Phase-matched Second-harmonic Generation Research Articles

Second-harmonic generation via double topological valley-Hall kink modes in all-dielectric photonic crystals

Nonlinear topological photonics, which explores topics common to the fields of topological phases and nonlinear optics, is expected to open up a new paradigm in topological photonics. Here, we demonstrate second-harmonic generation (SHG) via nonlinear interaction of double topological valley-Hall kink modes in all-dielectric photonic crystals (PhCs). We first show that two topological frequency bandgaps can be created around a pair of frequencies, $\omega_0$ and $2\omega_0$, by gapping out the corresponding Dirac points in two-dimensional honeycomb PhCs. Valley-Hall kink modes along a kink-type domain wall interface between two PhCs placed together in a mirror-symmetric manner are generated within the two frequency bandgaps. Importantly, through full-wave simulations and mode dispersion analysis, we demonstrate that tunable, bi-directional phase-matched SHG via nonlinear interaction of the valley-Hall kink modes inside the two bandgaps can be achieved. In particular, by using Stokes parameters associated to the magnetic part of the valley-Hall kink modes, we introduce a new concept, SHG directional dichroism, which is employed to characterize optical probes for sensing chiral molecules. Our work opens up new avenues towards topologically protected nonlinear frequency mixing and active photonic devices implemented in all-dielectric material platforms.

Second harmonic generation in gallium phosphide nano-waveguides.

We designed, fabricated and tested gallium phosphide (GaP) nano-waveguides for second harmonic generation (SHG). We demonstrate SHG in the visible range around 655 nm using modal phase matching. We observe phase matched SHG for different combinations of interacting modes by varying the widths of the waveguides and tuning the wavelength of the pump. We achieved a normalized internal SHG conversion efficiency of 0.4% W-1cm-2 for a continuous-wave pump at wavelength of 1283.5 nm, the highest reported in the literature for a GaP waveguide. We also demonstrated temperature tuning of the SHG wavelength with a slope of 0.17 nm/°C. The presented results contribute to the development of integrated photonic platforms with efficient nonlinear wave-mixing processes for classical and quantum applications.

Introduction of Li into Ag-Based Noncentrosymmetric Sulfides for High-Performance Infrared Nonlinear Optical Materials.

The understanding of the structure-performance relationship is beneficial to establish the balance between the second-harmonic generation (SHG) response and laser-induced damage threshold (LIDT) in exploring novel nonlinear optical (NLO) materials. In this work, a facile strategy of partial Li co-occupation with Ag is employed to investigate the structures and NLO performances of the four Ag-based noncentrosymmetric chalcogenides Ag2In2SiS6 (1), (Li0.79Ag1.21)In2SiS6 (2), (Li1.12Ag0.88)In2SiS6 (3), and (Li1.44Ag0.56)In2SiS6 (4). Both the SHG intensity and LIDT are improved as the Li element is introduced, among which 4 exhibits the optimal phase-matchable SHG response of ∼0.8 × AgGaS2 (AGS) and a high LIDT of 4 × AGS. These NLO activities are explained by theoretical calculation and the dipole moment analysis.

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).

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.

Backward Phase-Matched Second-Harmonic Generation from Stacked Metasurfaces.

We demonstrate phase-matched second-harmonic generation (SHG) from three-dimensional metamaterials consisting of stacked metasurfaces. To achieve phase matching, we utilize a novel mechanism based on phase engineering of the metasurfaces at the interacting wavelengths, facilitating phase-matched SHG in the unconventional backward direction. Stacking up to five metasurfaces,we obtain a phase-matched SHG signal, which scales superlinearly with the number of layers. Our results motivate further investigations to achieve higher conversion efficiencies also with more complex wave fronts.

Investigation of the second-order nonlinear optical property of Sr6Sb6S17

Metal chalcogenides are extensively studied as infrared second-order nonlinear optical (NLO) materials. Among the candidates, ones containing cations with lone-pair electrons in the structure have received great interest in view of the lone-pair electrons’ special contribution to the NLO property. Here, we revisit such a sulfide Sb6Sb6S17 and study its NLO property, which represents a novel structure-type of NLO material. It exhibits phase-matchable second-harmonic generation activity, and the powder sample shows a laser-induced damage threshold (LIDT) around 3.2 times that of AgGaS2 when both have the particle sizes of 75–110 ​μm. Its optical energy gap is measured to be 1.92 ​eV and the powder sample has a transparency range from 0.65 to 25 ​μm. The electronic structure and optical property calculations are also performed.

