Second Harmonic Generation Conversion Efficiency Research Articles

Femtosecond-laser-inscribed cladding waveguides in KTiOPO4 crystal for second-harmonic generation and Y-branch splitters

In this study, the fabrication of straight- and Y-branch-cladding waveguides in KTiOPO4 crystals using femtosecond laser-direct writing is described. The second-harmonic generation (SHG) of green light through the cladding waveguides was realized using a pulsed-wave pump at 1,064 nm. The guiding properties of straight-cladding waveguides fabricated by varying the laser-writing conditions were investigated. The minimum insertion loss was approximately 2.0 dB at 1,064 nm. Confocal micro-Raman spectroscopy was used to investigate the lattice micro-modifications. The maximum SHG conversion efficiency of the straight-cladding waveguide reached 31.4% with a maximum output power of 79.29 mW from an input power of 252.6 mW. The performances of the Y-branch splitters with splitting angles ranging from 0.5° to 2.6° were characterized at 1,064 nm, showing excellent properties, including symmetrical output ends and approximately equal splitting ratios. An SHG conversion efficiency of 13.4% and maximum output power of 33.63 mW were achieved in the Y-branch splitter with a splitting angle of 1.0°. These findings support potential applications in constructing compact-frequency converters by using femtosecond laser-written KTiOPO4 depressed-cladding waveguides.

Temperature-Switch-Controlled Second Harmonic Mode Sensor for Brain-Tissue Detection.

Identifying brain-tissue types holds significant research value in the biomedical field of non-contact brain-tissue measurement applications. In this paper, a layered metastructure is proposed, and the second harmonic generation (SHG) in a multilayer metastructure is derived using the transfer matrix method. With the SHG conversion efficiency (CE) as the measurement signal, the refractive index ranges that can be distinguished are 1.23~1.31 refractive index unit (RIU) and 1.38~1.44 RIU, with sensitivities of 0.8597 RIU-1 and 1.2967 RIU-1, respectively. It can distinguish various brain tissues, including gray matter, white matter, and low-grade glioma, achieving the function of a second harmonic mode sensor (SHMS). Furthermore, temperature has a significant impact on the SHG CE, which can be used to define the switch signal indicating whether the SHMS is functioning properly. When the temperature range is 291.4~307.9 Kelvin (K), the temperature switch is in the "open" state, and the optimal SHG CE is higher than 0.298%, indicating that the SHMS is in the working state. For other temperature ranges, the SHG CE will decrease significantly, indicating that the temperature switch is in the "off" state, and the SHMS is not working. By stimulating temperature and using the response of SHG CE, the temperature-switch function is achieved, providing a new approach for temperature-controlled second harmonic detection.

Boosting Second Harmonic Generation Efficiency and Nonlinear Susceptibility via Metasurfaces Featuring Split-Ring Resonators and Bowtie Nanoantennas.

This work investigates a metasurface design to achieve remarkable second harmonic generation (SHG) conversion efficiency and enhance effective nonlinear susceptibility using the finite element method. The elements of the designed structure are composed of a rectangular split-ring resonator Ag film, a bowtie-shaped Ag nanoantenna, and a pair of Bi bars that induce nonlinear optical phenomena due to the nonuniform distribution of the electric and magnetic fields within the device surface. The simulation results agree perfectly with the theory and demonstrate outstanding achievements in terms of SHG conversion efficiency (η) and effective nonlinear susceptibility (χeff(2)). Specifically, the metasurface reaches a peak η value of 4.544×10-8 and an effective nonlinear susceptibility of 3.4×104 pm/V. This work presents a novel and versatile design to achieve high η and χeff(2) in an SHG metasurface.

Double-pass second-harmonic generation of picosecond pulses with custom-poled KTP crystal

We present the double-pass second-harmonic generation (SHG) of picosecond pulses with a custom-poled potassium titanyl phosphate (KTP) nonlinear crystal. The average output power of 466 mW at central wavelength of 515.7 nm is obtained with the input of 1.2 W fundamental laser pulses. Compared to the highest conversion efficiency of 29.1% in the single-pass SHG, the conversion efficiency in the double-pass SHG is increased to 38.8%. Moreover, the average RMS stability of 0.67% in 2 hours and high beam quality (M2 < 1.10) of the second-harmonic pulses is observed. The presented results provide an efficient method to enhance the conversion efficiency of SHG for picosecond pulses.

Modulating Polarization in Second Harmonic Generation through Symmetry Evolution in Plasmonic Lattices.

