Anisotropic Strain-Tailoring Nonlinear Optical Response in van der Waals NbOI2.

Two-dimensional (2D) NbOI2 demonstrates significant second-harmonic generation (SHG) with a high conversion efficiency. To unlock its full potential in practical applications, it is desirable to modulate the SHG behavior while utilizing the intrinsic lattice anisotropy. Here, we demonstrate direction-specific modulation of the SHG response in NbOI2 by applying anisotropic strain with respect to the intrinsic lattice orientations, where more than 2-fold enhancement in the SHG intensity is achieved under strain along the polar axis. The strain-driven SHG evolution is attributed to the strengthened built-in piezoelectric field (polar axis) and the enlarged Peierls distortions (nonpolar axis). Moreover, we provide quantifications of the correlation between strain and SHG intensity in terms of the susceptibility tensor. Our results demonstrate the effective coupling of orientation-specific strain to the anisotropic SHG response through the intrinsic polar order in 2D nonlinear optical crystals, opening a new paradigm toward the development of functional devices.

Manipulation of Chiral Nonlinear Optical Effect by Light-Matter Strong Coupling.

Light-matter strong coupling (LMSC) is an intriguing state in which light and matter are hybridized inside a cavity. It is increasingly recognized as an excellent way to control material properties without any chemical modification. Here, we show that the LMSC is a powerful state for manipulating chiral nonlinear optical (NLO) effects through the investigation of second harmonic generation (SHG) circular dichroism. At the upper polariton band in LMSC, in addition to the enhancement of SHG by more than 1 order of magnitude, the responsivity to the handedness of circularly polarized light was largely modified, where sign inversion and increase of the dissymmetry factor were achieved. Quarter waveplate rotation analysis revealed that the LMSC clearly influenced the coefficients associated with chirality in the NLO process and also contributed to the enhancement of nonlinear magnetic dipole interactions. This study demonstrated that LMSC serves as a great platform for controlling chiral and magneto-optics.

Time-resolved study on signaling pathway of photoactivated adenylate cyclase and its nonlinear optical response.

Photoactivated adenylate cyclases (PACs) are multidomain BLUF proteins that regulate the cellular levels of cAMP in a light-dependent manner. The signaling route and dynamics of PAC from Oscillatoria acuminata (OaPAC), which consists of a light sensor BLUF domain, an adenylate cyclase domain, and a connector helix (α3-helix), were studied by detecting conformational changes in the protein moiety. Although circular dichroism and small-angle X-ray scattering measurements did not show significant changes upon light illumination, the transient grating method successfully detected light-induced changes in the diffusion coefficient (diffusion-sensitive conformational change (DSCC)) of full-length OaPAC and the BLUF domain with the α3-helix. DSCC of full-length OaPAC was observed only when both protomers in a dimer were photoconverted. This light intensity dependence suggests that OaPAC is a cyclase with a nonlinear light intensity response. The enzymatic activity indeed nonlinearly depends on light intensity, that is, OaPAC is activated under strong light conditions. It was also found that both DSCC and enzymatic activity were suppressed by a mutation in the W90 residue, indicating the importance of the highly conserved Trp in many BLUF domains for the function. Based on these findings, a reaction scheme was proposed together with the reaction dynamics.

Application to nonlinear optical properties of the RSX-QIDH double-hybrid range-separated functional.

The effective calculation of static nonlinear optical properties requires a considerably high accuracy at a reasonable computational cost, to tackle challenging organic and inorganic systems acting as precursors and/or active layers of materials in (nano-)devices. That trade-off implies to obtain very accurate electronic energies in the presence of externally applied electric fields to consequently obtain static polarizabilities ( α i j ) and hyper-polarizabilities ( ß i j k and γ i j k l ). Density functional theory is known to provide an excellent compromise between accuracy and computational cost, which is however largely impeded for these properties without introducing range-separation techniques. We thus explore here the ability of a modern (double-hybrid and range-separated) Range-Separated eXchange Quadratic Integrand Double-Hybrid exchange-correlation functional to compete in accuracy with more costly and/or tuned methods, thanks to its robust and parameter-free nature.

