Large Second-order Optical Nonlinearity Research Articles

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.

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.

Giant Enhancement and Directional Second Harmonic Emission from Monolayer WS2 on Silicon Substrate via Fabry-Pérot Micro-Cavity.

Two-dimensional transition metal dichalcogenides (TMDs) possess large second-order optical nonlinearity, making them ideal candidates for miniaturized on-chip frequency conversion devices, all-optical interconnection, and optoelectronic integration components. However, limited by subnanometer thickness, the monolayer TMD exhibits low second harmonic generation (SHG) conversion efficiency (<0.1%) and poor directionality, which hinders their practical applications. Herein, we proposed a Fabry-Pérot (F-P) cavity formed by coupling an atomically thin WS2 film with a silicon hole matrix to enhance the SH emission. A maximum enhancement (∼1580 times) is achieved by tuning the excitation wavelength to be resonant with the microcavity modes. The giant enhancement is attributed to the strong electric field enhancement in the F-P cavity and the oscillator strength enhancement of excitons from suspended WS2. Moreover, directional SH emission (divergence angle ∼5°) is obtained benefiting from the resonance of the F-P microcavity. Our research results can provide a practical sketch to develop both high-efficiency and directional nonlinear optical devices for silicon-based on-chip integration optics.

Enhancement of the second harmonic generation from monolayer WS2 coupled with a silica microsphere

Monolayer transition metal dichalcogenides (TMDs) are widely used for integrated optical and photoelectric devices. Owing to their broken inversion symmetry, monolayer TMDs have a large second-order optical nonlinearity. However, the optical second-order nonlinear conversion efficiency of monolayer TMDs is still limited by the interaction length. In this work, we theoretically study the second harmonic generation (SHG) from monolayer tungsten sulfide (WS2) enhanced by a silica microsphere cavity. By tuning the position, size, and crystal orientation of the material, second-order nonlinear coupling can occur between the fundamental pump mode and different second harmonic cavity modes, and we obtain an optimal SHG conversion efficiency with orders of magnitude enhancement. Our work demonstrates that the microsphere cavity can significantly enhance SHG from monolayer 2D materials under flexible conditions.

Second-order nonlinear polarizability of "Push-Pull" chromophores. A decade of progress in donor-π-acceptor materials.

Fine tuning and switching of nonlinear optical response of ferrocene chromophores has been an area of considerable significance as evidenced by a large number of reports in the current literature. In this personal account, we present linear/nonlinear behavior and structure-activity relationships of several classes of donor-π-acceptor designs using organometallic and organic materials, developed by our research group during the last decade. The results especially the electronic absorption spectral and the hyper-Rayleigh scattering have been supported by theoretical calculations. Exploiting the redox behavior of ferrocene donor, we have demonstrated switching of quadratic nonlinear optical responses with reversible redox chemistry, which is a useful attribute of nonlinear optical materials. Based on the ease in synthesis, structure diversification and structure-based large and switchable second-order optical nonlinearity, these materials are potential candidates for electro-optic applications.

The second-order NLO property of a photoswitchable heteroditpioc ion-pair receptor based on 2-pyridyl acylhydrazone linking with 2,6-pyridine bisamide: The impacts of metal cations and anions

A photoswitchable heteroditpioc ion-pair receptor E-1 and its isomeride Z-1 (without the anion binding site), that are based on the 2-pyridyl acylhydrazone linking 2,6-pyridine bisamide, have brought our attention to systematically explore the second-order nonlinear optical (NLO) properties by the density functional theory (DFT). In this work, we mainly studied the influences of metal cations (M = Na+, K+, Mg2+, Ca2+, Hg2+ and Pb2+), anions (X = Cl−, Br− and I−) and ion-pair (NaCl, NaBr and NaI) on NLO responses for the receptor. In addition, the impacts of isomerization and poto-switching processes on NLO response for these systems also have been discussed detailedly. The results show that the isomerization process does not effectively adjust the NLO properties for our studied systems. But the poto-switching process that was triggered by light to capure or release ions plays an important role in improving the NLO properties. The receptors E-1 and Z-1 are excellent candidates to effectively detect metal cation Pb2+, because the first hyperpolarizability (βtot) values of E∗Pb2+ and Z∗Pb2+ increased by 13 times and 20 times relative to that of receptors E-1 (188.06 a.u.) and Z-1 (270.21 a.u.), respectively. In addition, the receptor E-1 has the possibility to detect anion I− due to the larger βtot values compared with other anion-complexes. However, the changes of NLO responses for ion-pair complexes are not obvious compared with corresponding anion-complexes. We are looking forward to the research would be beneficial for further theoretical and experimental studies on recognizing metal cations and anions based on large second-order NLO difference.