We report a strategy for preparing cost-effective plasmonic square lattices with tunable unit structures of circles, crosses, and circle-cross pairs on a centimeter scale. The asymmetrical electromagnetic (EM) field distribution of the lattice enhances second harmonic generation (SHG) under oblique incidence. The SHG signals are progressively strengthened as the unit symmetry decreases from C∞v (circle) to C4v (cross) to C2v (circle-cross pair). The peak SHG signal is observed from the plasmonic lattice with a circle-cross pair, showcasing a conversion efficiency of 1.0 × 10-2, which is a 7.3-fold enhancement relative to the dielectric lattice comprised of circle units. This notably high conversion efficiency of SHG is on par with that of phase-matched bulk nanostructures under normal incidence, benefiting from the Bloch-surface plasmon polariton (Bloch-SPP) modes associated with the distribution of the photonic local density of states (LDOS). Furthermore, the SHG emission exhibits distinctive directional and polarization characteristics as the unit symmetry is reduced. This work offers valuable insights into a structural symmetry-dependent SHG in plasmonic lattices and the way forward for the design of functional nonlinear plasmonic devices.

Efficient second harmonic generation from nonlinear one-dimensional meta-grating metasurface under vertically incident pump laser excitation

The emergence of new materials will drive the development of a generation of devices. The purpose of this study is to realize the nonlinear optical frequency conversion in the novel material structure - metasurface structures. We study the second harmonic generation (SHG) from the all-dielectric resonant meta-grating metasurfaces with high quality (Q) factors. The quasi-bound states in the continuum (Q-BIC) modes can be easily induced using the linearly polarized plane wave with normal incidence. Two types of asymmetric one-dimensional meta-grating metasurfaces with square air notch defect and different widths based on AlGaAs are numerically investigated. When the symmetry of the AlGaAs meta-grating structures is broken, the BIC modes are transformed into the Q-BIC modes, which support strong local field confinement in the AlGaAs meta-grating array, thereby promoting the SHG conversion. When the pump laser intensity is about 5 MW/cm2, the two type structures provide the SHG conversion efficiencies about 0.059 and 0.166. The investigation results will promote the application of nonlinear metasurfaces in integrated nonlinear optical systems.

Analytical solution to incident angle quasi-phase-matching engineering for second harmonic generation in a periodic-poled lithium niobate crystal

Phase matching or quasi-phase matching (QPM) is of significant importance to the conversion efficiency of second harmonic generation (SHG) in artificial nonlinear crystals like lithium niobate (LN) crystal or microstructured nonlinear crystals like periodic-poled lithium niobate (PPLN) crystals. In this paper, we propose and show that the incident angle of pump laser light can be harnessed as an alternative versatile tool to engineer QPM for high-efficiency SHG in a PPLN crystal, in addition to conventional means of period adjusting or temperature tuning. A rigorous model is established and analytical solution of the nonlinear conversion efficiency under the small and large signal approximation theory is obtained at different incident angles. The variation of phase mismatching and walk-off length with incident angle or incident wavelength are also explored. Numerical simulations for a PPLN crystal with first order QPM structure are used to confirm our theoretical predictions based on the exact analytical solution of the general large-signal theory. The results show that the narrow-band tunable SHG output covers a range of 532 nm–552.8 nm at the ideal incident angle from 0° to 90°. This theoretical scheme, fully considering the reflection and transmission at the air-crystal interface, would offer an efficient theoretical system to evaluate the nonlinear frequency conversion and help to obtain the maximum SHG conversion efficiency by selecting an optimum incident wavelength and incident angle in a specially designed PPLN crystal, which would be very helpful for the design of tunable narrow-band pulse nanosecond, picosecond, and femtosecond laser devices via PPLN and other microstructured LN crystals.

A kinetic theory approach to second harmonic generation by high power laser in magnetized plasma with nonextensive distribution

The present study presents a kinetic theory approach to second-harmonic generation (SHG) through high-power laser in collisionless plasma with nonextensively-distributed electrons. The study applied a relativistic Vlasov equation to obtain nonlinear current density and second-harmonic (SH) conversion efficiency. It was observed that the power conversion efficiency (PCE) of second-harmonic generation depends on plasma and laser parameters such as dimensionless cyclotron frequency, plasma density, normalized thermal velocity, the nonextensive q-parameter, and laser intensity. Moreover, cyclotron frequency associated with plasma electrons has been found to play an important role in increasing the power conversion efficiency of SH generation.

