Harmonic Generation Efficiency Research Articles

2D sheets of MoSe2 for waveguide based second harmonic generation

Two-dimensional transition metal dichalcogenides show a strong second-order nonlinear response, which can only be exploited if the interaction between light and 2D material is maximized as it happens in waveguiding structures. Such an approach also allows the addition of a second order nonlinear response to linear integrated optics components by just placing a sheet of transition metal dichalcogenides material on top. Here, we analyze the second harmonic generation in MoSe2 sheets combined with silicon nitride ridge and thin-film waveguides for both modal phase matching and quasi-phase-matching. The calculated second harmonic generation efficiency is comparable with that in lithium niobate waveguides and can further be enhanced by increasing the number of MoSe2 layers on top of the waveguides. This also holds true for flux correlated photons generated by spontaneous parametric down conversion as we show by quantitative analysis.

Chemical Tuning of Second Harmonic Generation Efficiency in Aminothiophene-Based Molecular Microcrystals

Chemical Tuning of Second Harmonic Generation Efficiency in Aminothiophene-Based Molecular Microcrystals

44-fs, 1-MHz, 70-µJ Yb-doped fiber laser system for high harmonic generation

We report the development of a robust Yb-doped fiber laser system based on chirped-pulse amplification (CPA), generating 44-fs laser pulses with up to 70-µJ pulse energy at a 1-MHz repetition rate. It consists of a Yb-doped nonlinear polarization evolution (NPE) mode-locked fiber oscillator, a chirped fiber Bragg grating (CFBG) stretcher, a wave-shaper for manipulating the spectrum of the signal, cascaded fiber amplifiers, and two compression units. The output pulse duration of 44 fs for efficient high harmonic generation (HHG) was achieved by a multi-pass multi-plate Herriott-type non-linear compression unit.

Enhanced high-harmonic generation in a polarization-insensitive all-dielectric metasurface exploiting dual-Fano resonance

Nonlinear optics demands efficacious techniques for nonlinear properties engineering. Metasurfaces, as a flat technology with easy fabrication in various arrangements and without the need for phase-matching conditions, are suitable platforms for nonlinear optics. This study proposes a silicon metasurface with special geometry that provides high conversion efficiency by exploiting Fano resonances and the excitation of magnetic and toroidal dipoles. The results indicate the high efficiencies of third harmonic generation of 5.17×10−3 W−2 and fifth harmonic generation of 7.92×10−9 W−4 under 1 GW/cm2 pump intensity in the near-infrared regime. More specifically, linear and nonlinear optical responses of the structure are completely polarization-independent.

The functional moieties impact on optical, thermal, and nonlinear properties of chalcone derivatives. A comprehensive study on FT2MP

This study explains the intricate interplay between functional groups and the single crystal structure of the compound 1-(furan-2-yl)-3-(2,4,6-trimethoxyphenyl)prop-2-en-1-one (FT2MP) using Density Functional Theory (DFT) calculations. Notably, geometry optimization at B3LYP using 6-311G+(2d,p) closely aligned with experimental distances from X-ray diffraction (XRD) upon comparison. A Q-switched, frequency-doubled pulsed Nd. YAG laser (532 nm, 7 ns pulses), a 25 cm focal length lens, and a 0.001 mol/L FT2MP solution in Dimethylformamide was used to measure third-order nonlinear optical (NLO) parameters and subsequently the origin of second/third harmonic generation efficiency is discussed. The third-order nonlinear parameters of FT2MP were found to be Δɸ = 0.95, n2 = −9.605 × 10−9 cm2/W, β = 2.74 × 10−6 cm/W, and χ(3) = 5.58 × 10−7 esu. Information about the electronic structure and reactivity of the molecule is provided via the addition of Global Chemical Reactivity Descriptors (GCRD), molecular electrostatic potential (MEP) and Frontier Molecular Orbitals (FMOs) for electronic structure and reactivity insights. Hirshfeld surface analysis was used to study intermolecular interactions. This investigation indicates the potential of FT2MP for third harmonic generation, providing a comprehensive understanding of its molecular structure, reactivity, and intermolecular interactions.

Efficient Second-Harmonic Generation in Adapted-Width Waveguides Based on Periodically Poled Thin-Film Lithium Niobate.

