Effective Nonlinear Optical Coefficient Research Articles

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand.

Chiral hybrid organic-inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second-order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self-trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second-order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1-ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V-1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second-order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand

AbstractChiral hybrid organic–inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second‐order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self‐trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second‐order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1‐ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V−1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second‐order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

Chiral Hybrid Germanium(II) Halide with Strong Nonlinear Chiroptical Properties.

Due to the pronounced anisotropic response to circularly polarized light, chiral hybrid organic-inorganic metal halides have been regarded as promising candidates for the application in nonlinear chiroptics, especially for the second-harmonic generation circular dichroism (SHG-CD) effect. However, designing novel lead-free chiral hybrid metal halides with large anisotropy factors and high laser-induced damage thresholds (LDT) of SHG-CD remains challenging. Herein, we develop the first chiral hybrid germanium halide, (R/S-NEA)3 Ge2 I7 ⋅H2 O (R/S-NGI), and systematically investigated its linear and nonlinear chiroptical properties. S-NGI and R-NGI exhibit large anisotropy factors (gSHG-CD ) of 0.45 and 0.48, respectively, along with a high LDT of 38.46 GW/cm2 ; these anisotropy factors were the highest values among the reported lead-free chiral hybrid metal halides. Moreover, the effective second-order nonlinear optical coefficient of S-NGI could reach up to 0.86 pm/V, which was 2.9 times higher than that of commercial Y-cut quartz. Our findings facilitate a new avenue toward lead-free chiral hybrid metal halides, and their implementation in nonlinear chiroptical applications.

The Kurtz-Perry Powder Technique Revisited: A Study of the Effect of Reference Selection on Powder Second-Harmonic Generation Response.

The accurate evaluation of nonlinear optical (NLO) coefficient, the main parameter affecting light conversion efficiency, plays a crucial role in the development of NLO materials. The Kurtz-Perry powder technique can evaluate second-harmonic generation (SHG) intensity in pristine powder form, saving a significant amount of time and energy in the preliminary screening of materials. However, the Kurtz-Perry method has recently been subject to some controversy due to the limitations of the Kurtz-Perry theory and the oversimplified experimental operation. Therefore, it is very meaningful to revisit and develop the Kurtz-Perry technique. In this work, on the basis of introducing the light scattering effect into the original Kurtz-Perry theory, the theoretical expression of second-harmonic generation intensity with respect to band gap and refractive index are analyzed. In addition, the reference-dependent SHG measurements were carried out on polycrystalline LiB3O5 (LBO), AgGaQ2 (Q = S, Se), BaGa4Q7 (Q = S, Se), and ZnGeP2 (ZGP), and the results of SHG response emphasize the importance of using appropriate references to the Kurtz-Perry method. In order to obtain reliable values of nonlinear coefficients, two criteria for selecting a reference compound were proposed: (1) it should possess a band gap close to that of the sample to be measured and (2) it should possess a refractive index close to that of the sample to be measured. This work might shed light on improvements in accuracy that can be made for effective NLO coefficients obtained using the Kurtz-Perry method.

Investigations on the Synthesis, Crystal Structure, Linear- and Nonlinear-Optical Properties of the Zinc Germanate Rb2ZnGe2O6.

A quaternary nonlinear-optical (NLO) zinc germanate, Rb2ZnGe2O6 (RZGO), was prepared from its stoichiometric melts, which belongs to the noncentrosymmetric space group C2221 (No. 20) and features a 3D framework formed by GeO4 tetrahedra, ZnO4 tetrahedra, and Rb+ cations filling into the void space. RZGO displays a good phase-matchable second-harmonic-generation (SHG) intensity similar to that of KH2PO4. The UV cutoff edge for RZGO was found to be approximately 0.37 μm with a broad optical band gap of 3.22 eV, indicating a large laser-induced damage threshold. Further characterizations suggested that this compound displays an excellent thermal stability (1014 °C) and melts congruently. Besides, theoretical analysis was also implemented to investigate the electronic structures and effective NLO coefficients as well as the origin of the observed SHG response for RZGO. The present study can enrich the diversity of acentric germanate structures and pave the way for the synthesis of new NLO compounds.

