Second-order Nonlinear Optical Research Articles

Nonlinear Optical Activity of a Chiral Organic-Inorganic ([(NH3CH2CH2)3NH])2[MnBr5]Br5 Photoluminescent and Piezoelectric Crystal.

The family of Mn-based organic-inorganic hybrids has greatly expanded due to their advantages in applications. They also show superior bright and size-tunable photoluminescence and can be considered a perfect alternative to toxic lead-based compounds. In this work, we present the detailed structural, optical, and electrical characterization of ([(NH3CH2CH2)3NH])2[MnBr5]Br5. The title compound exhibits a unique type of inorganic arrangement created by the trigonal bipyramids. It crystallizes in noncentrosymmetric space group R32, indicating its optical activity, piezoelectricity, and second-order optical nonlinearity proven by the second harmonic of light measurements. The studied crystals exhibit intense photoluminescence originating from the Mn(II) ion 4T1(G) → 6A1 transition. The measured lifetime of the photoluminescence emission is ≤1.5 ms, while the measured quantum yield for both powder and crystal samples reaches ∼70%.

Boosted Second Harmonic Generation and Cascaded Sum Frequency Generation from a Surface Crystallized Glass Ceramic Microcavity.

The development of novel materials and structures for efficient second-order nonlinear micro/nano devices remains a significant challenge. In this study, we demonstrate the remarkable enhancement of second-harmonic generation (SHG) and even cascaded sum frequency generation (CSFG) in whispering gallery mode microspheres made of surface-crystallized glass with a 6-μm Ba2TiSi2O8 crystal layer. Attributed to the core-shell design, the Ba2TiSi2O8 located on the surface can be efficiently coupled with whispering gallery modes, resulting in a highly efficient micron-scale cavity-enhanced second-order optical nonlinearity. Greatly enhanced SHG of the microcavity was observed, which is up to 80 times stronger than that of non-resonant bulk sample. Furthermore, owing to the wavelength non-selectivity of random quasi-phase matching, we demonstrated ultra-wideband SHG with a strong response ranging from 860nm to 1600nm and high-contrast polarization characteristics. The glass-ceramic based microsphere cavity also boosts the cascading optical nonlinearity, manifested by a two-magnitude enhancement of CSFG. This work delineates an efficient strategy for boosting nonlinear optical response in glass-ceramics, which will open up new opportunities for applications in photonics and optical communications. This article is protected by copyright. All rights reserved.

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand.

Chiral hybrid organic-inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second-order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self-trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second-order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1-ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V-1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second-order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

Theoretical Understanding of Nonlinear Optical Properties in Solids: A Perspective.

Nonlinear optical (NLO) crystals have become a hot topic in chemical science and material physics, due to their essential role in laser technology, optical information, optoelectronics, and precision measurements. In this Perspective, we provide an overview of recent advances in second-order nonlinear optics, with a focus on two critical topics: second harmonic generation (SHG) and the bulk photovoltaic effect (BPVE). For SHG, we discuss recent progress in deep-ultraviolet (DUV) materials, highlighting their structural characteristics and nonlinear groups that contribute to their exceptional performance. For BPVE, we concentrate on the emerging field of low-dimensional materials, emphasizing their potential in a shift current. Additionally, we discuss the development of regulation approaches for NLO materials, which is vital for their practical application. Finally, we address the outlook for the field, including the challenges that must be overcome to further advance NLO materials research.

Two Acentric Zn-Based l-Tartrates with Moderate Second Harmonic Generation Responses and Large Birefringence.

