Near-field second-harmonic microscopy of thin ferroelectric films

We present a near-field optical technique for second-harmonic imaging by use of tapered optical fiber tips externally illuminated with femtosecond laser pulses. Enhancement of the electric field at the tip of the fiber results in enhanced second-harmonic (SH) generation from the sample region near the tip. This SH emission is collected by the same tapered fiber. The spatial distribution and polarization properties of SH generation from thin ferroelectric films and a poled single crystal of BaTiO(3) have been studied. A spatial resolution of the order of 80 nm was achieved. Symmetry properties of the near-field SH signal allow us to recover the local poling direction of individual ferroelectric domains in the film. Thus the technique provides a novel tool for nanometer-scale crystal analysis of polycrystalline samples.     

Highly efficient second-harmonic nanosource for near-field optics and microscopy

A nanometric source of second-harmonic (SH) light with unprecedented efficiency is demonstrated; it exploits the grazing-incidence illumination of a metal tip, which is conventionally used for atomic force microscopy, by 25-fs laser pulses of a high-energy Ti:sapphire oscillator. Tip scanning around the beam focus shows that the SH generation is strongly localized at its apex. The polarization dependence of the SH light complies with the model of an on-axis nonlinear oscillating dipole.     

基频倍频光群速度对超短脉冲三倍频的影响分析

在超短脉冲的谐波转换过程中, 由于不同波包之间的群速度失配的存在, 非线性晶体的有效厚度、转换效率及带宽等将相互制约, 不能获得理想结果。基于耦合波方程, 在平面波和小信号近似条件下推导出了关于基频、倍频光群速度关系的解析表达式。通过对200 fs超短脉冲在II类KDP晶体中的混频过程的模拟分析, 发现在满足相位匹配的前提下, 三次谐波转换效率随着基频、倍频光群速度倒数的差值的平方呈现出指数衰减的关系, 与理论结果吻合。得出的关于基频光、倍频光群速度的表达式, 作为三倍频群速度匹配关系式的必要补充, 对寻找合适的色散晶体和选择有效的匹配措施, 实现宽带三次谐波转换具有理论指导意义。     

在周期性极化铌酸锂波导中的波长转换与光谱分析

实验研究了在准相位匹配LiNbO₃波导中级联的倍频和差频波长转换,通过对转换频谱分析和波形对比分析,讨论了在脉冲泵浦情况下走离效应对光脉冲之间波长转换的影响：由于波导的色散性能,不同波长的光脉冲在波导中传播具有不同的群速度,处在0.8 μm光波段的倍频光脉冲比处在1.55 μm光波段的泵浦光脉冲的传播速度慢,导致了倍频光脉冲与泵浦光脉冲在传播的过程中发生走离;因此倍频脉冲在频域被压缩而在时域被展宽,并遗传给差频过程使得转换脉冲的谱宽变窄、脉宽增加、占空比变大,表明脉冲光泵浦波长转换对信号频谱及波形不是严格透明的转换。实验同时实现了一对多通道的波长转换,表明连续控制光的线宽对转换信号的信噪比有较大的影响。     

Phase matching limitation of high-efficiency second-harmonic generation in both phase- and group-velocity-matched structures

In this paper, we studied efficient second-harmonic generation (SHG) of femtosecond pulses in both phase- and group-velocity-matched structures. Obtained results show that phase matching becomes more critical under conditions required for high levels of conversion efficiency. And the imperfect phase mismatch caused by mismatched group-velocity dispersion (GVD) will limit conversion efficiency as well as bandwidth of generated second-harmonic (SH) pulses. The spectral characteristics of the generated SH pulses and its conversion efficiency in the strong pump regime are investigated in detail. The acceptance bandwidth of nonlinear crystal in the high-efficiency SHG is redefined in the paper, and the definition is much closer to the practical application of design.     

利用群速度匹配的级联二阶非线性实现超短激光脉冲压缩

提出了一种采用倾斜脉冲的级联二阶非线性来实现超短激光脉冲压缩的方法. 对基于单块BBO晶体中基频光与倍频光群速度匹配的级联二阶非线性的脉冲压缩方案进行了理论分析. 对比研究了群速度匹配与失配情况下利用级联二阶非线性进行脉冲压缩的效果, 并模拟分析了基频光与倍频光的位相失配量、非线性晶体长度、 基频光初始峰值光强和初始脉宽等因素对脉冲压缩效果的影响. 结果表明, 基频光与倍频光的群速度匹配将会大幅度改善压缩脉冲的时间波形和频谱分布. 通过对位相失配量、晶体长度、初始光强等参数的优化和选取可获得较理想的压缩效果. 采用倾斜脉冲的级联二阶非线性的脉宽压缩方法, 针对中心波长800 nm、脉宽100 fs, 峰值光强为50 GW/cm²的基频光脉冲, 采用25 mm厚BBO晶体, 当基频光与倍频光位相失配量Δk=60 mm^-1(对应失谐角1.98°), 晶体外部脉冲前沿倾斜角γ₀=74°时, 计算获得了质量较好的20 fs剩余基频光, 并同时产生了14 fs的倍频光. 关键词： 倾斜波 级联二阶非线性 群速度失配 脉冲压缩     