Recalibration of the nonlinear optical coefficients of BaGa4Se7 crystal using second-harmonic-generation method

Nonlinear optical coefficients are important parameters for nonlinear optical crystals. Based on the previously disclosed BaGa4Se7 crystal coefficients, the optimum phase-matching direction with the largest effective nonlinear optical coefficient appears in the second octant (90∘<θ<180∘, 0∘<ϕ<90∘). In this study, a recalibration was performed using the second-harmonic-generation method, and it was confirmed that the optimum phase-matching direction of the BaGa4Se7 crystal is located in its XZ principal plane (ϕ=0∘). Therefore, this study will serve as a good reference for future applications of this excellent crystal.

Determination of nonlinear optical coefficients and efficient type-II second-harmonic-generation for TmCOB crystals

By Maker Fringe (MF) method, all of the nonlinear optical (NLO) coefficients including d11, d12, d13, d31, d32 and d33 of TmCa4O(BO3)3 (TmCOB) crystals were measured to be 0.19, 0.26, −0.61, −0.30, 1.66 and −1.25 pm/V, respectively. Based on which, the second-harmonic-generation (SHG) phase-matching (PM) characteristics of TmCOB crystals in two-dimensional principal planes and three-dimensional non-principal planes (space) were theoretically calculated, such as PM tuning curves, distribution of effective NLO coefficient (deff), beam walk-off angle, and angular acceptance bandwidth. At the fundamental waves of 1064 nm, the type-II optimal PM angles with largest deff for principal planes and space were decided to be (52.8°, 90.0°) (deff = 0.24 pm/V) and (118.0°, 77.2°) (deff = 0.35 pm/V), respectively. The type-II SHG of TmCOB crystals was found to present large acceptance angle bandwidth (~2–12 times of type-I values) and small beam walk-off angle (~0.2–0.4 times of type-I values). At a high input energy of 6.89 mJ (35 ps, 10 Hz, 1064 nm), the SHG transformation efficiency for an uncoated TmCOB crystal sample cut along type-II optimal angle (118.0°, 77.2°) with 12 mm in length was measured to be 55.9%, which achieved the same conversion efficiency grade as type-I optimal PM.

Broadband Quasi-Phase-Matched Harmonic Generation in an On-Chip Monocrystalline Lithium Niobate Microdisk Resonator.

We reveal a unique broadband natural quasi-phase-matching (QPM) mechanism underlying an observation of highly efficient second- and third-order harmonic generation at multiple wavelengths in an x-cut lithium niobate (LN) microdisk resonator. For light waves in the transverse-electric mode propagating along the circumference of the microdisk, the effective nonlinear optical coefficients naturally oscillate periodically to change both the sign and magnitude, facilitating QPM without the necessity of domain engineering in the micrometer-scale LN disk. The second-harmonic and cascaded third-harmonic waves are simultaneously generated with normalized conversion efficiencies as high as 9.9%/mW and 1.05%/mW^{2}, respectively, thanks to the utilization of the highest nonlinear coefficient d_{33} of LN. The high efficiency achieved with the microdisk of a diameter of ∼30 μm is beneficial for realizing high-density integration of nonlinear photonic devices such as wavelength convertors and entangled photon sources.

Enhanced Second Harmonic Generation from Ferroelectric HfO2-Based Hybrid Metasurfaces.

Integrated nonlinear metasurfaces leading to high-efficiency optical second harmonic generation (SHG) are highly desirable for optical sensing, imaging, and quantum photonic systems. Compared to traditional metal-only metasurfaces, their hybrid counterparts, where a noncentrosymmetric nonlinear photonic material is incorporated in the near-field of a metasurface, can significantly boost SHG efficiency. However, it is difficult to integrate such devices on-chip due to material incompatibilities, thickness scaling challenges, and the narrow band gaps of nonlinear optical materials. Here, we demonstrate significantly enhanced SHG in on-chip integrated metasurfaces by using nanometer thin films of ferroelectric Y:HfO2. This material has the merit of CMOS compatibility, ultraviolet transparency up to 250 nm, and significant scalability down to sub-10 nm when deposited on silicon. We observe a 20-fold magnitude enhancement of the SHG intensity from the hybrid metasurface compared to a bare ferroelectric HfO2 thin film. Moreover, a 3-fold SHG enhancement is observed from the hybrid metasurface compared to a control structure using nonferroelectric HfO2, demonstrating a major contribution to the SHG signal from ferroelectric Y:HfO2. The effective second-order nonlinear optical coefficient χ(2) of Y:HfO2 is determined to be 6.0 ± 0.5 pm/V, which is comparable to other complex nonlinear photonic oxide materials. Our work provides a general pathway to build an efficient on-chip nanophotonic nonlinear light source for SHG using ferroelectric HfO2 thin films.

