Strong Second Harmonic Generation Intensity Research Articles

Mixed-anion square-pyramid [SbS3I2] units causing strong second-harmonic generation intensity and large birefringence

Mixed-anion square-pyramid [SbS3I2] units causing strong second-harmonic generation intensity and large birefringence

Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 : Promising Multifunctional Nonlinear Optical Materials Created by Partial Fluorination Strategy under Corrosion Resistant Supercritical Reactions.

It has historically been exceedingly challenging to create physically and chemically stable lanthanide compounds with strong second harmonic generation (SHG) due to their strong preference to central symmetry. In this work, five new non-centrosymmetric lanthanide selenites, namely, Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 (Ln = Sm, Eu, Gd, Tb and Dy), are achieved by partial fluorination of the lanthanide oxygen polyhedron. An HF corrosion resistant supercritical hydrothermal method is developed, which is a facile and universal method for HF corrosion and high-temperature high-pressure environment. The title compounds displayed a novel 3D framework composed of 1D molybdenum selenite chains bridged by Ln2 F2 O12 (OH2 ) dimers. Their powder SHG responses showed a large difference, ranging from 1.0 to 9.0 × KH2 PO4 (KDP) at 1064nm. The half-filled Gd compound exhibited very strong SHG efficiency of up to 1.2 × KTP (KTiOPO4 ) at 2050nm. Compounds Tb and Gd are the first lanthanide selenites with SHG intensity reaching KTP level, which is very rare in this system. Furthermore, these compounds can also possess excellent physicochemical stability and strong luminescence emission, indicating that they are promising multifunctional nonlinear optical materials. This work offered an effective way for design and synthesis of multifunctional and high-performant nonlinear optical materials.

Emission Dipole and Pressure‐Driven Tunability of Second Harmonic Generation in vdWs Ferroelectric NbOI2

Abstract2D in‐plane ferroelectric NbOI2 exhibits strong second harmonic generation (SHG) and ultrahigh effective susceptibility. To push forward their applications in nonlinear photonics and optoelectronics, it is highly desirable to understand the emission dipole orientation and tunability of SHG, which is not achieved. Here, by integrating tight focusing from parabolic mirror with back focal plane (BFP) imaging technique, for the first time it is demonstrated that SHG emission of NbOI2 presents purely in‐plane dipole orientation in consistent with numerical simulations, suggesting the in‐plane components of the SHG susceptibility tensor in NbOI2 dominate the emission. Moreover, with the aid of ab‐initio calculations, it is found that the hydrostatic pressure can dramatically change the structure and resultant SHG intensity of NbOI2. Explicitly, SHG intensity endures a slight increase due to the distortion of octahedral at low pressure pressure, and then monotonously decreases due to the improvement of structural symmetry with further increasing pressure, and drastically quenching resulting from the ferroelectric to paraelectric phase transition. This work unambiguously demonstrates the dipole emission behavior of SHG and the relationship between structural evolution and SHG intensity, which provides an avenue for tunable nonlinear optics and optoelectronics.

Superior ferroelectricity and nonlinear optical response in a hybrid germanium iodide hexagonal perovskite

Abundant chemical diversity and structural tunability make organic–inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO3, their ferroelectric key properties, including large spontaneous polarization (Ps), low coercive field (Ec), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI3 (DMA = Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large Ps of 24.14 μC/cm2 (on a par with BaTiO3), a low Ec below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH2PO4 (KDP)). Revealed by the first-principles calculations, its large Ps originates from the synergistic effects of the stereochemically active 4s2 lone pair of Ge2+ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low Ec. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3 )2 and Y3 F(SeO3 )4.

