Second Harmonic Generation Effect Research Articles

High-Performance Second-Harmonic-Generation (SHG) Materials: New Developments and New Strategies.

ConspectusSecond-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ2 = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO33--based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO33- and NO3-) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO4-based compounds, highly polarizable cations such as Hg2+ and Bi3+ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I5+ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO6 octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.

Second harmonic generation of MoSi2N4 -type layers

The recently discovered two-dimensional (2D) layered semiconductor MoSi2N4 has aroused great interest due to its unique 2D material characteristics. In this Letter, we found that differences in the structural details for MoSi2N4 may lead to differences in the intensity of second harmonic generation (SHG) and its response to strain. Accordingly, SHG can be used as a simple technique to identify the structural details of this system. We further calculated the SHG effects of MoSi2N4 derivatives and investigated their strain-regulation mechanism, especially including the anomalous SHG responses under strain for MoSi2P4 and MoGe2P4, differing from other known 2D materials. The studies may have forward-looking significance for the research of nonlinear optics and optoelectronics in this novel 2D material system.

α-Ca2CdP2 and β-Ca2CdP2: Two Polymorphic Phosphide-Based Infrared Nonlinear Crystals with Distorted NLO-Active Tetrahedral Motifs Realizing Large Second Harmonic Generation Effects and Suitable Band Gaps.

Two polymorphic phosphide-based infrared (IR) nonlinear optical (NLO) crystals, α-Ca2CdP2 and β-Ca2CdP2, were obtained by combining alkaline-earth metals and d10 transition metals using metal flux and metal salt flux methods, respectively. The crystal structure of α-Ca2CdP2 is orthorhombic in the space group Cmc21 (no. 36), while the structure of β-Ca2CdP2 is monoclinic in the space group Cm (no. 8). Both are two-dimensional layered structures that are composed of CdP4 tetrahedra layers via sharing vertices, which stack along the b-axis and the c-axis, respectively. The second harmonic generation (SHG) measurements manifest that α-Ca2CdP2 and β-Ca2CdP2 exhibit strong SHG intensities (1.41 and 3.28× that of AgGaS2 at a 2050 nm laser, respectively). Other optical measurements indicate that α-Ca2CdP2 and β-Ca2CdP2 have suitable band gaps (1.98 and 1.55 eV, respectively), high laser-induced damage thresholds (4.5 and 3.1× that of AgGaS2 at 1064 nm laser, respectively) and appropriate birefringence (0.12 and 0.20 at 2050 nm, respectively) in addition to covering wide infrared transparent regions. The research on α-Ca2CdP2 and β-Ca2CdP2 demonstrates that they are potential IR NLO candidates. Theoretical calculations uncover that their SHG effects are from distorted CdP4 tetrahedra, highlighting that tetrahedral motifs, including d10 transition metals, would be ideal NLO-active building blocks.

Alloy Engineering of a Polar (Si,Ge)2N2O System for Controllable Second Harmonic Performance.

Although silicon oxynitrides are important semiconductors for many practical applications, their second-order nonlinear optical (NLO) properties have never been systemically explored. Using the first-principles calculations, in this article, we discover that the sinoite (e.g., silicon oxynitride Si2N2O) can simultaneously exhibit wide optical band gap, strong second-harmonic generation (SHG) effect, and large birefringence, which are further confirmed by our preliminary experimental measurement. Importantly, we propose that alloying engineering can be further applied to control the balanced NLO properties in the Si2N2O system. Based on first-principles cluster expansion theory, we demonstrate that alloying Ge into Si2N2O can form low formation-energy Si2(1-x)Ge2xN2O alloys, which can in turn achieve controllable phase-matching harmonic output with high SHG efficiency at different energy ranges. Therefore, alloy engineering could provide a unique approach to effectively control the NLO performance of Si2(1-x)Ge2xN2O, making this polar alloy system hold potential applications in tunable laser conversion and controllable optical devices.

