Second Harmonic Generation Efficiency Research Articles

Boosting second harmonic generation by merging bound states in the continuum

Ultracompact devices for second harmonic generation (SHG) have broad applications in various fields. In this study, we numerically demonstrate efficient SHG from a merged bound state in the continuum (BIC) based on a lithium niobate (LN) freestanding metasurface. Such merged BIC efficiently reduces the radiative loss and strongly enhances the quality factor (Q factor). We achieved 1% conversion efficiency under the ${5}^{\ensuremath{\circ}}$ incident angle with 2 $\mathrm{kW}/{\mathrm{cm}}^{2}$ input intensity, which surpasses the performances of other nanostructures supported by traditional isolated BICs. Our work paves a promising way to enhance the interactions between light and matters at nanoscales, and provide a feasible idea in designing ultracompact SHG light sources.

Highly Efficient Ultraviolet Harmonic Generation at a Nonlinear Crystal Interface

High conversion efficiency is vital for the acquisition of UV light through nonlinear processes. Here, we investigate an alternative method to realize highly efficient second-harmonic generation (SHG) based on the interface of $\ensuremath{\beta}$ barium metaborate ($\ensuremath{\beta}\mathrm{BBO}$) and magnesium-doped lithium niobate ($\mathrm{MgO}$:LN). Theoretical simulations indicate that the SHG efficiency of our method is significantly higher than other phase-matching conditions. In the experiment, we demonstrate that higher conversion efficiency can be realized compared with only employing a single BBO crystal. Besides, the shortest output wavelength of SHG can reach 205 nm in our method. The method proposed here can open another way for the integrated ultraviolet laser and may promote the development of UV micromachining in the future.

Efficient Second- and Third-Harmonic Generations in Er3+/Fe2+-Doped Lithium Niobate Single Crystal with Engineered Surficial Cylindrical Hole Arrays.

Herein, significant enhancement of second- and third-harmonic generation efficiencies in a 1 mol% Er3+ and 0.07 mol% Fe2+-doped lithium niobate single-crystal plate were achieved after ablating periodic cylindrical pit arrays on the surface. Enhanced absorption and reduced transmittance of light were measured when the incident light signal passed through the patterned sample. Enhanced photoluminescence and two-photon-pumped upconversion emission spectra were also explored to obtain more details on the efficiency gains. The excitation-energy-dependent second-harmonic generation efficiency was measured, and an enhancement as high as 20-fold was calculated. The conversion efficiency of second-harmonic generation is 1 to 3 orders higher than that from other lithium niobite metasurfaces and nanoantennas. This work provides a convenient and effective method to improve the nonlinear conversion efficiency in a thin lithium niobite plate, which is desirable for applying to integrated optical devices.

Second-harmonic generation in polycrystalline ZnSe in the absence of phase matching

The work is devoted to the study of the second-harmonic generation (SHG) process as a result of random quasi-phase-matching of thulium laser radiation in ZnSe polycrystals. A linear dependence of the SHG efficiency on the nonlinear medium length has been observed in the experiment. The efficiencies of SHG in ZnSe polycrystals with average grain sizes from 35[Formula: see text][Formula: see text]m to 380[Formula: see text][Formula: see text]m were compared. The obtained results are in good agreement with the theory in the case of a Gaussian grain size distribution.

Two-dimensional semiconducting SnP2Se6 with giant second-harmonic-generation for monolithic on-chip electronic-photonic integration

Two-dimensional (2D) layered semiconductors with nonlinear optical (NLO) properties hold great promise to address the growing demand of multifunction integration in electronic-photonic integrated circuits (EPICs). However, electronic-photonic co-design with 2D NLO semiconductors for on-chip telecommunication is limited by their essential shortcomings in terms of unsatisfactory optoelectronic properties, odd-even layer-dependent NLO activity and low NLO susceptibility in telecom band. Here we report the synthesis of 2D SnP2Se6, a van der Waals NLO semiconductor exhibiting strong odd-even layer-independent second harmonic generation (SHG) activity at 1550 nm and pronounced photosensitivity under visible light. The combination of 2D SnP2Se6 with a SiN photonic platform enables the chip-level multifunction integration for EPICs. The hybrid device not only features efficient on-chip SHG process for optical modulation, but also allows the telecom-band photodetection relying on the upconversion of wavelength from 1560 to 780 nm. Our finding offers alternative opportunities for the collaborative design of EPICs.