Enhanced second-harmonic generation by metal surfaces with nanoscale roughness: nanoscale dephasing, depolarization, and correlations

On the basis of spectral-expansion Green's function theory, we theoretically describe the topography, polarization, and spatial-coherence properties of the second-harmonic (SH) local fields at rough metal surfaces. The spatial distributions of the fundamental frequency and SH local fields are very different, with highly enhanced hot spots of the SH. The spatial correlation functions of the amplitude, phase, and direction of the SH polarization all show spatial decay on the nanoscale in the wide range of the metal fill factors. This implies that SH radiation collected from even nanometer-scale areas is strongly depolarized and dephased, i.e., has the nature of hyper-Rayleigh scattering, in agreement with recent experiments. The present theory is applicable to nanometer-scale nonlinear-optical illumination, probing, and modification.     

Spatial optimization of filaments

We demonstrate the spatial homogenization of intense laser pulses by adaptive minimization of spatial chirp of the spectrally broadened output pulses of a filament. A liquid-crystal-based two-dimensional spatial light modulator is used to control the spatial phase of the driver pulse. An evolutionary algorithm finds the optimal spatial laser phase distribution that introduces minimal distortions during filamentation and enhances the beam quality of the output pulse. A homogeneous intensity distribution favours efficient temporal compression close to the bandwidth limit without the need for spatial filtering after the filament. PACS 42.65.-k; 42.65.Re; 41.85.Ct     

Noncolinear Second-Harmonic Generation Pairs and Their Scatterings in Nd~(3+):SBN Crystals with Needle-Like Ferroelectric Domains

Second-harmonic generation in Nd~(3+):SBN crystal with needle-like ferroelectric with aperiodic domain structures is investigated. Two pairs of second harmonic(SH) waves appearing in lines are observed in unpoled Nd~(3+):SBN crystals with aperiodic needle-like domains. A pair of SH waves emit from the exit face, whose intensities are angle-dependent. The angular dependence is corresponding to the spatial frequency spectrum of the aperiodic domain structure. Another pair of SH waves emit from both the side surfaces, which are mainly the scattered SH waves by needle-like domain walls and obey the theory of Rayleigh scattering.     

Frequency-domain interferometric second-harmonic spectroscopy

We report a new spectroscopic technique to measure simultaneously the intensity and the phase of second-harmonic (SH) radiation over a broad spectral range without laser tuning. Temporally separated SH pulses from two sources, excited by the same broad-bandwidth 15-fs Ti:sapphire fundamental pulse, interfere in a spectrometer to yield frequency-domain interference fringes. We demonstrate the technique by measuring the strongly bias-dependent phase of SH radiation from a Si/SiO(2)/Cr metal-oxide-semiconductor capacitor in the spectral range of the SiE(1) critical point.     

Second harmonic generation in Ga nanoparticle monolayers

Ga nanoparticle monolayers formed by evaporation-condensation in ultrahigh vacuum and embedded in a transparent SiO_x matrix generate second harmonic (SH) signals in transmission and reflection when illuminated by a 150 fs, 800 nm laser pulses. The observed SH light exhibits a critical dependence on input and output polarizations, angle of incidence and azimuthal orientation of the samples. The results lead to a consistent picture of shape and orientation of the nanoparticles. Linear transmittance spectra in the visible range support these findings and the observed size dependence of the SH signal. Received 29 November 2000     

Single SH guided wave mode generation method for PPM EMATs

Using periodic permanent magnet(PPM) electromagnetic acoustic transducers(EMATs), different shear horizontal(SH) guided wave modes can form simultaneously in some situations, which can interfere with the inspection. The main cause of this phenomenon(typically named multiple modes) is related to the frequency bandwidth of excitation signals and the transducer spatial bandwidth. Simply narrowing the frequency bandwidth cannot effectively limit the number of different SH modes. Previous researches showed that unnecessary SH wave modes can be eliminated by using dual EMATs.However, in practical applications, it is more convenient to change the excitation frequency than to use dual EMATs. In this paper, the stress boundary conditions of the PPM-EMAT are analyzed, the analytical expression of SH guided wave is established, and the magnitude of SH guided wave mode under continuous tone and tone-burst input is obtained. A method to generate a single SH mode by re-selecting an operating point is proposed. Furthermore, the influence of the frequency bandwidth of the tone-burst signal is analyzed. Finally, a single SH mode excitation is achieved with tone-burst input.     

