Second Harmonic Generation Effect Research Articles

(C5H12N2S)Hg(NO3)2: A Mercury Nitrate Nonlinear Optical Crystal Motivated by Heteroatom Coordination Strategy.

Effective design and synthesis of second-order nonlinear optical (NLO) materials hold immense significance in driving modern science and technology advancements. In this study, we synthesized a new acentric mercury nitrate, (C5H12N2S)Hg(NO3)2, by regulating the coordination of the Hg atom through the introduction of a heteroatom. It exhibits an unprecedented [(C5H12N2S)2Hg2(NO3)4]∞ chain composed of Hg2+, NO3-, and organic molecule C5H12N2S. Notably, (C5H12N2S)Hg(NO3)2 demonstrates an unprecedented HgO3S unit and a second harmonic generation (SHG) intensity of 1.3 × KDP at 1064 nm, presenting the second-order nonlinear mercury nitrate constructed by organic molecule. Theoretical calculations suggest that the HgO3S unit and organic molecule C5H12N2S significantly contribute to the SHG effect. This study demonstrates that the incorporation of heteroatoms is an effective strategy for the development of new NLO materials.

Second-harmonic generation effect enhanced by Li substitution for Na in Li1(LiXNa5−X)Mo9O30

Large single crystals of the solid-solution series ▪ were grown by the Czochralski method by varying the Li substitution for Na. Structure analyses with X-ray single-crystal diffraction data elucidated statistic disorder of Li with Na at three independent Na sites. The resulting Li substitution X = 0, 1.2, and 2.8 could be confirmed by neutron activation analysis. Second-harmonic generation (SHG) light intensities and non-linear optical (NLO) coefficients were determined in three crystallographic main directions by Maker fringe measurements. DFT calculations delivered absolute NLO magnitudes. UV/Vis optical spectra revealed that the transmission rises from X = 0 to X = 2.8. However, the SHG effect is at largest in ▪ (X = 1.2), so far investigated in this study.

Two Short-Wave UV Beryllium Selenites Exhibiting Diverse Optical Properties Stemming from Functional Group Arrangements.

The arrangement of functional groups exerts a crucial role in determining the characteristics of compounds. In this study, we synthesized two novel short-wave ultraviolet (UV) nonlinear optical (NLO) crystals: KBe2(SeO3)2(OH)·H2O and K2Be(SeO3)2. Interestingly, the two compounds show the same SeO3 triangular pyramids and K-O polyhedra. However, the two compounds exhibit distinct beryllium-oxygen anion groups: BeO3(OH) for KBe2(SeO3)2(OH)·H2O and BeO4 for K2Be(SeO3)2. This results in the SeO3 groups within the structure having different orientations, ultimately leading to the two compounds exhibiting completely different optical properties. KBe2(SeO3)2(OH)·H2O displays a large second harmonic generation (SHG) effect equivalent to 2× KH2PO4 (KDP), coupled with a large birefringence of 0.078 at 546 nm. In contrast, the SHG effect and birefringence of K2Be(SeO3)2 are only 0.33× that of KDP and 0.024 at 546 nm, respectively. Structural analyses and theoretical calculations indicate that these pronounced differences in optical properties stem from variations in the arrangement of the SeO3 functional groups. This study not only sheds light on the correlation between crystal structure and optical behavior but also presents a hopeful avenue for the advancement of materials in the short-wave UV spectrum.

Synergic action of linear dispersion, second-order nonlinearity, and third-order nonlinearity in shaping the spectral profile of a femtosecond pulse transporting in a lithium niobate crystal

We present a detailed theoretical and numerical analysis on the temporal-spectral-spatial evolution of a high-peak-power femtosecond laser pulse in two sets of systems: a pure lithium niobate (LN) plate and a periodically poled lithium niobate (PPLN) plate. We develop a modified unidimensional pulse propagation model that considers all the prominent linear and nonlinear processes and carried out the simulation process based on an improved split-step Fourier transformation method. We theoretically analyze the synergic action of the linear dispersion effect, the second-order nonlinearity (2nd-NL) second-harmonic generation (SHG) effect, and the third-order nonlinearity (3rd-NL) self-phase modulation (SPM) effect, and clarify the physical mechanism underlying the peculiar and diverse spectral broadening patterns previously reported in LN and PPLN thin plate experiments. Such analysis and discussion provides a deeper insight into the synergetic contribution of these linear and nonlinear effects brought about by the interaction of a femtosecond laser pulse with the LN nonlinear crystal and helps to draw a picture to fully understand these fruitful optical physical processes, phenomena, and laws.