Polysulfide Anions [Sx]2- (x = 2, 3, 4, 5): Promising Functional Building Units for Infrared Nonlinear Optical Materials.

Infrared nonlinear optical (IR NLO) materials play significant roles in laser technology. The novel functional building units (FBUs) are of great importance in constructing NLO materials with strong second harmonic generation (SHG). Herein, polysulfide anion [Sx]2- (x = 2, 3, 4, 5) units are investigated on NLO-related properties and structure-performance relationships. Theoretical calculations uncover that the [Sx]2- (x = 2, 3, 4, 5) units are potential IR NLO FBUs with large polarizability anisotropy (δ), hyperpolarizability (ß) and wide HOMO-LUMO gap. Fourteen crystals including [Sx]2- (x = 2, 3, 4, 5) units are calculated and analyzed. The results show that these units can result in a wide IR transmittance range, significant SHG effects, wide band gap Eg (Na2S4: Eg = 3.09 eV), and large birefringence Δn [BaS3 (P21212): Δn = 0.70]. More importantly, it is highlighted that the crystal materials including with [Sx]2- (x = 2, 3, 4, 5) groups are good candidates for the exploration of the outstanding IR NLO materials.

Polaronic Nonlinear Optical Response and All-Optical Switching Based on an Ionic Metal Oxide.

It has been well-established that light-matter interactions, as manifested by diverse linear and nonlinear optical (NLO) processes, are mediated by real and virtual particles, such as electrons, phonons, and excitons. Polarons, often regarded as electrons dressed by phonons, are known to contribute to exotic behaviors of solids, from superconductivity to photocatalysis, while their role in materials' NLO response remains largely unexplored. Here, the NLO response mediated by polarons supported by a model ionic metal oxide, TiO2, is examined. It is observed that the formation of polaronic states within the bandgap results in a dramatic enhancement of NLO absorption coefficient by over 130 times for photon energies in the sub-bandgap regions, characterized by a 100 fs scale ultrafast response that is typical for thermalized electrons in metals. The ultrafast polaronic NLO response is then exploited for the development of all-optical switches for ultrafast pulse generation in near-infrared (NIR) fiber lasers and modulation of optical signal in the telecommunication band based on evanescent interaction on a planar waveguide chip. These results suggest that the polarons supported by dielectric ionic oxides can fill the gaps left by dielectric and metallic materials and serve as a novel platform for nonlinear photonic applications.

Ba4B14O25: A Deep Ultraviolet Transparent Nonlinear Optical Crystal with Strong Second Harmonic Generation Response Achieved by a Boron-Rich Closed-Loop Strategy.

Discovering new deep ultraviolet (DUV) nonlinear optical (NLO) materials is the current research hotspot. However, how to perfectly integrate several stringent performances into a crystal is a great challenge because of the natural incompatibility among them, particularly wide band gap and large NLO coefficient. To tackle the challenge, a boron-rich closed-loop strategy is supposed, based on which a new barium borate, Ba4B14O25, is designed and synthesized successfully via the high-temperature solid-state melting method. It features a highly polymeric 3D geometry with the closed-loop anionic framework [B14O25]8- constructed by the fundamental building blocks [B14O33]24-. The high-density π-conjugated [BO3]3- groups and the fully closed-loop B-O-B connections make Ba4B14O25 possess excellent NLO properties, including short UV cutoff edge (<200 nm), large second harmonic generation response (3.0 × KDP) and phase-matching capability, being a promising DUV-transparent NLO candidate material. The work provides a creative design strategy for the exploration of DUV NLO crystals.

AIB3 IIC2 IIIQ6 VIXVII: A Thioborate Halide Family for Developing Wide Bandgap Infrared Nonlinear Materials by Coupling Planar [BS3] and Polycations.