Influence of phase transitions on the second harmonic generation in metal nanoparticles

In this work we use nonlinear optical properties of Ga nanoparticles monolayers of different average sizes embedded in dielectric matrices to investigate the liquid-solid phase transitions in these materials. Ga nanoparticles, formed by exploiting the partial wetting of liquid Ga over a SiO_x surface are irradiated with fs laser pulses from a Ti:sapphire source. The resulting Second Harmonic (SH) generated in the reflection and transmission directions is measured along the phase transitions by cooling the sample from 320 K down to liquid nitrogen temperature. Hysteresis cycles are observed in the nonlinear transmission, which exhibit a strong amplification from the solid to liquid values as compared to the linear optical results. A simple model for SH generation, based on Mie scattering calculations which includes the effect of surface plasmon resonance provides a fair key for the interpretation of the observed effects.     

Generation and weak beam control of two-dimensional multicolored arrays in a quadratic nonlinear medium

We report on the first experimental observation of 2D multicolored transverse arrays in a quadratic nonlinear medium under the pump of two crossly overlapped femtosecond beams. The 2D reproducible patterns are caused by cascaded noncollinear quadratic nonlinear couplings between the input pulses and quadratic spatial solitons originated from spatial breakup of one of the input beams with spatial ellipticity. A seed supercontinuum pulse is then diffracted and amplified with phase preservation, resulting in the formation of up-converted multicolor 2D transverse arrays. By seeding with weak second harmonic pulses, the 2D multicolored transverse patterns are suppressed due to weak beam control of the induced quadratic spatial solitons.     

Adaptive compression of tunable pulses from a non-collinear-type OPA to below 16 fs by feedback-controlled pulse shaping

The self-controlled compression of widely tunable pulses in the visible generated by a non-collinear-type optical parametric amplifier is accomplished by a pulse shaper based on a 4f setup with a pixeled mask in the Fourier plane which is controlled by an evolutionary algorithm in a feedback loop. Pulse durations below 16 fs are achieved by shaping the pulses such that their second-harmonic signal is maximized. The optimization process generally requires less than five minutes. It is shown that the algorithm eventually determines the shaper settings which produce the global optimum for the SH signal. Moreover, pulses having propagated through a disturbing medium which introduced additional group velocity dispersion have been recompressed to below 16 fs. An acceptable value for the phase difference between two adjacent pixels of the liquid crystal mask is experimentally found to be 1.6. The described setup provides a powerful tool for delivering ultrashort tunable pulses to any location within an experiment, as well as tailored sub-20-fs pulses for optimal control studies.     

Generation of RF oscillations in the interaction of an electromagnetic shock with a synchronous backward wave

A new mechanism for the transformation of video pulses into radio pulses during their propagation along a nonlinear transmission line with spatial dispersion—synchronism with a backward wave—is considered. Numerical simulations demonstrate that a substantial advantage of this mechanism over the interaction with a forward wave is the possibility of generating longer radio pulses at higher frequencies.     

Two-color pulse compression in aperiodically-poled lithium niobate

We experimentally demonstrate engineerable compression of two-colored pulses in a linearly-chirped quasi-phase-matching grating. Quadratic solitons generated from fundamental input are reshaped through cascaded parametric processes of second-harmonic generation (SHG) and the back-conversion. We use type-I (e: o + o) SHG geometry in a 50-mm-long aperiodically-poled MgO:LiNbO₃ device to satisfy the group-velocity matching condition. Simultaneously compressed fundamental and SH pulses of about 55-fs duration with small pedestal are generated from the fundamental input pulses of 95-fs duration.     

Collisions of unipolar subcycle pulses in a nonlinear resonantly absorbing medium

Collisions of unipolar subcycle pulses in a nonlinear resonant medium that coherently interact with it are studied theoretically. The dynamics of spatial polarization structures and population difference that the pulses induce in the medium are analyzed. A surprising feature is that the medium is capable of “remembering” the result of the interaction with the pulses and their collisions during times comparable to the polarization relaxation time. An analysis of the dynamics of light-induced structures makes it possible to judge the parameters of subcycle pulses at the times longer than the pulse duration, which, in the future, can be useful in their detection.     

Near-field second-harmonic generation induced by local field enhancement

The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes.     

The thermal properties of a single-heterostructure laser diode supplied with short current pulses

The analysis of the heat spreading in the single-heterostructure GaAs-Ga₁-x Al _x As laser diode supplied with short current pulses (in the case, however, when the adiabatic approximation is no longer valid) at room temperature is presented in this paper. Relations are derived, describing the time-dependent temperature rise within the volume of the laser diode. The calculations are carried out for a typical SH laser diode. It turns out that in the duration of the short current pulses (t _I=200 ns,j=1.5 × 10⁴A cm^–2) the increase in junction temperature of the typical SH laser diode amounts to about 6.1 K. This increase leads to an increase of about 9% in the threshold current, to a decrease of about 18% in the laser radiation intensity, and to a shift of the spontaneous radiation band and of the stimulated radiation modes of about 1.9 nm and 0.22 nm, respectively, during each current pulse.