Frequency conversion process based on periodically poled thin-film lithium niobate (PPTFLN) has been widely recognized as an important component for quantum information and photonic signal processing. Benefiting from the tight confinement of optical modes, the normalized conversion efficiency (NCE) of nanophotonic waveguides is improved by orders of magnitude compared to their bulk counterparts. However, the power conversion efficiency of these devices is limited by inherent nanoscale inhomogeneity of thin-film lithium niobate (TFLN), leading to undesirable phase errors. In this paper, we theoretically present a novel approach to solve this problem. Based on dispersion engineering, we aim at adjusting the waveguide structure, making local waveguide width adjustment at positions of different thicknesses, thus eliminating the phase errors. The adapted waveguide width design is applied for etched and loaded waveguides based on PPTFLN, achieving the ultrahigh power conversion efficiency of second harmonic generation (SHG) up to 2.1 × 104%W-1 and 6936%W-1, respectively, which surpasses the power conversion efficiency of other related works. Our approach just needs standard periodic poling with a single period, significantly reducing the complexity of electrode fabrication and the difficulty of poling, and allows for the placing of multiple waveguides, without individual poling designs for each waveguide. With the advantages of simplicity, high production, and meeting current micro-nano fabrication technology, our work may open a new way for achieving highly efficient second-order nonlinear optical processes based on PPTFLN.

Efficient High-Order Harmonic Generation from the van der Waals Layered Crystal Copper Indium Thiophosphate.

Layered metal thio- and selenophosphates (MTPs) are a family of van der Waals gapped materials that exhibit a multitude of functionalities in terms of magnetic, ferroelectric, and optical properties. Despite the recent progress in terms of understanding the material properties of these compounds, the potential of MTPs as a material class yet needs further scrutiny, especially in terms of nonlinear optical properties. Recent reports of efficient low-order harmonic generation and extremely high third-order nonlinear optical properties in MTPs suggest the potential application of these materials in integrated nanophotonics. In this article, we investigate the high-order nonlinear response of bulk and exfoliated thin-film crystals of copper indium thiophosphate (CIPS) to intense mid-infrared fields through experimental and computational studies of high-order harmonic generation (HHG). From a driving laser source with a 3.2 μm wavelength, we generate odd and even harmonics up to the 10th order, exceeding the bandgap of the material. We note conversion efficiencies as high as 10-7 measured for the fifth and seventh harmonics and observe that the harmonic intensities follow a power law scaling with the driving laser intensity, suggesting a perturbative nonlinear optical origin of the observed harmonics for both bulk and thin flakes. Furthermore, first-principles calculations suggest that the generation of the highest harmonic orders results from electron-electron interactions, suggesting a correlation-mediated enhancement of the high-order optical nonlinearity.

Enhancement of growth perfection and optical transmission in rhodamine B-doped KDP single crystal for photonic applications

In recent years, because of the rapid development persisting in the modern electronic industries, materials with the ability to produce highly efficient harmonic generation have acquired great demand as they are suitable for high-speed devices and the progressing field of optical communication systems. The present work reports the growth, diffraction and spectroscopic features of the pure and Rhodamine B (RB)-doped potassium dihydrogen phosphate (KDP) crystals. The crystals were grown by employing the conventional slow evaporation technique. The XRD and UV–visible spectral studies have been carried out to examine the structural and optical behavior of the grown crystals. An optical microscopic technique has been utilized to explore the surface features of the crystals. Based on the growth features and the observed analytical results, it is authenticated that the RB-doped KDP crystal possesses a better crystalline nature and optical transmittance than the pure KDP crystal thereby it is strongly suggested for the applications of photonic devices.

Spectral, optical, thermal, dielectric, micro hardness analysis and computational calculations of a novel crystal, 2-amino-3-methylpyridinium 2, 4-dinitrobenzoate

By adapting slow evaporation technique novel single crystals of 2-Amino-3- Methylpyridinium 2, 4-Dinitrobenzoate (AMPD) were synthesized. The grown crystal was subjected to Single crystal X‒Ray Diffraction technique to explicate the crystal structure. The spectral analyses were carried out to identify the vibrational modes of various functional groups. To explore the optical transparency of the synthesized crystal UV‒Visible study has been performed and its cut-off wavelength was found to be 209 nm. By Kurtz‒Perry powder technique, the assessment of second harmonic generation efficiency of the crystal has been calculated. Using TG/DTA analysis, the thermal behavior of the crystal was studied. The mechanical strength of the crystal was examined by Vicker′s micro hardness test and the work hardening coefficient was 3.59, affirming that the grown crystal belongs to soft material category. Optimized structural geometry, vibrational wavenumbers, frontier molecular orbitals energy, chemical reactivity indices and density of states were also computed using DFT method. Nonlinear optical property of the molecule was explored by the first‒order hyperpolarizability calculation. These experimental and theoretical studies reveal that the molecules can be suitable to use in optoelectronic devices.