Straight Z and twisted E isomers from triphenylamine derivatives: Intramolecular charge transfer and second-order nonlinear optical response

Intramolecular charge transfer (CT) in push-pull systems plays an important role in enhancing nonlinear optical (NLO) response. The main purpose of this work is to explore the effect of straight Z and twisted E isomers on the second-order NLO properties, two types of push-pull systems (Z(E)-A and D-Z(E)-A) based on the triphenylamine are designed and investigated. Significantly, the first hyperpolarizability (βtot) results show that the βtot value of straight Z-A is larger than that of twisted E-A. It is noteworthy that Z conformation modified by acceptor units can effectively adjust the second-order NLO properties. Further, D-Z-A and D-E-A systems combining multiple donor and acceptor units exhibit large second-order NLO properties. Obviously, D3-Z-A3 shows the largest βtot values of 2.46 × 104 au. Moreover, the trend of the transition energy (ΔE) value is inconsistent with the trend of βtot value, which explains the variation tendency of the βtot value. The presented correlation between conformation and the second-order NLO properties can provide a simple predictive tool to design highly NLO material based on triphenylamine derivatives.

Rational structure design of isoxazolone-based crystals with large second-order optical nonlinearity

Two new acentric isoxazolone-based organic crystals of C24H17NO3 (MPFI) and C18H15NO3S (MPMI) were designed and synthesized by a structure fine-tuning strategy.

Electronic structure and second-order nonlinear optical properties of lemniscular [16]cycloparaphenylene compounds.

Chiral organic compounds are excellent second-order nonlinear optical (NLO) materials due to their inherent non-symmetric electronic structures combined with the advantages of organic compounds. At present, density functional theory (DFT) has become a powerful tool for predicting the properties of novel materials. In this paper, based on chiral lemniscular [16]cycloparaphenylene, three novel compounds are designed by introduction of donor/acceptor units and their combinations. The geometrical/electronic structure, electronic absorption, and the second-order NLO properties of these compounds have been systematically investigated by DFT/TDDFT theory. The simulated UV-Vis/CD spectra of compound 1 are in good agreement with the experimental ones, enabling us to assign their electronic transition characteristics and absolute configuration with high confidence. The investigations show that energy gaps, absorption wavelength and second-order NLO response may be effectively tuned by the introduction of the donor or acceptor units or their combinations. For instance, the second-order NLO value of compound 4 is about 207 times as large as the average second-order polarizability of the organic molecule urea. Thus, the studied compounds are expected to be potential large second-order NLO materials.

Switchable second-order nonlinear optical response of platinum-sensitized dithienylethenes

The electronic structure and the nonlinear optical (NLO) properties of a series of platinum-sensitized dithienylethenes (DTEs) were investigated by using the density functional theory (DFT) method. DFT calculations reveal that the second-order NLO properties of complexes significantly increase with the DTE ligand being directly linked with terpyridine–Pt(II) complexes. Due to the good [Formula: see text]-conjugated characteristics, closed-ring complexes possess much larger second-order NLO properties than the corresponding open-ring complexes. The computational [Formula: see text] values are in the order of 3c ([Formula: see text] esu) [Formula: see text] 3o ([Formula: see text] esu) [Formula: see text] 4c ([Formula: see text] esu) [Formula: see text] 4o ([Formula: see text] esu) [Formula: see text] 1 ([Formula: see text] esu) [Formula: see text] 2c ([Formula: see text] esu) [Formula: see text] 2o ([Formula: see text] esu). Among all calculated platinum-sensitized dithienylethenes, 3c has the largest second-order NLO properties. The 4c and 4o have almost the same [Formula: see text] values and the [Formula: see text] value of 4o is slightly larger than that of 2o because the ether bonds can significantly prevent charge transfer within the complexes.

