Harmonic Generation Effect Research Articles

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).

A plastically bendable and polar organic crystal

An organic crystal of the polar space groupPcthat is capable of plastic bending is reported, and its high dielectric constant and strong second-order harmonic generation (SHG) effect have been demonstrated.

Generation of highly nonlinear irregular waves in a wave flume experiment: Spurious harmonics and their effect on the wave spectrum

Spurious harmonics generation can occur in wave basins when strongly nonlinear waves are generated with linear wave maker theory. For regular long crested waves, the phenomena is known to cause a beating pattern for the amplitude of the higher harmonics, while for irregular waves the effect is often thought to average out. We here report on recent experimental results for spurious harmonic generation for highly nonlinear irregular waves. Long crested waves at two depths are considered. We find a systematic distortion of the spectral shape which is non-uniform in space. The effect occurs in the linear and super harmonic region of the spectrum and is strongest at the smaller depth. We analyze the phenomenon through re-computations in a fully nonlinear wave model and the further application of a four-phase harmonic separation. We next extend the analysis to allow separation of the third- and first-order content of the first-harmonic wave field, by computation at two different amplitudes. The numerical results resembles the measured spectrum closely and the harmonic separation shows that the spectral distortion in the super-harmonic region is to large extent a property of the second-harmonic wave field. Further, the match between the first-harmonic spectrum and the original target spectrum is shown to improve strongly by the removal of the extracted third-order contribution. The observed spectral distortion in the linear frequency region can thus be explained by this third-order field.Returning to the second-order wave field, we demonstrate that the extracted second-order wave field with associated spectral distortion can be reproduced by application of the Sharma and Dean theory with inclusion of the reconstructed spurious wave field from the linear wave generation. Finally, the effect of the spurious second-order harmonic generation on the measured forces is discussed and quantified. We find that the force peak exceedance probability is less influenced by the spurious waves than the spectral shape. We link this to the smaller phase speed of the spurious waves, that makes them arrive later than the main wave packet and thus leads to a limited enhancement of the local peaks.

Propagation effects in multipass high harmonic generation from plasma surfaces

Multipass high harmonic generation from plasma surfaces is a promising technique to enhance the efficiency of the generation process. In this paper it is shown that there is an optimal distance between two targets where the efficiency is maximized, depending on the laser and plasma parameters. This can be explained by the Gouy phase shift, which leads to the relative phase between the colours being changed with propagation in free space. A simple model is used to mimic the propagation of light from one target to another and to observe this effect in 1D particle-in-cell (PIC) simulations. The results are also verified using 2D PIC simulations.

Direct Observation of Incommensurate-Commensurate Transition in Graphene-hBN Heterostructures via Optical Second Harmonic Generation.

Commensurability effects play a crucial role in the formation of electronic properties of novel layered heterostructures. The interest in these moiré superstructures has increased tremendously since the recent observation of a superconducting state (Nature 2018, 556, 43-50) and metal-insulator transition (Nature 2018, 556, 80-84) in twisted bilayer graphene. In this regard, a straightforward and efficient experimental technique for detection of the alignment of layered materials is desired. In this work, we use optical second harmonic generation, which is sensitive to the inversion symmetry breaking, to investigate the alignment of graphene/hexagonal boron nitride heterostructures. To achieve that, we activate a commensurate-incommensurate phase transition by a thermal annealing of the sample. We find that this structural change in the system can be directly observed via a strong modification of a nonlinear optical signal. Unambiguous interpretation of obtained results reveals the potential of a second harmonic generation technique for probing of structural changes in layered systems.