Developing high-performance infrared (IR) nonlinear optical (NLO) materials is urgent but challenging due to the competition between NLO coefficient and bandgap in one compound. Herein, by coupling NLO-active [BS3] planar units and halide-centered polycations, six new metal thioborate halides ABa3B2S6X (A = Rb, Cs; X = Cl, Br, I) composed of zero-dimensional [XBamRbn/Csn] polycations and [BS3] units, belonging to a new A I B 3 II C 2 III Q 6 VI X VII ${\mathrm{A}}^{\mathrm{I}}{\mathrm{B}}_{3}^{\mathrm{II}}{\mathrm{C}}_{2}^{\mathrm{III}}{\mathrm{Q}}_{6}^{\mathrm{VI}}{\mathrm{X}}^{\mathrm{VII}}$ family, are rationally designed and fabricated. The compounds show an interesting structural transition from Pbcn (ABa3B2S6Cl) to Cmc21 (ABa3B2S6Br and ABa3B2S6I) driven by the clamping effect of polycationic frameworks. ABa3B2S6Br and ABa3B2S6I are the first series metal thioborate halide IR NLO materials, and the introduction of [BS3] unit effectively widens the bandgap of planar unit-constructed chalcogenides. ABa3B2S6Br and ABa3B2S6I, exhibiting wide bandgaps (3.55-3.60 eV), high laser-induced damage thresholds (≈ 6 × AgGaS2), and strong SHG effects (0.5-0.6 × AgGaS2) with phase-matching behaviors, are the promising IR NLO candidates for high-power laser applications. The results enrich the chemical and structural diversity of boron chemistry and give some insights into the design of new IR NLO materials with planar units.

Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd): Mixed Metal Thioborate-Thiosilicates as Well-Performed Infrared Nonlinear Optical Materials.

The first examples of thioborate-thiosilicates, namely Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P63 mc and feature a novel three-dimensional crystal structure in which the discrete [BS3 ]3- and [SiS4 ]4- anionic groups are linked by Ca2+ and Ln3+ cations occupying the same atomic site. Remarkably, all three compounds show comprehensive properties required as promising infrared nonlinear optical materials, including phase-matchable strong second harmonic generation (SHG) responses at 2.05 µm (1.1-1.2 times that of AgGaS2 ), high laser-induced damage thresholds (7-10 times that of AgGaS2 ), wide light transmission range (0.45-11 µm), high thermal stabilities (>800 °C), and large calculated birefringence (0.126-0.149 @1064 nm), which justify the material design strategy of combining [BS3 ]3- and [SiS4 ]4- active units. Theoretical calculations suggest that their large SHG effects originate mainly from the synergy effects of the LnS6 , BS3 , and SiS4 groups. This work not only broadens the scope of research on metal chalcogenides but also provides a new synthetic route for mixed anionic thioborates.

Excellent Nonlinear Optical M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, Rb) Achieved by Unusual Cationic Substitution Strategy.

The typical chalcopyrite AgGaQ2 (Q = S, Se) are commercial infrared (IR) second-order nonlinear optical (NLO) materials; however, they suffer from unexpected laser-induced damage thresholds (LIDTs) primairy due to their narrow band gaps. Herein, what sets this apart from previously reported chemical substitutions is the utilization of an unusual cationic substitution strategy, represented by [[SZn4 ]S12 + [S4 Zn13 ]S24 + 11ZnS4 ⇒ MS12 + [M4 Cl]S24 + 11GaS4 ], in which the covalent Sx Zny units in the diamond-like sphalerite ZnS are synergistically replaced by cationic Mx Cly units, resulting in two novel salt-inclusion sulfides, M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, 1; Rb, 2). As expected, the introduction of mixed cations in the GaS4 anionic frameworks of 1 and 2 leads to wide band gaps (3.04 and 3.01 eV), which exceeds the value of AgGaS2 , facilitating the improvement of high LIDTs (9.4 and 10.3 × AgGaS2 @1.06 µm, respectively). Furthermore, compounds 1 and 2 exhibit moderate second-harmonic generation intensities (0.84 and 0.78 × AgGaS2 @2.9 µm, respectively), mainly originating from the orderly packing tetrahedral GaS4 units. Importantly, this study demonstrates the successful application of the cationic substitution strategy based on diamond-like structures to provide a feasible chemical design insight for constructing high-performance NLO materials.

