Poly(DR-1/MMA)聚合物波导中Cerenkov倍频辐射的实验研究

本文报道采用电晕极化技术和旋转涂布方法制备Poly(DR-1/MMA)型有机共聚物薄膜波导生成二次谐波(SH)的实验研究.Q开关Nd:YAG激光器的1.064μm波长的输入光在极化聚合物波导中经Cerenkov型相位匹配产生0.532μm的倍频辐射光,总的转换效率达1.25×10^-3%.     

Review on second-harmonic generation of ultrasonic guided waves in solid media(Ⅰ): Theoretical analyses

Considering the high sensitivity of the nonlinear ultrasonic measurement technique and great advantages of the guided wave testing method, the use of nonlinear ultrasonic guided waves provides a promising means for evaluating and characterizing the hidden and/or inaccessible damage/degradation in solid media. Increasing attention on the development of the testing method based on nonlinear ultrasonic guided waves is largely attributed to the theoretical advances of nonlinear guided waves propagation in solid media. One of the typical acoustic nonlinear responses is the generation of second harmonics that can be used to effectively evaluate damage/degradation in materials/structures. In this paper, the theoretical progress of second-harmonic generation(SHG) of ultrasonic guided wave propagation in solid media is reviewed. The advances and developments of theoretical investigations on the effect of SHG of ultrasonic guided wave propagation in different structures are addressed. Some obscure understandings and the ideas in dispute are also discussed.     

Review on second-harmonic generation of ultrasonic guided waves in solid media (I):Theoretical analyses

Considering the high sensitivity of the nonlinear ultrasonic measurement technique and great advantages of the guided wave testing method, the use of nonlinear ultrasonic guided waves provides a promising means for evaluating and characterizing the hidden and/or inaccessible damage/degradation in solid media. Increasing attention on the development of the testing method based on nonlinear ultrasonic guided waves is largely attributed to the theoretical advances of nonlinear guided waves propagation in solid media. One of the typical acoustic nonlinear responses is the generation of second harmonics that can be used to effectively evaluate damage/degradation in materials/structures. In this paper, the theoretical progress of second-harmonic generation (SHG) of ultrasonic guided wave propagation in solid media is reviewed. The advances and developments of theoretical investigations on the effect of SHG of ultrasonic guided wave propagation in different structures are addressed. Some obscure understandings and the ideas in dispute are also discussed.     

Second-harmonic generation in periodically poled nonlinear polymer waveguides

We demonstrate quasi-phase-matched (QPM) second-harmonic generation (SHG) at the optical communication wavelengths with side-chain polymer waveguides. A ridge waveguide structure is designed to support fundamental mode guiding at both the pump and the second harmonics, leading to a high field overlap integral of the guided modes. The nonlinearity contrast in the +/0 type QPM waveguide is maximized under a QPM poling electrode width of nearly half the coherence length. Using these configurations, we record a normalized SHG efficiency of 2.2% W(-1) cm(-2) in the polymer waveguide.     

有机环形光纤中二次谐波产生的研究

本文从经典电磁理论出发，分析了有机环形光纤中二次谐波产生（ＳＨＧ）的转换效率，着重讨论利用环形光纤中模式的色散特性实现相位匹配等问题。报道了采用ＬＢ膜方法，制成半花菁环形光纤，由高灵敏度光检测计测出在强泵光（１．０６μｍ，ＹＡＧ激光）作用下该光纤中的二次谐波产生的实验结果。     

Influence of Change in Inner Layer Thickness of Composite Circular Tube on Second-Harmonic Generation by Primary Circumferential Ultrasonic Guided Wave Propagation

The influence of change in inner layer thickness of a composite circular tube is investigated on second-harmonic generation(SHG) by primary circumferential ultrasonic guided wave(CUGW) propagation. Within a secondorder perturbation approximation, the nonlinear effect of primary CUGW propagation is treated as a second-order perturbation to its linear response. It is found that change in inner layer thickness of the composite circular tube will influence the efficiency of SHG by primary CUGW propagation in several aspects. In particular, with change in inner layer thickness, the phase velocity matching condition that is originally satisfied for the primary and double-frequency CUGW mode pair selected may no longer be satisfied. This will remarkably influence the efficiency of SHG by primary CUGW propagation. Theoretical analyses and numerical results show that the effect of SHG by primary CUGW propagation is very sensitive to change in inner layer thickness, and it can be used to accurately monitor a minor change in inner layer thickness of the composite circular tube.     