DFT Studies on Second-order Nonlinear Optical Response of Ir(C^N)2(pic) Complexes

Density functional theory(DFT) was employed to calculate the geometrical structures, UV-Vis absorption spectra and second-order nonlinear optical(NLO) properties of a family of iridium(III) complexes, which possess of different cyclometallated ligands(C^N) and ancillary ligands[pyridine-2-carboxylate(pic)]. It was found that the modification of the LUMO energy levels was achieved by changing pic ligands and the energy gaps between the HOMO and LUMO were notably increased or decreased. In addition, the degree of conjugation was significantly changed with the substituent groups varied, which led to that the first hyperpolarizability β could be effectively modulated. Through the analysis of time-dependent DFT(TD-DFT) results, we predicted that these studied complexes with π→π* charge transfer was beneficial to the large second-order NLO properties. Therefore, we hope that these studied iridium( III) complexes can be considered as versatile second-order NLO materials.

Unprecedented Large Hyperpolarizability of Twisted Chromophores in Polar Media.

Twisted intramolecular charge transfer (TICT) chromophores exhibit uniquely large second-order optical nonlinearities ( μβ). However, their promise as electro-optic (E-O) materials is yet untapped, reflecting a strong tendency to aggregate in low-polarity media, leading to a dramatic fall in μβ. Until now, TICT chromophores in deaggregating polar solvents suffered decreased response due to polarity-driven changes in electronic structure. Here we report a new series of benzimidazolium-based TICT chromophores with interaryl torsional angles in the range of 64-77°. The most twisted, B2TMC-2, exhibits a large μβvec = -26,000 × 10-48 esu (at 1907 nm) in dilute nonpolar CH2Cl2 solution, which is maintained in polar DMF ( μβvec= -20,370 × 10-48 esu) as measured by DC electric field-induced second harmonic generation (EFISH). Sterically enforced interaryl torsional angles are confirmed by single-crystal X-ray diffraction and solution phase Nuclear Overhauser Effect (NOE) NMR, and spectroscopic characterization reveals a zwitterionic/aromatic ground state electronic structure associated with the high μβ. We show that increasingly disrupted conjugation is correlated with increased μβ even at intermediate twist angles. The excellent performance and reduced aggregation in polar solvents opens new avenues for bridging microscopic and macroscopic chromophore performance.

DFT/TDDFT, NPA, and AIM-based study of the molecular switching properties of photocyclization and metallochromism of the DAE complexes

Diarylethene (DAE), the ubiquitous photochromic unit, is one of the most promising candidates for well-recognized optoelectronic applications. In this paper, the relationship between reversible photoisomerization/metallochromic properties and second-order nonlinear optical (NLO) responses for a new photochromism based on DAE with a benzyl-linked 8-aminoquinoline-2-aminomethylpyridine unit (1O) has been systematically investigated by using density functional theory. Our results show that closed-ring (1C) comprising DAE unit was shown to be capable of exhibiting both large static first hyperpolarizabilities βtot (1.075 × 10−28 esu) and large switch ratio βratio ~ 31 with corresponding open-ring because of the better delocalization of the π-electron system, the more obvious degree of charge transfer and the smaller transition energy in the closed-ring. When the metal ions have been injected in open-ring (1O*M), we observed a larger NLO response compared with the corresponding open-ring 1O. However, it can slightly decrease the βtot values in 1C*M. Natural bonding orbital and atom in molecules analysis provide a better understanding for the property differences in metallochromic process. We hope this research would be beneficial for further theoretical and experimental studies on large second-order NLO switches applications.