Theoretical Prediction of Monolayer BeP2O4H4 with Excellent Nonlinear-Optical Properties in Deep-Ultraviolet Range.

Most 2D nonlinear optical (NLO) materials do not have an ultrawide bandgap, therefore, they are unsuitable for working in the deep-ultraviolet spectral range (< 200 nm). Herein, the theoretical prediction of an excellent monolayer BeP2O4H4 (ML-BPOH) is reported. DFT analyses suggest a low cleavage energy (≈45 meV per atom) from a naturally existed bulk-BPOH material, indicating feasible exfoliation. This novel 2D material exhibits excellent properties including an ultrawide bandgap (Eg) of 7.84 eV, and a strong second-order nonlinear susceptibility ( d b u l k e f f $d_{bulk}^{eff}$ = 0.43 pm V-1), which is comparable to that of benchmark bulk-KBBF crystal (d16 = 0.45 pm V-1). The wide bandgap and large SHG effect of ML-BPOH are mainly derived from the (PO2H2)- tetrahedron. Notably, ML-BPOH exhibits an outstanding 50% variation in dsheet under minor stress stimuli (±3%) due to rotation of structurally rigid (PO2H2)- tetrahedron. This indicates significant potential for application in material deformation monitoring.

Breaking Through the Trade-Off Between Wide Band Gap and Large SHG Coefficient in Mercury-Based Chalcogenides for IR Nonlinear Optical Application.

It is substantially challenging for non-centrosymmetric (NCS) Hg-based chalcogenides for infrared nonlinear optical (IR-NLO) applications to realize wide band gap (Eg > 3.0 eV) and sufficient phase-matching (PM) second-harmonic-generation intensity (deff > 1.0 × benchmark AgGaS2 ) simultaneously due to the inherent incompatibility. To address this issue, this work presents a diagonal synergetic substitution strategy for creating two new NCS quaternary Hg-based chalcogenides, AEHgGeS4 (AE = Sr and Ba), based on the centrosymmetric (CS) AEIn2 S4 . The derived AEHgGeS4 displays excellent NLO properties such as a wide Eg (≈3.04-3.07 eV), large PM deff (≈2.2-3.0 × AgGaS2 ), ultra-high laser-induced damage threshold (≈14.8-15 × AgGaS2 ), and suitable Δn (≈0.19-0.24@2050 nm), making them highly promising candidates for IR-NLO applications. Importantly, such excellent second-order NLO properties are primarily attributed to the synergistic combination of tetrahedral [HgS4 ] and [GeS4 ] functional primitives, as supported by detailed theoretical calculations. This study reports the first two NCS Hg-based materials with well-balanced comprehensive properties (i.e., Eg > 3.0 eV and deff > 1.0 × benchmark AgGaS2 ) and puts forward a new design avenue for the construction of more efficient IR-NLO candidates.

LiGa(IO3 )4 : An Excellent NLO Material with Unprecedented 2D [Ga(IO3 )4 ]∞ - Layer Synthesized by Aliovalent Substitution.