Magnetic Order and Symmetry in the 2D Semiconductor CrSBr

The advent of two-dimensional (2D) magnets offers unprecedented control over electrons and spins. A key factor in determining exchange coupling and magnetic order is symmetry. Here, we apply second harmonic generation (SHG) to probe a 2D magnetic semiconductor CrSBr. We find that monolayers are ferromagnetically ordered below 146 K, an observation enabled by the discovery of a large magnetic dipole SHG effect in the centrosymmetric structure. In multilayers, the ferromagnetic monolayers are coupled antiferromagnetically, and in contrast to other 2D magnets, the Néel temperature of CrSBr increases with decreasing layer number. We identify magnetic dipole and magnetic toroidal moments as order parameters of the ferromagnetic monolayer and antiferromagnetic bilayer, respectively. These findings establish CrSBr as an exciting 2D magnetic semiconductor and extend the SHG probe of magnetic symmetry to the monolayer limit, opening the door to exploring the applications of magnetic-electronic coupling and the magnetic toroidal moment.

π-Conjugated Trigonal Planar [C(NH2)3]+ Cationic Group: A Superior Functional Unit for Ultraviolet Nonlinear Optical Materials.

Employing π-conjugated anionic groups in molecular construction has been proven to be an effective strategy to find superior ultraviolet (UV) nonlinear optical (NLO) crystals over the decades. Herein, unlike the traditional π-conjugated anionic groups, we identify that a π-conjugated cationic group, viz., [C(NH2)3]+, is also an excellent UV NLO-active functional group in theory. Furthermore, we identify a [C(NH2)3]+-containing compound, C(NH2)3ClO4, as a promising UV NLO candidate due to its short UV cutoff edge (200 nm), remarkable second-harmonic generation effect (∼3 × KDP), and moderate birefringence of 0.076@1064 nm. Additionally, C(NH2)3ClO4 has excellent ferroelectric properties and reversal of domains, which also enables it to produce ultraviolet coherent light as short as 200 nm by a quasi-phase matching technique with a periodically poling method. Our study may provide not only a promising UV NLO crystal but also a new π-conjugated functional unit, [C(NH2)3]+, which will open a path to finding new classes of high-performance UV NLO crystals.

Structural Modulation from Cu3PS4 to Cu5Zn0.5P2S8: Single-Site Aliovalent-Substitution-Driven Second-Harmonic-Generation Enhancement.

Diamond-like (DL) chalcophosphates, which possess the merits of impressive second-harmonic-generation (SHG) responses, strong laser-induced damage thresholds, and low melting points, are highly desirable for IR nonlinear-optical (NLO) applications. Herein, a new quaternary DL chalcophosphate, Cu5Zn0.5P2S8, is successfully discovered, taking known Cu3PS4 as the template via a single-site aliovalent-substitution strategy. It crystallizes in the orthorhombic system with noncentrosymmetric space group Pmn21, and the 3D DL structure is built by corner-shared [(Cu/Zn)S4], [CuS4], and [PS4] tetrahedra. Compared with its parent Cu3PS4, Cu5Zn0.5P2S8 exhibits a good phase-matching capability and a sharply enhanced SHG effect (10Cu3PS4) benefiting from partial Zn substitution. Moreover, the structure-performance relationships have been illustrated by means of theoretical investigations. Such an aliovalent-substitution strategy based on known DL semiconductors might be widely applied for the discovery of high-performance IR NLO crystals.

New Solids in As-O-Mo, As(P)-O-Mo(W) and As(P)-O-Nb(W) Systems That Exhibit Nonlinear Optical Properties.

Interactions between well-mixed fine powders of As2O3, P2O5, MoO3, WO3 and Nb2O5 at different stoichiometry in quartz ampoules under vacuum at ~1000 °C in the presence of metallic molybdenum (or niobium), over several weeks, led to shiny dichroic crystalline materials being formed in cooler parts of the reaction vessel. An addition of small quantities of metals-Mo or Nb-was made with the aim of partially reducing their highly oxidized Mo(VI), W(VI) or Nb(V) species to corresponding Mo(V), W(V) and Nb(IV) centers, in order to form mixed valence solids. Sublimed crystals of four new compounds were investigated using a variety of techniques, with prime emphasis on the X-ray analysis, followed by spectroscopy (diffusion reflectance, IR, Raman and EPR), second harmonic generation (SHG), thermal analysis under N2 and air atmosphere, and single crystals electrical conductivity studies. The results evidenced the formation of new complex solids of previously unknown compositions and structures. Three out of four compounds crystallized in non-centrosymmetric space groups and represent layered 2D polymeric puckered structures that being stacked on each other form 3D lattices. All new solids exhibit strong second-harmonic-generation (SHG effect; based on YAG 1064 nm tests with detection of 532 nm photons), and a rare photosalient effect when crystals physically move in the laser beam. Single crystals’ electrical conductivity of the four new synthesized compounds was measured, and the results showed their semiconductor behavior. Values of band gaps of these new solids were determined using diffusion reflectance spectroscopy in the visible region. Aspects of new solids’ practical usefulness are discussed.

