Harmonic Generation Conversion Efficiency Research Articles

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.

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.

Accurate derivation of THG conversion efficiency in periodically poled nonlinear medium and optimizing conversion parameters

We have derived a novel equation to accurately predict the third harmonic generation (THG) conversion efficiency in hybrid periodically poled nonlinear medium. Our equation considers the general case that takes both depleted pump regime and phase mismatching cases, resulting in more precise predictions of efficiency. This level of accuracy is crucial for certain applications like high-power THG lasers. Moreover, accurate calculation of THG power density is essential to prevent exceeding the crystal damage threshold. We applied our equation on hybrid MgO:PPLN crystal to determine the optimal SHG region length corresponding to two different power densities, namely, 0.25 and 0.5MW/cm2. The effect of crystal temperature on efficiency was also studies. Furthermore, a comparison between the derived equation and the commonly used nondepleted pump regime equation was performed. We found that the latter equation is significantly less accurate, particularly at high power densities, with the efficiency of the depleted pump regime being 50.6% less than nondepleted one. To demonstrate the effectiveness of the equation, our results were compared with experimental data, and we observed a good agreement between them.

Three orders of magnitude enhancement of second and third harmonic generation in the visible and ultraviolet ranges from plasmonic gold nanogratings

We report experimental observations and numerical simulations of second and third harmonic generation from a gold nanograting, which exhibits a plasmonic resonance in the near infrared. The resonance is tunable, with a spectral position that depends on the angle of incidence. All things being equal, the enhancement of nonlinear optical processes produced by the field localization in the nanograting when compared with a flat gold mirror manifests itself dramatically from the ultraviolet to the visible range: second harmonic generation conversion efficiencies increase by more than three orders of magnitude, while we report a third harmonic generation conversion efficiency enhancement factor of 3200, both in excellent agreement with our theoretical predictions. The clear inferences one may draw from our results are that our model describes the dynamics with unprecedented accuracy and that much remains to be revealed in the development of nonlinear optics of metals at the nanoscale.

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.

Enriched second-harmonic generation in meta-phase barium titanate nanostructures synthesized by sol-gel hydrothermal method

This study reports on the second harmonic generation (SHG) of barium titanate (BaTiO3) nanoparticles synthesized by a low-cost and easily-accessible sol-gel hydrothermal method at different temperatures (100–250 °C). According to the powder X-ray diffraction (XRD) pattern, all of the prepared samples had meta-phase without a center of symmetry, and the crystal structure changes from simple cubic to rhombohedral at different temperatures. In addition to XRD, differential scanning calorimetry (DSC) confirms the meta-phase in BaTiO3 by showing a significant phase shift indicating structural distortion. The variation in particle size was observed from transmission electron microscope (TEM) and the selected area electron diffraction (SAED) pattern as well supports the presence of structural distortion in the nanomaterials. The phase variation in BaTiO3 has not only affected its structure but also creates a disorder in its magnetic properties by changing it from paramagnetic to ferromagnetic for different synthesizing temperatures. This was determined with a vibrating sample magnetometer (VSM) and electron paramagnetic resonance (EPR) techniques. As a consequence of this structural deformation, particle size variation, and magnetic phase transition the second harmonic generation (SHG) conversion efficiency of BaTiO3 has been improved. Hence, this technique for synthesizing nanomaterials could be a promising candidate for fabricating bio-compatible imaging probes and coherent bio-markers in the field of medicine and electronics.

Dual‐Band Chiral Nonlinear Metasurface Supported by Quasibound States in the Continuum

AbstractThis paper presents an all‐dielectric chiral metasurface composed of an array of two‐layer distorted square structures with nanoholes. This metasurface can simultaneously generate dual‐band opposite chiral responses, which can be attributed to the bound states in the continuum (BICs) and the distorted chiral structure. By combining the BICs with chirality, a dual‐band chiral nonlinear metasurface and a chiral‐sensing method are proposed. The simulated results show that the third harmonic generation (THG) conversion efficiency can reach the order of 10−4 for two peaks with a peak‐pump intensity of approximately 5.3 GW cm−2 in the near‐infrared region. Moreover, the third‐harmonic circular dichroism for each peak can reach up to 0.99 and ‐0.94, respectively. Furthermore, it is theoretically demonstrated that this metasurface can be used for chiral sensing with high sensitivities. These results provide new insights for linear and nonlinear spin‐dependent applications.

