Noncentrosymmetric Crystals Research Articles

Free-space quasi-phase matching.

We report a new, to the best of our knowledge, approach to phase matching of nonlinear materials based on the free-space multipass cells. This technique is applicable to noncentrosymmetric nonlinear crystals, including crystals that cannot be birefringent phase-matched or quasi-phase matched by periodic poling. Notably, by using this approach, the crystalline quartz is quasi-phase matched with the demonstrated increase of the second harmonic generation efficiency by a factor of 40. The method can be extended toward UV and THz ranges. This promises to revolutionize experimental nonlinear optics and all applications by increasing the number of available crystals for quasi-phase matching by at least one order of magnitude and brings fresh motivation for developing novel nonlinear materials.

Enhanced photogalvanic effect in MoSSe monolayer with grain boundaries

The photogalvanic effect (PGE) in two-dimensional materials has emerged as a fascinating mechanism for generating photocurrent in non-centrosymmetric crystals without semiconductor p–n junction or bias voltage. In this paper, the impact of grain boundaries (GBs) on the performance of photogalvanic device with Janus MoSSe monolayer is theoretically investigated by quantum transport simulations. Two 4|8 GBs along armchair direction are taken into consideration. Under the illumination of linearly polarized light, we observe a significant enhancement of the PGE photocurrent in the visible light region, which can be attributed to the reduction of device's symmetry. The averaged enhancement ratios reach around 20 and 13 for two 4|8 GBs, respectively.

Design of High-Performance Organic Nonlinear Optical and Terahertz Crystals by Controlling the van der Waals Volume.

In the development of new organic crystals for nonlinear optical and terahertz (THz) applications, it is very challenging to achieve the essentially required non-centrosymmetric molecular arrangement. Moreover, the resulting crystal structure is mostly unpredictable due to highly dipolar molecular components with complex functional substituents. In this work, new organic salt crystals with top-level macroscopic optical nonlinearity by controlling the van der Waals volume (VvdW ), rather than by trial and error, are logically designed. When the VvdW of molecular ionic components varies, the corresponding crystal symmetry shows an observable trend: change from centrosymmetric to non-centrosymmetric and back to centrosymmetric. All non-centrosymmetric crystals exhibit an isomorphic P1 crystal structure with an excellent macroscopic second-order nonlinear optical response. Apart from the top-level macroscopic optical nonlinearity, new organic crystals introducing highly electronegative fluorinated substituents with strong secondary bonding ability show excellent performance in efficient and broadband THz wave generation, high crystal density, high thermal stability, and good bulk crystal growth ability.

Autonomous self-healing organic crystals for nonlinear optics

Non-centrosymmetric molecular crystals have a plethora of applications, such as piezoelectric transducers, energy storage and nonlinear optical materials owing to their unique structural order which is absent in other synthetic materials. As most crystals are brittle, their efficiency declines upon prolonged usage due to fatigue or catastrophic failure, limiting their utilities. Some natural substances, like bone, enamel, leaf and skin, function efficiently, last a life-time, thanks to their inherent self-healing nature. Therefore, incorporating self-healing ability in crystalline materials will greatly broaden their scope. Here, we report single crystals of a dibenzoate derivative, capable of self-healing within milliseconds via autonomous actuation. Systematic quantitative experiments reveal the limit of mechanical forces that the self-healing crystals can withstand. As a proof-of-concept, we also demonstrate that our self-healed crystals can retain their second harmonic generation (SHG) with high efficiency. Kinematic analysis of the actuation in our system also revealed its impressive performance parameters, and shows actuation response times in the millisecond range.

Thermal expansion anomalies in polar and ferroelectric crystals

The physical nature of the phenomenon of negative thermal expansion coefficient seen in the polar crystals and ferroelectrics is considered. Usually it is observed at low temperatures but especially evident in vicinity of ferroelectric phase transitions. This phenomenon is associated with fundamental nature of non-centrosymmetric crystals, caused by electronegativity difference of adjoined ions forming polar crystal. Negative thermal expansion coefficient indicates a change in the nature of forces of the attraction at close proximity of ions forming polar bond. The thermal expansion minimum is specified by the configurational entropy, conditioned by polar bonds self-ordering in pyroelectrics and ferroelectrics.