Na6 MQ4 (M=Zn, Cd; Q=S, Se): Promising New Ternary Infrared Nonlinear Optical Materials.

Four sodium-based ternary IR nonlinear optical (NLO) materials, Na6 MQ4 (M=Zn, Cd; Q=S, Se), were prepared through a high-temperature flux method. The crystal structure of the compounds is built up of isolated [MQ4 ] tetrahedra and a 3D framework formed by the NaQn (n=4, 5) units. The two selenides, Na6 MSe4 (M=Zn, Cd), as promising IR NLO materials, show moderate second-harmonic generation (SHG) responses (0.9 and 0.5×AgGaS2 ) with good phase-matching behavior, as well as high laser damage thresholds (2 and 1.9×AgGaS2 ). The two sulfides, Na6 MS4 (M=Zn, Cd), exhibit higher laser damage thresholds (13 and 4×AgGaS2 ), but smaller SHG responses (0.3 and 0.2×AgGaS2 ). Theoretical calculations and statistical analyses indicate that the SHG effect and band gap in the compounds originate mainly from the distorted NaQ4 NLO-active units with a short Na-S bond length, which provides a new insight into the design of novel IR NLO materials.

Spatial coherence effects in second-harmonic generation of scalar light fields

We consider the spectral spatial coherence characteristics of scalar light fields in second-harmonic generation in an optically non-linear medium. Specifically, we take the fundamental-frequency (incident) field to be a Gaussian Schell-model (GSM) beam with variable peak spectral density and different coherence properties. We show that with increasing intensity the overall degree of coherence of both the fundamental and the second-harmonic field in general decreases on passage through the non-linear medium. In addition, the spectral density distributions and the two-point degree of coherence may, for both beams, deviate significantly from those of the GSM, especially at high intensities. Propagation in the non-linear medium is numerically analyzed with the Runge–Kutta and the beam-propagation methods, of which the latter is found to be considerably faster. The results of this work provide means to synthesize, via non-linear material interaction, random optical beams with desired coherence characteristics.

Nonlinear Optical Oxythiophosphate Approaching the Good Balance with Wide Ultraviolet Transparency, Strong Second Harmonic Effect, and Large Birefringence.

Ultraviolet (UV) transparency, second harmonic effect and optical birefringence are three vital but mutually restrictive factors in the application of UV nonlinear optical (NLO) materials. It is difficult for traditional phosphates to achieve a good balance among these factors. In this communication, we propose that the structural evolution of the NLO motif from traditional phosphate to oxythiophosphate would enhance the birefringence and second harmonic generation (SHG) effect while maintaining wide UV transparency, which is confirmed by first-principles calculations and preliminary experimental measurements. Following this strategy, we predict that, compared with the well-known NLO phosphate KH2 PO4 , the oxythiophosphate KH2 PO3 S exhibits better balance for the UV NLO performance, including wide UV transparency (UV cutoff down to 203 nm), strong SHG effect (ca. 0.9 pm V-1 ), and large birefringence (ca. 0.1 at 1 μm) with short phase-matching SHG output wavelength (≈214 nm).

Nonlinear Optical Oxythiophosphate Approaching the Good Balance with Wide Ultraviolet Transparency, Strong Second Harmonic Effect, and Large Birefringence

AbstractUltraviolet (UV) transparency, second harmonic effect and optical birefringence are three vital but mutually restrictive factors in the application of UV nonlinear optical (NLO) materials. It is difficult for traditional phosphates to achieve a good balance among these factors. In this communication, we propose that the structural evolution of the NLO motif from traditional phosphate to oxythiophosphate would enhance the birefringence and second harmonic generation (SHG) effect while maintaining wide UV transparency, which is confirmed by first‐principles calculations and preliminary experimental measurements. Following this strategy, we predict that, compared with the well‐known NLO phosphate KH2PO4, the oxythiophosphate KH2PO3S exhibits better balance for the UV NLO performance, including wide UV transparency (UV cutoff down to 203 nm), strong SHG effect (ca. 0.9 pm V−1), and large birefringence (ca. 0.1 at 1 μm) with short phase‐matching SHG output wavelength (≈214 nm).