Influence of l-alanine doping on zinc (TRIS) thiourea sulphate (ZTS) single crystals

ZTS has been grown using a slow evaporation technique at ambient temperature, and the l-alanine-doped ZTS was likewise prepared. Powder X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and Vickers’s micro hardness test have all been used to analyse growing crystals. Both pure ZTS and l-Alanine-doped ZTS crystals have an orthorhombic structure. l-alanine doped ZTS had greater second harmonic generation (SHG) conversion efficiency. l-alanine doped ZTS crystals had a higher hardness than a pure ZTS sample. UV spectral analysis indicates that both crystals have excellent UV and visible transmission. Doping with l-alanine resulted in an increase in the melting point of the pure ZTS crystal.Graphical abstract

High-order vortex harmonics generation by bi-circular Laguerre-Gaussian laser fields with relativistic plasmas

Vortex beams with ultra-high brilliance can greatly enrich the light and matter interaction process and even shed light on the unexpected information in relativistic nonlinear optics. Thus, we propose a scheme for relativistic intense vortex harmonic radiation by use of bi-circular Laguerre–Gaussian lasers irradiating relativistic plasmas. Three-dimensional particle-in-cell simulation results show that the emitted harmonics own controllable spin and orbital angular momentum simultaneously, which can be attributed to the vortex mirror model and the related conservation laws. Meanwhile, the conversion efficiency of harmonic generation can be tuned through adjusting the intensity ratio of the two driving field components.

Effect of laser focus in two-color synthesized waveform on generation of soft x-ray high harmonics

Synthesis of multi-color laser pulses has been developed as a promising way to improve low conversion efficiency of high-order harmonic generation (HHG). Here we systematically study the effect of laser focus in a two-color waveform on generation of macroscopic HHG in soft x-rays. We find that the dependence of HHG yields on laser focus at low or high gas pressure is sensitive to the characteristics of single-atom harmonic response, in which “short”- or “long”-trajectory emissions can be selectively controlled by changing the waveform of two-color synthesized laser pulse. We uncover the phase-matching mechanism of HHG in the gas medium by examining the propagation of the two-color waveform and the evolution of time-frequency emissions of high-harmonic field. We further reveal that the nonlinear effects, such as geometric phase, atomic dispersion, and plasma defocusing, are responsible for modification of two-color waveform upon propagation. This work can be used to find better macroscopic conditions for generating soft x-ray HHG by employing two-color optimized waveforms.

Giant Enhancement and Directional Second Harmonic Emission from Monolayer WS2 on Silicon Substrate via Fabry-Pérot Micro-Cavity.

Two-dimensional transition metal dichalcogenides (TMDs) possess large second-order optical nonlinearity, making them ideal candidates for miniaturized on-chip frequency conversion devices, all-optical interconnection, and optoelectronic integration components. However, limited by subnanometer thickness, the monolayer TMD exhibits low second harmonic generation (SHG) conversion efficiency (<0.1%) and poor directionality, which hinders their practical applications. Herein, we proposed a Fabry-Pérot (F-P) cavity formed by coupling an atomically thin WS2 film with a silicon hole matrix to enhance the SH emission. A maximum enhancement (∼1580 times) is achieved by tuning the excitation wavelength to be resonant with the microcavity modes. The giant enhancement is attributed to the strong electric field enhancement in the F-P cavity and the oscillator strength enhancement of excitons from suspended WS2. Moreover, directional SH emission (divergence angle ∼5°) is obtained benefiting from the resonance of the F-P microcavity. Our research results can provide a practical sketch to develop both high-efficiency and directional nonlinear optical devices for silicon-based on-chip integration optics.