Giant enhancement of second harmonic generation from monolayer 2D materials placed on photonic moiré superlattice

Abstract We numerically investigate second harmonic generation (SHG) from a monolayer of 2D-material placed on photonic moiré superlattice fabricated by dielectric materials. The greatly enhanced local field at the resonance modes of moiré superlattice can dramatically boost the SHG response in 2D materials. Considering a typical 2D-material MoS2 monolayer placed on a photonic moiré superlattice of a twist angle 9.43°, the maximum SHG conversion efficiency reaches up to 10−1 at a relatively low intensity of fundamental light 1 kW/cm2, which is around 14 orders of magnitude larger than that from the monolayer placed on a flat dielectric slab without moiré superlattices. The SHG conversion efficiency from the monolayer can be further enhanced with the decrease of the twist angles of moiré superlattice due to the even more confinement of local field. The flat bands in the moiré superlattices formed by the small twist angles can particularly ensure the efficiency even under wide-angle illuminations. The results indicate that photonic moiré superlattice which can tightly confine light is a promising platform for efficient nonlinear optics.

Resonant nonlinear nanostructured grating in an unstructured lithium niobate on insulator platform

Lithium niobate (LN) is a versatile material for applications in integrated nonlinear optics; however, due to its stable physical and chemical properties, efficient nonlinear conversion with nanostructured LN is not easy to implement. Here, we propose a resonant nanostructure composed of a SiO2 grating and a lithium niobate on insulator (LNOI) layer, which has been proven to support bound states in the continuum (BIC) modes through simulations. By experimentally measuring the angle-resolved transmittance spectra of the fabricated sample, we confirm that our proposed nanostructure exhibits quasi-BIC modes transitioning from BIC modes, resulting in strong localized field confinement. In the nonlinear experiment, our results show the significant potential of the nanostructure in enhancing second harmonic generation (SHG). By pumping at the resonant wavelength, we achieve a SHG enhancement factor of 6400, compared to a flat LNOI thin film under a fundamental wave (FW) peak intensity of 460 MW/cm2. The SHG conversion efficiency reaches up to 1.741 × 10−7, leading to a normalized conversion efficiency of 3.785 × 10−7 cm2/GW. Although the experimental results are two orders of magnitude lower than the theoretical maximum SHG conversion efficiency of approximately 10−5, we believe that our proposed etchless LNOI nanostructure highlights the tremendous potential for applications in nonlinear resonant nanophotonics.

Highly efficient second harmonic generation assisted by the quasi-bound states in the continuum from AlGaAs meta-gratings

Due to the design freedom of metasurfaces in tailoring wavefronts, they have become a general platform for miniaturized functional nonlinear optics. Recently, with the help of the quasi-bound states in the continuum (Q-BIC), the all-dielectric resonance metasurfaces with high quality factor (Q) boost the interaction between pump light and dielectric structure, which have become an effective way to achieve high nonlinear efficiency. Here, an asymmetric grating structure composed of AlGaAs is utilized to enhance the second harmonic generation (SHG) near Q-BIC. By breaking the symmetry of the grating structure, a simple normal incidence linearly polarized plane wave can be treated to excite stronger localized fields within the metasurfaces, allowing strong light–matter​ interactions in nonlinear metasurfaces. Numerical investigations depict that the Q-factor and resonant field enhancement noticed on the metasurface are intimately related to the asymmetric structure of the system. In addition, the high Q-factor resonance facilitates the SHG conversion process in AlGaAs metasurfaces. Especially, for the AlGaAs metasurface with Q-factor of 4.0314 × 104, the conversion efficiency of SHG can reach 10.5% when the pumping intensity is 5 MW/cm 2. This study is of great importance for analyzing the efficient nonlinear metasurfaces with high Q-factors.

High-power two-color Kerr frequency comb generation on the gallium phosphide-on-insulator platform at SWIR and MIR spectra

