Characteristics Of Second Harmonic Generation Research Articles

Giant second-harmonic generation in monolayer MoS2 boosted by dual bound states in the continuum

Abstract Dielectric metasurfaces open new avenues in nonlinear optics through their remarkable capability of boosting frequency conversion efficiency of nonlinear optical interactions. Here, a metasurface consisting of a square array of cruciform-shaped silicon building blocks covered by a monolayer MoS2 is proposed. By designing the metasurface so that it supports optical bound states in the continuum (BICs) at the fundamental frequency and second harmonic, nearly 600× enhancement of the second-harmonic generation (SHG) in the MoS2 monolayer as compared to that of the same MoS2 monolayer suspended in air is achieved. To gain deeper insights into the physics of the metasurface-induced enhancement of nonlinear optical interactions, an eigenmode expansion method is employed to analytically investigate the main characteristics of SHG and the results show a good agreement with the results obtained via full-wave numerical simulations. In addition, a versatile nonlinear homogenization approach is used to highlight and understand the interplay between the BICs of the metasurface and the efficiency of the SHG process. This work suggests a promising method to enhance the nonlinear optical processes in two-dimensional materials, enabling the development of advanced photonic nanodevices.

Laser ablation induced impulse study for removal of space debris mission using small satellite

In recent years, many plans have arisen for building constellations in low Earth orbit, some of which have already provided commercial services. The number of satellites that form these constellations will exceed 10^4, and a number of these could become space debris due to accidental failure, leading to serious problems for human activities in space. Laser ablation-induced propulsion achieved by remote irradiation from the service spacecraft has been proposed as one effective method to remove such space debris and it involves effectively generating a propulsion impulse from a laser. Because most of the high-power lasers currently utilized in space applications are Nd:YAG lasers, in this study, we evaluate the characteristics of second harmonic generation (SHG), which can be generated using nonlinear crystals. The momentum coupling coefficient may be dependent on the irradiated laser wave length which has a significant difference in the ablation process such as plasma generation and heating. These effects have been investigated using an impulse measurement instrumental setup with a KD*P nonlinear crystal. As an efficient impulse generation method, the irradiation of the fundamental and SHG beams of the Nd:YAG laser at the same point was found to be more effective than the fundamental beam with the same total energy. SHG does not require additional power-consuming equipment such as additional exciters or amplifiers, but requite temperature-controlled nonlinear crystals, and impulses can be increased with minimal additional power. It turned out to be advantageous for space applications, which will require minimum power operation. Additionally, we found an additional nonlinear laser-induced impulses through the interaction of the beams.

Modeling of second-harmonic generation of circumferential guided wave propagation in a composite circular tube

This paper investigated modeling of second-harmonic generation (SHG) of circumferential guided wave (CGW) propagation in a composite circular tube, and then analyzed the influences of interfacial properties on the SHG effect of primary CGW. Here the effect of SHG of primary CGW propagation is treated as a second-order perturbation to its linear wave response. Due to the convective nonlinearity and the inherent elastic nonlinearity of material, there are second-order bulk driving forces and surface/interface driving stresses in the interior and at the surface/interface of a composite circular tube, when a primary CGW mode propagates along its circumference. Based on the approach of modal expansion analysis for waveguide excitation, the said second-order driving forces/stresses are regarded as the excitation sources to generate a series of double-frequency CGW modes that constitute the second-harmonic field of the primary CGW propagation. It is found that the modal expansion coefficient of each double-frequency CGW mode is closely related to the interfacial stiffness constants that are used to describe the interfacial properties between the inner and outer circular parts of the composite tube. Furthermore, changes in the interfacial stiffness constants essentially influence the dispersion relation of CGW propagation. This will remarkably affect the efficiency of cumulative SHG of primary CGW propagation. Some finite element simulations have been implemented of response characteristics of cumulative SHG to the interfacial properties. Both the theoretical analyses and numerical simulations indicate that the effect of cumulative SHG is found to be much more sensitive to changes in the interfacial properties than primary CGW propagation. The potential of using the effect of cumulative SHG by primary CGW propagation to characterize a minor change in the interfacial properties is considered.