Design and characterization of novel broadband chalcone-based THz optical crystals

By combining extended π-conjugated structures and halogen substituents, two novel chalcone-based optical THz crystals were designed: CBC (C22H17ClO2, Pc) and TFBC (C23H17F3O2, P21). These crystals were synthesized and successfully grown using the solution evaporation method. The effects of different types and quantities of halogen substituents on hydrogen bonding interactions, molecular alignment, and nonlinear strength were analyzed. The second harmonic generation (SHG) efficiency, thermal stability, transmission spectra, dielectric constant, and THz generation of the crystals were characterized. Theoretical calculations were conducted to determine the hyperpolarizability, band gap, density of states, and nonlinear optical coefficients of the crystals. The results showed that the synthesized crystals exhibited higher transparency (≥85%) and SHG efficiencies of 9.5 times and 6.2 times that of standard potassium dihydrogen phosphate (KDP), respectively, compared to previously reported chalcone NLO crystals. For the first time, the THz generation efficiency of the two crystals was confirmed through the optical rectification method, with TFBC demonstrating a wider THz spectrum range compared to CBC.

Nucleation, crystal growth, XRD, optical, electrical, PC, SEM, SHG and Z-scan analysis of LHPP for NLO and optical limiting application

High-quality organic L-histidinium 4-nitrophenolate 4-nitrophenol (LHPP) single crystals were grown using an aqueous solution. The crystals were grown using slow evaporation solution technique (SEST) at a temperature of 40°C using a constant temperature bath (CTB). Nucleation growth rates were monitored for various durations using an optical microscope. The crystalline perfection was confirmed through sharp, high-intensity diffraction peaks observed in the theoretical and experimental powder X-ray diffraction (PXRD) analysis, which also confirmed the monoclinic crystal structure with the noncentrosymmetric P21 space group. The crystallite size and strain were calculated using Williamson–Hall coefficient. Vibrational frequency assignments of LHPP were determined using FTIR spectroscopic analysis. Optical properties were studied using UV–Vis NIR spectroscopic studies, which showed that the crystals have very low absorption and high transmittance in the entire visible region. Additionally, the optical energy bandgap (Eg) was calculated and found to be 6.15[Formula: see text]eV. Luminescence properties were studied using fluorescence (FL) analysis. Dielectric investigations were subjected to varying frequencies to study the suitability of the crystal for transistor application. The positive photoconductive (PC) nature of the crystal was confirmed using photoconductivity studies to confirm suitability for photodetector devices that measure the intensity of light rays. Etching studies revealed the growth pattern and the formation of different kinds of etch pits. The surface morphology was studied using scanning electron microscopy (SEM) and electrochemical studies were also carried out. The relative second harmonic generation (SHG) efficiency of the material was also examined and found to be 1.34 times that of standard KDP. The optical limiting and higher-order nonlinear optical (NLO) properties of the grown crystals were studied by the [Formula: see text]-scan method using a Nd: YAG laser functioning at 532[Formula: see text]nm.

Polarization-controlled third harmonic generation by quasi-bound states in the continuum in silicon nanopillar metasurfaces

All-dielectric metasurfaces have received widespread attention due to their low intrinsic loss compared to the metallic counterparts. However, relatively high radiation loss and low quality (Q) factor of the dielectric nanostructures may significantly degrade their nonlinear optical efficiency. Here, we demonstrate that silicon-nanopillar-based metasurfaces supporting quasi bound states in the continuum (BICs) can greatly enhance the efficiency of third harmonic generation (THG). We show that symmetry-protected BICs form at the Γ-point of doubly degenerate band or nondegenerate band can be selectively transformed into quasi BIC through symmetry breaking of our silicon metasurfaces. Benefiting from the high Q and resonance enhancement by the quasi BICs, the THG conversion efficiencies of the silicon metasurface reaches 4.3 × 10-4 with a peak pump intensity of 1.3 GW/cm2, which can be four orders higher than that of a silicon planar film with the same thickness. In addition, the THG intensity is polarization-dependent for the single quasi BIC while it is polarization-independent for the doublet quasi BIC. Our results provide a new strategy for the design of high-efficiency silicon-based optical nonlinear devices.