Optical frequency combs (OFCs) covering multiple spectral windows are of great interest as broadband coherent light sources. Pushing into high powers for traditional single OFCs as well as nonlinear frequency translated OFCs led to the narrowing of their bandwidths. Here, we present a hybrid integrated solution on the gallium phosphide-on-insulator (GaP-OI) platform to generate high-power two-color Kerr frequency combs at both the short-wave infrared (SWIR) and the mid-infrared (MIR) spectra. The design consists of a GaP-OI resonator with a partially etched gap for frequency comb generation at the two colors and a modal phase-matched strip waveguide for a second-harmonic generation (SHG). The resonator has a 3.25 µm wide anomalous dispersion window, which is enabled by mode hybridization and higher-order modes waveguide dispersion. The pump light at 1550 nm wavelength is frequency doubled from the 3100 nm wavelength light source, with a normalized SHG conversion efficiency of 793%W−1cm−2. We also propose the ring-bus coupler design to efficiently deliver optical power into the resonator while suppressing the leakage out of the resonator. The simulated two-color combs show a bandwidth of 87 nm above the −30dBm power level at the SWIR spectrum and a bandwidth of 749 nm above the same power level at the MIR spectrum. Our proposed two-color OFC generation scheme levitates the ceiling in terms of high power and broad bandwidth simultaneously on a single platform, paving the way toward monolithic solutions to integrated broadband coherent sources.

Second harmonic generation in plasmonic metasurfaces enhanced by symmetry-protected dual bound states in the continuum.

We numerically investigate linear and nonlinear optical responses in metasurfaces consisting of Au double-gap split ring resonators (DSRRs). Symmetry-protected dual bound states in the continuum (BICs) in such plasmonic metasurfaces are observed at the near-infrared optical regime. Efficient second harmonic generation (SHG) is obtained at the quasi-BIC models due to the symmetry broken. The optimized SHG responses are obtained at the critical couplings between radiation and nonradiation processes at the linearly x- and y-polarized light, respectively. High conversion efficiency of SHG of a value 10-6 is arrived at the fundamental intensity of 10 GW/cm2 at the quasi-BIC wavelength under the y-polarized illumination. Large extrinsic and tunable chirality of linear and nonlinear optical responses empowered by quasi-BICs is acquired in asymmetry metasurfaces at oblique circularly polarized incidence. The results indicate that the plasmonic metasurfaces of symmetry-protected BICs at the near-infrared optical regime have great potential applications in the on-chip efficient frequency conversion, and the linear and nonlinear chiral manipulation.

High-Efficiency Second-Harmonic Generation Using Quasi-Bound State in LiNbO3 Metasurface

We numerically demonstrated a high-efficiency second-harmonic generation (SHG) using quasi-bound state in the continuum (quasi–BIC) in thin film LiNbO3 (TFLN) metasurface. The TFLN possessed exceptionally high second-order nonlinear coefficients, contributing to the enhanced SHG performance. An eccentric cylinder unit cell was presented to achieve high Q–factor resonances associated with the asymmetric parameter introduced. Simulations showed that the high efficiency of the second-harmonic conversion was obtained by using the high Q–factor of the asymmetric dielectric cylinder metasurface, and it achieved a high SHG efficiency of 6.5% at pump intensities as low as 1 MW/cm2 at a normal incident. Furthermore, the simulation results indicated that breaking the symmetry through oblique incidence was more effective in achieving a higher Q–factor compared to altering the structural parameters. Specifically, under 1° oblique incidences, the conversion efficiency could reach 1.2% at an incident power of 1 kW/cm². We have proposed a method to achieve a high conversion efficiency of second-harmonic generation in low-refractive-index materials. Our work not only offers theoretical support but also provides valuable insights for the advancement of efficient nonlinear frequency doubling technology, optical communication, and sensing applications.

Single-Crystalline Li2Sn(IO3)6 Microwires: Combining Optical-Waveguiding and Frequency-Doubling Functions.

High-quality single-crystalline Li2Sn(IO3)6 microwires (MWs) have been successfully prepared by using a facile hydrothermal method. The as-synthesized Li2Sn(IO3)6 MWs exhibit regular hexagonal prism morphology, excellent surface smoothness, and remarkable diameter uniformity. The optical propagation loss has been determined to be as low as 0.026 dB μm-1 at 785 nm wavelength, implying the low-loss optical waveguiding capability of the Li2Sn(IO3)6 MWs. The effective frequency-doubling conversions of the fundamental frequency light source in the wavelength range from 916 to 1560 nm have been observed, and the second-harmonic generation (SHG) conversion efficiency has been measured to be 2.1% with a 1560 nm fundamental pump source (pulse duration of 10 ns, and average power of 9.06 nW) transmitted through a 1.32-μm-diameter and 300-μm-length Li2Sn(IO3)6 MW. These intriguing optical waveguiding and strong SHG conversion capabilities of the Li2Sn(IO3)6 MWs suggest its potential applications for photonic devices in micrometer scale.

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.