The discovery of novel organic-inorganic hybrid nonlinear optical (NLO) crystal materials holds great potential in advancing laser science and technology as they offer a wide range of benefits including easy synthesis, structural versatility, and high hyperpolarizability. Herein, the integration of l-tartaric acid (L-C4H6O6) and ZnSO4 gave rise to two acentric Zn-based organic-inorganic hybrid crystals, namely, Zn2(H2O)2(C4H4O6)2·3H2O (Zn-LT) and Zn2(H2O)(C4H4O6)(C4H6O6)(SO4)·4H2O (Zn-LTS). They both feature layered structures constructed by [ZnO6] octahedron, l-C4H6O6/l-C4H4O62-, water molecule, or [SO4] tetrahedron. Interestingly, they possess moderate second-order NLO effects of 0.28 × KH2PO4 (Zn-LT) and 0.57 × KH2PO4 (Zn-LTS), large birefringence of 0.075 (Zn-LT) and 0.069 (Zn-LTS), suggesting that the introduction of [SO4] groups with intrinsically weak polarizability and weak optical anisotropy induces the enhancement of the NLO effect while without reducing birefringence much. In addition, both of them display UV cutoff edges near 210 nm, indicating their potential as NLO crystals applied in the UV and solar-blind region.

DC Voltage Induces Quadratic Optical Nonlinearity in Ion-Exchanged Glasses at Room Temperature

We demonstrate that applying DC voltage at room temperature to an ion-exchanged glass induces quadratic optical nonlinearity in a subsurface region of the glass. We associate this with the EFISH (Electric-Field-Induced Second Harmonic) effect due to the Maxwell–Wagner charge accumulation in the subsurface region of the glass, in which a conductivity gradient forms as a result of the ion exchange processing. The second harmonic (SH) signal from the soda–lime glass subjected to potassium-for-sodium ion exchange is comparable with one from the same glass after thermal poling. The signal linearly increases with the duration of the ion exchange. The lower mobility of the potassium ions results in a higher SH signal from the potassium-for-sodium exchanged glass than that from the silver-for-sodium ion-exchanged one. This phenomenon is resistant to thermal annealing: only a 500 °C anneal caused noticeable degradation of the SH signal after “charging” the specimen. The phenomenon found is of interest for characterizing graded conductivity regions and providing and controlling second-order optical nonlinearity in transparent isotropic media.

Ba10In2Mn11Si3O12S18: First Hexanary Oxychalcogenide Containing an Infrequent Three-Dimensional Noncentrosysmmetric Framework.

Noncentrosymmetric (NCS) oxychalcogenides have attracted great attention in recent years due to their immense potential as candidates for IR nonlinear-optical (NLO) applications. Despite notable advancements in this field, the discovery of oxychalcogenides with three-dimensional (3D) framework structures remains a formidable challenge. In this study, we report the discovery of the first hexanary oxychalcogenide, Ba10In2Mn11Si3O12S18, exhibiting second-order NLO activity, using a high-temperature solid-phase method. This compound showcases a novel structure type, featuring an uncommon NCS 3D [In2Mn11Si3O12S18]20- framework formed by vertex-sharing [(Mn/In)S6] octahedra, [(Mn/In)OS3] tetrahedra, and [SiO4] tetrahedra, with charge-balanced Ba2+ cations occupying the channels. Our study serves as a source of inspiration for researchers to further investigate the synthesis of novel NLO-active oxychalcogenides with 3D frameworks.

Electric field induced out-of-plane second-order optical nonlinearity in monolayer transition metal dichalcogenides

Electric field induced out-of-plane second-order optical nonlinearity in monolayer transition metal dichalcogenides

High-throughput virtual screening of organic second-order nonlinear optical chromophores within the donor-π-bridge-acceptor framework.