Characterization of inner layer thickness change of a composite circular tube using nonlinear circumferential guided wave: A feasibility study* *Project supported by the National Natural Science Foundation of China (Grant Nos. 12074050, 11834008, and 11704410).

The feasibility of using the nonlinear effect of primary circumferential guided wave (CGW) propagation for characterizing the change of inner layer thickness of a composite circular tube (CCT) has been investigated. An appropriate mode pair of the fundamental and double-frequency CGWs (DFCGWs) has been selected to enable the second harmonics of primary wave mode in the given CCT to accumulate along the circumferential direction. When changes in the inner layer thickness (described as the equivalent inner layer thickness) take place, the corresponding nonlinear CGW measurements are conducted. It is found that there is a direct correlation between change of equivalent inner layer thickness of the CCT and the relative acoustic nonlinearity parameter (Δβ) measured with CGWs propagating through one full circumference, and that the effect of second-harmonic generation (SHG) is very sensitive to change in the inner layer thickness. The experimental result obtained demonstrates the feasibility for quantitatively assessing the change of equivalent inner layer thickness in CCTs using the effect of SHG by primary CGW propagation.

A2(TeO)P2O7 (A = K, Rb, Cs): Three new tellurite-pyrophosphates with large birefringence

Three new tellurite-pyrophosphates K2(TeO)P2O7, Rb2(TeO)P2O7, and Cs2(TeO)P2O7 have been synthesized through high-temperature solution method and their structures have been determined on single crystals. The three title compounds exhibit two-dimensional layered structures and composed of two different [TeO5] square pyramids and [P2O7] units. The compounds show quite large birefringences of Δn = 0.070, 0.090 and 0.060 for K2(TeO)P2O7, Rb2(TeO)P2O7 and Cs2(TeO)P2O7, respectively, as indicated by first-principles theoretical calculations based on density functional theory (DFT). In addition, K2(TeO)P2O7 and Rb2(TeO)P2O7 show a weak second-harmonic generation (SHG) effects, with strengths about 0.1 × KDP. The deformation charge density indicate that the stereo-active lone-pair electrons on the Te(IV) atoms play an important role for the optical properties. Furthermore, the title compounds show noticeable thermal stabilities up to 602 °C, 609 °C and 568 °C, respectively.

Sb6 O7 (SO4 )2 : A Promising Ultraviolet Nonlinear Optical Material with an Enhanced Second-Harmonic-Generation Response Activated by SbIII Lone-Pair Stereoactivity.

The stereochemical activity of lone pairs (SCALP) in a cation favors the formation of acentric materials and can enhance the second-harmonic-generation (SHG) response and/or the birefringence. By introducing functional SbIII into sulfates, an anhydrous sulfate of Sb6 O7 (SO4 )2 (1) is explored. Sb3+ cations are in seesaw configurations and in-phase aligned in a 3D asymmetric dense structure. Compound 1 exhibits an enhanced phase-matching SHG response, a moderate birefringence, a wide transparency window, and considerable environmental stabilities, which result in it being a promising UV nonlinear optical (NLO) material. Theoretical studies reveal that the stereoactive lone pairs on Sb3+ cations make the predominant contribution to the SHG effect. This work will attract more interest from scientists for research into SCALP-cation-based NLO materials.

Effect of inhomogeneous magnetization in optical second harmonic generation from layered nanostructures.

Magnetic nanostructures reveal unique interface induced properties that differ from those of bulk materials, thus magnetization distributions in interface regions are of high interest. Meanwhile, direct measurement of magnetization distribution in layered nanostructures is a complicated task. Here we study magnetic field induced effects in optical second harmonic generation (SHG) in three-layer ferromagnetic / heavy metals nano films. For a certain experimental geometry, which excludes the appearance of magnetooptical effects for homogeneously magnetized structures, magnetization induced SHG intensity variation is observed. Symmetry analysis of the SHG intensity dependencies on external magnetic field shows that the nonlinear source terms proportional to the out-of-plane gradient component of magnetization govern the observed effect.