铌酸锂微纳波导中的二阶非线性光学效应研究进展

<p indent="0mm">Lithium niobate (LiNbO₃, LN) crystal has a large second-order nonlinear coefficient (<italic>d</italic>₃₃=25.2 pm/V@1064 nm), wide transparent window (0.35–5 μm), and stable periodic microdomain structure preparation, which is a good platform for the study and application of second-order nonlinear optical effects. Lithium niobate thin film (LNTF) is regarded as a promising integrated photonics platform due to its excellent linear and nonlinear optical properties. As the basic unit of an integrated optical system, lithium niobate micro/nano waveguide has been studied as a transmission and control device, and it shows excellent second-order nonlinear optical characteristics. The second-order nonlinearity plays an important role in modern optics, including second-harmonic generation (SHG), sum- and difference-frequency generation, parametric down conversion, and parametric oscillation. Efficient and compact wavelength converters based on second-order nonlinearity are key components for a wide range of applications, including entangled photon sources, optical parametric oscillators, and optical parametric amplifiers. The second-order nonlinear polarization intensity is proportional to the square of the electric field intensity. Micro/nano optical waveguides can improve the light intensity in the device through the local characteristics of the spatial light field, to improve the conversion efficiency of the nonlinear processes. And significantly enhanced electric field strength makes the normalized nonlinear conversion efficiency of LNTF waveguides exceed that of the reverse proton<bold> </bold>exchange lithium niobate waveguides (frequency conversion efficiency 150% W^–1 cm^–2@1550 nm) by one order of magnitude. The nonlinear conversion efficiency of LNTF micro/nano waveguides can be further improved through mode phase matching (MPM) or quasi-phase matching (QPM) to optimize the spatial overlap between the eigenmodes involved in the nonlinear process. The detailed theory, fabrication and application were demonstrated in the manuscript as follows: (1) The theory of second harmonic generation. As an example of second-order nonlinear process, the factors affecting the conversion efficiency of second harmonic generation were analyzed, and can be summarized as nonlinear coefficient, mode special overlap, mode area and phase-matching condition. (2) The fabrication of periodically poled lithium niobate waveguides and periodically poled technics. Based on the micro fabrication processes such as electron beam lithography, dry etching and chemico-mechanical polishing, the propagation loss could be reduced to <sc>0.027 dB/cm.</sc> With the combination of low loss waveguides and the electrode polarization process, periodically poled lithium niobate microstructures and the detailed fabrication processes were discussed in the manuscript. (3) Efficient second harmonic generation based on MPM and QPM. It was reported that the normalized efficiency can reach 1000% W^–1 cm^–2, and we described broadband and tunable SHG in detail. (4) Spontaneous parametric down conversion. With the development of quantum information technology and its application in the fields of secure communication and precision measurement, quantum photon sources with high speed, high brightness and high purity have attracted more and more attention. Due to the strong optical confinement, the micro/nano waveguides reduce the pump power and improve the quality of generated photon pairs, which is helpful to realize the integrated optical path of quantum information processing. In summary, this paper reviews the recent development of second-order nonlinear optical effects in LNTF micro/nano waveguides. Firstly, the theory of second harmonic conversion is introduced, and then the fabrication and periodically poled techniques of LNTF micro/nano waveguides are discussed. As an example, the efficient SHG and parametric down conversion of LNTF waveguides were detailedly demonstrated. The processes of mode phase matching and quasi-phase matching technics, tunable SHG and wideband frequency doubling effect were emphatically investigated.

Sub-50 fs pulses at 2050 nm from a picosecond Ho:YLF laser using a two-stage Kagome-fiber-based compressor

The high-energy few-cycle mid-infrared laser pulse beyond 2 μm is of immense importance for attosecond science and strong-field physics. However, the limited gain bandwidth of laser crystals such as Ho:YLF and Ho:YAG allows the generation of picosecond (ps) long pulses and, hence, makes it challenging to generate few-cycle pulse at 2 μm without utilizing an optical parametric chirped-pulse amplifier (OPCPA). Moreover, the exclusive use of the near-infrared wavelength has limited the generation of wavelengths beyond 4 μm (OPCPA). Furthermore, high harmonic generation (HHG) conversion efficiency reduces dramatically when driven by a long-wavelength laser. Novel schemes such as multi-color HHG have been proposed to enhance the harmonic flux. Therefore, it is highly desirable to generate few-cycle to femtosecond pulses from a 2 μm laser for driving these experiments. Here, we utilize two-stage nonlinear spectral broadening and pulse compression based on the Kagome-type hollow-core photonic crystal fiber (HC-PCF) to compress few-ps pulses to sub-50 fs from a Ho:YLF amplifier at 2 μm at 1 kHz repetition rate. We demonstrate both experimentally and numerically the compression of 3.3 ps at 140 μJ pulses to 48 fs at 11 μJ with focal intensity reaching 10 13 W / cm 2 . Thereby, this system can be used for driving HHG in solids at 2 μm. In the first stage, the pulses are spectrally broadened in Kagome fiber and compressed in a silicon-based prism compressor to 285 fs at a pulse energy of 90 μJ. In the second stage, the 285 fs pulse is self-compressed in air-filled HC-PCF. With fine-tuning of the group delay dispersion (GDD) externally in a 3 mm window, a compressed pulse of 48 fs is achieved. This leads to a 70-fold compression of the ps pulses at 2050 nm. We further used the sub-50 fs laser pulses to generate white light by focusing the pulse into a thin medium of YAG.