Nonlinear optical (NLO) crystals are indispensable for the solid-state lasers for their ability to expand wavelength spectral to the regions where the directing lasing is difficult or even impossible, yet the rational design of a high-performance NLO crystal remains a great challenge owing to the severe structural and properties' requirements. Herein, a new noncentrosymmetric (NCS) and polar gallium iodate, LiGa(IO3 )4 , with a novel 2D anionic layer, is successfully designed and synthesized by the aliovalent substitution strategy based on classic α-LiIO3 . The 2D [Ga(IO3 )4 ]∞ - layer in LiGa(IO3 )4 is built from the GaO6 octahedra and highly polarizable units IO3 . Compared with its parent compound, the partial replacement of A-site Li+ cation with main group Ga3+ cation facilitates LiGa(IO3 )4 to possess excellent NLO properties, including the large second-harmonic generation (SHG) response (14 × KH2 PO4 (KDP) @ 1064 nm), wide bandgap (4.25 eV), large birefringence (0.23 @ 1064 nm), and wide optical transparency from UV to mid-IR. These reveal that LiGa(IO3 )4 will be a promising NLO crystal.

Sr2[B5O8(OH)]2 ⋠[B(OH)3] ⋠H2O: A Strontium Borate That Shows Deep-Ultraviolet-Transparent Nonlinear Optical Properties.

A new noncentrosymmetric strontium borate, P1-Sr2[B5O8(OH)]2 ⋅ [B(OH)3] ⋅ H2O (1), has been synthesized under the hydrothermal condition. The P1-Sr2[B5O8(OH)]2 ⋅ [B(OH)3] ⋅ H2O shows a layered B-O network with 9-ring windows in the ab plane. Sr2+ cations, H3BO3, and H2O molecules are located in the voids of layers and interlayers, respectively. The P1-Sr2[B5O8(OH)]2 ⋅ [B(OH)3] ⋅ H2O is the first synthetic phase of veatchite, while the other three polymorphs are found in different natural minerals. This strontium borate is a potential deep-ultraviolet-transparent nonlinear-optical (NLO) crystal whose second-harmonic-generation (SHG) intensity is 1.7 times that of KH2PO4 (KDP) and is phase-matchable. The short wavelength cutoff edge of compound 1 is below 190 nm. Density functional theory (DFT) calculations show that the B-O units are responsible for the nonlinear optical property.

Perfect Polar Alignment of Parallel Beloamphiphile Layers: Improved Structural Design Bias Realized in Ferroelectric Crystals of the Novel "Methoxyphenyl Series of Acetophenone Azines".

An improved design is described for ferroelectric crystals and implemented with the "methoxyphenyl series" of acetophenone azines, (MeO-Ph, Y)-azines with Y=F (1), Cl (2), Br (3), or I (4). The crystal structures of these azines exhibit polar stacking of parallel beloamphiphile monolayers (PBAMs). Azines 1, 3, and 4 form true racemates whereas chloroazine 2 crystallizes as a kryptoracemate. Azines 1-4 are helical because of the N-N bond conformation. In true racemates the molecules of opposite helicity (M and P) are enantiomers A(M) and A*(P) while in kryptoracemates they are diastereomers A(M) and B*(P). The stacking mode of PBAMs is influenced by halogen bonding, with 2-4 showcasing a kink due to directional interlayer halogen bonding, whereas fluoroazine 1 demonstrates ideal polar stacking by avoiding it. Notably, (MeO-Ph, Y)-azines display a stronger bias for dipole parallel alignment, attributed to the linearity of the biphenyl moiety as compared to the phenoxy series of (PhO, Y)-azines with their non-linear Ph-O-Ph moiety. The crystals of 1-4 all feature planar biphenyls and this synthon facilitates their crystallization through potent triple T-contacts and enhances their nonlinear optical (NLO) performance by increasing conjugation length and affecting favorable chromophore conformations in the solids.

A Curious Case of Two-Photon Absorption in n-Helicene and n-Phenylene, n=6-10: Why n=7 is Different?

n-Helicenes and n-Phenylenes are interesting examples of twisted molecules, where although the atoms are connected through conjugated π ${\pi }$ -bonds, the π ${\pi }$ -conjugation is largely hindered by the twisted nature of the bonds. Such structures provide a unique opportunity to study the effect of twisted π ${\pi }$ -system on non-linear optical properties. In this work, we studied the two-photon absorption in donor-acceptor substituted n-helicenes and n-phenylenes employing the state-of-the-art RI-CC2 method and reported a unique feature we observed in n=7 systems. We found that both 7-helicene and 7-phenylene systems exhibit largest two-photon absorption than other members in their respective classes. Furthermore, using generalized few-state model, we provided a detailed microscopic mechanism of this unique observation involving participation of different transition dipole moment vectors and their relative orientations.

