radial distribution of the LiNbO₃ crystallites in spherulite domains is at the origin of the isotropic bulk second order optical property. It is suggested that spherulitic crystallization in glass‐ceramic is a challenging method to elaborate isotropic nonlinear optical properties in inorganic materials.     

New Opportunities: Second Harmonic Generation of Boron‐Doped Graphene Quantum Dots for Stem Cells Imaging and Ultraprecise Tracking in Wound Healing

Second harmonic generation (SHG) has recently emerged, having the advantages of no bleaching, no blinking, and no signal saturation, as well as a high signal to‐noise ratio compared to fluorescence. Existing SHG probes are based on heavy metal or organic dye molecules, which have the shortcomings of toxicity, a large size, or photoinstability. To address the urgent need for long‐term tracking and imaging in organisms, boron‐doped graphene quantum dots (B‐GQDs), a highly biocompatible graphene based with a strong and photostable SHG signal is first synthesized and is further applied for stem cell imaging and tracking in wounds. The results demonstrate the possibility of stem cell internalization of B‐GQDs as a SHG probe and show no hindering of the stem cell's central physiological activities such as differentiation. Most importantly, B‐GQDs are successfully tracked in skin tissue in vivo after the labeled mouse mesenchymal stem cells being implanted over 35 days. This work will inspire the development of doped graphene quantum dot materials and promote the broad use of B‐GQDs in future molecular imaging, drug delivery, and stem cell therapy.     

Cover Picture: A Hierarchically Structured Ni(OH)2 Monolayer Hollow‐Sphere Array and Its Tunable Optical Properties over a Large Region (Adv. Funct. Mater. 4/2007)

The fabrication of hierarchically structured Ni(OH)₂ monolayer hollow‐sphere arrays with the shell composed of building blocks of nanoflakelets is reported on p. 644 by Weiping Cai and co‐workers. The morphology can be easily controlled by the synthesis parameters, and the arrays show a tunable optical transmission stop band. Tuning can be achieved by changing the size or morphology of the hollow spheres. Such arrays may have potential applications in optical devices, photonic crystals, and as sensors for gas detection. The fabrication of a hierarchically structured Ni(OH)₂ monolayer hollow‐sphere array with the shell composed of building blocks of nanoflakelets is demonstrated based on a colloidal monolayer and electrochemical deposition. The morphology can be easily controlled by the colloidal monolayer and deposition parameters. Importantly, such monolayer hollow‐sphere array shows a morphology‐ and size‐dependent tunable optical transmission stop band. This stop band can be easily tuned from 455–1855 nm by changing the size of the hollow spheres between 1000 and 4500 nm, and also fine‐adjusted by changing the deposition time. The array exhibits a nearly incident‐angle‐independent position of the stop band that 3D photonic crystals do not possess. This structure may have potential applications in optical devices, photonic crystals, and sensors for gas detection.     

BaMgF4: An Ultra‐Transparent Two‐Dimensional Nonlinear Photonic Crystal with Strong χ(3) Response in the UV Spectral Region

Ferroelectric patterning is often used in advanced photonics and optoelectronic devices to increase their operational bandwidth and functionality, providing novel and unique performances. However, the extension of the ferroelectric structures to two‐dimensional geometries is currently limited to very few oxides and phosphates systems, which constrains its current and future applications. Here, careful processing based on e‐beam lithography and poling is employed to fabricate the first example of a two‐dimensional nonlinear photonic crystal in Barium Magnesium Fluoride, BaMgF₄, a ferroelectric fluoride crystal with an extraordinary transparency ranging from the deep ultraviolet (≈126 nm) to the mid infrared (≈13 μm). The optical characterization shows the possibility of obtaining simultaneously up to three different Cerenkov‐type second harmonic generation processes distributed in a conical geometry via χ⁽²⁾‐quasi‐phase‐matching technique. Additionally, the remarkably high χ⁽³⁾ nonlinear response of BaMgF₄ crystal in the UV spectral region is exploited to demonstrate what is believed to be the highest direct UV‐third harmonic generation conversion efficiency in a solid state system via pure χ⁽³⁾ nonlinear process. Together, the results highlight the outstanding opportunities offered by nonlinear photonic structures as innovative avenues to manipulate the light generation and control with reliable multifunctional optical character.