Electronic structure and optical properties of phosphate bis-guanidinoacetate crystal containing guanidine phosphate interaction

L-arginine phosphate monohydrate (LAP) crystal is an excellent nonlinear optical material, its effective nonlinear optical coefficient is about 2−3.5 times that of potassium dideuterium phosphate (KDP) crystal, and its conversion efficiency can achieve up to 90%. The deuterated crystal of LAP has a very high laser damage threshold. Thus, once it was considered as a preferred material to replace KDP crystal for laser inertial confinement fusion and other fields. In addition, the LAP crystal has a much higher stimulated Brillouin scattering (SBS) reflectivity than quartz crystal and also has a lower SBS threshold. Moreover, it exhibits a special reversible phase-change in the variable temperature process, and shows an ultra-long spin-lattice relaxation time at solid-state NMR. In a word, the LAP crystal has shown its uniqueness under the action of energy such as light, heat and magnetic field. However, for these special phenomena, there is no reasonable explanation. Phosphate arginine is responsible for the biological energy storage and transfer in invertebrates as an important phosphorus source, which has a similar chemical composition to that of LAP crystal. The special electrostatic or hydrogen bonding interaction between guanidine and phosphate plays an important role in protein molecule interaction and their biochemical functions. Moreover, the conformational transitions of L-arginine molecule in phosphoric acid solution at different energies have been reported, and the fluorescence emission of L-arginine molecule aggregates can be changed by the interaction between phosphoate and guanidine group. The interaction between phosphoate and guanidine group in crystal structure is also studied as a model of biomolecular interaction. In order to further study the mechanism of interaction between phosphoate and guanidine group and the crystal macroscopic properties, phosphate bis-guanidinoacetate (PBGA) crystal containing the similar phosphoate and guanidine groups has been synthesized and reported. In this paper, the geometry parameters, band structure, electronic density of states, and optical properties of PBGA crystal are investigated by first-principles based the density functional theory. The energy gap of PBGA crystal is 4.77 eV, much smaller than 5.96 eV of KDP crystal. Therefore, the photon transition becomes easier and the corresponding photon absorption is relatively large in PBGA crystal. The top states of crystal valence band are mainly composed of the N-2p of guanidine and the O-2p of carboxyl and phosphate groups. There exists the electron interaction among guanidine, carboxyl and phosphate groups. The optical properties of PBGA crystal are similar in the [100] and [010] orientation, whose linear optical properties are better than those of [001] when the incident photon energy is less than 10 eV. The strong energy loss peak at 9.46 eV in the [001] orientation is due to the electronic transition of N-2p on guanidine group in the valence band, and its distribution is narrow. Thus the optical properties of [001] orientation are limited. The present research establishes a good foundation for further understanding and studying the intergroup interactions and optical properties in PBGA crystal.

β-BaB2O4 with special cut-angle applied to single crystal cascaded third-harmonic generation**Project supported by the Natural Science Foundation of Shandong Province, China (Grant Nos. ZR2017MF031 and ZR2018BF029).

High-efficiency single crystal cascaded third-harmonic generation (THG) was realized in β-BaB2O4 (BBO) material with special cut-angle. By analyzing effective nonlinear optical coefficient (deff) of the cascaded THG process, which was composed by type-II frequency doubling and type-I sum-frequency, the optimum phase matching (PM) direction in BBO crystal was determined to be (θ = 32.1°, ϕ = 11°). With an optimized 9-mm long sample which was processed along this direction, the highest cascaded THG conversion efficiency reached 42.3%, which is much superior to the similar components reported previously, including ADP, KDP, and GdxY1−xCOB crystals.

Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions

Multiple optical harmonic generation-the multiplication of photon energy as a result of nonlinear interaction between light and matter-is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency,andthe generation of frequency combs.Owing to the unique electronic band structure of graphene, which features massless Dirac fermions1-3, it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies4-6. However, these predictionshave yet to be confirmed experimentally under technologically relevant operation conditions. Here we report the generation of terahertz harmonics up to the seventh order in single-layer graphene at room temperature and under ambient conditions, driven by terahertz fields of only tens of kilovolts per centimetre, and with field conversion efficiencies in excess of 10-3, 10-4 and 10-5 for the third, fifth and seventh terahertz harmonics, respectively. These conversion efficienciesare remarkably high, given that the electromagnetic interaction occurs in a single atomic layer. The key to such extremely efficient generation of terahertz highharmonics in graphene is the collective thermal response of its background Dirac electrons to the driving terahertz fields. The terahertz harmonics, generated via hot Dirac fermion dynamics, were observed directly in the time domain as electromagnetic field oscillations at these newlysynthesized higher frequencies. The effective nonlinear optical coefficients of graphene for the third, fifth and seventh harmonics exceed the respective nonlinear coefficients of typical solids by 7-18 orders of magnitude7-9. Our results provide a direct pathway to highly efficient terahertz frequencysynthesis using the present generation of graphene electronics, which operate at much lower fundamental frequencies of only a few hundreds of gigahertz.