Structural, optical, second harmonic, and mechanical studies on zinc chloride added alpha-glycine crystals

The slow evaporation solution method was employed to synthesize and crystallize zinc chloride doped α-glycine (ZCAG) single crystals. The single crystal X-ray diffraction studies of pure α-glycine single crystals revealed that the structure belong to primitive monoclinic lattice. Powder X-ray diffraction studies of ZCAG showed the well-defined sharp peaks and formation of centrosymmetric structure with space group P21/n. The increase in lattice volume of ZCAG suggested that the access of zinc chloride into the α-glycine crystal and the entry of zinc chloride to the α-glycine crystal was also established from the energy-dispersive X-ray analysis. Fourier transform infrared spectrum of ZCAG revealed that the functional groups of α-glycine was not transformed due to the introduction of zinc chloride into the lattice. The optical transparency of ZCAG was observed in the UV–visible region, and it exhibited a superior transmittance in the visible region. Energy bandgap value was determined and found to changes while increasing the dopant concentration. An emission peak was observed in the violet region from the photoluminescence spectra. The centrosymmetric ZCAG exhibited second harmonic generation efficiency of 1.44 times higher than standard potassium dihydrogen phosphate, possibly due to local non-centrosymmetry in the lattice, which is further augmented by lattice distortion induced by the dopant. Low dielectric constant and loss of the grown ZCAG crystals showed the remarkable application of non-linear optical. The Meyer index indicates that the grown ZCAG are categorized as soft materials.

Amplified linear and nonlinear chiral sensing assisted by anapole modes in hybrid metasurfaces

The interaction between chiral molecules and circularly polarized light is largely influenced by the local optical chirality density. This interaction prompts substantial demand of the design of nanophotonic platforms capable of enhancing such effects across large and accessible volumes. Such a magnification requires nanostructures that provide strong electric and magnetic field enhancements while preserving the phase relation of circular light. Dielectric nanostructures, particularly those able to support resonances, are ideal candidates for this task due to their capacity for high electric and magnetic field enhancements. On the other hand, efficient third harmonic generation requires strong electric field resonances within dielectric materials, a feature often boosted by incorporating plasmonic materials into hybrid systems. In this work, we numerically propose a coupled silicon disk-gold ring system that can exploit the anapole-induced field confinement to provide a broadband magnified circular dichroism under realistic conditions, reaching values up to a 230-fold enhancement. We also demonstrate that this structure can be employed as an efficient third harmonic generator, which, when integrated with chiral media, enables an 800-fold enhancement in circular dichroism. Furthermore, we show that pulsed illumination at intensities up to 10 GW/cm2 does not induce temperature increments that could potentially damage the samples. These findings suggest that this system can be a promising and versatile approach toward ultrasensitive chiral sensing.

Numerical experiment of phase-matched ion-based high-order harmonic generation in the water-window x-ray spectral region via electromagnetic envelope equation.

We present a phase-matching scheme for efficient high-order harmonic generation in the water-window x-ray spectral region using a 405-nm driving pulse. A high-intensity pulse (∼1016 W/cm2) is used to produce He1+ ions as the target medium, increasing the cutoff photon energy to the water-window x-ray spectral region. By adjusting the driving pulse divergence, the positive dipole phase variation balances the negative plasma dispersion and geometrical phase shift, achieving phase matching. Using the electromagnetic envelope equation coupled with the Keldysh ionization model, numerical experiments identify the optimal conditions. Results show that the relative conversion efficiencies of the 95th harmonic (4.26 nm) and the 169th harmonic (2.4 nm) reach 66% and 79% of the perfect phase-matching conditions, respectively.

Strong enhancement of third harmonic generation from a Tamm plasmon multilayer structure with WS2.

Improving the conversion efficiency is particularly important for the generation and applications of harmonic waves in optical microstructures. Herein, we propose to enhance the efficiency of third harmonic generation by integrating a monolayer WS2 with the metal/dielectric/photonic crystal multilayer structure. The numerical simulations show that the multilayer structure enables to generate the Tamm plasmon mode between the metal film and photonic crystal around the telecommunication wavelength, which is consistent with the experimental result. By measuring with a self-built nonlinear optical micro-spectroscopy system, we find that the third harmonic signal can be reinforced by 16-fold through inserting the monolayer WS2 in the dielectric spacer. This work will provide a new way for improving nonlinear optical response, especially THG in multilayer photonic microstructures.