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y3 F(SeO3 )4 , has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO3 )2 . The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO3 )2 has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y3 F(SeO3 )4 can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y3 F(SeO3 )4 is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3)2 and Y3F(SeO3)4

AbstractIt is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y3F(SeO3)4, has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO3)2. The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO3)2 has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non‐centrosymmetric Y3F(SeO3)4 can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut‐off edge (204 nm) and high thermal stability (690 °C). So, Y3F(SeO3)4 is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

Noncentrosymmetric γ-Cs2I4O11 Obtained from IO4 Polyhedral Rearrangements in the Centrosymmetric β-Phase

We report the synthesis and optical properties of noncentrosymmetric (NCS) γ-Cs2I4O11 that was obtained through IO4 polyhedral rearrangements from centrosymmetric (CS) β-Cs2I4O11. Trifluoroacetic acid (TFA) acts as a structure-directing agent and plays a key role in the synthesis. It is suggested that the function of TFA is to promote rearrangement reactions found in the organic synthesis of stereoisomers. γ-Cs2I4O11 crystallizes in the NCS monoclinic space group P21 (No. 4) and exhibits a strong second-harmonic-generation (SHG) response of 5.0 × KDP (KH2PO4) under 1064 nm laser radiation. Additional SHG experiments indicate that the material is type I phase matchable. First-principles calculations show that SHG intensity mainly comes from its d34, d21, and d23 SHG tensor components. The synthetic strategy of discovering γ-Cs2I4O11 provides a new way for designing novel NCS SHG materials.

Optical second-harmonic generation of Janus MoSSe monolayer

The transition metal dichalcogenides (TMD) monolayers have shown strong second-harmonic generation (SHG) owing to their lack of inversion symmetry. These ultrathin layers then serve as the frequency converters that can be intergraded on a chip. Here, taking MoSSe as an example, we report the first detailed experimental study of the SHG of Janus TMD monolayer, in which the transition metal layer is sandwiched by the two distinct chalcogen layers. It is shown that the SHG effectively arises from an in-plane second-harmonic polarization under paraxial focusing and detection. Based on this, the orientation-resolved SHG spectroscopy is realized to readily determine the zigzag and armchair axes of the Janus crystal with an accuracy better than ±0.6°. Moreover, the SHG intensity is wavelength-dependent and can be greatly enhanced (∼ 60 times) when the two-photon transition is resonant with the C-exciton state. Our findings uncover the SHG properties of Janus MoSSe monolayer, therefore lay the basis for its integrated frequency-doubling applications.

Nonlinear Optical Benzoindolium-Based Single Crystals: H-Contacts Induced Noncentrosymmetric Alignment Approaching Efficient Terahertz-Wave Generation

Ionic-type nonlinear optical (NLO) crystals, which belong to noncentrosymmetric (NCS) space groups, have been the core device materials in the field of laser technology, especially in the field of terahertz-wave generation. In this study, three iodide benzoindolium-based crystals, namely, MBI (C24H24NOI, Cc), m-OHBI (C23H22NOI, P21), and p-OHBI (C23H22NOI, P21/c), with different substitution sites of oxygen-containing donor groups have been successfully synthesized and grown. Acentric MBI and m-OHBI crystals exhibit transparent edges below 639 nm, good environmental stabilities, and a strong second-harmonic generation (SHG) response close to typical OH1 (d33 = 120 pm/V at 1.9 μm), and the oxygen-containing donor groups are analyzed to be effective for inducing noncentrosymmetric alignment of the functional building units. Notably, for the first time, a widely tunable terahertz wave has been realized through an MBI prototype device by the difference frequency generation method. This work provides a feasible strategy for a new NLO crystal design to achieve strong SHG intensity and efficient terahertz-wave generation.

Mid-Infrared Nonlinear Frequency Conversion Using Monolithic Barium Titanate on Silicon-on-Insulator

Efficient nonlinear frequency conversion in the mid-infrared (mid-IR) region was demonstrated by a barium titanate (BTO) on a silicon-on-insulator (SOI) substrate. The BTO thin film was epitaxially grown by pulsed-laser deposition (PLD). The tensorial second order optical nonlinearity and the ferroelectric domain property of the BTO were characterized by the polarimetric mid-IR second harmonic generation (SHG) measurements. Azimuthal-dependent polarized SHG was modeled at different combinations of the nonlinear coefficient <formula><tex>$d_{ij}$</tex></formula> and the domain fraction factor <formula><tex>$\Delta D^{Y}/\Delta D^{X}$</tex></formula>. Strong SHG intensity was experimentally observed over a broad spectrum at &#x03BB; = 3.1 &#x03BC;m - 3.7 &#x03BC;m. A characteristic linear dependence between the SHG signal and the square of the pumping laser intensity was observed. The monolithic integration between the epitaxial BTO and the Si provides a compact platform for efficient on-chip light generation and quantum photonic technologies.