基于准相位匹配的掺铒飞秒光纤激光器倍频特性研究

基于准相位匹配的掺铒飞秒光纤激光器倍频特性研究

Thermal characteristics of second harmonic generation by phase matched calorimetry.

We analyze a solution of the heat equation for second harmonic generation (SHG) with a focused Gaussian beam and simulate the temperature rise in SHG materials as a function of the second harmonic power and the focusing conditions. We also propose a quantitative value of the heat removal performance of SHG devices, referred to as the effective heat capacity Cα in phase matched calorimetry. We demonstrate the inverse relation between Cα and the focusing parameter ξ, and propose the universal quantity of the product of Cα and ξ for characterizing the thermal property of SHG devices. Finally, we discuss the strategy to manage thermal dephasing in SHG using the results from simulations.

Experimental characterization of efficient second harmonic generation of Lamb wave modes in a nonlinear elastic isotropic plate

This research experimentally characterizes the efficiency of Lamb wave mode pairs to generate the cumulative second harmonic in an undamaged aluminum plate. Previous research developed the theoretical framework for the characteristics of second harmonic generation of Lamb waves in nonlinear elastic plates, and identified five mode types where the amplitude of the measured second harmonic should increase linearly with ultrasonic wave propagation distance. The current research considers one of these five mode types, Lamb wave mode pairs at the longitudinal velocity, and experimentally confirms the theoretically predicted ratios of the rate of accumulation of the second harmonic amplitude versus propagation distance for two different Lamb wave mode pairs. By comparing these rates of accumulation, these experimental results are used to characterize the measurement efficiency of the mode pairs under consideration.

Characteristics of second harmonic generation of Lamb waves in nonlinear elastic plates

This paper investigates the characteristics of the second harmonic generation of Lamb waves in a plate with quadratic nonlinearity. Analytical asymptotic solutions to Lamb waves are first obtained through the use of a perturbation method. Then, based on a careful analysis of these asymptotic solutions, it is shown that the cross-modal generation of a symmetric second harmonic mode by an antisymmetric primary mode is possible. These solutions also demonstrate that modes showing internal resonance-nonzero power flux to the second harmonic mode, plus phase velocity matching-are most useful for measurements. In addition, when using finite wave packets, which is the case in most experimental measurements, group velocity matching is required for a cumulative increase in the second harmonic amplitude with propagation distance. Finally, five mode types (which are independent of material properties) that satisfy all three requirements for this cumulative increase in second harmonic amplitude-nonzero power flux, plus phase and group velocity matching-are identified. These results are important for the development of an experimental procedure to measure material nonlinearity with Lamb waves.

Noise Analysis of Second‐Harmonic Generation in Undoped and MgO‐Doped Periodically Poled Lithium Niobate

Noise characteristics of second‐harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using the quasiphase matching (QPM) technique are analyzed experimentally. In the experiment, a0.78 μm second‐harmonic (SH) wave was generated when a 1.56 μm fundamental wave passed through a PPLN crystal (bulk or waveguide). The time‐domain and frequency‐domain noise characteristics of the fundamental and SH waves were analyzed. By using the pump‐probe method, the noise characteristics of SHG were further analyzed when a visible light (532 nm) and an infrared light (1090 nm) copropagated with the fundamental light, respectively. The noise characterizations were also investigated at different temperatures. It is found that for the bulk and waveguide PPLN crystals, the SH wave has a higher relative noise level than the corresponding fundamental wave. For the same fundamental wave, the SH wave has lower noise in a bulk crystal than in a waveguide, and in MgO‐doped PPLN than in undoped PPLN. The 532 nm irradiation can lead to higher noise in PPLN than the 1090 nm irradiation. In addition, increasing temperature of device can alleviate the problem of noise in conjunction with the photorefractive effect incurred by the irradiation light. This is more significant in undoped PPLN than in MgO‐doped one.

Phase Matching Characteristics of Second-Harmonic Generation in Periodic 90°-Domain Structures of KNbO3

Phase matching characteristics of second-harmonic generation (SHG) in periodic 90°-domain structures of KNbO3 single crystal were extensively analyzed. Important parameters including types of interaction and the corresponding effective nonlinear optical coefficients, grating periods for quasi-phase matching (QPM), spectral bandwidths, and group velocity mismatches were estimated. The results show that KNbO3 with a specific internal structure of periodic 90°-domains can be used for efficient narrowband second-harmonic generation and group-velocity-matched broadband optical frequency doubling near the optical communication window.