Anisotropy of nonlinear optical properties in monoclinic SmxY1-xCa4O(BO3)3 crystals

The angular noncritical phase matching (A-NCPM) second-harmonic-generation (SHG) properties of SmxY1-xCOB (x = 0, 0.44, 0.55 and 1) nonlinear optical (NLO) crystals were investigated. The type-I A-NCPM SHG wavelengths along the Y and Z axes were found to be tunable in the ranges of 725–900 nm and 826–1116 nm, respectively, by adjusting the ratio of the rare-earth elements Sm and Y in SmxY1-xCOB crystals. Taking advantages of the fundamental laser pulses generated from a mode-locked Ti:Sapphire laser and a long Y-cut Sm0.44Y0.55COB crystal sample, the single-pass SHG average output power of ∼436 mW at 397 nm was obtained with a conversion efficiency of 41.5%. In addition, the SHG phase-matching (PM) curve and the effective NLO coefficients (deff) of SmCOB crystal for 1064 nm laser were studied, where the optimal type-I SHG PM direction in principal planes was determined to exist in the XY principal plane i.e. (90°, 52.4°), rather than the ZX plane i.e. (θº, 180°), which was different with those of TmCOB (32.5°, 180°), YCOB (30.7°, 180°), TbCOB (22.6°, 180°), GdCOB (19.7°, 180°) and LaCOB (15.1°, 180°) crystals in the same crystal family.

Pressure-induced transformations of multiferroic relaxor PbFe0.5Nb0.5O3

The effects of hydrostatic pressure at ambient temperature on the structural and dielectric properties of PbFe0.5Nb0.5O3 (PFN) were investigated using second harmonic generation (SHG) and powder x-ray diffraction measurements to 31 GPa. The results demonstrate that PFN undergoes a pressure-induced structural transition from the R3m ferroelectric to the R3¯m paraelectric phase. SHG measurements showed a continuous decrease in the signal with pressure and complete disappearance at 7.1 GPa. Effective nonlinear optical coefficients were determined from the SHG data, and their pressure behavior was used to infer the nature of the transition. The loss of the SHG signal is accompanied by drastic changes in line widths of Bragg reflections, but no discontinuous change in volume was observed. The pressure-volume data were fit to various equations of state, and a bulk modulus K0 = 136 (±2) GPa, bulk modulus pressure derivative K0′ = 4.0 (±0.2), and initial volume V0 = 64.5 (±0.1) Å3 were obtained.

Cascaded third-harmonic-generation converter based on a single ADP crystal**Project supported by the National Natural Science Foundation of China (Grant Nos. 61178060, 51202129, and 91022034), the Independent Innovation Foundation of Shandong University, China (Grant No. 2011GN056), and the Natural Science Foundation for Distinguished Young Scholar of Shandong Province, China (Grant No. 2012JQ18).

Based on a specially designed optical structure, an efficient cascaded third-harmonic-generation (THG) output of a 1064-nm, pico-seconds pulse laser is successively realized by using an NH4H2PO4 (ADP) crystal that acts as the second-harmonic-generation component and sum-frequency-generation component. The maximum THG output is 1.61 mJ, and the highest conversion efficiency from 1064 nm to 355 nm reaches 35%, which are obviously superior to the results obtained using a KDP crystal under the same circumstance. The further phase-matching analysis indicates that this THG configuration of ADP crystal can be applied to various fundamental wavelengths in a range of 1 μm–1.1 μm. Compared with the previously reported KDP THG converter, which is based on a similar principle, the present ADP THG converter is favorable for large-energy, high-efficiency operation because of the larger effective nonlinear optical coefficient deff and higher laser damage threshold.

Angular non-critical phase-matching second-harmonic-generation characteristics of RECOB (RE = Tm, Y, Gd, Sm, Nd and La) crystals.