Retrieving Linear and Nonlinear Optical Dispersions of Matter: Combined Experiment-Numerical Ellipsometry in Silicon, Gold and Indium Tin Oxide

The predominant methods currently used to determine nonlinear optical constants like the nonlinear refractive index n2 or the third order susceptibility χ(3) rely mostly on experimental, open and closed z-scan techniques and beam deflection methods. While these methods work well when the linear absorption is relatively small or negligible, the retrieval process is more complicated for a strongly scattering, dispersive or absorbing medium. The study of optics at the nanoscale in the picosecond or femtosecond laser pulsed regimes demands the development of new theoretical tools, and diverse experimental approaches, to extract and verify both linear and nonlinear optical dispersions exhibited by matter, especially when material constituents are fashioned into nanostructures of arbitrary shape. We present a practical, combined experimental and theoretical approach based on the hydrodynamic model that uses experimental results of harmonic generation conversion efficiencies to retrieve complex, nonlinear dispersion curves, not necessarily only for third order processes. We provide examples for materials that are of special interest to nanophotonics, for example, silicon, gold, and indium tin oxide (ITO), which displays nonlocal effects and a zero-crossing of the real part of the dielectric constant. The results for silicon and gold compare well with analytical predictions of nonlinear dispersion based on the nonlinear oscillator model. Based on our assessment of third harmonic generation conversion efficiencies in silicon, we predict χω(3) and χ3ω(3) are of order 10−17 (m/V)2 in the visible and near IR ranges, with respective peaks of 10−14 (m/V)2 and 10−16 (m/V)2 in the UV range. Similarly, gold’s χω(3) and χ3ω(3) are of order 10−17–10−16 (m/V)2, and predict χω(3)∼10−17(m/V)2 and χ3ω(3)∼10−18(m/V)2 for ITO. These results clearly suggest that judicious exploitation of the nonlinear dispersion of ordinary semiconductors has the potential to transform device physics in spectral regions that extend well into the UV range.

Crystal growth, structural, optical, thermal, dielectric, mechanical and NLO studies of L-tyrosine zinc carbonate single crystals

L-Tyrosine Zinc Carbonate (LTZC) a semi-organic nonlinear optical single crystal has been grown by slow evaporation solution technique. The lattice parameters of the grown crystals are found out by single crystals X- ray diffraction technique. The modes of vibrations of different functional groups present were identified by using Fourier Transform Infrared (FTIR) spectroscopic analysis. The UV–Visible optical absorption spectrum shows that the crystal has high transmission with lowest UV cutoff wavelength of 230 nm. The calculated band gap of LTZC crystal was found to be 5.13 eV. Dielectric constant and dielectric loss for various frequencies and temperatures were performed on LTZC crystal and the results were discussed in detail. The frequency dependence of ac conductivity for different temperatures was studied. The photoconductivity studies confirm that the crystal LTZC has negative photo conducting nature. The mechanical and thermal properties of the grown crystals have been studied. Surface morphological changes are observed in the grown LTZC crystal. The Non-linear optical second harmonic generation conversion efficiency of LTZC was determined using Q-switched Nd: YAG laser. The second harmonic generation conversion efficiency of LTZC is 2.12 times that of KDP.