Syntheses, characterization and calculations of LimAnM6O15 (A=Rb, Cs; M=Si, Ge; m+n=6)

Alike phosphates, silicates and germanates exclusively containing tetrahedral basic building units (BBUs) can also exhibit ultraviolet (UV) even deep-UV transitions. They are important for the design of new UV or deep-UV nonlinear optical (NLO) materials. In this paper, four new alkali metal silicates and germanates, Li2Rb4Si6O15, Li2Cs4Si6O15, Li3Rb3Ge6O15 and Li3Cs3Ge6O15 were successfully synthesized by a high temperature solid state reaction. They obey the general formula of LimAnM6O15 (A=Rb, Cs; M=Si, Ge; m+n=6) and all exhibit the Sr2Be2B2O7 (SBBO)-like structures. More importantly, Li3Rb3Ge6O15 and Li3Cs3Ge6O15 crystallize in the noncentrosymmetric (NCS) structures and exhibit remarkable phase-matched second harmonic generation (SHG) effect, 0.8×KH2PO4 (KDP) and 1×KDP, respectively. These indicate that they are potential as UV or deep-UV NLO materials. Furthermore, their optical and NLO properties as well as thermal properties were measured. The structure-property relationships were studied by the dipole moment calculations and the first-principles calculations.

Two Rare-Earth Molecular Ferroelectrics with High Curie Temperatures, Large Spontaneous Polarization, Switchable Second Harmonic Generation Effects, and Strong Photoluminescence.

Smart multifunctional molecular ferroelectrics bearing high Curie temperatures and diverse excellent physical properties, such as second harmonic generation (SHG) responses, luminescence, and semiconductivity, among others, have significant applications but have seldom been documented. Herein, the rare-earth metals Nd and Pr are introduced into a simple molecular system (nBu4 N)3 [M(NO3 )x (SCN)y ] (nBu4 N= tetrabutyl ammonium, M=rare-earth metal, nBu=CH3 CH2 CH2 CH2 ), and two new multifunctional molecular ferroelectrics are obtained: (nBu4 N)3 [Nd(NO3 )4 (SCN)2 ] (1) and (nBu4 N)3 [Pr(NO3 )4 (SCN)2 ] (2). Their distinct heat and dielectric anomaly dependence on temperature verifies that compounds 1 and 2 experience high-temperature para-ferroelectric phase transitions at 408 and 413 K, respectively. Strikingly, both molecular ferroelectrics possess large spontaneous polarization with Ps values of 9.05 and 8.50 μC cm-2 , respectively, and are further characterized by the appearance of multiple intersecting non-180° domains and polarization switching behavior. In particular, compounds 1 and 2 show good stability with only a small decrease in SHG intensity after switching cycles, suggesting that they have great potential for application in nonlinear optical (NLO) switches. Simultaneously, the rare-earth compounds 1 and 2 present bright yellow-red and bright green fluorescence, respectively, at room temperature.

A Novel Reference Current Extraction Technique with Multi-Functional Capability for Shunt Active Filter

The exponential growth of non-linear loads results the generation of harmonics in electrical distribution system. Major Harmonic generation effects due to non-linear loads are energy loss, nuisance tripping, equipment failure/mal-operation, and transformer saturation, overheating of neutral conductors and/or capacitors, interference with communication circuits, excess reactive power burden and lowering the true power factor. Improving the current quality is the prime responsibility of the consumers whereas utility is accountable for minimizing the supply voltage distortions. Shunt active filters (SAFs) were being used for many years to mitigate the current harmonics typically up to 25th order. Mainly, time and frequency domain reference current extraction control techniques were being used for SAF. Time-domain reference current extraction techniques were proved efficient than frequency-domain techniques due to their faster response. Time-domain reference current extraction techniques such as real and reactive (p-q) and synchronous reference frame (SRF) are having their own merits and demerits. The p-q control does not perform well under distorted and unbalanced supply voltage conditions whereas SRF control is limited to balanced loads. In this paper a novel reference current extraction control strategy is proposed i.e. inverse matrix averaging pq-SRF (IMApq-SRF) control. IMApq-SRF control extract the features of both conventional p-q and SRF control, based on inverse matrix average. The mathematical modeling of the proposed IMApq-SRF control is tested with MATLAB Simulink environment and it validates that the proposed IMApq-SRF control gives the superior performance as compared with conventional p-q and SRF techniques under balanced, unbalanced and distorted supply voltage and dynamic loading conditions.