Structural elucidation, solvent (polar and non-polar) effect on electronic characterization, non-covalent charge interaction nature, topology and pharmacological studies on NLO active l-argininium methanesufonate

Reaction of l-arginine (l-Arg) with methanesulfonic acid (l-ArgH) (CH3SO3) in aqueous solution was investigated. The method of synthesis of (l-ArgH)(CH3SO3) was solvothermal. The precursors l-arginine and methanesulfonic acid was dissolved in aqueous solution. The solution was allowed to slowly evaporate, leading to the formation of non-centrosymmetric crystals of (l-ArgH)(CH3SO3). The crystal structure was determined using X-ray diffraction analysis. Using Hirshfeld surfaces and 2D-fingerprint plots, the interactions between the crystal structures were studied. The structure is optimized using density functional theory (DFT) at the B3LYP/ 6–31 + G* level in gas phase which in turn compared with the empirically determined molecular structure in the solid state. Vibrational spectrum analyses were also used to characterise the (l-ArgH)(CH3SO3) salt. To ascertain the energy gap, the HOMO-LUMO behaviour in several solvents (polar and non-polar) has been performed. The HOMO-LUMO energies from various liquids (polar and non-polar) indicate that the molecules are interacting via charge transfer. The interaction of the lone pair, bonding, and anti-bonding molecular orbitals has been evaluated by NBO. Topology investigations also shed light on the molecule's molecular structure and reactivity. The molecular electrostatic potential illustrates the potential for electrophilic and nucleophilic sites in a molecule for different solvents. A few polar and non-polar solvents were computed for NLO characteristics such the dipole moment, total polarizability, and first order hyper polarizability in addition to the gas phase. The SHG efficiency of (l-ArgH)(CH3SO3) salt relative to KDP was found to be 0.9. SwissADME is performed to get the drug likeness of the grown (l-ArgH)(CH3SO3).

Concurrent Terahertz Generation via Quantum Interference in a Quadratic Media

AbstractThe strive for efficiency in the generation of terahertz (THz) waves motivates intense research on novel field–matter interactions. Presently, THz generation via quadratic crystals remains the benchmark thanks to its simple and practical deployment. An interesting problem is whether new mechanisms can be exploited to elicit novel generation approaches and forms of control on the THz output in existing systems. THz generation via quantum interference (QI) leverages a third‐order nonlinear response under resonant absorption, and it has been recently explored to access surface generation in centrosymmetric systems. Its deployment in standard THz quadratic sources can potentially create a physical setting with the concurrence of two different mechanisms. Here, THz generation via QI in noncentrosymmetric crystals concurrent with phase‐matched quadratic generation in a bulk‐transmission setting is demonstrated. Beyond investigating a new physical setting, it is demonstrated that conversion efficiencies much larger than those typically associated with the medium become accessible for a typically adopted crystal, ZnTe. An inherent control on the relative amplitude and sign of the two generated THz components is also achieved. This approach provides disruptive boost and management of the optical‐to‐THz conversion performance of a well‐established technology, with significant ramifications in emerging spectroscopy and imaging applications.

Anomalous Positron Lifetime in Single Crystal of Weyl Semimetal CoSi

The positron annihilation lifetimes were measured using a 48V positron source in noncentrosymmetric cubic single crystals of CoSi, FeSi and MnSi. The following lifetimes were determined from the positron annihilation time spectra: 168(1) ps for CoSi, 114(1) ps for FeSi and 111(1) ps for MnSi. For single-crystal CoSi, the positron annihilation lifetime was also determined with a 22Na positron source. For CoSi, the lifetimes obtained from different positron sources are consistent. The differences in the positron annihilation lifetimes in MnSi and FeSi, on the one hand, and in the Weyl semimetal CoSi, on the other hand, are possibly caused by the formation of a positron + electron bound state (positronium).

Li3B8O13X (X = Cl and Br): Two New Noncentrosymmetric Crystals with Large Birefringence Induced by BO3 Units

Enthusiasm for the exploration of nonlinear alkali metal borates remains high. Focusing on the Li-B-O-X (X = Cl and Br) system, two examples of noncentrosymmetric borates, Li3B8O13Cl and Li3B8O13Br, were obtained using a high-temperature solution method under vacuum conditions. Structurally, the Li3B8O13X crystals exhibit two independent alternately arranged three-dimensional B-O network structures formed by the basic building block unit B8O16. The performance measurements demonstrate their short ultraviolet cutoff edges. The theoretical calculation indicates that the BO3 units dominate the contribution to their large optical anisotropy with the birefringence, 0.094 and 0.088@1064 nm for Li3B8O13Cl and Li3B8O13Br, respectively.