Numerical Perspective of Second-Harmonic Generation of Circumferential Guided Wave Propagation in a Circular Tube

The effect of second-harmonic generation(SHG) by primary(fundamental) circumferential guided wave(CGW)propagation is investigated from a numerical standpoint.To enable that the second harmonic of the primary CGW mode can accumulate along the circumferential direction,an appropriate mode pair of primary and double frequency CGWs is chosen.Finite element simulations and evaluations of nonlinear CGW propagation are analyzed for the selected CGW mode pair.The numerical simulations performed directly demonstrate that the response of SHG is completely generated by the desired primary CGW mode that satisfies the condition of phase velocity matching at a specific driving frequency,and that the second harmonic of the primary CGW mode does have a cumulative effect with circumferential angles.The numerical perspective obtained yields an insight into the complicated physical process of SHG of primary CGW propagation unavailable previously.     

Investigation on the nonlinear optical properties of the DCNP/PEK-c guest–host polymer films

The polyetherketone (PEK-c) guest-host polymer thin films doped with 3-(1,1-dicyanothenyl)-1-phenyl-4,5-dihydro-1H-pryazole (DCNP) were prepared. The polymer films were investigated with in situ second-harmonic generation (SHG) measurement. The corona poling temperature was optimized by the temperature dependence of the in situ SHG signal intensity under the poling electric field applying. The temporal and temperature stability of the second-order properties of the poled polymer film were measured by the in situ SHG signal intensity probing. The second-order NLO coefficient L₃₃⁽²⁾=32.65 pm/V at 5=1064 nm was determined by using the Maker fringe method after poling under the optimal poling condition. The dispersion of the NLO coefficient of the guest-host polymer system was determined by the measured value of L₃₃⁽²⁾ at 1064 nm and the two-level model.     

Characterization of inner layer thickness change of a composite circular tube using nonlinear circumferential guided wave:A feasibility study

The feasibility of using the nonlinear effect of primary circumferential guided wave (CGW) propagation for characterizing the change of inner layer thickness of a composite circular tube (CCT) has been investigated. An appropriate mode pair of the fundamental and double-frequency CGWs (DFCGWs) has been selected to enable the second harmonics of primary wave mode in the given CCT to accumulate along the circumferential direction. When changes in the inner layer thickness (described as the equivalent inner layer thickness) take place, the corresponding nonlinear CGW measurements are conducted. It is found that there is a direct correlation between change of equivalent inner layer thickness of the CCT and the relative acoustic nonlinearity parameter (Δβ) measured with CGWs propagating through one full circumference, and that the effect of second-harmonic generation (SHG) is very sensitive to change in the inner layer thickness. The experimental result obtained demonstrates the feasibility for quantitatively assessing the change of equivalent inner layer thickness in CCTs using the effect of SHG by primary CGW propagation.     

Surface ferroelectric phase transition in multilayer polymer Langmuir films

The ferroelectric phase transition on the free surface of a polymer ferroelectric Langmuir-Blodgett film was studied by the optical second harmonic generation (SHG) technique. A hysteresis in the temperature dependence of the SHG intensity observed for a multilayer film of a poly(vinylidene fluoride)-trifluoroethylene copolymer in the vicinity of T≈15°C is a manifestation of the first-order ferroelectric phase transition in the topmost surface monolayer of the film.     