Linear and nonlinear optical properties, pyroelectricity and vibrational spectroscopy of polar guanidinium hydrogen phosphite, GuH2PO3, and hydrogen selenite, GuHSeO3

For the non-centrosymmetric guanidinium compounds GuH2PO3 and GuHSeO3 with polar symmetry P21 a fundamental characterization as optical materials is given, including X-ray crystal structure analysis and a vibrational spectroscopy (Raman and IR) study. In addition, for both polar crystals the pyroelectric coefficients and their temperature dependence are evaluated. Using large single crystals grown from aqueous solution, refractive indices and their dispersion in the wavelength range 435–1083 nm are determined by the prism method. From the data phase matching conditions for collinear second harmonic generation (SHG) are analysed. Both crystals allow the realization of phase matching for SHG of both, type I and type II processes. Quantification of SHG properties was performed for both powdered salts and GuH2PO3 single crystals. The measurements of the pyroelectric coefficients in the temperature range 100–340 K show pronounced pyroelectricity but no indication for ferroelectric behaviour of the two crystals.

Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br): Halides with Strong Second-Harmonic Generation Response Induced by Acetate Groups.

Nonlinear optical (NLO) crystals are the key component in solid-state lasers. New second-harmonic generation (SHG) effective halides, Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br), have been rationally synthesized by introducing acetate groups into lead halide perovskites under moderate solvothermal conditions. Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br) feature highly oriented anionic chains constructed by distorted [PbX4 O2 ] (X=I, Br) polyhedrons. Both the theoretical studies and the dipole moment calculations uncover that their SHG effects are mainly originated from the distorted [PbX4 O2 ] (X=I, Br) polyhedrons. Remarkably, Cs3 Pb2 (CH3 COO)2 I5 exhibits strong phase-matching SHG response (8×KDP) and large birefringence (0.26@ 1064 nm). Moreover, Cs3 Pb2 (CH3 COO)2 X5 (X=I, Br) also possess high laser damage threshold (LDT) which are almost 27 and 41 times that of AgGaS2 , respectively.

AMnAs3S6 (A = Cs, Rb): Phase-Matchable Infrared Nonlinear Optical Functional Motif [As3S6]3- Obtained via Surfactant-Thermal Method.

Exploration of a new nonlinear optical (NLO)-active functional motif is important in the rational design of promising infrared (IR) NLO materials. Compared with typical tetrahedral MQ4 (M = IIB, III, IV metals; Q = S, Se) motifs, MQ3 (M = As, Sb) pyramids favor high second-harmonic generation (SHG) efficiency while frequently hindering phase matching (PM) because of excessively large optical anisotropy. The surfactant-thermal method was first adopted to achieve PM in MQ3-containing systems and synthesize mixed covalent-ionic IR NLO materials. Two new thioarsenates of AMnAs3S6 (A = Cs, Rb) exhibiting strong PM SHG efficiencies comparable to commercial AGS and laser-induced damage thresholds of one order higher than AGS were obtained. The [As3S6]3- unit in their structures is an unprecedented NLO-active functional motif, which can be useful in designing new IR NLO compounds with large SHG efficiency. In addition, the surfactant-thermal method provides a new general strategy for synthesizing new IR NLO materials.

Finding Optimal Mid-Infrared Nonlinear Optical Materials in Germanates by First-Principles High-Throughput Screening and Experimental Verification.

Owing to wide infrared (IR) transparency ranges, high laser damage thresholds, and being easy to grow in open air, germanates are emerging as promising mid-infrared (mid-IR) nonlinear optical (NLO) materials. However, the germanates as NLO materials have not been investigated comprehensively and the crystals with large second harmonic generation (SHG) response have not been identified. Herein, we used the first-principles high-throughput screening pipeline for NLO materials to search for excellent NLO crystals from germanates collected in the inorganic crystal structure database. After two steps of screening, three crystals are picked out from 128 structures based on their predicted energy gaps, birefringences, and SHG coefficients. Subsequently, the three germanates are synthesized and measured. The results show that Pb3Ga2Ge4O14 and Ba2TiGe2O8 exhibit a wide energy gap (>3.1 eV) and a strong phase-matchable SHG intensity that are comparable to the benchmark AgGaS2 (0.8 and 1.2 × AgGaS2, respectively). In addition, the statistical analyses of different categories classified according to their cations show that the d0-transition metal and lone pair cations are more conducive to achieving a larger SHG response and birefringence compared to other cations in germanates. It gives a guideline for exploring new mid-IR NLO materials.