In Situ Chiral Template Approach to Synthesize Homochiral Lead Iodides for Second-Harmonic Generation.

Homochiral halide perovskites have gained increasing attention because of their fascinating optoelectronic properties and prospective applications in laser technologies. However, the limited choice of chiral organic templates severely restricts their structural diversity and second-harmonic generation (SHG) effects. Here, we present an in situ chiral template approach for the synthesis of one-dimensional (1D) homochiral lead iodides. A chiral imine (L-ipp) template was generated in situ by reacting L-proline (L-pro) and acetone under ambient conditions. Notably, L-ipp can cooperate with L-pro to direct the formation of a homochiral lead iodide with dual chiral templates, which is unprecedented in crystalline metal halides. The homochiral lead iodide containing both L-ipp and L-pro shows a strong SHG response of 8.0 times that of KH2 PO4 (8.0×KDP). The SHG efficiency is one of the largest values reported to date for any homochiral lead halides under 1064 nm laser irradiation. A comparative study shows that homochiral 1D lead iodides containing either L-ipp or L-pro exhibit relatively weak SHG responses (≤1.0×KDP). This work demonstrates the advantage of using two different chiral templates over a single chiral template in enhancing the SHG responses of halide materials.

A UV non-hydrogen pure selenite nonlinear optical material for achieving balanced properties through framework-optimized structural transformation.

For non-centrosymmetric (NCS) oxides intended for ultraviolet (UV) nonlinear optical (NLO) applications, achieving a wide band gap, large second harmonic generation (SHG) intensity, and sufficient birefringence to satisfy phase matching is a significant challenge due to their inherent incompatibility. To address this issue, this study proposes a strategy called framework-optimized structural transformation. Building upon centrosymmetric (CS) NaGa(SeO3)2 as a foundation, an original UV selenite NLO material, NaLu(SeO3)2, was successfully synthesized. The derived NaLu(SeO3)2 exhibits a balanced comprehensive performance, including a band gap (5.3 eV), an SHG response (2.7 × KDP), a UV cut-off edge (210 nm), a laser-induced damage threshold (LIDT) (151.69 MW cm-2), birefringence (Cal: 0.138@546 nm, Exp: 0.153@546 nm), thermal stability (∼575 °C) and environmental stability. Notably, its SHG effect, band gap, LIDT, and birefringence are all the largest among UV non-hydrogen pure selenite materials. Such progress can be attributed to the successful arrangement of the SeO3 groups by optimizing the cations on the framework of the parent compound.

Chromium iodate: the structure and origin of optical second harmonic generation.

Iodate crystal materials are some of the important candidates for mid-infrared nonlinear optical (NLO) crystals. Almost all the structures and NLO properties of monometallic iodates have been studied, except for a few difficult to synthesize crystalline phases. In this work, crystalline Cr(IO3)3 was synthesized and its optical properties were studied. Cr(IO3)3 has a strong powder second-harmonic generation (SHG) effect (1.3 × AgGaS2) and a large birefringence (0.24) at 2100 nm. Theoretical calculations indicate that the 3d orbital of Cr3+ is strongly involved in the SHG process, and the contribution of the metal cation centered unit CrO6 to the SHG response is much greater than that of the non-metal cation centered unit IO3. This study provides a new case among the few NLO materials in which the SHG response is dominated by metal cation centered groups.

Plasmon-enhanced second harmonic generation of metal nanostructures.

As the most common nonlinear optical process, second harmonic generation (SHG) has important application value in the field of nanophotonics. With the rapid development of metal nanomaterial processing and chemical preparation technology, various structures based on metal nanoparticles have been used to achieve the enhancement and modulation of SHG. In the field of nonlinear optics, plasmonic metal nanostructures have become potential candidates for nonlinear optoelectronic devices because of their highly adjustable physical characteristics. In this article, first, the basic optical principles of SHG and the source of surface symmetry breaking in metal nanoparticles are briefly introduced. Next, the related reports on SHG in metal nanostructures are reviewed from three aspects: the enhancement of SHG efficiency by double resonance structures, the SHG effect based on magnetic resonance and the harmonic energy transfer. Then, the applications of SHG in the sensing, imaging and in situ monitoring of metal nanostructures are summarized. Future opportunities for SHG in composite systems composed of metal nanostructures and two-dimensional materials are also proposed.

(C5H6N2)2H](Sb4F13): a polyfluoroantimonite with a strong second harmonic generation effect.