In view of the theoretical importance and huge application potential of second-order nonlinear optical (NLO) materials, it is of great significance to conduct high-throughput virtual screening (HTVS) on a compound library to find candidate NLO chromophores. Under the donor-π-bridge-acceptor structural framework, a virtual compound library (size = 27 090) was constructed by enumeration of structural fragments. The kernel property adopted for optimization is the static first hyperpolarizability (β0). By combining machine learning and quantum chemical calculations, we have performed an HTVS procedure to sieve NLO chromophores out, and the response mechanism of the selected optimal NLO chromophores was examined. We have found: (a) The multi-layer perceptron/extended connectivity fingerprint combination with 20% selection ratio gives the highest prediction accuracy for the studied systems. (b) The two optimal donors are bis(4-diphenylaminophenyl)aminyl and bis(4-tert-butylphenyl)aminyl; the optimal π-bridges are composed of two thiophenyl, selenophenyl or furanyl units; and the two optimal acceptors are tri-s-triazinyl and 2,3-dicyanopyrazinyl. (c) The no. 1 candidate molecule can exhibit a calculated β0 equal to 8.55 × 104 a.u. (d) The difference in NLO responses of the optimal 16 molecules comes from the synergistic interaction of ES1, Δμ and f, by employing the two-level model. In addition, the sizable Δμ and f allow the studied optimal molecules to obtain a large NLO response in the meantime keeping a not-too-low excitation energy (retaining good optical transparency in the restricted range of the visible spectrum region). (e) With further modification on the acceptor, the designed DPA-π-TRZ-A' (A' = CN or NO2, π = oligo-thiophenyl or selenophenyl) systems can exhibit a rather large NLO response (maximum β0 = 3.17 × 105 a.u.), hence should have considerable potential as second-order NLO chromophores. With the above observations, we expect to provide some insight for the research community into the HTVS of organic second-order NLO chromophores.

Interband second-order nonlinear optical susceptibility of asymmetric coupled quantum wells

Asymmetric molecular bonds possess a microscopic second-order nonlinear optical polarizability p(2). Crystals built from them possess a macroscopic second-order nonlinear optical susceptibility, χ(2), if their structure lacks centrosymmetry. χ(2) can be enhanced by introducing additional asymmetry at the meta-structural level. Here, we use a dipole matrix formalism to calculate χ(2) of asymmetric GaAs/AlGaAs coupled quantum well structures at telecommunication frequencies, for which interband (rather than previously considered intersubband) optical transitions govern optical nonlinearities. Using unit cell and envelope wavefunctions and considering all possible transitions between two bound electron and two bound hole states, we predict tenfold enhancement in χ(2) in previously underexplored ranges of quantum well asymmetry and coupling barrier thickness. This work paves the way toward enhanced, tailorable second-order optical nonlinearities for semiconductor digital alloy and superlattice structures.

Second-order optical nonlinearity of two 1,3,4-oxadiazole derivatives: An experimental and theoretical study

Measurements and theoretical modeling of the second-order optical nonlinearity of 2-(4-(dodecyloxy)phenyl)-5-(pyridin-3-yl)-1,3,4-oxadiazole (Oxa-4-Py) and 2-(4-(dodecyloxy)phenyl)-5-(4-(5-(thiophen-2-yl)-1,3,4-oxadiazol-2-yl)phenyl)-1,3,4-oxadiazole (DiOxaBn-2-Tio) are reported here. The hyper-Rayleigh scattering technique was applied to measure the dynamic first hyperpolarizability, βexpλ=1064, of the samples. The excitation source was a Nd:YAG laser operating at 1064 nm, 6 ns and 10 Hz, and a two-level approximation was applied to estimate the static first hyperpolarizability, βexpλ=0, of the samples. The obtained values for βexpλ=1064 were 173×10−30 esu (Oxa-4-Py) and 226×10−30 esu (DiOxaBn-2-Tio), and the calculated values for static first hyperpolarizability, βexpλ=0 were 106×10−30 esu (Oxa-4-Py) and 126×10−30 esu (DiOxaBn-2-Tio). The βexpλ=1064 value determined for the DiOxaBn-2-Tio is among the largest experimental values reported so far for a 1,3,4-oxadiazole derivative. The calculations based on a density functional method confirmed the trend of the βexpλ=1064 values among the samples, i.e. the first hyperpolarizability of DiOxaBn-2-Tio is approximately 30% larger than βexpλ=1064 for Oxa-4-Py.