Магнитоиндуцированный нелинейно-оптический отклик плeнок на основе нанослоeв тяжeлых и ферромагнитного металлов

We present the results of the experimental studies of magnetization-induced effects in optical second harmonic generation (SHG) from three-layer W/Co/Pt films with different thicknesses of the cobalt layer (2-10 nm) influenced by longitudinal dc magnetic field. The intensity effect in the p-polarized second harmonic that is odd in magnetization was observed, which reveals an increase in the corresponding magnetic contrast of the SHG intensity with an increase of the cobalt layer thickness from 2 nm to 10 nm. A phenomenological description of the observed effects was carried out when considering the bulk nonlinear polarization at the SHG frequency, which is proportional to the magnetization gradient in the direction normal to the structure, as well as the interface contribution, which is proportional to the square of the magnetization of the films.

In-situ modal inspection based on transverse second harmonic generation in single CdS nanobelt

Based on the transverse second-harmonic generation (TSHG) effect, we demonstrate a method for in-situ modal inspection of nonlinear micro/nanowaveguides. Pumping lights are equally split and coupled into two ends of a single CdS nanobelt (NB). As pumping light counter-propagates along the NB, transverse second-harmonic (TSH) interference patterns are observed. The influence of multimode interaction on the TSHG effect is discussed in detail. Using fast Fourier transform, TSH interference patterns are analyzed, indicating the existence of at least four modes inside the NB. Experimental beat lengths are found to be in agreement with calculated results.

Significant second-harmonic generation and bulk photovoltaic effect in trigonal selenium and tellurium chains.

One-dimensional (1D) selenium and tellurium crystalize in helical chainlike structures and thus exhibit fascinating properties. By performing first-principles calculations, we have researched the linear and nonlinear optical (NLO) properties of 1D Se and Te, and find that both systems exhibit pronounced NLO responses. In particular, 1D Se is found to possess a large second-harmonic generation coefficient with the χ value being up to 7 times larger than that of GaN, and is even several times larger than that of the bulk counterpart. On the other hand, 1D Te also produces significant NLO susceptibility χ which exceeds that of bulk GaN by 5 times. Furthermore, 1D Te is shown to possess a prominent linear electro-optic coefficient rxxx(0). In particular, the Te chain exhibits a large shift current response and the maximum is twice as large as the maximal photovoltaic responses obtained from BaTiO3. Therefore, 1D Se and Te may find potential applications in solar energy conversion, electro-optical switches, and so on. Finally, the much stronger NLO effects of 1D Se and Te are attributed to their one-dimensional structures with high anisotropy, strong covalent bonding and lone-pair electrons. These findings will contribute further to experimental studies and the search for excellent materials with large NLO effects.

Chiral Switchable Low-Dimensional Perovskite Ferroelectrics.

Low-dimensional hybrid organic-inorganic perovskites (HOIPs) possess more localized electronic states and narrower conduction and valence bands to promote self-trapping of excitons and stronger exciton emission; therefore, they are widely used as building blocks for various applications in the fields of optoelectronics, photovoltaics, light-emitting diodes, luminescence, fluorescence, and so forth. Despite the past decades of intensive study, the discovered low-dimensional chiral HOIPs are rare as of the 1D chiral HOIP single crystals reported in 2003, as well as the low-dimensional chiral HOIP ferroelectrics are particularly scarce since the first chiral two-dimensional (2D) and/or one-dimensional (1D) HOIP ferroelectrics reported. Herein, two new low-dimensional HOIPs with the same conformational formula [R-MPA]2CdCl4 (R-MPA+ = (R)-(-)-1-methyl-3-phenylpropylamine) were successfully synthetized by means of regulating the stoichiometric proportion of R-MPA and CdCl2 in two ways of 1:1 (1) and 2:1 (2). By combining single-crystal X-ray diffraction, circular dichroism (CD) spectroscopy, differential scanning calorimetry, temperature-dependent dielectric constant, temperature-dependent second-harmonic generation (SHG) effect, polarization-dependent SHG response, and P-E hysteresis loop, we reveal that 1 is a 1D nonchiral molecular ferroelectric and 2 is the first zero-dimensional (0D) chiral ferroelectric with distinct CD signals; meanwhile, 2 exhibits increased properties of high-Tc, large dielectric constant, SHG isotropy, and ferroelectricity than that of 1. These results not only shed light on the high tunability of the low-dimensional HOIP ferroelectrics but also open up an avenue to explore multifunctional chiral ferroelectrics.