Effects of P2O7 clusters arrangement on second harmonic generation responses of pyrophosphates

Phosphates with rich structural diversities and novel physicochemical properties can be regarded as the indispensible sources of optical functional materials. Therefore, the extensive research has been performed to search the suitable nonlinear optical (NLO) materials and unveil the relationship among the crystal structures, properties and functions of phosphates system. Herein, two pyrophosphates, namely, Na4P2O7 and β-Ca2P2O7 have been obtained and characterized. Results show that the two title compounds can achieve the coexistence of deep-ultraviolet (UV) cutoff edges (<190 nm) and observable second harmonic generation (SHG) effects (0.2 × and 0.5 × KH2PO4 (KDP)). Besides, the positive roles of alkaline and alkaline earth metals with larger radii or higher ratios of M/P2O7 on SHG effects of pyrophosphates can be concluded. Theoretical analysis reveals that their NLO effects mainly originate from O-2p and P-2p orbitals near to the Fermi level. In addition, their thermal properties, infrared spectra and elemental analysis were also discussed.

First-principles Studies of Second-Order Nonlinear Optical Properties of Organic-Inorganic Hybrid Halide Perovskites

Organic-inorganic hybrid halide perovskites have ignited tremendous interests for photovoltaic applications. However, their nonlinear optical response has not been studied although many of these structures lack the centrosymmetry and exhibit ferroelectricity. In this work, we employ our developed large-scale parallel, first-principles simulation tool (ArchNLO) to explore second-order nonlinear optical properties of a typical family of organic-inorganic hybrid halide perovskites, CH3NH3MX3 (M= Ge, Sn, Pb; X=Cl, Br, I). We find that these hybrid perovskites exhibit second harmonic generation and linear electro-optic effect. The nonlinear optical effects are strongly influenced by the types and positions of cations/anions and corresponding band gaps. Particularly, the distorted cubic phase, which is essentially triclinic, of CH3NH3SnI3 shows significant second harmonic generation and electro-optic effect, which are comparable with those widely used materials, such as GaAs. These second-order optical properties of organic-inorganic hybrid halide perovskites and their low-temperature, solution-based fabrication pave the way for achieving and implementing nonlinear optical devices with low cost.

Quadratic Non-Linear Optical Properties of the poly(2,5-bis(but-2-ynyloxy) Benzoate Containing the 2-(ethyl(4-((4-nitrophenyl)buta-1,3-diynyl)phenyl)amino)ethanol) Chromophore.

Excellent quadratic non-linear optical (ONL-2) properties of the poly(2,5-bis(but-2-ynyloxy) benzoate, containing a polar diacetylene as a chromophore, were found. According with the Maker fringes method, oriented polymer films showing an order parameter of ∼0.23 can display outstanding and stable Second Harmonic Generation (SHG) effects under off-resonant conditions (SHG-532 nm). Also, the macroscopic non-linear optical (NLO)-coefficients were evaluated under the rod-like molecular approximation, obtaining: χzzz(2) and χzxx(2) in the order of 280 ± 10 and 100 ± 10 pm V−1, respectively. The mechanical and chemical properties, in addition to the large ONL-2 coefficients exhibited by this polymer, make it a promising organic material in the development of optoelectronic/photonic devices.

Nonlinear optical ASnX (A = Na, H; X = N, P) nanosheets with divalent tin lone electron pair effect by first-principles design.

We propose a series of novel ASnX (A = Na, H; X = N, P) layered structures based on first-principles modeling and simulation. These nanosheet materials are all derived from the rational design for the parent structures of NaSnX (X = N, P) with unique divalent tin lone electron pairs. First-principles calculations show that the polar divalent tin effect can lead to strong second-order nonlinear optical (NLO) and large birefringence response in bulk materials, and can also greatly enhance the second harmonic generation (SHG) effect in the corresponding two-dimensional (2D) nanostructures by hydrogen passivation or strain engineering. The correlation between structural evolution and bandgap is illustrated through detailed analysis of the electronic structure and charge transfer model. The SHG effect and its origin can be further evaluated based on the SHG-weighted density scheme, which will facilitate the NLO research of 2D materials in the nitride and phosphide systems.

Helix-constructed polar rare-earth iodate fluoride as a laser nonlinear optical multifunctional material.