New Functional Groups Design toward High Performance Ultraviolet Nonlinear Optical Materials.

ConspectusThe invention of the laser is a pivotal milestone in the evolution of modern science and technology. Second-order nonlinear optical (NLO) crystals, which possess the ability to convert frequencies, have found widespread applications in laser science, information transmission, industrial Internet, and other cutting-edge fields within materials and optics. As modern science and technology continue to advance at a rapid pace, existing ultraviolet (UV) and deep ultraviolet (DUV) NLO crystals struggle to meet the ever-growing demands of various applications. Consequently, the development of novel UV and DUV NLO crystals has become an urgent necessity. For a UV NLO crystal to be considered outstanding in the UV/DUV range, it must exhibit three fundamental yet crucial properties: large second-order NLO coefficients, suitable birefringence, and short UV cutoff edge corresponding to a wide band gap. However, these key factors often conflict with one another, making it challenging to achieve a harmonious balance within a single crystal. It is widely believed that these mutually constrained optical properties are codetermined by microscopic NLO-active units and macroscopic structure features. Therefore, how to design high performance UV NLO-active groups to balance these three key properties is an essential scientifically question and serious challenge. In this Account, we present three strategies for designing high-performance UV NLO-active groups: (1) The "tetrahedron partial substitution" strategy by employing various substituents to replace one or more atoms in the traditional nonpolar tetrahedral groups, might achieve the aim of increasing the polarizability anisotropy and hyperpolarizability of the newly formed polar tetrahedral functional groups, such as from SO4 to SO3NH2 or SO3CH3 groups. (2) The "structure-analogue" strategy to develop a range of organic functional groups exhibiting more strong polarizability anisotropy and hyperpolarizability by using inorganic π-conjugated groups, such as BO3 and B3O6 groups, as templates. (3) The "two in one" strategy for integrating groups featuring planar triangle configurations and tetrahedrons to create NLO-active functional groups possessing large band gaps, strong hyperpolarizability, and moderate polarizability anisotropy. These three strategies successfully guide us to design and explore various kinds of organic-inorganic composite NLO crystal materials with excellent performances, like Ba(SO3CH3)2, M(SO3NH2)2 (M = Sr, Ba), C(NH2)3SO3F, KLi(HC3N3O3)·2H2O, KLi(C3H2O4)·H2O, and so on. Finally, we briefly conclude these strategies and propose some prospects for exploring new excellent UV/DUV NLO materials with practical applications. These findings could inspire novel thoughts for researchers designing new UV/DUV NLO materials and providing abundant materials used in UV/DUV regions.

Terahertz radiation by quantum interference of excitons in a one-dimensional Mott insulator

Nearly monocyclic terahertz waves are used for investigating elementary excitations and for controlling electronic states in solids. They are usually generated via second-order optical nonlinearity by injecting a femtosecond laser pulse into a nonlinear optical crystal. In this framework, however, it is difficult to control phase and frequency of terahertz waves. Here, we show that in a one-dimensional Mott insulator of a nickel-bromine chain compound a terahertz wave is generated with high efficiency via strong electron modulations due to quantum interference between odd-parity and even-parity excitons produced by two-color femtosecond pulses. Using this method, one can control all of the phase, frequency, and amplitude of terahertz waves by adjusting the creation-time difference of two excitons with attosecond accuracy. This approach enables to evaluate the phase-relaxation time of excitons under strong electron correlations in Mott insulators. Moreover, phase- and frequency-controlled terahertz pulses are beneficial for coherent electronic-state controls with nearly monocyclic terahertz waves.