Broad transparency and wide band gap achieved in a magnetic infrared nonlinear optical chalcogenide by suppressing d-d transitions.

Magnetic infrared (IR) nonlinear optical (NLO) materials, particularly those containing d-block metals, have attracted considerable attention due to the contributions of d-orbitals to large NLO efficiency. However, the d-d transitions from the d-block metals lead to strong optical absorption and narrow band gap, seriously hindering their practical applications. The structural flexibility of salt-inclusion systems provides a good opportunity for modulating the crystal field of magnetic ions to suppress the d-d transitions but allowing the NLO-active d-s and d-p transitions. These ideas afford a new salt-inclusion sulfide [K3Cl][Mn2Ga6S12], which features a rare nanoporous [MnGa3S6]- framework with tunnels of inner diameter of 9.0 Å and possesses a broad transparency (0.39-25.0 μm) and the widest band gap (3.17 eV) among all magnetic IR NLO chalcogenides. Remarkably, it exhibits a strong phase-matchable second-harmonic generation intensity (0.8 × AgGaS2 at 1910 nm and 3.1 × AgGaS2 at 1064 nm) and a high laser-induced damage threshold (12.5 × AgGaS2 at 1064 nm), achieving the important criteria of an advanced IR NLO material.

Layered oxide B2S2O9 with a deep-ultraviolet band gap and a strong and robust second-harmonic generation

Two-dimensional (2D) layered semiconductors with both ultrawide bandgap and strong second harmonic generation (SHG) are essential for expanding the nonlinear optical (NLO) applications to deep-ultraviolet (DUV) region in nanoscale. Unfortunately, these materials are rare in nature and have not been discovered until now. In this Letter, we predict the B2S2O9 (BSO), an existing layered oxide, can exhibit both DUV bandgap and strong SHG effects, comparable to the best known DUV NLO bulks. The strong SHG intensities in BSO, originated from the ordered arrangement of polar SO4 and BO4 tetrahedra forming planar structure, are linearly tunable by the layer thickness. Surprisingly, the spontaneous rotations of rigid tetrahedra under strains can induce the (nearly) zero Poisson's ratios in BSO, which simultaneously result in the robust SHG effects against large strains, fundamentally differing from other known 2D NLO semiconductors. The discovery of BSO may provide an unprecedented opportunity to explore DUV NLO physics and applications in 2D limit.

Second harmonic generation in two-dimensional transition metal dichalcogenides with growth and post-synthesis defects

Strong second harmonic generation (SHG) in monolayer transition metal dichalcogenides demonstrates great promise for nonlinear photonic applications. However, to utilize these materials in photonic applications, it is imperative to investigate the robustness of the nonlinear optical properties against various defects introduced during or post synthesis. Here we study the SHG in hexagonal monolayer WS2 with synthesis defects, in triangular monolayer WS2 with focused ion beam induced structural defects, and in WS2 under ambient and aqueous environmental conditions. We find that WS2 hexagonal monolayers exhibit significantly more uniform SHG intensity across the sample with a coefficient of variation, than the varying photoluminescence (PL) intensity (). We also show that introducing structural defects in WS2 monolayers leads to diminishing, but not vanishing SHG. To further explore the robustness of SHG in comparison with PL, we show that aged WS2 monolayers have greatly varying PL across different samples (), while exhibiting much more uniform SHG (). Lastly, we find that WS2 monolayers submerged in deionized water shows similar intensity in SHG signal. To understand these experimental results, density functional theory calculations have been performed. Our analysis suggests that potential defects and water introduced to the WS2 monolayers produce relatively little change in the second-order nonlinear susceptibility, whereas nonradiative recombination caused by these defects can significantly affect the PL. These results show a good qualitative agreement with the experimental results.

Solid-State Nonlinear Optical Switch with the Widest Switching Temperature Range Owing to Its Continuously Tunable Tc.