The first trivalent rare-earth iodate fluoride nonlinear optical (NLO) crystal, Y(IO3)2F (YIF), was successfully designed and synthesized, featuring polarization-favorable helical chains constructed from trans-YO6F2 polyhedra and IO3 groups. It exhibited a suitable balance of a wide transparency range of 0.26-10.0 μm, high laser damage threshold (LDT) of 39.6 × AgGaS2, and moderate second harmonic generation (SHG) effect of 2 × KDP. A series of doped RE:YIF (RE = Pr, Nd, Dy, Ho, Er, Tm, and Yb) crystals were easily synthesized benefiting from the spring-shaped helix structure, which possess wide absorption and emission peaks as well as long lifetime, especially in the visible and near-infrared regions. Particularly, the remarkable fluorescence properties of Nd and Yb doped YIF crystals are comparable to and even better than those of traditional self-frequency doubling (SFD) crystals such as YAB, YCOB, and GdCOB. Thus, these RE-doped YIF crystals are promising laser SFD crystals. This work also indicated that constructing helical chains should be an effective strategy for the design of inorganic polar materials.

BaGe2Pn2 (Pn = P, As): Two Congruent-Melting Non-chalcopyrite Pnictides as Mid- and Far-Infrared Nonlinear Optical Materials Exhibiting Large Second Harmonic Generation Effects

Two ternary non-chalcopyrite pnictide-based nonlinear optical (NLO) crystals, BaGe2P2 (BGP) and BaGe2As2 (BGA) with the non-centrosymmetric tetragonal space group P42mc (NO.105), were successfully ...

Designing excellent mid-infrared nonlinear optical materials with fluorooxo-functional group of d0 transition metal oxyfluorides

Exploration of new infrared (IR) nonlinear optical (NLO) materials is still in urgency owing to the indispensable roles in optoelectronic devices, resource exploration, and long-distance laser communication. The formidable challenge is to balance the contradiction between wide band gaps and large second harmonic generation (SHG) effects in IR NLO materials. In the present work, we proposed new kinds of NLO active units, d0 transition metal fluorooxo-functional groups for designing mid-IR NLO materials. By studying a series of d0 transition metal oxyfluorides (TMOFs), the influences of fluorooxo-functional groups with different d0 configuration cations on the band gap and SHG responses were explored. The results reveal that the fluorooxo-functional groups with different d0 configuration cations can enlarge band gaps in mid-IR NLO materials. The first-principles calculations demonstrate that the nine alkali/alkaline earth metals d0 TMOFs exhibit wide band gaps (all the band gaps > 3.0 eV), large birefringence Δn (> 0.07), and two W/Mo TMOFs also exhibit large SHG responses. Moreover, by comparing with other fluorooxo-functional groups, it is found that introducing fluorine into building units is an effective way to enhance optical performance. These d0 TMOFs with superior fluorooxo-functional groups represent a new exploration family of the mid-IR region, which sheds light on the design of mid-IR NLO materials possessing large band gap.

Simulation of Nonlinear Resonance, Amplitude-Frequency, and Harmonic Generation Effects in SAW and BAW Devices.

Nonlinearly coupled sets of piezoelectric field equations in the frequency domain were derived for the nonlinear propagation of finite-amplitude waves in piezoelectric bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices. To verify their accuracy, we have embedded these sets of equations in the finite-element method (FEM) of COMSOL Multiphysics software and compared the FEM results with both the analytical and experimental results found in the published literature. The nonlinear frequency responses for both plano- and contoured-plate resonators of AT-cut quartz were investigated under various voltage drives, circuit resistances, and quality factors. The proposed equations with FEM have also been employed to study 33.3-MHz very-high-frequency (VHF) quartz resonators, showing that under different conditions of how well the third overtone mode (f3) matches the third harmonic (3f) and how the fractional frequency shift of the third overtone mode (f3) occurs as a function of the fundamental mode current. Furthermore, we have studied the nonlinear harmonic generation of an 840-MHz 128° Y-cut X-propagating (128° YX) LiNbO3 SAW resonator. The second-harmonic (H2) and third-harmonic (H3) modes were observed to occur, respectively, at two-time (2f) and three-time (3f) frequencies of fundamental frequency (f) when such resonators were driven with high power. The effects of substrate thickness, bottom surface conditions of the substrate, and different circuit connections on the H2 and H3 generations were simulated and compared with available measurements. Current proposed sets of equations are general and could be used for the study of nonlinear resonance, amplitude-frequency effect, and harmonic generation in any piezoelectric devices, provided that the necessary nonlinear material constants are known.