Effect of cycloalkanone ring size on second and third order nonlinear optical properties of bischalcones

With the objective of establishing the effect of cycloalkanone ring size on the NLO properties of bischalcones para-substituted diarylideneacetone and substituted diarylidenecycloalkanone derivatives were synthesized by the Claisen–Schmidt condensation reaction and characterized by using UV–Vis, FT-IR, 1H NMR, 13C NMR and mass spectrometry. The second harmonic generation (SHG) efficiencies and third-order nonlinear optical (NLO) properties of the synthesized compounds were studied using the Kurtz-Perry method and the Degenerate Four Wave Mixing (DFWM) technique respectively. Among all the synthesized compounds that we have reported here, 2,6-bis-(4-methoxydibenzylidene) acetone showed the highest SHG efficiency (12.96 times that of urea) and 2, 6-bis-(4-methylthiobenzylidene) cyclopentanone showed the highest third order NLO property with χ (3) value of 2.650 ± 0.023 × 10−13 esu. Though diarylideneacetones exhibited the highest SHG efficiency, among the diarylidenecycloalkanones SHG efficiency decreased with an increase in the cycloalkanone ring size. Diarylidenecyclopentanone series showed better third order NLO activity followed by diarylideneacetone, diarylidenecycloheptanone and diarylidenecyclohexanone series. The results indicate that the cycloalkanone ring size as well as the presence of substituents on the aryl rings affect the NLO properties. The experimental findings correlated well with the results of in silico studies with a correlation factor, R2 = 0.9285.

Doubly resonant photonic crystal cavity using merged bound states in the continuum

In this work, a doubly resonant photonic crystal (PhC) cavity using the merged bound states in the continuum (BICs) is proposed to obtain a higher second harmonic generation (SHG) efficiency. Firstly by scanning geometry parameters the accidental BICs and a band-edge mode outside the light cone can be obtained. Then as the lattice constant or the thickness of the slab is adjusted the accidental BICs will merge. A supercell with large and small holes is constructed and the band-edge mode outside the light cone can be mode-matched with the merged BICs mode. Finally the heterostructure PhC cavity is designed. The merged BICs show a high quality factor for the photonic crystal with finite size. Consequently, the SHG efficiency of the lattice constant near merged BICs of ~6000% W-1 is higher than the one of the isolated BIC.

Elucidating the structure-nonlinear optical property relationship of Te2O4(OH)2

A mixed-valent tellurium oxide hydroxide, Te2O4(OH)2, containing both tetravalent (IV) and hexavalent (VI) tellurium cations has been synthesized using a hydrothermal reaction method. Single crystal X-ray diffraction reveals that the compound crystallizes in the polar noncentrosymmetric space group, Pna21 (no. 33). The reported compound has a layered structure consisting of Te(VI)O4(OH)2 octahedra and Te(IV)O4 seesaw, in which each layer is aligned through interlayer hydrogen bonds. In addition to the structural refinements, the structure and optical properties of the material have been studied and compared with Te2O5, which is dehydrated residue of the title compound. Second harmonic generation (SHG) measurements indicate that the Te2O4(OH)2 exhibits a large SHG efficiency of ca. 4.7 times that of KH2PO4. The ultraviolet–visible diffuse reflectance spectrum shows that the material has a band gap of ca. 3.5 eV, corresponding to a UV cutoff edge of 354 nm. These values suggest that the compound has expanded its band gap while maintaining its SHG response, compared to Te2O5. Theoretical calculations using the density functional theory have been also performed to clarify the structure-property relationship of the reported material more effectively.