Interest on the nonlinear optical (NLO) switches that turn on/off the second-harmonic generation (SHG) triggered by the external stimulus (such as heat) have continuously grown, especially on the solid-state NLO switches showing superior stability, reversibility, and reproducibility. Herein, we discover (NH4)2PO3F, as an entirely new solid-state NLO switch showing outstanding switch contrast and reversibility as well as strong SHG intensity (1.1 × KH2PO4 (KDP)) and high laser-induced damage threshold (2.0 × KDP), undergoes a unique first-order phase transition that originates from a reversible hydrogen-bond rearrangement and needs to overcome an energy barrier. Accordingly, we put forward a strategy to continuously modify such an energy barrier by reducing the number of hydrogen bonds per unit cell via an isoelectronic replacement of NH4+ by K+ with a similar size yet incapability of providing any hydrogen bond. Consequently, Kx(NH4)2-xPO3F (x = 0-0.3) exhibiting excellent switching performance are obtained. Remarkably, Kx(NH4)2-xPO3F not only realizes a continuously tunable Tc spanning from 270 to 150 K, representing the widest NLO switching temperature range ever known but also indicates the first solid-state NLO switch example with continuous Tc. Intrinsically, such a Tc decline depends on the weakening degree of the hydrogen-bonding interactions in the unit cell. These new insights will shed useful light on the future material design and open new application possibilities.

2(C3H7N6)+·2Cl-·H2O: an ultraviolet nonlinear optical crystal with large birefrigence and strong second-harmonic generation.

Large birefringence (Δn = 0.277 at 546 nm) and strong second-harmonic generation (SHG) intensity (4.3 × KDP) achieved a remarkable balance in a potential ultraviolet nonlinear optical crystal: 2(C3H7N6)+·2Cl-·H2O. Theoretical calculations showed that planar benzene-like π-conjugated [C3N6] groups had important optical properties.

Phase Identification and Strong Second Harmonic Generation in Pure ε-InSe and Its Alloys.

Two-dimensional material indium selenide (InSe) has offered a new platform for fundamental research in virtue of its emerging fascinating properties. Unlike 2H-phase transition-metal dichalcogenides (TMDs), ε phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers, and predicted to have a strong second harmonic generation (SHG) effect. In this work, we find that the as-prepared pure InSe, alloyed InSe1- xTe x and InSe1- xS x ( x = 0.1 and 0.2) are ε phase structures and exhibit excellent SHG performance from few-layer to bulk-like dimension. This high SHG efficiency is attributed to the noncentrosymmetric crystal structure of the ε-InSe system, which has been clearly verified by aberration-corrected scanning transmission electron microscopy (STEM) images. The experimental results show that the SHG intensities from multilayer pure ε-InSe and alloyed InSe0.9Te0.1 and InSe1- xS x ( x = 0.1 and 0.2) are around 1-2 orders of magnitude higher than that of the monolayer TMD systems and even superior to that of GaSe with the same thickness. The estimated nonlinear susceptibility χ(2) of ε-InSe is larger than that of ε-GaSe and monolayer TMDs. Our study provides first-hand information about the phase identification of ε-InSe and indicates an excellent candidate for nonlinear optical (NLO) applications as well as the possibility of engineering SHG response by alloying.

New Metal Chalcogenides Ba4CuGa5Q12 (Q = S, Se) Displaying Strong Infrared Nonlinear Optical Response

A series of new metal chalcogenides Ba4CuGa5Q12 (Q = S, S0.75Se0.25, Se) were synthesized using KBr flux at 750 °C. The three compounds are isostructural and adopt the noncentrosymmetric space group P4̅21c. Crystal data are as follows: Ba4CuGa5S12, 1, a = 13.040(1) Å, c = 6.304(1) Å, and Z = 2; Ba4CuGa5S9.00(1)Se2.92(1), 2, a = 13.1585(2) Å, c = 6.3520(2) Å, and Z = 2; Ba4CuGa5Se12, 3, a = 13.598(1) Å, c = 6.527(1) Å, and Z = 2. The three-dimensional framework in 1 is constructed by infinite columns ∞1[CuGa4S10]7– that surround the discrete GaS4 tetrahedra situated on a 4̅ axis. The discrete GaS4 tetrahedra on the stacking (112) planes and canted oriented edge-sharing CuS4 tetrahedra within the columns may account for the occurrence of strong second-harmonic generation (SHG) responses. Compounds 1–3 are transparent in the mid-infrared range and have the absorption edges at 2.82, 2.05, and 1.45 eV, respectively. The new nonlinear optical (NLO) materials are type-I nonphase matching at 693 nm and display strong SHG intensities that are ∼2.7, ∼2.6, and ∼1.1 times that of AgGaSe2 at 808 nm. Raman spectroscopic characterization of the compounds is reported.