Theoretical investigation on second-order nonlinear optical properties of ruthenium alkynyl–dihydroazulene/vinylheptafulvene complexes

Ru metal acetylide electron donor-acceptor complexes have important applications in the field of nonlinear optics. Herein, in this work, a series of half-sandwich ruthenium-based Cp*(dpe)Ru ([Ru*]) metal complexes with the dihydroazulene/vinylheptafulvene (DHA/VHF) have been investigated by density functional theory (DFT) calculations. The results showed that the position of the [Ru*] acetylide functionality, either para or meta on the phenylene ring to the DHA/VHF core (1c/1o and 2c/2o), and additional a p-phenylene spacer (3c/3o) had a great influence on the second-order nonlinear optical (NLO) responses. The systems 1 and 3 can significantly increased second-order NLO responses compared with system 2. It was attributed to the more obvious charge transfer along y-axis, which is from [Ru*] acetylide functionality to DHA, accompanied by a significant decrease of the transition energy according electron density difference maps and time-dependent DFT calculations. The βvec values of the open-ring complexes were larger than the corresponding closed-ring complexes owing to the smaller HOMO−LUMO gap in the open-ring complexes. It was also because of the smaller BLA values in open-ring complexes, which had stronger π-conjugation. Especially, the change ratio of βvec value of system 2 was the largest due to the fact that their charge transfers degree varied greatly. In addition, the frequency-dependent NLO properties of the studied complexes were evaluated at 0.0239 a.u. and 0.0340 a.u. The calculation results demonstrated that the magnitude of the frequency-dependent first hyperpolarizability increased with the increasing frequency. We believe that our present work will be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes.

ZIF-8 as Nonlinear Optical Material: Influence of Structure and Synthesis

Metal–organic framework ZIF-8, from the zeolitic imidazolate framework family, shows a large intrinsic second-order nonlinear optical (NLO) response. In addition, ZIF-8 is a stable, inexpensive material that is transparent in the visible (vis) and near-infrared (NIR) window. This is crucial for NLO applications. The second-order NLO activity is due to the noncentrosymmetric octupolar symmetry of the material. We found that fast syntheses lead to a lower second-order NLO response. Consistent with polarized second-harmonic generation (SHG) microscopy measurements, we ascribe this to defects that create local centers of centrosymmetry but do not affect the orientation of the crystal lattice. Syntheses with slow nucleation lead to quasi-perfect crystals with a large average second-order NLO coefficient ⟨deff⟩ of 0.25 pm/V, which is explained and supported by ab initio theoretical calculations.

Novel Fullerene Platinum Alkynyl Complexes with High Second-Order Nonlinear Optical Properties as a Springboard for NLO-Active Polymer Films

The synthesis and second-order nonlinear optical (NLO) properties of seven novel fullerene-C60 derivatives are reported. In these donor–bridge–acceptor systems, the fullerene unit (acceptor) is connected through a cyclopropane ring to an ethynyl thienyl fragment (bridge) which binds a trimethylsilyl or platinum alkynyl fragment (donor). All derivatives are NLO-active, as determined by the electric field induced second harmonic generation technique, but the fullerene platinum alkynyl complexes are characterized by a particularly large second-order NLO response. Substitution of the thienyl fragment by a terthiophene leads to an increase of the NLO performance by using trimethylsilyl as a donor group, whereas no effect is observed in the case of platinum derivatives. Remarkably, fullerene platinum alkynyl complexes can be dispersed in poly(methyl methacrylate) or polystyrene, affording NLO-active polymer films.