Universal epitaxy of non-centrosymmetric two-dimensional single-crystal metal dichalcogenides

The great challenge for the growth of non-centrosymmetric 2D single crystals is to break the equivalence of antiparallel grains. Even though this pursuit has been partially achieved in boron nitride and transition metal dichalcogenides (TMDs) growth, the key factors that determine the epitaxy of non-centrosymmetric 2D single crystals are still unclear. Here we report a universal methodology for the epitaxy of non-centrosymmetric 2D metal dichalcogenides enabled by accurate time sequence control of the simultaneous formation of grain nuclei and substrate steps. With this methodology, we have demonstrated the epitaxy of unidirectionally aligned MoS2 grains on a, c, m, n, r and v plane Al2O3 as well as MgO and TiO2 substrates. This approach is also applicable to many TMDs, such as WS2, NbS2, MoSe2, WSe2 and NbSe2. This study reveals a robust mechanism for the growth of various 2D single crystals and thus paves the way for their potential applications.

Crystal Engineering to Avoid Pairing Dipolar Moments: The Case of 5-Nitrouracil, a Highly Polarizable Molecule

5-nitrouracil is a polarizable molecule with a permanent electric dipole moment. Its molecular properties caught the attention of physicists working on nonlinear optics or optoelectronics, but the translation of the molecular assets to the crystalline solid has not been straightforward. This review compares all the known crystal structures incorporating the neutral or ionic 5-nitrouracil, or the two species concomitantly, discussing the effect of the packing in the optimization of the crystalline optical properties. Two new centrosymmetric 5-nitrouracilate salts are also reported for the first time, showing extensive hydrogen bonding between anions and cations. This review also gathers data from nonlinear optical measurements of non-centrosymmetric crystals and thermal stabilities of known polymorphs, showing that a neutral 5-nitrouracil molecule in acentric crystalline environment allows efficient blue-light generation.

Polarization-dependent second harmonic generation in peptide crystals: effects of molecular packing.

A series of chiral peptide luminophores containing the coumarin moiety was synthesized via a simple and efficient solution-based procedure. The peptides, containing either L-Phe, or L-Ala, or L-Leu (designated, respectively, as p1, p2, and p3), self-aggregate to form anti-parallel sheet-like structures. The self-assembly of the peptide luminophores leads to non-centrosymmetric crystals which display significant second harmonic generation (SHG). The dependence of the SHG intensity on the input laser polarization revealed a strong correlation between the SHG and the crystal packing. In the polar plots, the SHG intensity as a function of the linear polarization orientation of the input laser beam gave a four-petal pattern for p1, a predominantly two-petal pattern for p2, and a dumbbell-shaped pattern for p3. This reflects the dependence of the second order optical susceptibility tensor on the crystal symmetry. The polar plots can be fitted very well with the theoretical expressions derived from the second order polarization equation after incorporating crystal symmetry in the second order optical susceptibility tensor. The strong polarization-dependent SHG from organic crystals may be interesting for polarization controlled nonlinear optical switches, sensors, and actuators.

Exfoliable and self-healable two-dimensional materials from wurtzite zinc chalcogenides as building blocks of nanodevices.

With the advent of graphene, two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. The most anticipated 2D materials have been synthesized and exploited for novel applications. Multilayered zinc chalcogenides (ZnX) are the best precursors for obtaining atomic layer two-dimensional materials by exfoliation. Therefore, we carry out a detailed density functional theory-based study to achieve an exfoliation process of ZnX non-van der Waals sheets by straining and provide a microscopic understanding of the ferroelectric, optic, and spin behaviors of ZnX systems and the corresponding self-healable two-dimensional ZnX devices. The results revealed that 2D ZnX sheets can be obtained when strain is 14% for ZnS and ZnSe, and the peak values of exfoliation energy have a similar order of magnitude to those of traditional 2D materials, indicating the possibility of obtaining 2D ZnX monolayers. For intrinsic 2D ferroelectric materials with in-plane electric polarization, the direction of ZnX sheets can be reversed using an electric field with an energy barrier of ∼0.175 eV per atom for ZnSe, offering a promising functional basis for their application in ferroelectric nanodevices. The first absorption of photons for polarization perpendicular to the monolayer plane occurs in a high energy range of photons, facilitating their application in LEDs. The spin splitting in non-centrosymmetric ZnX crystals exhibits a Rashba spin-texture according to first-principles calculations. The self-healable two-dimensional nanodevices have a smooth curve from -0.5 to 0.5 eV. This work indicates the potential value of non-van der Waals ZnX 2D materials for their application in photoelectric and spintronic nanodevices.