Spectral characteristics of autofluorescence and second harmonic generation from ex vivo human skin induced by femtosecond laser and visible lasers

The spectral properties of one-photon, two-photon excited autofluorescence and second harmonic generation (SHG) from ex vivo human skin induced by a femtosecond (fs) laser and three visible lasers in backscattering geometry are systematically investigated. Our experimental results indicate that peak position of autofluorescence spectra from the dermis and epidermis shift toward long wavelengths, and the fluorescent intensity decreases when the excitation wavelength increases due to an effect of the excitation wavelength on autofluorescence signals. However, the intensity of the SHG signal in collagen has the maximal value of 800 nm excitation wavelength. This may be the result that the energy of the SHG signal is in resonance with an electronic absorption band. The two-photon excited autofluorescence and SHG intensity all obey a quadratical dependence on the excitation power. Compared with the two-photon excited fluorescence and SHG, the one-photon excited fluorescence in the dermis and epidermis exhibits different spectral characteristics. The investigation of the spectral characteristics of autofluorescence and SHG from ex vivo human skin can provide new insights into morphologic structures and biochemical components of tissues, which are vital for improving the application of laser-induced autofluorescence and SHG spectroscopy technique for noninvasive in vivo tissue diagnostics.

40-GHz picosecond-pulse second-harmonic generation in an MgO-doped PPLN waveguide

Second-harmonic generation (SHG) of 40-GHz picosecond optical pulses with different pulsewidths, pulse energies, and central wavelengths in a MgO-doped periodically poled lithium niobate (PPLN) waveguide is studied experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40-GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulsewidth can be tuned from 2 to 7 ps. The second-harmonic (SH) pulses are generated when the picosecond pump pulses pass through the MgO-doped PPLN waveguide. Dependences of SHG on pump pulsewidth, average pump power, and pump central wavelength are then investigated systematically. Meanwhile, dynamic behaviors of both pump and SH pulses in propagation are simulated numerically. Based on the temporal and spectral characteristics of SHG, a quantitative and systematic analysis on SHG efficiencies in terms of both pulse energy and spectral peak is presented. The simulation results are in good agreement with the measured data

Theoretical investigation on dispersion effect and two-photon resonance enhancement of molecular first hyperpolarizability

The calculations of the first hyperpolarizability frequency dispersion effect of hemicyanine derivative chromophore have been performed employing time-dependent Hartree-Fock and sum-over-states method respectively. Two-photon resonance enhancement characteristics of second harmonic generation β(-2ω；ω，ω) have also been investigated using the two-level model (TLM), which was originated by Ouder and Chemla. The results indicate that the first excited state dominates the nonlinear optical properties of hemicyanine derivatives. The first hyperpolarizability β(-2ω；ω，ω) increases with the decrease of fundamental wavelength and gives rise to two-photon resonance near the fundamental wavelength of 950 nm, in accordance with the known experimental facts. In addition, the relaxation effect of the excited states should be considered for TLM in the resonance regime. These researches provide theoretical references for experimental measurements.

The effect of MgO doping on optical properties of LiNbO 3 single crystals

Undoped and MgO doped Lithium niobate single crystals were grown by the Czochralski technique. Comparative study of the optical properties of undoped and 7 mol% MgO doped LiNbO 3 crystal was undertaken. The effect of doping on refractive indices as well as second harmonic generation has been experimentally analyzed. The results of the polarization characteristics of second harmonic generation (SHG) support the major contribution of Li–O bonds to optical nonlinearity. MgO doping reduces the number of localized excitons and the grown LiNbO 3 crystal approaches the stoichiometric composition. This causes blue shift in the absorption edge of the crystals.