Unveiling the Superior Optical Properties of Novel Melamine-Based Nonlinear Optical Material with Strong Second-Harmonic Generation and Giant Optical Anisotropy.

Two melamine-based metal halides, (C3 N6 H7 )(C3 N6 H6 )HgCl3 (I) and (C3 N6 H7 )3 HgCl5 (II), are synthesized by incorporating the heavy d10 cation, Hg2+ , and the halide anion, Cl- . The noncentrosymmetric structure of I results from two unique attributes: large asymmetric secondary building units produced by direct covalent coordination of melamine to Hg2+ and a small dihedral angle between melamine molecules. The former makes inorganic modules locally acentric, while the latter prevents planar organic groups from forming deleterious antiparallel arrangement. The unique coordination in I results in an enlarged band gap of 4.40eV. Due to the large polarizability of the heavy Hg2+ cation and the π-conjugated system of melamine, I exhibits a strong second-harmonic generation efficiency of 5 × KH2 PO4 , larger than any reported melamine-based nonlinear optical materials to date. Density functional theory calculations indicate that I possesses giant optical anisotropy, with a birefringence of 0.246@1064nm.

Intercalating-induced second-harmonic generation in centrosymmetric multilayer graphene

Second-harmonic generation (SHG) is a nonlinear optical process, where two incident photons coherently combine into one photon with double the energy, which is important for laser frequency conversion. Graphene has huge potential in various on-chip optical applications including laser sources, waveguide structures, and detection-sensitive units. However, efficient SHG is commonly present in systems with broken inversion symmetry but absent from centrosymmetric structures such as graphene. The study of graphene SHG signal has gradually become a hot research topic. In our work, H2SO4-intercalation tunable method was developed to tune the lattice distortion of multilayer graphene and lead to charge transfer from graphene layers to H2SO4 molecules. Thus, noncentrosymmetric electric dipoles can be acquired and enhance SHG response. Furthermore, the change of SHG is reversible after deintercalation, because the lattice distortion can be restored. Our research on the intercalating-induced SHG effect may lead to a better design of graphene-based nonlinear optical devices in the future.

Sum-frequency generation in a high-quality thin film lithium niobate microdisk via cyclic quasi-phase matching

Whispering-gallery-mode (WGM) microresonators can dramatically boost the interaction between light and matter, making them an ideal platform for studying nonlinear optics. Efficient wavelength conversion by nonlinear parametric processes is key to integrated optics in many applications, where WGM microresonators play an important role. Here, we demonstrate efficient second-harmonic generation (SHG) and sum-frequency generation (SFG) in an x-cut thin film lithium niobate microdisk with a high intrinsic quality factor over 106. The normalized conversion efficiency of cyclic quasi-phase-matched (CQPM) SHG and SFG is measured to be 1.53%/mW and 2.52×10−4/mW, respectively. Our work expands nonlinear optics applications of CQPM WGM microresonators as efficient wavelength convertors for light upconversion.

Acoustic frequency multiplication and pure second-harmonic generation of phonons by magnetic transducers

We predict frequency multiplication of surface acoustic waves in dielectric substrates via the ferromagnetic resonance of adjacent magnetic transducers when driven by microwaves. We find pure second-harmonic generation (SHG) without any linear and third-harmonic components by a magnetic nanowire. The SHG and linear phonon pumping are switched by varying the saturated magnetization direction of the wire, or resolved directionally when pumped by a magnetic nanodisk. We address the high efficiency of SHG with comparable magnitude to that of the linear response, as well as a unique nonreciprocal phonon transport that is remarkably distinct in different phonon harmonics. Such an acoustic frequency comb driven by microwaves should bring exceptional tunability for miniaturized phononic and spintronic devices.