For the first time, the angular non-critical phase-matching (A-NCPM) second-harmonic-generation (SHG) characteristics of a family of monoclinic oxoborate crystals, RECa4O(BO3)3 (RECOB, RE = Tm, Y, Gd, Sm, Nd and La), were comprehensively investigated. For all of the realizable A-NCPM SHG styles, the feature parameters including PM wavelength, angular, wavelength and temperature acceptance bandwidths, have been derived from the theory and verified by the experiments. We discovered that the closer the ion radius between RE3+ and Ca2+, the smaller the birefringence, and the better the A-NCPM SHG properties. As a result, for the Type-I SHG on Y-axis which has the largest effective nonlinear optical coefficient (deff) among the three realizable A-NCPM styles, NdCOB crystal presents the longest PM wavelength (927 nm), the largest angular acceptance bandwidth (Δθ⋅l1/2 = 84.3 mrad·cm1/2, Δϕ⋅l1/2 = 58.8 mrad·cm1/2), and the broadest wavelength acceptance bandwidth (8.7 nm). This discovery will contribute to the design of new NCPM materials, at the same time the parameter formula will be helpful for the theoretical prediction of NCPM performance.

The optimal phase-matching angles for third harmonic generation of low symmetry nonlinear optical crystal La2CaB10O19

The signs of nonlinear optical (NLO) coefficients of La2CaB10O19 (LCB) crystal were determined by Maker fringe method and phase-matching method. Compared with d22, the signs of d21 and d23 are found by Maker fringe method to be negative and positive, respectively, and the sign of d14 is found to be positive by phase-matching method. Phase-matching angles for third harmonic generation (THG) in LCB crystal were calculated and verified by experiments. The distribution of effective NLO coefficient (deff) and the optimal phase-matching angles for THG with LCB crystal are determined for the first time, with optimal phase-matching angles of (48.7°, 72.5°) and (65.3°, 37.5°) for type-I and type-II THG, respectively. The results show that LCB is an excellent NLO crystal for THG application.

Phase-matching condition of four-wave mixing in cylindrical nonlinear optical media

Propagation of cylindrical waves in homogeneous and inhomogeneous nonlinear optical media has been investigated theoretically. The coupled-wave equations for a homogeneous and inhomogeneous nonlinear media have been investigated numerically. For homogeneous media, calculations show that phase-matching condition is not fulfilled and efficiency of output wave is very low, but for inhomogeneous media, phase-matching condition is nearly fulfilled and the generated wave amplitude of the four-wave mixing (FWM) is much larger than the homogeneous media. The effect of effective nonlinear optical coefficient [Formula: see text] on the generated wave amplitude for inhomogeneous media has been investigated.

Effect of size and indium-composition on linear and nonlinear optical absorption of InGaN/GaN lens-shaped quantum dot**Project supported by the Ministry of Higher Education and Scientific Research in Iraq, Ibnu Sina Institute and Physics Department of Universiti Teknologi Malaysia (UTM RUG Vote No. 06-H14).

Based on the Schrödinger equation for envelope function in the effective mass approximation, linear and nonlinear optical absorption coefficients in a multi-subband lens quantum dot are investigated. The effects of quantum dot size on the interband and intraband transitions energy are also analyzed. The finite element method is used to calculate the eigenvalues and eigenfunctions. Strain and In-mole-fraction effects are also studied, and the results reveal that with the decrease of the In-mole fraction, the amplitudes of linear and nonlinear absorption coefficients increase. The present computed results show that the absorption coefficients of transitions between the first excited states are stronger than those of the ground states. In addition, it has been found that the quantum dot size affects the amplitudes and peak positions of linear and nonlinear absorption coefficients while the incident optical intensity strongly affects the nonlinear absorption coefficients.

Second order nonlinear optical properties of Cs_2TeW_3O_12 single crystal

We report the linear and nonlinear optical (NLO) properties of a polar crystal Cs2TeW3O12 (CTW). The second-order NLO coefficients were determined by Maker Fringe (MF) techniques for the first time. CTW belong to hexagonal system with point group P63, the two independent NLO coefficients of d32 and d33 were determined to be 6.2 and 4.3 pm/V, respectively. CTW can realize phase-matching (PM) and in the direction of θ = 39.6° the effective NLO coefficient is 4.0 pm/V at 1064 nm. To illustrate the good NLO properties of CTW, the distortions of polyhedron structures were also discussed in detail.