Controllable synthesis and spontaneous phase transition of photonic coordination polymer to produce a strong second-harmonic generation response

Herein, a novel photonic coordination polymer material was constructed by aggregation-induced emission luminogen (AIEgen) containing a tripyridyl moiety used as the linking ligand. It displayed a spontaneous direct centrosymmetric to noncentrosymmetric phase transition in a single crystal. The two crystals, before and after the phase transition, were both controllably synthesized and characterized by single-crystal X-ray diffraction. After being exposed to air, the centrosymmetric metastable phase (1-α) transitioned to a new stable phase with a noncentrosymmetric structure (1-β). Interestingly, the 1-β structure exhibited a strong phase-matching second-harmonic generation (SHG) response, about 4.5 times higher than that of KH2PO4 (KDP). In order to better understand the relationship between the structure and the nonlinear optical properties, the dipole moments were calculated and discussed. Remarkably, the noncentrosymmetric phase with high thermal stability for 1-β retained and improved the initial photoluminescent properties of the AIEgen ligand after the structural phase transition from 1-α, and simultaneously produced the excellent SHG property, which are beneficial for the design and construction of excellent optical materials.

Phase-matched frequency conversion in waveguides by means of transverse wavevector projections

We present a versatile technique for modal phase-matched second-harmonic generation in a waveguide, which relies on the incident electromagnetic field having transverse wavevector components on the input facet of the waveguide. Such a requirement is satisfied by using an angular-coupling arrangement, which allows for the excitation of modes having an odd spatial distribution that contribute to modal phase-matched second-harmonic generation. The strength of the second-harmonic signal is directly related to the angle at which the incident field is coupled into these modes. This is critical for on-chip nonlinear frequency conversion processes in the areas of optical communication, optical computing, and quantum computing.

(Na0.60Ba0.70)Ga2Se4: An Infrared Nonlinear Optical Crystal Designed using AgGaSe2 as the Template.

A novel quaternary selenide (Na0.60Ba0.70)Ga2Se4 (1) obtained by a high-temperature method crystallizes in the non-centrosymmetric space group I4cm, and its structure features {[GaSe2]-}∞ chains. It exhibits phase-matchable second-harmonic generation responses with the largest intensity of 0.3 times that of AgGaS2, and its powder sample has a laser-induced damage threshold of ∼2.4 times that of AgGaS2. Discussion about its relationship with other highly related structures are addressed as well. Theoretical calculations are performed to analyze its electronic structure and optical properties.

Ca2B5O9Cl and Sr2B5O9Cl: Nonlinear Optical Crystals with Deep-Ultraviolet Transparency Windows.

M2B5O9X is a prominent family with excellent nonlinear optical (NLO) responses, just as the Pb2B5O9I crystal with a large second harmonic generation (SHG) of 13.5 times that of KH2PO4. However, most of these compounds are limited to ultraviolet and visible regions because of their long absorption edge (small band gap). Here, we report two members of this family, which change the situation. Using a high-temperature solution method, we obtain Ca2B5O9Cl and Sr2B5O9Cl crystals, which exhibit a deep-ultraviolet (DUV) absorption edge of 170 nm (band gap ≈ 7.29 eV). It is an important breakthrough in the DUV transparency of the M2B5O9X family. Furthermore, Ca2B5O9Cl crystals display a phase-matching SHG response under a 1064 nm laser, which is further confirmed by the balance between the suitable birefringence and the small dispersion of refractive indexes in the wavelength range of 1064-532 nm. Therefore, they are promising DUV transparency windows and NLO candidates.

Phase-matched second-harmonic generation in coupled nonlinear optical waveguides

Phase matching is crucial for effective nonlinear frequency conversion. Recent advances in photonic miniaturization greatly promote implementations of efficient nonlinear frequency conversion in integrated photonic devices. Here, we investigate second-harmonic generation in coupled nonlinear waveguides. We develop a nonlinear optical coupled-mode theory and implement the concept of supermode to analyze the phase-matching condition, which greatly facilitates deep understanding of physics therein and optimization of device design. We have successfully identified six phase-matching conditions for the first time. The dynamic nonlinear optical processes are analytically modeled, which explicitly demonstrates that the phase-matching condition can be realized by extra momentum compensating due to the interwaveguide coupling. The influences of input pump amplitude and relative phase difference are successively investigated. The theoretical model presented here should be readily generalized to other nonlinear optical processes (e.g., difference frequency generations) and will remain valid in asymmetric coupled waveguide structures or even beyond a weakly coupling regime. Our results will be useful for constructing highly efficient integrated nonlinear optical conversion devices with extended functionality.