Efficient second harmonic generation of femtosecond laser at 1 �m

By using spectrally noncritical phase-matching in a partially deuterated KDP around its retracing point of phase-matching, we have experimentally and numerically investigated the characteristics of second-harmonic generation (SHG) with femtosecond laser at 1 microm for the first time. This phase-matching configuration can support efficient SHG over 20nm bandwidth of the fundamental laser at 1 microm in a 10-mm-long crystal. Efficiency of harmonic conversion as high as 55% has been demonstrated.

Characteristics of second-harmonic generation including third-order nonlinear interactions

A theoretical analysis is presented for second-harmonic generation in nonlinear dielectric media. Mathematical expressions are derived for both the amplitude and phase evolution of optical waves for second-harmonic generation, wherein both second- and third-order nonlinear interactions are taken into consideration. Based on the results, numerical examples of second-harmonic generation in LiNbO/sub 3/ are presented, and the effects of third-order interactions on the frequency conversion efficiency and the intensity-dependent phase-matching condition are discussed. The derived result is amenable to a rigorous analysis of second-harmonic generation with a high-intensity incidence to nonlinear dielectric media, where the intensity-dependent optical parameters cannot he neglected.

Microwave harmonics due to hot electrons in a magnetic field

The characteristics of Second Harmonic Generation (SHG) of microwaves in a covalent semiconductor at low lattice temperatures are studied here in the presence of a transverse, static magnetic field. The characteristics are estimated in the general case of any mechanism of energy and momentum scattering of hot carriers in the strong and weak magnetic field limits. Calculations are carried out with the approximations of a parabolic law of dispersion and a scalar effective mass. For some definite scattering mechanisms the characteristics are obtained in the presence of any finite value of the magnetic field. The presence of the magnetic field brings in significant changes in both qualitative and quantitative aspects of the characteristics. The results are useful to the determination of the predominant relaxation processes of the hot electrons under any prevalent condition.

Electric field response of second order optical nonlinearity in dye doped poled polymer

The optical nonlinearity induced by electrical poling decays gradually in poled polymers. This suggests that the molecular alignment can be reoriented in the polymer matrix at room temperature by the external electric field. We measured the frequency responses and the temperature characteristics of second-harmonic generation (SHG) in a dye doped polymer in order to clarify the electric field dependence of the nonlinearity. The experimental results show that there is a slow response of the SHG coefficient variation due to the reorientation of nonlinear molecules and another fast response. This characteristic can be applied to develop a new type of nonlinear optical devices by dynamically controlling the nonlinearity of the material by the external electric field.

Harmonic generation of microwaves in semiconductors in a magnetic field

The characteristics of second harmonic generation due to hot electrons in semiconductors have been investigated in the presence of a magnetic field. The calculations are carried out for a parabolic law of dispersion and a scalar effective mass. Numerical results for n-Ge indicate the interesting effects the magnetic field may have on the harmonic generation.

Second-harmonic generation from metal surfaces: Beyond jellium

The generation of Second-Harmonic (SH) radiation from simple metals has been the subject of numerous investigations for almost three decades. The use of clean metal surfaces in the experiments and increasingly sophisticated jellium-based theoretical models has led to a convergence on the magnitude and some of the dispersion characteristics of SH generation in free-electron metals like aluminum. However, the sensitivity of the SH-generation process to crystal symmetry, surface morphology, and surface electronic states indicates that nonlinear optical properties are influenced by processes which cannot be explained using a model based on an isotropic electron gas. Here, we review some recent experimental work indicating where deviation from jellium-based models occurs and attempts to understand these.

Some characteristics of second harmonic generation under conditions of strong energy exchange between interacting waves

A theoretical analysis is made of the phase-matching curve of second harmonic generation for ultrahigh-power subnanosecond laser pulses. The conversion efficiency for a plane wave and the integrated conversion efficiency for a homocentric quasiplane beam are calculated as a function of the reduced mismatch and power density of the fundamental-frequency radiation over a wide range of introduced generalized parameters. It is shown that pulling of the zeros of the phase-matching curve toward the origin and an increase in the side maxima with increasing power density of the fundamental-frequency radiation results in an abrupt nonmonotonic dependence of the integrated conversion efficiency on this power density and possibly causes a shift of the optimal phase-matching direction. This should be distinguished from the self-interaction effects in media with a cubic nonlinearity.