It is of great difficulty to create a new antimonite with second-harmonic-generation (SHG) intensity larger than 6 times that of KDP. In this study, a polyfluoroantimonite strategy has been proposed to explore fluoroantimonites with large nonlinear optical (NLO) coefficients. Under the cooperation of chemical (highly asymmetric π-conjugated organic amine) and physical (viscous reaction medium ethylene glycol) methods, two novel polyfluoroantimonites, namely, (3PC)2(Sb4F14) and (3AP)2(Sb4F13), have been achieved. Interestingly, these two structures contain two new polyfluoroantimonite groups respectively, an isolated (Sb4F14)2- four-member polyhedral ring and an infinite [Sb4F13]∞- helical chain. More importantly, the polar (3AP)2(Sb4F13) displays a strong SHG intensity of 8.1 × KDP, a large birefringence of 0.258@546 nm and a high laser-induced damage threshold (LIDT) value of 149.7 MW cm-2. Theoretical calculations indicated that its strong SHG effect stems from the synergistic effect of the helical [Sb4F13]∞- polyfluoroantimonite chain and π-conjugated 3AP+ cation, with a contribution ratio of 48.93% and 50.77% respectively. This work provides a new approach for the design and synthesis of high-performance fluoroantimonites.

Mid-infrared second-harmonic generation and its red-shift signal induced by cross-phase modulation in an AsSe2 soft glass microstructured optical fiber

In this study, we investigate an AsSe2 soft glass microstructured optical fiber (MOF) featuring a hexagonal arrangement of six holes. The MOF is subjected to femtosecond laser pumping, employing a pump wavelength range spanning from 3950 nm to 4100 nm. Our experimental results reveal an intriguing outcome: a tunable mid-infrared second-harmonic generation (SHG) effect is observed, yielding a wavelength range of ∼ 1980 nm to ∼ 2203 nm. The maximum conversion efficiency is achieved at 4100 nm with a power of 130 mW, which can be up to 9.2E-5. The observed SHG is primarily due to the surface nonlinearity polarization at the core-cladding interface. Additionally, we increase the additional degrees of freedom of the SH signal by strong third-order nonlinearity, and demonstrate that strong self-phase modulation (SPM) of pump and cross-phase modulation (XPM) of SH signal help broaden the SH signal by means of generating new wavelength lobes. To the best of our knowledge, this is the first time that strong XPM-based SH signal is observed in an optical fiber in the mid-infrared wavelength region. The realized strong second-order and third-order nonlinearity might expand the fundamental physical mechanism of SHG in optical fiber especially in mid-infrared region, which would have important applications in telecommunications, laser technology, and quantum photonics.

Na2CeF6: A Highly Laser Damage-Tolerant Double Perovskite Type Ce(IV) Fluoride Exhibiting Strong Second-Harmonic Generation Effect.

Perovskite structure compounds are significant candidates for designing new optical function materials due to their structural variability. Here, an inorganic tetravalent cerium fluoride, Na2CeF6, is derived from the perovskite structure through double-site cation co-substitution. Na2CeF6 crystalizes in the non-centrosymmetric space group . Edge-sharing connected NaF9 and CeF9 polyhedra build the whole 3D structure of Na2CeF6. Importantly, it represents the first Ce(IV) fluoride nonlinear optical (NLO) crystal and can produce a strong and phase-matchable second-harmonic generation (SHG) effect of ≈2.1 times that of KH2PO4 (KDP), making it the strongest among non-lone pair electron metal fluoride system. Further, it exhibits a high laser-induced damage threshold (LIDT) of 74.65-76.25MWcm-2, which is over 20 times that of AgGaS2. It also exhibits a wide transparent region (0.5-14.3µm). This work provides a facile route for exploring high-performance halide NLO materials.

Classification of second harmonic generation effect in magnetically ordered materials

The relationship between magnetic order and the second harmonic generation (SHG) effect is a fundamental area of study in condensed matter physics with significant practical implications. In order to gain a clearer understanding of this intricate relation, this study presents a comprehensive classification scheme for the SHG effect in magnetically ordered materials. This framework offers a straightforward approach to connecting magnetic order and the SHG effect. The characteristics of the SHG tensors in all magnetic point groups are studied using the isomorphic group method, followed by a comprehensive SHG effect classification scheme that includes seven types based on the symmetries of the magnetic phases and their corresponding parent phases. In addition, a tensor dictionary containing the SHG and linear magneto-optic (LMO) effect is established. Furthermore, an extensive SHG database of magnetically ordered materials is also built up. This classification strategy exposes an anomalous SHG effect with even characteristics (i.e., invariant) under time-reversal symmetry, which is solely contributed by magnetic structure. Moreover, the proposed classification scheme facilitates the determination of magnetic structures through the SHG effect.