Femtosecond laser induced damage threshold and incubation in l-threonine aminoacid crystal

l-threonine aminoacid crystals exhibit high second-order optical nonlinearities that can be exploited for second harmonic generation, optical parametric amplification, and optical parametric oscillation. In addition, it possesses a large transparency window and low refractive index, making them an attractive material for photonics devices. Among several processing methods used to develop photonic integrated micro-devices, fs-laser micromachining has stood out for its high resolution, either on the surface or in the bulk, as well as to its flexibility to be used with various types of materials. Although organic crystals present relevant linear and nonlinear optical features, studies on fs-laser processing in such materials are yet scarce. Thus, this work presents a study of the fs-laser incubation and damage threshold fluence determination in l-threonine crystals at 515 nm and 1030 nm. The damage threshold fluence was determined for one up to 105 pulses. For the single pulse regime, we obtained Fth = (0.94 ± 0.04) J/cm2 at 1030 nm and Fth = (0.31 ± 0.01) J/cm2 at 515 nm. Such difference is explained by the number of photons involved in each case; 5-photons at 1030 nm and 3-photons at 515 nm. Also, a slower incubation dynamic was observed at 515 nm. Still, micro-Raman spectroscopy revealed that no structural modifications were induced by the fs-laser pulses on the sample upon micromachining. Such results provide relevant data for the processing of l-threonine crystals via fs-laser micromachining to achieve organic photonic integrated devices.

DFT molecular simulations for static, dynamic and solvent-dependent nonlinear optical properties of triphenylamine-carbazole-based organic dyes with D-D-A framework

Nonlinear optical (NLO) materials are vital for advancing cutting-edge optoelectronic technology. Herein, triphenylamine-carbazole-based organic dyes with a D-D-A framework are quantum chemically designed with acceptor modifications, and their geometric, electronic, static, dynamic, and solvent-dependent NLO properties are explored using density functional theory (DFT) simulations. The IEFPCM model is employed to investigate the solvent effect on the NLO properties of proposed compounds (BSA-1 to BSA-9). Interestingly, the dipole moment (μtot), average linear polarizability (αtot), and first hyperpolarizability (βtot) were determined for all proposed compounds in gas phase and water (ε = 78.36), DMSO (ε = 46.83), methanol (ε = 32.61), tetrahydrofuran (ε = 7.43), chloroform (ε = 4.71), and benzene (ε = 2.27) solvents. Natural population analysis (NPA), transition density matrix (TDM), UV–Vis investigation, frontier molecular orbital (FMO), and topological analysis (MEP, ELF, and LOL) were carried out to explain the NLO findings. The topological study suggests that BSA and BSA-1 to BSA-9 are chemically active and may have NLO implications. Moreover, the findings demonstrate that BSA-9 is the most promising second-order NLO candidate because of the highest λmax (418.60 nm) in dichloromethane (DMF) and the lowest energy gap (1.49 eV) among all the derivatives. It was found that BSA-2 and BSA-9 demonstrated the giant static αo (7.26 × 102) and βo (9.91 × 104) values among the proposed compounds for use in a wide range of solvent-dependent NLO applications. The solvent effect on NLO in all proposed compounds is noticeable, especially in BSA-9, which exhibited βo = 1.89 × 105 in benzene, 2.80 × 105 in THF, 3.21 × 105 in methanol, 3.25 × 105 in DMSO and 3.28 × 105 in water. Furthermore, Electro-optical Pockels effect (EOPE) β(-ω,ω,0) and the second harmonic generation (SHG) β(-2ω,ω,ω) results computed at ω = 532 nm (0.0856 au) and ω = 1064 nm (0.0428 au) frequencies confirm that all proposed compounds are excellent NLO candidates and should be targeted for future static, frequency-dependent and solvent-dependent NLO applications.