Guided waves in anisotropic and quasi-isotropic aerospace composites: Three-dimensional simulation and experiment

Three-dimensional (3D) elastic wave simulations can be used to investigate and optimize nondestructive evaluation (NDE) and structural health monitoring (SHM) ultrasonic damage detection techniques for aerospace materials. 3D anisotropic elastodynamic finite integration technique (EFIT) has been implemented for ultrasonic waves in carbon fiber reinforced polymer (CFRP) composite laminates. This paper describes 3D EFIT simulations of guided wave propagation in undamaged and damaged anisotropic and quasi-isotropic composite plates. Comparisons are made between simulations of guided waves in undamaged anisotropic composite plates and both experimental laser Doppler vibrometer (LDV) wavefield data and dispersion curves. Time domain and wavenumber domain comparisons are described. Wave interaction with complex geometry delamination damage is then simulated to investigate how simulation tools incorporating realistic damage geometries can aid in the understanding of wave interaction with CFRP damage. In order to move beyond simplistic assumptions of damage geometry, volumetric delamination data acquired via X-ray microfocus computed tomography is directly incorporated into the simulation. Simulated guided wave interaction with the complex geometry delamination is compared to experimental LDV time domain data and 3D wave interaction with the volumetric damage is discussed.     

Response features of nonlinear circumferential guided wave on early damage in inner layer of a composite circular tube

A theoretical model to analyze the nonlinear circumferential guided wave(CGW) propagation in a composite circular tube(CCT) is established. The response features of nonlinear CGWs to early damage [denoted by variations in third-order elastic constants(TOECs)] in an inner layer of CCT are investigated. On the basis of the modal expansion approach, the second-harmonic field of primary CGW propagation can be assumed to be a linear sum of a series of double-frequency CGW(DFCGW) modes. The quantitative relationship of DFCGW mode versus the relative changes in the inner layer TOECs is then investigated. It is found that the changes in the inner layer TOECs of CCT will obviously affect the driving source of DFCGW mode and its modal expansion coefficient, which is intrinsically able to influence the efficiency of cumulative second-harmonic generation(SHG) by primary CGW propagation. Theoretical analyses and numerical simulations demonstrate that the second harmonic of primary CGW is monotonic and very sensitive to the changes in the inner layer TOECs of CCT, while the linear properties of primary CGW propagation almost remain unchanged. Our results provide a potential application for accurately characterizing the level of early damage in the inner layer of CCT through the efficiency of cumulative SHG by primary CGW propagation.     

Detection of fatigue-induced micro-cracks in a pipe by using time-reversed nonlinear guided waves: a three-dimensional model study

Localization of fatigue-related micro-cracks in pipelines is of increasing importance in industrial applications. A three-dimensional (3D) fatigue-crack imaging technique combining nonlinear guided waves with time reversal is proposed in this paper for potential applications in pipeline inspections. By using this method, the non-classical nonlinear guided waves generated from micro-cracks with hysteretic behavior are recorded, and the third harmonic waves are used to reconstruct the fatigue-crack images in a pipe by using a time reversal (TR) process. The feasibility of this method is examined by the imaging simulations for a steel pipe with varied defect areas. A finite-difference time-domain (FDTD) code is programmed to solve the wave equations under cylindrical coordinates, and simulate the experimental process of wave propagation. The results show that: (1) the proposed technique has excellent spatial retrofocusing capability; (2) the accuracy of defect localization and sizing depends on the crack orientation and the adopted guided wave mode; and (3) different displacement/stress components have varied sensitivities to the crack orientation.     

非扫描衰减全反射探测法监控光漂白过程

提出了一种基于衰减全反射原理的实时探测方法。其实验框架由半导体激光器、空间滤波器、聚焦透镜、耦合棱镜、CCD接受装置等组成。激光束先经空间滤波器扩束，再由透镜聚焦，以一定角宽度的光束聚焦于耦合棱镜底面，由于棱镜底面的波导结构，在反射光斑中会出现若干条吸收峰，即显示出标志波导模式的一组黑线，若外界条件变化时(如紫外光照射、电极化等)，波导薄膜某些参量会受外界条件的变化而变化，表现为黑线的移动，可根据黑线的移动量，精确测量每一时刻波导薄膜的折射率和膜厚等参量。利用这种技术，可以广泛地用于条波导制作、镀膜监控、薄膜极化监控、工业气体浓度监控等领域。     