Structural, electronic, linear, and nonlinear optical properties of undoped and Mo (I, II)-doped LiNbO3 crystal

In this paper, density functional theory (DFT) calculations used to investigate the structural, electronic, linear and nonlinear optical properties of pure lithium niobate crystal (LiNbO3) and doped LiNbO3 by Mo (I) and Mo (I). The lattice constants were calculated by analysing the optimised unit cell, and the results were found to be in good agreement with both the experimental and theoretical values that were reported. The results indicate a clear distinction in the electronic characteristics of LiNbO3 and doped LiNbO3 (Mo-Nb), the band structures that have been computed indicate that the band gap of LiNbO3 can be narrowed by adding Mo (I) and Mo (II) dopants to 2.068 eV and 0.476 eV, respectively. Interesting results were obtained by this doping process, as these results showed that the effect of doping led to an improvement in some optical properties, such as the absorption spectrum of the material, and also led to an increase in the effect of second harmonic generation (SHG), which is considered a basic requirement for developing optical devices in many photonic applications such as laser, electro-optic applications and optical switches.

Eu2MGe2OS6 (M = Mn, Fe, Co): Three Melilite-Type Rare-Earth Oxythiogermanates Exhibiting Balanced Nonlinear-Optical Behaviors.

Metal oxychalcogenides as candidates for novel mid-infrared nonlinear-optical materials have attracted great interest due to the distinctive advantages of oxides and chalcogenides in this field. Herein, the first melilite-type rare-earth (RE) oxythiogermanates Eu2MGe2OS6 [M = Mn (1), Fe (2), Co (3)] are obtained by combining RE metals with localized f electrons, magnetic transition metals with delocalized d electrons, and the highly distorted mixed anionic group [GeOS3] into one structure. They belong to the tetragonal P4̅21m space group, and highly distorted [EuOS7] bicapped trigonal prisms bridge adjacent {[MGe2OS6]4-}∞ layers to build the three-dimensional network. Their optical band gaps are determined as 2.40, 2.11 and 2.14 eV, and they show moderate second-harmonic-generation (SHG) responses (0.3, 0.3 and 0.5 × AGS) and large laser-induced damage thresholds (2.77-8.31 × AGS). Theoretical calculation results indicate that the synergistic effect of [EuOS7] and [MS4] units acts on the SHG effect. This work enriches the crystal chemistry of melilite-structure materials.

AllHgMIVS4 (All = Sr, Ba, MIV = Si, Ge): A Series of Materials with Large Second Harmonic Generation Response and Wide Band Gaps

AbstractWide band gaps and large second harmonic generation (SHG) response are two crucial yet contradictory parameters for nonlinear optical (NLO) crystals. Herein, through exploring the influence of anion groups on structural crystallization dimension and space group, a series of new NLO Hg‐based chalcogenides AIIHgMIVS4 (AII = Sr, Ba, MIV = Si, Ge) with wide band gaps and large SHG responses have been successfully designed and synthesized. The structures of these compounds consist of 2D [HgMS4]2− (M = Si and Ge) layers composed of distorted (HgS4 and MS4) tetrahedra arranged alternately, and the layers are separated by eight‐coordinated A2+ (A = Sr and Ba) cations. Their band gaps and SHG responses meet the basic requirements (SHG > 1 × AgGaS2; Eg > 2.33 eV). Especially, SrHgSiS4 and SrHgGeS4 exhibit both large SHG effects (2 and 2.4 × AgGaS2) and wide band gaps (3.06 and 2.97 eV), respectively, indicating they are promising IR NLO crystals. This work opens a new avenue for new NLO material design to achieve the balance between strong SHG response and large band gaps.

Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides

Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbOX2 (X = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl2 can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl2 and a reversible ferroelectric-to-paraelectric phase transition in NbOI2 under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbOX2 for nonlinear optical device applications.

External field intensity and wavelength dependency from IR to deep UV of linear and nonlinear optical properties of 1-butyl-3-methylimidazolium dicyanamide ionic liquid