Enhanced Photon-Pair Generation Based on Thin-Film Lithium Niobate Doubly Resonant Photonic Crystal Cavity

In this work, a doubly resonant photonic crystal (PhC) cavity is proposed to enhance second harmonic generation (SHG) efficiency and photon pair generation rate (PGR). Through the exploration of geometry parameters, a band-edge mode within the light cone is identified as the first harmonic (FH) mode, and a band-edge mode outside the light cone is designated as the second harmonic (SH). Subsequently, by increasing the layers of the core region, a heterostructure PhC cavity is designed. The results showcase a doubly resonant PhC cavity achieving a 133/W SHG efficiency and a photon pair generation rate of 3.7 × 108/s. The exceptional conversion efficiency is attributed to the high quality factors Q observed in the FH and SH modes with values of approximately 280,000 and 2100, respectively. The remarkably high Q factors compensate for nonlinear efficiency degradation caused by detuning, simultaneously making the manufacturing process easier and more feasible. This work is anticipated to provide valuable insights into efficient nonlinear conversion and photon pair generation rates.

The growth aspects and experimental techniques for the characterization of amino acid L-histidine hybrid crystals for nonlinear optical device applications

The development of crystal growth and measurement methods opens new possibilities for studying materials with remarkable nonlinear optical features. The development of bulk crystals is a constant concern to be beneficial for device applications. This study focuses on the development of nonlinear optical crystals of l-histidine complexes, characterization approaches, and prospective applications. The solution growth approach, a straightforward, inexpensive, and low-temperature growth technique, was adopted broadly. Recent studies show that l-histidine derivatives have potential optical, SHG, thermal, dielectric, and mechanical properties, making them an excellent choice for nonlinear optical devices. In this work, structural, spectroscopic, chemical composition, linear and nonlinear optical analyses, thermal, and dielectric characterizations of l-histidine derivatives were conducted. Most of the l-histidine complexes were found to be crystallized in orthorhombic and monoclinic crystal systems with P212121 and P21 space groups. The crystalline perfections of the samples were analyzed by the HR-XRD method. The energy-dispersive X-ray (EDX) spectroscopic technique was applied to evaluate the chemical content of the compounds. FT-IR and FT-Raman techniques were used to recognize the molecular vibrations and functional groups of the l-histidine derivatives. The UV-visible and photoconductivity studies were done to examine the optical behaviors of the developed crystals. Most of the crystals were found to have excellent optical transparency and wide bandgaps. It has been observed that l-histidine hydro-fluoride dehydrate and l-histidine tetrafluoroborate are found to have the highest second harmonic generation (SHG) efficiencies. The dielectric analyses were utilized to study the dielectric constant and dielectric loss variations with the applied frequency. According to the thermal analyses, most of the crystals were found to have substantial thermal stabilities suitable for device applications.

Effect of Laser Beam Amplitude Profile on Second Harmonic Generation Efficiency

Effect of Laser Beam Amplitude Profile on Second Harmonic Generation Efficiency

Meta-structure enhanced second harmonic S0 waves for material microstructural changes monitoring

Cumulative second harmonic Lamb waves in nonlinear media feature increasing amplitudes with propagation distance, conducive to the monitoring of material microstructural changes in structures. The phenomenon can be readily generated by zero-order symmetric (S0) mode waves in the low-frequency range. However, in a practical piezoelectric-transducer-activated system, both S0 and A0 (zero-order antisymmetric) mode Lamb waves are inevitably excited, while only the former is responsible for cumulative effects. The generation efficiency of the cumulative second harmonics is then affected by the presence of the A0 waves. To tackle the problem, this study develops a metamaterial structure, referred to as a meta-structure, to tactically enhance the cumulative second harmonic S0 Lamb waves by converting the A0 mode components into S0 mode waves. Topology optimization is conducted to design the meta-structure, which is surface-mounted onto the structure under inspection, to achieve high-efficiency A0-to-S0 wave mode conversion. Through tuning the parameters and constraints of the optimization, the designed single-sided meta-structure breaks the structural symmetry in the thickness direction, while facilitating its practical implementation. Typical scenarios with different meta-structure materials are discussed. Numerical simulations demonstrate that the strain amplitudes of the fundamental S0 mode waves can be increased by 60% with the deployment of the meta-structure, alongside an enhancement of the second harmonic S0 mode waves at different sensing distances. Finally, the designed meta-structure is fabricated via 3D printing technique and tested experimentally on an aluminum plate subjected to thermal aging treatment for monitoring the heating-induced microstructural changes inside the structure. Experimental results confirm an increase in the wave amplitudes of the linear S0 mode waves with the assistance of the meta-structure. The developed system improves the sensitivity of nonlinear Lamb wave-based monitoring methods in characterizing material microstructural changes, which shows great promise for detecting incipient damage in practical structural health monitoring applications.