Centrosymmetric Rb2Mg3(SO4)4 and non-centrosymmetric Cs2Mg3(SO4)4 with a phase-matching nonlinear optical response

Non-centrosymmetric structure is the prerequisite of a nonlinear optical (NLO) material. However, most inorganic compounds tend to crystallize in centrosymmetric structures. Here, two new sulfates, Rb2Mg3(SO4)4 (1) and Cs2Mg3(SO4)4 (2), were successfully prepared by high-temperature solution method. Due to cationic size difference, 1 and 2 crystallize in centrosymmetric P21/c and chiral P212121, respectively. As a result, only 2 is NLO-active and exhibits a phase-matching second-harmonic generation response. Furthermore, diffuse reflectance spectra show that both 1 and 2 have short cutoff edges below 200 nm. In addition, they are congruently melting compounds with melting points higher than 780 °C. Calculations of dipole moment are carried out to explain the origin of NLO response. Cation-tuned structural strategies will shed a useful light on the development of new NLO materials.

Second-harmonic phase-matching based on zero refractive index materials

We provide a two-dimensional honeycomb photonic crystal, exhibiting accidental triply-degenerate points for each polarization at two frequencies with 2 times relationships. Using method and group theory, we demonstrate the zero refractive index phenomenon at two different polarizations. Without considering the additional loss, the numerical simulation shows that the honeycomb crystal exhibits greatly enhanced second harmonic generation (SHG) conversion efficiency over the traditional nonlinear materials. Our study has established the prerequisites for the photonic structure to achieve the phase-matching SHG naturally.

Triclinic Pedial Sarcosinium Hydrogen l(+)‐Tartrate—Crystal Growth, Pyroelectricity, Thermal Expansion, and Linear Optical Properties

AbstractSingle crystals of optical quality and dimensions up to 25 × 25 × 40 mm3 of sarcosinium hydrogen l(+)‐tartrate and up to 15 × 20 × 30 mm3 of sarcosinium hydrogen d(−)‐tartrate are grown from aqueous solution during growth periods of 10–14 weeks. For sarcosinium hydrogen l(+)‐tartrate refractive indices and their dispersion in the wavelength range 365–1083 nm are determined. The biaxial positive crystals show marked optical anisotropy with maximum birefringence at 365 nm of Δn = 0.089 and only small orientation dispersion of the principal axes of the optical indicatrix. Based on the linear optical data, phase matching conditions for collinear second harmonic generation (SHG) are analyzed. The optical properties allow SHG phase matching for both, type I and type II processes. Temperature‐dependent coefficients of the tensor of thermal expansion [αij] between 145 and 300 K show pronounced anisotropy and moderate orientation dispersion of thermal expansion, with principal values at 293 K of = −3.3(1) × 10−6 K−1, = 35.7(1) × 10−6 K−1, and = 66.6(1) × 10−6 K−1. The absolute value of the pyroelectric vector |pσ| of sarcosinium hydrogen l(+)‐tartrate amounts to about 20 µC K−1 m−2 and varies only slightly with temperature in the temperature range 100–320 K.

Second-harmonic generation microscopy analysis reveals proteoglycan decorin is necessary for proper collagen organization in prostate.

.Collagen remodeling occurs in many prostate pathologies; however, the underlying structural architecture in both normal and diseased prostatic tissues is largely unexplored. Here, we use second-harmonic generation (SHG) microscopy to specifically probe the role of the proteoglycan decorin (Dcn) on collagen assembly in a wild type (wt) and Dcn null mouse (). Dcn is required for proper organization of collagen fibrils as it regulates size by forming an arch-like structure at the end of the fibril. We have utilized SHG metrics based on emission directionality (forward–backward ratio) and relative conversion efficiency, which are both related to the SHG coherence length, and found more disordered fibril organization in the . We have also used image analysis readouts based on entropy, multifractal dimension, and wavelet transforms to compare the collagen fibril/fiber architecture in the two models, where all these showed that the prostate comprised smaller and more disorganized collagen structures. All these SHG metrics are consistent with decreased SHG phase matching in the and are further consistent with ultrastructural analysis of collagen in this model in other tissues, which show a more random distribution of fibril sizes and their packing into fibers. As Dcn is a known tumor suppressor, this work forms the basis for future studies of collagen remodeling in both malignant and benign prostate disease.