Electric-field-induced quasi-phase-matched three-wave mixing in silicon-based superlattice-on-insulator integrated circuits

We present a theoretical investigation, based on the tight-binding Hamiltonian, of efficient electric-field-induced three-waves mixing (EFIM) in an undoped lattice-matched short-period superlattice (SL) that integrates quasi-phase-matched (QPM) SL straight waveguides and SL racetrack resonators on an opto-electronic chip. Periodically reversed DC voltage is applied to electrode segments on each side of the strip waveguide. The spectra of χxxxx(3) and of the linear susceptibility have been simulated as a function of the number of the atomic monolayers for “non-relaxed” heterointerfaces, and by considering all the transitions between valence and conduction bands. The large obtained values ofχxxxx(3) make the (ZnS)3/(Si2)3 short-period SL a good candidate for realizing large effective second-order nonlinearity, enabling future high-performance of the SLOI PICs and OEICs in the 1000-nm and 2000-nm wavelengths ranges. We have made detailed calculations of the efficiency of second-harmonic generation and of the performances of the optical parametric oscillator (OPO). The results indicate that the (ZnS)N/(Si2)M QPM is competitive with present PPLN technologies and is practical for classical and quantum applications.

Extremely high-Q quasi-BICs induced by simultaneously broken out-of- and in-plane symmetries in dielectric metasurfaces of low-index materials

Generating high-Q resonances is a difficult task to realize with low-index materials as the building blocks of metasurfaces, despite of its significance. Up to date, various schemes for obtaining strong resonances with low-index materials have been reported, while the highest record of Q-factor still remains in the order of several thousands. This work shows that quasi-bound states in the continuum (quasi-BICs) with extremely high Q-factors in the orders of 105∼107 can be obtained in the low-index metasurfaces, taking lithium niobate (LN) metasurfaces supported by silica substrate as examples. In such metasurfaces, BICs with inversion symmetric distributions evolve to quasi-BICs with extremely high Q-factors by simultaneously breaking out-of- and in-plane symmetries due to multiple symmetry protectedness. Dramatically high enhancement of mode intensity in the order of 104∼106 due to the high Q-factor enables highly efficient nonlinear frequency conversion: the results based on the self-consistent approach show that the total efficiency of second-harmonic generation reaches around 9.4% for pump intensity of 0.5 W/cm2. As the first report on extremely high-Q quasi-BICs in low-index metasurfaces, the presented results pave the way towards broad applications for ultraviolet nanophotonics.

Polar Bismuth Selenite Iodate Oxide BiSeIO6 with Three Types of Lone Pair Cations in One Structure.

Novel bismuth selenite iodate oxide BiSeIO6 was synthesized in a mild hydrothermal condition. BiSeIO6 was crystallized in the polar space group Pna21 of an orthorhombic system. The crystal structure features a three-dimensional framework composed of three types of lone pair cations with distorted BiO7 polyhedra, SeO3 pyramids, and IO3 pyramids in one structure. Interestingly, BiSeIO6 exhibits a strong and phase-matchable second-harmonic generation (SHG) of ∼6 times that of KH2PO4 (KDP). Dipole moment analysis shows that all three local acentric groups of BiO7, SeO3, and IO3 cooperatively contribute to the large macroscopic polarization and thereby strong SHG efficiency of BiSeIO6. In addition, BiSeIO6 has a broad transparency range from 0.35 to 11 μm, indicating its promising nonlinear optical applications from visible to mid-infrared bands.

Efficient second-harmonic generation through cascaded optically poled fibers.

We report the experimental demonstration of efficient second-harmonic generation by splicing optically poled fiber segments. A device made from five segments each 20cm-long exhibits, at a fundamental average power of 4.2mW, a maximum increase of 5.5 in the conversion efficiency with respect to a single 20cm poled fiber, corresponding to a conversion efficiency of 5.4%. For any considered fundamental power, the conversion efficiency at the output of a five-segment device is larger than the efficiency of a single poled fiber having the same length.