A two-fold interpenetrated mixed-ligands acentric 3D coordination polymer Zn(OBA)(DIB)·0.5H2O: Topological analysis, fluorescent and second-order NLO properties

Second-order nonlinear optical (NLO) materials have important applications in the realms of photoelectronics and photonics whose microstructures need to be organized into a noncentrosymmetric (NCS) structure. In this work, a mixed-ligands three-dimensional (3D) noncentrosymmetric coordination polymer Zn(OBA)(DIB)·0.5H2O (1) was hydrothermally prepared from 4, 4′-oxydibenzoic acid (H2OBA), 1, 3-di(1H-imidazol-1-yl)benzene (DIB) and Zn(NO3)2·6H2O via spontaneous resolution approach, which crystallizes in a noncentrosymmetric space group Fdd2. Compound 1 was characterized by powder X-ray diffraction (XRD), thermogravimetric (TGA) and IR analyses. The structural analysis shows that 1 is a two-fold self-interpenetrated three-dimensional (3D) network material. Single crystal X-ray diffraction and the topological analysis manifests that the topological structure of 1 belongs to a cds topological net. The measurement of the fluorescent properties of 1 and the free DIB ligand show that the emission peak at 460 nm of 1 can be attributed to the charge transition from the п bond electron to the п anti-bond orbital of the DIB ligand. The second-harmonic generation (SHG) measurement shows that the second-order NLO effect of compound 1 is approximately equivalent to that of potassium dihydrogen phosphate (KDP). Compound 1 is high thermal stable and insoluble in water and common organic solvents, making it to be a potential candidate as a second-order NLO material.

Comparative study on the second-harmonic generation of thermally poled fused silica by multi-energy boron and argon ion implantations

The second-order optical nonlinearity (SON) of thermally poled fused silica was shown for the first time to be significantly improved by the implantation of multi-energy boron (B) ions, and the outcomes were compared with those obtained from the implantation of multi-energy argon (Ar) ions. Second-harmonic (SH) signals in both samples grew roughly linearly as the poling temperature rose. Samples with Ar implants showed a greater increase in SH signal with rising poling temperature. A qualitative explanation for the formation of [Formula: see text] in poled samples can be found in a model of charge hopping and migration through defects. The diminished SH signal in the B-implanted sample is probably caused by the negative charge trapping layer, as well as by B[Formula: see text] and E[Formula: see text] center defects.

New group of inorganic salts of 1,3-diaminoguanidinium(1+) cation – crystal structures, vibrational spectra, linear and nonlinear optical properties

In the search for new molecular crystals for nonlinear optics (NLO), nine inorganic salts containing the 1,3-diaminoguanidinium(1+) cation as a promising organic “Y-aromatic” carrier of NLO properties were prepared and studied. Crystal structures of eight new salts, namely 1,3-diaminoguanidinium(1+) chloride (P21/c), hypophosphite (P21/c), hydrogen phosphite (P-1), dihydrogen phosphate (P21/c), hydrogen phosphate (P21/m), nitrite (P-1), sulfite (Pbcm) and chlorate (P21/c) were solved by the single crystal X-ray diffraction. The crystal structures contain centrosymmetric arrangements of pairs of counterions assembled into hydrogen-bonded undulated sheets. The charge-assisted hydrogen bonds in the resulting hydrogen bond networks are accommodated by the conformation of the 1,3-diaminoguanidinium(1+) cation as a hydrogen bond donor. All salts were also characterized by powder X-ray diffraction. The vibrational spectra (FTIR and Raman) were studied for all newly prepared salts and the previously published non-centrosymmetric sulfate (P6122). The assignment of the vibrational bands of these hydrogen-bonded crystals is derived from quantum-chemical computations (B3LYP/6-311G+(d,p) along with the Potential Energy Distribution analysis) concerning isolated 1,3-diaminoguanidinium(1+) cation and the results of correlation analysis involving present inorganic anions. The linear and nonlinear optical properties of the only non-centrosymmetric salt – 1,3-diaminoguanidinium sulfate – were studied. This uniaxial positive crystal is optically transparent down to 230 nm, and refractive indices (ne = 1.65 and no = 1.56) have been determined at 532 nm. The dispersion curves of the refractive indices were also calculated by the Q(LFT) procedure in the 532–1907 nm region. The experimentally determined (using 800 and 1000 nm laser irradiation) very low efficiency of second harmonic generation (i.e., ∼1% of KDP), which is consistent with the Kleinman symmetry assumption for the P6122 space group, limits further use of this crystal for second-order NLO applications.