Magnetic Fano resonance enhanced second-harmonic generation in chiral hybrid bismuth halides

Magnetic Fano resonance provides a potential opportunity to control both linearity and nonlinearity of light for their low radiation loss and near-field enhancement. Previous investigations have demonstrated its significant enhancement of nonlinearity in a plasmonic structure and the 2D materials coupled to it. In this work, the enhancement of second harmonic generation (SHG) of hybrid bismuth halides at the important communication wavelength of 1550 nm with a magnetic Fano dip is theoretically studied. To this end, a hybrid system composed of two asymmetric silver square split rings (SSRs) and this chiral perovskite film is designed. The simulation results show that magnetic Fano-like resonance is induced at the destructive interface of two magnetic modes in the SSR dimer, which can be inherited to hybrid bismuth halides, thereby leading to the increase of four orders of magnitude in its SH near-field enhancement factor. With a peak intensity of 0.16 GW cm−2, the composite structure features a high SHG conversion efficiency of up to 1.6 × 10−3 at the Fano resonance position. By rotating the polarization angle of fundamental optical excitation, the emitted SHG signal is switched on–off. Our research provides a valuable thought for enhancing the nonlinear optical process of the perovskite films by coupling the magnetic modes.

Anisotropy-enhanced second-harmonic generation from graphene-wrapped nanoparticles

Due to the various applications of second-harmonic generation (SHG), achieving high SHG conversion efficiency is highly appreciated. In this paper, we achieve high SHG conversion efficiency from graphene-wrapped radial anisotropic nanoparticles by combining the strong second-order nonlinear effects of graphene and the field-enhancing effects of anisotropic materials. We find that the nonlinear conversion efficiency of radial anisotropic nanoparticles can be several orders of magnitude higher than that of isotropic counterpart. This is because the radial anisotropic provides the additional enhancement of field intensity near the boundary, and the graphene that provides SHG nonlinear effect is located at the particle boundary where the fundamental frequency field is significantly enhanced. Meanwhile, the highly tunable characteristic of graphene offers additional flexibility for manipulating the SHG spectrum in actual applications. Our results provide a promising method for improving the nonlinear conversion efficiency of photonic devices and the sensitivity of the nonlinear sensor.

Enhanced frequency-doubled pulse generation pumped by fundamental soliton molecules

To the best of our knowledge, this is the first experimental demonstration of enhanced pulse frequency doubling efficiency pumped by fundamental soliton molecules (SMs). A 1.0-µm polarization-maintaining (PM) ytterbium-doped oscillator-compressor system is first constructed to output various mode-locked pulse patterns, i.e., single-pulse-type dissipative solitons (DSs), noise-like pulses (NLPs) and soliton molecules (SMs), at a fundamental repetition rate of 62.7 MHz, respectively, by managing the intracavity dispersion parameters. Then, these pulse patterns are used as pump seeds for exciting frequency-doubled pulse generation at a wavelength of 515 nm based on second harmonic generation (SHG) in a commercial periodically polarized LiNbO3 (PPLN) crystal. Among the abovementioned pulse pump schemes, the SHG conversion efficiency is increased by up to 29 % when fundamental SMs are used as pump seeds for frequency-doubled pulse generation compared with the use of a conventional single-pulse-type DS pump scheme. Our work provides a promising technique for enhancing frequency-doubled pulse generation, which contributes to an in-depth understanding of frequency-doubled pulse generation with a coherent multipulse pump scheme.

General solution of phase mismatched second harmonic conversion efficiency in periodically poled nonlinear medium

In this work we provide for the first time a novel and more accurate second harmonic generation (SHG) conversion efficiency equation for periodically poled nonlinear medium. We believe the equation derived in this work is more useful than equations reported in previous works which are limited because it is considering the case of either nondepleted pump regime or perfect phase matching. The equation derived in this work tackles the general case: depleted pump regime and phase mismatching which makes it important for theoretical – experimental study. We used MgO:PPLN crystal as an example to illustrate SHG conversion efficiency as function of crystal length, laser power density, crystal temperature and beam divergence. We included fundamental and second harmonic waves reflection at entrance and exit surfaces respectively and performed numerical solution using fifth-order Runge-Kutta method. The numerical results show excellent match with analytical results obtained from the derived formula. We compared our theoretical results with experimental results reported in the literature and the comparison shows a good match. Finally, we performed a comparison between depleted and nondepleted pump regime. The results show a difference of 20% between two efficiencies making nondepleted efficiency equation, which used in many theoretical – experimental comparisons, not accurate at high power densities.