The electronic structure, inter-ionic interactions and charge transfer, polarizability, and first-order and second-order hyperpolarizabilities of 1-butyl-3-methylimidazolium dicyanamide ionic liquid have been explored via modeling at M06-2X/6-311++G(d,p) level of theory. External field intensity dependence of electronic properties was investigated by utilization of field intensity from 0.0001 to 0.002 au applied in the direction of anion-cation. Interionic interactions have been characterized through the reduced density gradient method to reveal and distinguish hydrogen bonding and other non-covalent interactions. Extraordinary bonding interactions were observed between the anion and ring system of the cation. Electronic excitation characteristics were analyzed based on the inter-fragment charge transfer method to distinguish charge transfer contributions from local excitations. Static and dynamic nonlinear optical parameters namely the total dipole moment μ, the mean polarizability α(0,0) and α(−ω; ω), the anisotropy of polarizability Δα(0,0) and Δα(−ω; ω) the first hyperpolarizability in the direction of dipole moment β(0,0,0), β(−ω; ω,0), and β (−2ω; ω,ω), the average second hyperpolarizability γ(0;0,0,0), γ(−ω; ω,0,0) and γ(−2ω; ω,ω,0) have been calculated for specific laser wavelengths corresponding to the UV, visible, and infrared region. Thus, a novel way of mapping the field intensity and wavelength dependence of NLO properties was achieved. The hyperpolarizability calculations indicate that 1-butyl-3-methylimidazolium dicyanamide has a significant potential to be used in electro-optics Pockels effect, second-harmonic generation, and electro-optical Kerr effect applications. NLO property maps indicate that the wavelength dependence in the IR and visible region is almost ignorable when compared with that of the UV region where the first hyperpolarizabilities can increase by ten thousand and the second hyperpolarizabilities can reach up to ten billion times.

Two Noncentrosymmetric Bismuth Phosphates: Rational Design Synthesis and Optical Properties.

Developing strategies to rational design noncentrosymmetric structure still attract much interest for their applications in nonlinear optical and piezoelectric materials. Two noncentrosymmetric (NCS) alkaline earth metal bismuth phosphates have been successfully achieved via partial replacement of Bi3+ with Ca2+ or Sr2+ ions. BiCa(H0.5PO4)2 (designated as CaBiPO) and BiSr(H0.5PO4)2 (designated as SrBiPO), together with their solid solution Bi(Sr1-xCax)(H0.5PO4)2 (0 < x ≤ 0.5), crystallize in the NCS space group C2. Both CaBiPO and SrBiPO exhibit ultraviolet nonlinear optical (NLO) properties, and their second-harmonic generation effects belong to type-II phase matching. Meanwhile, they could also act as photoluminescence hosts in which the Eu3+-doping samples SrBiPO:xEu3+ (x = 0.02-0.2) emit orange light. The effect of different radius ions on the derivative structures and the structure-NLO property relationship has also been discussed in detail.

α- and β-(C4H5N2O)(IO3)·HIO3: Two SHG Materials Based on Organic-Inorganic Hybrid Iodates.

Organic-inorganic hybrid nonlinear optical (NLO) materials are highly anticipated because of the integration of both merits of the organic and inorganic moieties. Herein, the 2-pyrimidinone cation (C4H5N2O)+ has been incorporated into the iodate system to form two polymorphic organic-inorganic hybrid iodates, namely, α- and β-(C4H5N2O)(IO3)·HIO3. They crystallize in different polar space groups (Ia and Pca21), and their structures feature one-dimensional (1D) chain structures composed of (C4H5N2O)+ cations, IO3- anions, and HIO3 molecules interconnected via hydrogen bonds. α- and β-(C4H5N2O) (IO3)·HIO3 exhibit strong and moderate second-harmonic-generation (SHG) responses of 6.4 and 0.9 × KH2PO4 (KDP), respectively, the same band gaps of 3.65 eV, and high powder laser-induced damage threshold (LIDT) values [51 and 57 × AgGaS2 (AGS)]. The results of theoretical calculations revealed that the large SHG effect of α-(C4H5N2O)(IO3)·HIO3 originated from the IO3 and HIO3 groups. This work indicates that (C4H5N2O)+ is a potential group for designing new NLO materials with brilliant optical performances.

Second-harmonic generation in fluctuating Ising superconductors

In a two-dimensional noncentrosymmetric transition metal dichalcogenide Ising superconductor in the fluctuating regime under the action of a uniform external electromagnetic field, a second-harmonic generation (SHG) effect takes place. There emerge two contributions to this effect, one conventional, which is due to the electron gas in its normal state, and the other one is of the Aslamazov–Larkin nature. Namely, it originates from the presence of fluctuating Cooper pairs in the system when the temperature approaches the temperature of the superconducting transition in the sample from above. Employing a usual approach to Ising superconductors, we lift the valley degeneracy by application of a weak out-of-plane external magnetic field, which produces a Zeeman effect. In calculations, we use the Boltzmann equations approach for the electron gas in the normal state, and the time-dependent Ginzburg–Landau equations for the fluctuating Cooper pairs, and show the emergence of SHG electric current characterized by a temperature-dependent broadening and a redshift.