Broadband Second Harmonic Generation in a z-Cut Lithium Niobate on Insulator Waveguide Based on Type-I Modal Phase Matching

We numerically investigate a second harmonic generation (SHG) in a z-cut lithium niobate on insulator (LNOI) waveguide based on type-I mode phase matching (MPM) between two fundamental modes. A mode overlap factor that is close to unity is achieved and the normalized SHG efficiency reaches up to 72.1% W−1cm−2 at the telecommunication band, together with a large spectral tunability of 2.5 nm/K. Moreover, a bandwidth of about 100 nm for the broad SHG in a 5 mm-long LNOI ridge waveguide is demonstrated with this scheme. This stratagem will inspire new integrated nonlinear frequency conversion methods for versatile applications.

Doped potassium dihydrogen phosphate single crystals with enhanced second-harmonic generation efficiency: an investigation of phase purity, nonlinear, piezoelectric, optical, and crystalline qualities

We investigated the impact of itaconic acid (ITA) on potassium dihydrogen phosphate (KDP) aimed at concentrations of 1, 3, and 5 mol % on Single Crystal x-ray Diffraction, Fourier Transform Infra-Red, absorption of optical charm, Powder XRD, improved second Harmonic Generation competency, and third order non-Linearity of fully grown solitary crystals formed by slow cooling (Low Temperature Solution Growth). The crystals were crushed into a powder with micron-sized crystals for x-ray diffraction research. The Powder XRD spectrum of different ITA doped KDP concentrations showed fewer peaks with higher intensities than pure KDP crystals. Doping in the KDP crystal matrix accelerate growth in the [200] plane. Despite vibration assignment modes of ITA doped KDP, the FT-IR spectrum shows doping materials in KDP crystalline media. ITA doped KDP showed 90% optical transmission in UV–vis-NIR spectroscopy, whereas pure KDP possessed 60%. The dielectric investigation found that doped KDP crystals had lower dielectric loss and greater dielectric constants than pure counterparts. In pure and doped crystals, piezoelectric effects and second harmonics are premeditated. Using the Z scan approach, we were able to assess the third order nonlinear optical behaviour of pure crystal samples as well as the effects of self-defocusing caused by doping of ITA in KDP. ITA doping contributes to the growth, nucleation, and induction period. Doping concentration initially accelerated growth. Photoluminescence study shows that fully formed crystals have fewer defects. High Resolution XRD study determined crystal quality.

Second harmonic generation from nano cavities milled in thin aluminum film

Second harmonic generation (SHG) is forbidden for centrosymmetric materials such metals. Yet, symmetry can be broken by introducing plasmonic nano-structures which lead to enhancement of the electromagnetic field at both the fundamental and the SH frequencies. Using non-linear microscopy, we experimentally demonstrate enhanced SHG from isosceles triangular cavities (~215 nm side length, 200 nm base) milled in thin aluminum film. Upon strong interaction between the cavities, they behave as a single unit and response in a coherent manner, i.e. SHG is observed from the coupled system. The hybridization between the cavities is not only dependent on the distance between them, but their spatial arrangement is found to be a crucial parameter. That is, the SHG efficiency can be enhanced upon different arrangement of the same cavities, holding the same distance between them. We characterize those cavities also by cathodoluminescence and show that the structure support plasmonic modes both at the fundamental and the SH frequencies.

Efficient second- and higher-order harmonic generation from LiNbO3 metasurfaces.

Lithium niobate (LiNbO3) is a material that has drawn great interest in nonlinear optics because of its large nonlinear susceptibility and wide transparency window. However, for complex nonlinear processes such as high-harmonic generation (HHG), which involves frequency conversion over a wide frequency range, it can be extremely challenging for a bulk LiNbO3 crystal to fulfill the phase-matching conditions. LiNbO3 metasurfaces with resonantly enhanced nonlinear light-matter interaction at the nanoscale may circumvent such an issue. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and HHG from a LiNbO3 metasurface enhanced by guided-mode resonance. We observe a high normalized SHG efficiency of 5.1 × 10-5 cm2 GW-1. Moreover, with the alleviated above-gap absorption of the material, we demonstrate HHG up to the 7th order with the shortest generated wavelength of 226 nm. This work may provide a pathway towards compact coherent white-light sources with frequency spanning into the deep ultraviolet region for applications in spectroscopy and imaging.