Plasmon-enhanced multi-photon excited photoluminescence of Au, Ag, and Pt nanoclusters.

In this work, we have studied the multi-photon excited photoluminescence from metal nanoclusters (NCs) of Au, Ag and Pt embedded in Al2O3matrix by ion implantation. The thermal annealing process allows to obtain a system composed of larger plasmonic metal nanoparticles (NPs) surrounded by photoluminescent ultra-small metal NCs. By exciting at 1064 nm, visible emission, ranging from 450 to 800 nm, was detected. The second and fourth-order nature of the multiphoton process was verified in a power-dependent study measured for each sample below the damage threshold. Experiments show that Au and Ag NCs exhibit a four-fold enhanced multiphoton excited photoluminescence with respect to that observed for Pt NCs, which can be explained as a result of a plasmon-mediated near-field process that is of less intensity for Pt NPs. These findings provide new opportunities to combine plasmonic nanoparticles and photoluminescent nanoclusters inside a robust inorganic matrix to improve their optical properties. Plasmon-enhanced multiphoton excited photoluminescence from metal nanoclusters may find potential application as ultrasmall fluorophores in multiphoton sensing, and in the development of solar cells with highly efficient energy conversion modules.

Thermal and Nonlinear Optical Properties of Sudan III.

We report the experimental and theoretical study of the diffraction patterns (DPs) and thermal properties of Sudan III. DPs are used in the calculation of the Sudan III nonlinear refractive index (NLRI), n 2 . As high as n 2 = 7.69 ×10-6 cm2/W is obtained. The study of the Sudan III thermal conductivity, TC, shows the reduction of the TC against the increase of the Sudan III temperature. The property, all-optical switching (AOS), is studied in details, both static and dynamic ones, using two, cw, visible, single mode laser beams of wavelengths 473 and 635 nm.

Solvent Polarity Dependent Ultrafast Relaxation Kinetics of ADS800AT Dye.

This work investigated the photoexcitation and relaxation kinetics of the ADS800AT dye dissolved in different solvents using transient absorption spectroscopy (TAS) with a white-light continuum probe. The dye was dissolved in various solvents, including dichloromethane (DCM), 1,2-dichlorobenzene (DCB), ethanol, and methanol, to study their impact on the dye's characteristics. The linear absorption peak varied from 835 to 809 nm, depending on the polarity of the solvent, and the pump wavelength for TAS was chosen accordingly. We observed ground-state bleaching and excited-state absorption after exciting the dye with the pump pulse. Global analysis was performed using Glotaran software to fit exponential decay curve models, allowing us to determine the relaxation time of the excited molecule. The relaxation time varied from 198 ps to 508 ps across the different solvents, decreasing as the polarity of the solvent increased. Additionally, we could experimentally correlate the dye molecule's nonlinear properties with the solvent's polarity.

Spontaneous helielectric nematic liquid crystals: Electric analog to helimagnets.

Recently, a type of ferroelectric nematic fluid has been discovered in liquid crystals in which the molecular polar nature at molecule level is amplified to macroscopic scales through a ferroelectric packing of rod-shaped molecules. Here, we report on the experimental proof of a polar chiral liquid matter state, dubbed helielectric nematic, stabilized by the local polar ordering coupled to the chiral helicity. This helielectric structure carries the polar vector rotating helically, analogous to the magnetic counterpart of helimagnet. The helielectric state can be retained down to room temperature and demonstrates gigantic dielectric and nonlinear optical responses. This matter state opens a new chapter for developing the diverse polar liquid crystal devices.