Growth and characterization of thiosemicarbazide barium chloride [Ba(NH2NHCSNH2)Cl2]crystals: Novel potential NLO material

Semiorganic Nonlinear optical Crystals of Thiosemicarbazide Barium Chloride was grown by slow evaporation solution growth technique. The unit cell parameters were estimated by subjecting the crystal to single crystal X- ray diffraction. The Grown crystals were subjected to powder X- ray diffraction for analyzing the crystalline nature of the sample. The transmission spectrum of grown crystal was recorded by UV–Vis–NIR Spectrometer in the range between 200 nm and 800 nm which addressed the high percentage of transmission of the sample in the entire visible region. The presence of functional groups was studied by FT- IR analysis. Thermal Stability and melting temperature of the grown crystal was identified from TG –DTA analysis Dielectric behaviour of the grown crystal was studied and the results are discussed in detail. Second harmonic generation (SHG) conversion efficiency has been estimated as 1.21 times greater than potassium dihydrogen phosphate crystal.

Ultimate conversion efficiency of second harmonic generation in all-dielectric resonators of quasi-BICs in consideration of nonlinear refraction of dielectrics

We investigate second harmonic generation (SHG) in all-dielectric resonance nanostructures of high-Q factors assisted by quasi-bound states in the continuum (quasi-BICs). The typical resonators, e.g., guided-mode resonance gratings and asymmetric metasurfaces, fabricated by AlGaAs were numerically studied with the consideration of nonlinear refraction of AlGaAs. The resonance peak and line-shape of linear transmission and SHG spectra in the resonators can be dramatically changed under intense pump intensities. The SHG conversion efficiency in the nanostructures working at quasi-BICs is much lower than the traditionally expected values without considering the nonlinear refraction of dielectrics. The ultimate SHG conversion efficiency is finally obtained. The investigation has the significance for the design and understanding of efficient nonlinear metasurfaces of high-Q factors.

Harmonic generation by an interaction of laser with an array of anharmonic carbon nanotubes

A scheme of second and third harmonic generation from laser irradiated anharmonic carbon nanotubes (CNTs) embedded in silica is proposed. An intense ultra short laser pulse interacting with the array of CNTs displaces its electrons. When this displacement becomes comparable to the radius of electrons of CNTs, the restoring force of electrons varies nonlinearly with the displacement. As a result, plasmon resonance gets broadened and the large resonance absorption of the laser by the electrons of CNTs occurs. The power conversion efficiency of harmonic generation due to the anharmonicity of CNTs is much higher than that due to the ponderomotive force nonlinearity. The effect of amplitude modulated parameter on the amplitude of second and third harmonic is studied. The high amplitude of generated harmonics can be realized with the amplitude modulated laser as compared to ordinary unmodulated laser.

High-energy, high-repetition-rate ultraviolet pulses from an efficiency-enhanced, frequency-tripled laser

Abstract In this study, a high-energy, temporally shaped picosecond ultraviolet (UV) laser running at 100 Hz is demonstrated, with its pulses boosted to 120 mJ by cascaded regenerative and double-pass amplifiers, resulting in a gain of more than 108. With precise manipulation and optimization, the amplified laser pulses were flat-top in the temporal and spatial domains to maintain high filling factors, which significantly improved the conversion efficiency of the subsequent third harmonic generation (THG). Finally, 91 mJ, 470 ps pulses were obtained at 355 nm, corresponding to a conversion efficiency as high as 76%, which, as far as we are aware of, is the highest THG efficiency for a high-repetition-rate picosecond laser. In addition, the energy stability of the UV laser is better than 1.07% (root mean square), which makes this laser an attractive source for a variety of fields including laser conditioning and micro-fabrication.

Realizing high efficiency 532 nm laser by optimizing the mode- and impedance-matching

Increasing the conversion efficiency of second harmonic generation (SHG) is an area of interest in research. We report a high-efficiency 532 nm laser generation, with a conversion efficiency of 94.04 ± 0.115% from the pump depletion of 98.1% ± 0.1%, by accurately quantifying the round-trip loss and the transmissivity of the input mirror using our proposed scheme. The optimal conversion efficiency of the cavity-enhanced frequency doubling process is independent of the waist and is determined by the pump depletion, round-trip loss, and transmissivity of the input mirror. These results show that the cavity-enhanced frequency doubling process is not necessary to set the focusing parameter at the optimal single-pass conversion. These results provide a guide for future research on high-efficiency SHG.