Structure Change Induced by Terminal Sulfur in Noncentrosymmetric La2Ga2GeS8 and Eu2Ga2GeS7 and Nonlinear-Optical Responses in Middle Infrared

Two new noncentrosymmetric quaternary sulfides, La(2)Ga(2)GeS(8) (1) and Eu(2)Ga(2)GeS(7) (2), have been synthesized by high-temperature solid-state reactions. The structure change on going from 1 to 2 to the known Li(2)Ga(2)GeS(6) (3) nicely shows that the reduced cation charge-compensation requirement causes a decrease in the number of terminal S atoms per formula, which is a key to determining the connectivity of the GaS(4) and GeS(4) building units. Powder sample 2 exhibits a strong second-harmonic-generation (SHG) response of about 1.6 times the benchmark AgGaS(2) at 2.05 μm laser radiation, a non type I phase-matchable behavior, and a comparable transparency region. The SHG intensities of these compounds originate from the electronic transitions from S 3p states to La/Eu/Li-S, Ga-S, and Ge-S antibonding states according to Vienna ab initio simulation package studies.

Nonlinear optical activity in dipolar organic–lanthanide complexes

A series of fifteen dipolar polymeric lanthanide complexes of trans-cinnamic acid, [Ln(C9H7O2)3]n, which crystallize in the non-centrosymmetric space groups R3c and P21, have been synthesized and the crystal structures of 14 of these have been determined. From the crystallographic data, two different coordination scenarios for these complexes have been found: Type I, CN = 9, Ln = La, Ce, Nd, Sm, Eu, Gd and Tb, and Type II, CN = 7, Ln = Y, Dy, Ho, Er, Tm, Yb and Lu. The linear and nonlinear photophysical properties of these complexes have been studied using various laser excitation wavelengths. The low temperature photoluminescence spectrum shows a distortion from the C3 site symmetry of the Eu3+ complex. Strong second harmonic generation (SHG) has been observed for both Types I and II in the solid state. The normalized SHG intensities of the members of the entire series of lanthanide complexes have been compared and the solution state hyperpolarizabilities have also been measured for the study of the effects of 4f-electrons on nonlinear photophysical properties. These SHG parameters show maximum values near the center of the lanthanide series, as opposed to previously reported fairly linear or independent variations.

Strongly Nonlinear Optical Glass Fibers from Noncentrosymmetric Phase-Change Chalcogenide Materials

We report that the one-dimensional polar selenophosphate compounds APSe(6) (A = K, Rb), which show crystal-glass phase-change behavior, exhibit strong second harmonic generation (SHG) response in both crystal and glassy forms. The crystalline materials are type-I phase-matchable with SHG coefficients chi((2)) of 151.3 and 149.4 pm V(-1) for K(+) and Rb(+) salts, respectively, which is the highest among phase-matchable nonlinear optical (NLO) materials with band gaps over 1.0 eV. The glass of APSe(6) exhibits comparable SHG intensities to the top infrared NLO material AgGaSe(2) without any poling treatments. APSe(6) exhibit excellent mid-IR transparency. We demonstrate that starting from noncentrosymmetric phase-change materials such as APSe(6) (A = K, Rb), we can obtain optical glass fibers with strong, intrinsic, and temporally stable second-order nonlinear optical (NLO) response. The as-prepared glass fibers exhibit SHG and difference frequency generation (DFG) responses over a wide range of wavelengths. Raman spectroscopy and pair distribution function (PDF) analyses provide further understanding of the local structure in amorphous state of KPSe(6) bulk glass and glass fiber. We propose that this approach can be widely applied to prepare permanent NLO glass from materials that undergo a phase-change process.