Electronic, nonlinear optical, UV–vis and NBO analysis of methyl methacrylate for optoelectronic and optical applications: DFT study and impact of conformation

Using Density Functional Theory (DFT) and time-dependent DFT (TD-DFT), we studied a compound widely used in daily life, namely: Methyl Methacrylate (MMA) monomer in its two stable isomers, cis-MMA and trans-MMA. The impact of conformation on the optical and electronic properties of MMA was highlighted. Structural parameters and thermodynamic properties were also assessed, and allowed to discuss the stability and reactivity of this compound. In addition, analysis of optoelectronic and electronic properties, global reactivity descriptors, natural bond orbital (NBO), and hole and electron reorganization energies was performed. The absorption and fluorescence properties, as well as the linear and nonlinear optical (NLO) properties of MMA were also investigated. The findings show that MMA is an insulator due to its wide band gap of 6.20 eV. It also exhibits good optoelectronic properties with high average electric field values of about 5.90 × 109 Vm−1 for cis-MMA and 5.42 × 109 Vm−1 for trans-MMA and high electric displacement values of at least 14.65 × 10-2 cm−2 for cis-MMA and 14.33 × 10-2 cm−2 for trans-MMA, suggesting a potential use for the design of piezoelectric and pyroelectric materials. In addition, trans-MMA has a dielectric constant close to that of good insulators; while cis-MMA exhibits a dielectric behaviour close to that of polymeric substances. Further, these materials are thermodynamically stable in its two conformations, with a good reactivity which can lead to good ability of polymerization. The analysis of the UV–vis spectra revealed that both forms of MMA absorb and emit mainly in the UV and that the Stokes shift of MMA is low, reducing its potential use in devices such as solar cells. At NLO level, MMA isomers exhibit weak optical properties such as second- and third-order optical susceptibility and cannot suitable for second and third harmonic generation; however, the conformation enhances second-order NLO response by at least 2 times when moving from cis- to trans-MMA, whereas the third-order response is almost unaffected. In addition, the conformation also affects the electronic and optoelectronic properties of MMA.

Theoretical Perspective on Two-State "ON-OFF" NLO Switch of Rh(III)-Azobenzene Complex with Considerable First Hyperpolarizability.

In this paper, a light-induced Rh(III)-azobenzene (azo) complex in its two conformations (cis- and trans-form) is analyzed via density functional theory methods, focusing on the geometrical and electronic structure, linear and second-order nonlinear optical (NLO) properties, as well as UV-vis absorption spectra. The results show that trans-Rh exhibits more obvious charge separation and more extensive π-conjugation than cis-Rh. The energy gaps (Egap) between the highest occupied molecular orbital and lowest unoccupied molecular orbital of cis-Rh and trans-Rh are as small as 0.28 and 0.36 eV, respectively. The unusual small Egap facilitates electronic transition and accompanies large NLO responses. In particular, cis-Rh and trans-Rh possess considerably large second-order NLO responses with the first hyperpolarizability (βtot) up to 7.0 × 104 a.u. ("OFF" state) and 8.5 × 104 a.u. ("ON" state), respectively. trans-Rh shows good π-conjugation and obvious electric delocalization relative to cis-Rh and thus a considerable βtot value. The switchable ratio of βtot (trans-Rh)/βtot (cis-Rh) is equal to 1.2, showing switch characteristics. UV-vis analyses indicate that the ON and OFF states can finely tune the absorption range due to the strongest absorption peaks of cis-Rh (301 nm) and trans-Rh (446 nm) lying in the UV and vis region, respectively. It is expected that the studied Rh(III)-azo complexes may be applied in the field of optoelectronics.