Engineering of Organic Chromophores with Large Second-Order Optical Nonlinearity and Superior Crystal Growth Ability

We present two D-π-A organic cationic core structures leading to highly efficient nonlinear optical (NLO) salts (E)-2-(4-(dimethylamino)styryl)-1,1,3-trimethyl-1H-benzo[e]indol-3-ium iodide (P–BI) and (E)-2-(2-(5-(dimethylamino)thiophen-2-yl) vinyl)-1,1,3 -trimethyl-1H-benzo[e]indol-3-ium iodide (S–BI). Single crystals of the above two materials were successfully obtained by the slow evaporation method. Two different polymorphic crystals of both P–BI and S–BI were obtained from different polar solvents. Kurtz powder tests revealed that the maximum second harmonic generation (SHG) efficiency of P–BI with monoclinic space group P21 is 1.14 times the benchmark DAST(4-N,N-dimethylamino-4,-N,-methylstilbazolium tosylate). Bulk single crystals of P–BI were obtained with size of up to 17.0 × 6.0 × 2.0 mm3 without using seed crystals. This demonstrates that this material exhibits great crystal growth ability along with a high second-order optical nonlinearity, making it a very attractive candidate for NLO applica...

Hybrid 2D Material Nanophotonics: A Scalable Platform for Low-Power Nonlinear and Quantum Optics

Two-dimensional, atomically thin materials offer unique physical properties, such as a large second-order optical nonlinearity. Integrating these materials into a scalable and silicon-compatible platform could provide a pathway toward realizing low-power nonlinear optics. We have presented a formalism to calculate the effective nonlinear optical coefficient for a cavity integrated 2D material system as well as the cavity enhanced absorptive loss, which can be used to guide future experiments. We have also shown that the few-photon regime of cavity QED is within reach with a cavity quality factor of ∼105–106, provided the loss at the second harmonic mode is minimized. As test applications, we have simulated the performances of (a) an optically bistable device and (b) a nanostructured device exhibiting single-photon blockade.

DFT studies on second-order nonlinear optical properties of a series of axially substituted bis(salicylaldiminato) zinc(II) Schiff-base complexes

Coordination of an axial ligand to metal center to enhance the second-order nonlinear optical(NLO) response of a two-dimensional bis(salicylaldiminato) zinc(II) Schiff-base complex is an unprecedented model. The second-order NLO responses of a series of axially substituted bis(salicylaldiminato) zinc(II) Schiff-base complexes were explored according to the finite field(FF) method at CAM-B3LYP/6-31+G(d) level(LANL2DZ basis set for metal atoms). The results show that the second-order NLO properties can be effectively tuned by exchanging the donor and accepter of the axial ligand and extending the length of the conjugated bridge along the axial direction. A system involving the electron acceptor along the appropriate direction has a large three-dimensional second-order NLO response. Meanwhile, time dependent density functional theory(TD-DFT) method was employed to calculate the physical parameters of excited states. The results show that the Y- and Z-polarized transitions of the zinc(II) Schiff-base complex are the first and second excited states, respectively, and have a low-lying excited energy. Although the X-polarized transition has a high excited energy, the large oscillator strength indicates that it will significantly contribute to the second-order NLO response.

Theoretical exploration to the cation effect on the second-order nonlinear optical properties of Strandberg-type polyoxometalates

The combination of cations with octahedral coordinated d0 transition metal ions has been proved to be an effective way for designing new polar materials. So we investigate the second-order nonlinear optical (NLO) properties of Strandberg-type polyoxometalates (POMs) with alkali metal cations M 6 Mo 5 X 2 O 23 ( M = K +, Rb +, Cs +; X = P , As ) and M 4 M o5 X 2 O 21 ( M = K +, Rb +, Cs +; X = S , Se , Te ) by density functional theory (DFT) method. The calculated results show that this kind of Strandberg-type POMs possesses remarkably large molecular second-order NLO polarizability, especially for the Cs 6 Mo 5 P 2 O 23 (system Ic), which has a computed β0 value of 12526 a.u. and might be an excellent second-order NLO material. Moreover, the cations have important impact on the second-order NLO polarizabilities. Therefore, a careful choice of appropriate cations may allow the control of the second-order NLO response on these Strandberg-type POMs, which may provide a new route to design efficient NLO materials.