Investigation of a new electro-optical guest–host polyetherketone polymer films

The polyetherketone (PEK-c) guest–host system thin films in which the range of the weight percent of 3-(1,1-dicyanothenyl)-1-phenyl-4, 5-dihydro-1H-pyrazole (DCNP) is from 20% to 50% were prepared. Their corona poling temperatures were optimized by using the in situ second-harmonic generation (SHG) measurement. The high predicted value of electro-optical (EO) coefficient γ₃₃=48.8 pm/v by using two-level model was obtained when the weight percent of DCNP in the polymer system is 40%, but EO coefficients are attenuated at higher chromophore loading than 40%. The temporal stability of the EO activity of the guest–host polymer was evaluated by probing the decay of the orientational order of the chromophores in the polymer system.     

Interactive length of fundamental wave and second harmonic generated on the surface of anomalous dispersion medium

In this Letter, a new method is presented to calculate the interactive length between the fundamental wave and second harmonic generation(SHG) for the configuration of total internal reflection on the inner surface of a nonlinear crystal. Three independent experiments are designed to measure the bandwidths of this second harmonic wave. The theoretical expression of the intensity of SHG is obtained through a nonlinear coupled wave equation. The interactive length of this phase-matched SHG can be calculated mathematically by utilizing the measured bandwidths and the intensity equation. There is no existing method to obtain the interactive length either from theoretical calculations or by experimental measurement. This method can be applied to estimate the extremely short interactive volume in nonlinear processes.     

管道弯头对低频纵向导波传播特性影响分析

管道弯头显著改变了导波传播特性,影响了对检测信号的解读,研究弯头对导波传播特性的影响是实现复杂管道系统导波检测的基础。采用半解析有限元法计算弯管导波频散曲线,分析了弯管导波频散曲线所呈现的不同特征,并基于弯管导波频散曲线,以低频L(0,1)模态导波为研究对象,实验研究了低频L(0,1)模态导波通过管道弯头时的模态变换特征。研究结果发现,当L(0,1)模态导波通过管道弯头时,不仅会发生L(0,1)到F(1,1)的模态变换,还会模态变换出反向L(0,1)模态导波,即弯头反射现象,且随着激励频率的降低和弯头弯曲半径的减小,弯头反射现象愈发明显。研究结果将深化对弯管导波传播特性的认识,推动导波检测技术在复杂管道系统检测中的应用。      

Enhanced second-harmonic generation for multiple wavelengths by defect modes in one-dimensional photonic crystals

We present an effective design of aperiodically stacked layers of nonlinear material and air, sandwiched by two truncated photonic crystals, in terms of the simulation annealing method. The constructed structure can achieve multiple-wavelength second-harmonic generation (SHG) at the preassigned wavelengths. We derive a general solution of SHG in 1D inhomogeneous systems and apply it to evaluate the SHG conversion efficiency. Numerical simulations show that the conversion efficiency of SHG can be significantly enhanced when the fundamental wave frequencies are assigned to the designed defect states.     

Electronic and Optical Properties of a Lens Shaped Quantum Dot under Magnetic Field: Second and Third-Harmonic Generation

In the present work, we have studied electronic and optical properties of a lens-shaped quantum dot under an external magnetic field. For this goal, we have calculated the energy levels and wave functions using the finite element method(FEM) for different values of magnetic field. We have also studied effect of magnetic field on second harmonic generation(SHG) and third-harmonic generation(THG) in the lens-shaped quantum dot. In this regard, we have obtained an analytic expression for the SHG and THG by a compact density matrix approach and an iterative procedure. According to the obtained results, it is found that the presence of the magnetic field affects the symmetry of the system. The SHG and THG are decreased with increasing the magnetic field. The magnetic field has a great influence on the energy levels, wave functions, the SHG and THG in a lens shaped quantum dot.