Quantum interference of multi-orbital effects in high-harmonic spectra from aligned carbon dioxide and nitrous oxide**Project supported by the National Natural Science Foundation of China (Grant Nos. 91750104, 11534004, 11874179, 11127403, 11474130) and the Natural Science Foundation of Jilin Province, China (Grant No. 20180101289JC).

We investigate experimentally multi-orbital effects in high-order harmonic generation (HHG) from aligned CO2 and N2O molecules by intense femtosecond laser fields with linear and elliptical polarizations. For either of the aligned molecules, a minimum in the harmonic spectrum is observed, the position of which shifts to lower-order harmonics when decreasing the intensity or increasing the ellipticity of the driving laser. This indicates that the minimum originates from the dynamic interference of different channels, of which the tunneling ionization and recombination are contributed via different molecular orbitals. The results show that both the highest occupied molecular orbital (HOMO) and low-lying HOMO-2 in CO2 (or HOMO-1 in N2O) contribute to the molecular HHG in both linearly and elliptically polarized strong laser fields. Our study would pave a way for understanding multi-electron dynamics from polyatomic molecules irradiated by strong laser fields.

The Nonlinear Drain–Source Capacitance Effect on Continuous-Mode Class-B/J Power Amplifiers

The effects of the nonlinear drain-to-source capacitance ( $C_{\mathrm {DS}}$ ), or the varactor effect, on the operation of continuous-mode class-B/J power amplifiers (PAs) are described in terms of the mathematical analysis as well as experimental validation. The nonlinear $C_{\mathrm {DS}}$ generates harmonic current when the class-B/J drain voltage is applied across it, modifying the theoretical load trajectories on the Smith chart. This is shown to significantly deviate from the continuous-mode assertion of constant power and efficiency. A novel measurement approach called voltage–pull is used to investigate the effects of the second harmonic generation by the varactor in a class-B/J PA mode. Theoretical and experimental results showed the nonlinear $C_{\mathrm {DS}}$ degrades the drain efficiency compared to class-B on one half of the class-B/J design space while improving the other half due to the energy upconversion and downconversion, respectively. This paper provides an approach for die nonlinear characteristic optimizations for transistor manufacturers.

Poly(difluorophosphazene) as the First Deep‐Ultraviolet Nonlinear Optical Polymer: A First‐Principles Prediction

AbstractA novel concept to obtain the deep‐ultraviolet (DUV) nonlinear optical (NLO) materials is proposed based on the assembling of one‐dimensional (1D) polar motifs into quasi‐1D polymer patterns. Based on the first‐principles calculations, we have successfully discovered an excellent DUV NLO polymer, i.e., poly(difluorophosphazene), with the chemical formula of (PNF2)n. Calculations reveal that PNF2 has a larger band gap, a stronger second harmonic generation effect, a larger birefringence, and a shorter phase‐matching cutoff than KBe2BO3F2. These findings not only demonstrate that the PNF2 is the first reported DUV NLO polymer, but also could open a new direction to discover novel DUV NLO materials in polymer systems.

Poly(difluorophosphazene) as the First Deep-Ultraviolet Nonlinear Optical Polymer: A First-Principles Prediction.

A novel concept to obtain the deep-ultraviolet (DUV) nonlinear optical (NLO) materials is proposed based on the assembling of one-dimensional (1D) polar motifs into quasi-1D polymer patterns. Based on the first-principles calculations, we have successfully discovered an excellent DUV NLO polymer, i.e., poly(difluorophosphazene), with the chemical formula of (PNF2 )n . Calculations reveal that PNF2 has a larger band gap, a stronger second harmonic generation effect, a larger birefringence, and a shorter phase-matching cutoff than KBe2 BO3 F2 . These findings not only demonstrate that the PNF2 is the first reported DUV NLO polymer, but also could open a new direction to discover novel DUV NLO materials in polymer systems.