The Use of Circularly Polarized Synchrotron Radiation in Diffraction and Spectral Studies of Noncentrosymmetric Crystals

The Use of Circularly Polarized Synchrotron Radiation in Diffraction and Spectral Studies of Noncentrosymmetric Crystals

Superconductivity and Local Inversion-Symmetry Breaking

Inversion and time reversal are essential symmetries for the structure of Cooper pairs in superconductors. The loss of one or both leads to modifications to this structure and can change the properties of the superconducting phases in profound ways. Superconductivity in materials lacking inversion symmetry, or noncentrosymmetric materials, has become an important topic. These materials show unusual magnetic and magnetoelectric properties and can host topological superconductivity. Recently, crystal structures with local, but not global, inversion-symmetry breaking have attracted attention. Here, superconductivity can exhibit phenomena not naively expected in centrosymmetric materials. In this review, we first introduce the concept of locally noncentrosymmetric crystals and different material realizations. We then discuss consequences of such local symmetry breaking on the normal state electronic structure and the classification of superconducting order parameters. Finally, we review the expected and, in parts, already observed phenomenology of unconventional superconductivity and possible topological superconducting phases.

Polymorphism of Purpurin and Low-level Detection of the Noncentrosymmetric form by Second Harmonic Generation Microscopy

Nonlinear optical imaging based on second harmonic generation (SHG) provides rapid and highly selective detection of polar crystals. Purpurin (PUR) is a natural product with multiple pharmacological activities. Two polymorphs of PUR show distinct crystal packing and structural symmetry, where form I crystallizes in a polar space group and form II crystallizes in a centrosymmetric crystal structure. The two polymorphs are monotropically related, with form I being the thermodynamically stable form, as suggested by slurry experiments, in-situ Raman spectroscopy and crystal structure prediction (CSP). The specificity of SHG to the polar crystals of form I allows rapid polymorphism detection at the limit of individual crystals. SHG is also able to detect low levels of form I in a tablet matrix dominated by amorphous excipients. This study shows that SHG microscopy can achieve the rapid and sensitive detection of noncentrosymmetric crystals in solid dosage forms, which is especially helpful for the early detection of unwanted polymorphic conversion or crystallization of amorphous drugs in formulations and final products.

Hardness and Young's modulus of BaGa4S7 and BaGa4Se7 nonlinear optical crystals

We report hardness and Young's modulus measurements of two recently developed non-chalcopyrite, non-centrosymmetric crystals, BaGa4S7 and BaGa4Se7, which seem very promising for three-wave parametric interactions in the mid-IR part of the spectrum between 5 and 18 μm. The obtained averaged values for the hardness from nanoindentation tests, converted into Vickers hardness number, are 556 kg/mm2 for the sulphide and 336 kg/mm2 for the selenide compound, respectively. Knowledge of these values will be essential for proper polishing and fabrication of optical elements, the wide use and commercialization of these novel non-oxide nonlinear crystals, which possess a number of other advantages over classical chalcopyrites operating in the same spectral window.

New N-pyrimidinyl stilbazolium crystals for second-order nonlinear optics

We report a new π-conjugated cationic chromophore based on phenolic N-pyrimidinyl stilbazolium and its corresponding non-centrosymmetric crystals. The new cationic chromophore, HPR (4-(4-hydroxy-3-methoxystyryl)-1-(pyrimidin-2-yl)pyridin-1-ium), consists of a strong electron donor, the 4-hydroxy-3-methoxyphenyl group based on two electron-donating groups, and a strong electron acceptor, the N-pyrimidinyl pyridinium group based on two electron-withdrawing groups. The HPR chromophore possesses large molecular optical nonlinearity with first hyperpolarizability of up to 264 × 10−30 esu, which is significantly larger than that of the benchmark stilbazolium chromophores. In solution, the HPR chromophore showed co-existence of phenol- and phenolate-like forms with two absorption bands (∼460 and ∼610 nm, respectively). In the crystalline state, all HPR-based derivatives with three different counter anions, N2S (naphthalene-2-sulfonate), VBS (4-vinylbenzenesulfonate), and NBS (3-nitrobenzenesulfonate), exhibit a macroscopic second-order nonlinear optical response. HPR-N2S crystals exhibit non-centrosymmetric monoclinic Pn space group symmetry and large off-diagonal effective macroscopic optical nonlinearity (94 × 10−30 esu), which is approximately four times higher than that of the benchmark stilbazolium crystals. Therefore, the newly developed HPR-based crystals are highly promising materials for second-order nonlinear optical applications.

AgGaGeSe4: An Infrared Nonlinear Quaternary Selenide with Good Performance

The symmetry of crystals is an extremely important property of crystals. Crystals can be divided into centrosymmetric and non-centrosymmetric crystals. In this paper, an infrared (IR) nonlinear optical (NLO) material AgGaGeSe4 was synthesized. The related performance analysis, nonlinear optical properties, and first-principle calculation of AgGaGeSe4 were also introduced in detail. In the AgGaGeSe4 structure, Ge4+ was replaced with Ga3+ and produced the same number of vacancies at the Ag+ position. The low content of Ge doping kept the original chalcopyrite structure and improved its optical properties such as the band gap. The UV-Vis diffuse reflection spectrum shows that the experimental energy band gap of AgGaGeSe4 is 2.27 eV, which is 0.48 eV larger than that of AgGaSe2 (1.79 eV). From the perspective of charge-transfer engineering strategy, the introduction of Group IV Ge elements into the crystal structure of AgGaSe2 effectively improves its band gap. The second harmonic generation (SHG) effect of AgGaGeSe4 is similar to that of AgGaSe2, and at 1064 nm wavelength, the birefringence of AgGaGeSe4 is 0.03, which is greater than that of AgGaSe2 (∆n = 0.02). The results show that AgGaGeSe4 possessed better optical properties than AgGaSe2, and can been broadly applied as a good infrared NLO material.

Supramolecular cis-“Bis(Chelation)” of [M(CN)6]3− (M = CrIII, FeIII, CoIII) by Phloroglucinol (H3PG)

Studies on molecular co-crystal type materials are important in the design and preparation of easy-to-absorb drugs, non-centrosymmetric, and chiral crystals for optical performance, liquid crystals, or plastic phases. From a fundamental point of view, such studies also provide useful information on various supramolecular synthons and molecular ordering, including metric parameters, molecular matching, energetical hierarchy, and combinatorial potential, appealing to the rational design of functional materials through structure–properties–application schemes. Co-crystal salts involving anionic d-metallate coordination complexes are moderately explored (compared to the generality of co-crystals), and in this context, we present a new series of isomorphous co-crystalline salts (PPh4)3[M(CN)6](H3PG)2·2MeCN (M = Cr, 1; Fe, 2; Co 3; H3PG = phloroglucinol, 1,3,5-trihydroxobenzene). In this study, 1–3 were characterized experimentally using SC XRD, Hirshfeld analysis, ESI-MS spectrometry, vibrational IR and Raman, 57Fe Mössbauer, electronic absorption UV-Vis-NIR, and photoluminescence spectroscopies, and theoretically with density functional theory calculations. The two-dimensional square grid-like hydrogen-bond {[M(CN)6]3−;(H3PG)2}∞ network features original {[M(CN)6]3−;(H3PG)4} supramolecular cis-bis(chelate) motifs involving: (i) two double cyclic hydrogen bond synthons M(-CN⋅⋅⋅HO-)2Ar, {[M(CN)6]3−;H2PGH}, between cis-oriented cyanido ligands of [M(CN)6]3− and resorcinol-like face of H3PG, and (ii) two single hydrogen bonds M-CN⋅⋅⋅HO-Ar, {[M(CN)6]3−;HPGH2}, involving the remaining two cyanide ligands. The occurrence of the above tectonic motif is discussed with regard to the relevant data existing in the CCDC database, including the multisite H-bond binding of [M(CN)6]3− by organic species, mononuclear coordination complexes, and polynuclear complexes. The physicochemical and computational characterization discloses notable spectral modifications under the regime of an extended hydrogen bond network.