基于群速度失配的超声兰姆波二次谐波的时域测量方法

考虑到发射和接收换能器对超声兰姆波时域二次谐波信号所带来的不可避免的影响,提出一种基于基频与二倍频兰姆波群速度失配的超声兰姆波二次谐波的时域测量方法。当基频与二倍频超声兰姆波的相速度匹配而群速度失配时,在超声兰姆波传播过程中所发生的二次谐波信号,在时域上可与源于斜劈换能器的二次谐波信号相分离。采用仅源自于基频兰姆波的时域二次谐波的积分振幅,定量描述兰姆波二次谐波的发生效率。以铝板中传播的兰姆波为例,给出了时域二次谐波的具体测量过程。本文提出的测量方法放宽了超声兰姆波二次谐波的测量条件,且扣除了换能器对二次谐波信号所带来的影响,所测得的二次谐波信号完全来自于基频兰姆波时域信号的二次谐波发生效应。      

First comparison of electric field induced second harmonic of near-infrared femtosecond laser pulses in reflection and transmission generated from Si/SiO<Subscript>2</Subscript> interfaces of a silicon membrane

For the first time electric field induced second harmonic (EFISH) generation of femtosecond (fs) laser pulses (λ=800 nm, τ=75±5 fs, rep. rate=80 MHz, E _pulse≤10 nJ) is observed in transmission through a thin free-standing silicon (Si) membrane of 10-μm thickness and compared to the well-known EFISH results in reflection by use of the z-scan technique. EFISH in reflection and transmission unequivocally originate from the front and rear Si/SiO₂ interfaces, respectively, with SiO₂ being the natural oxide on the Si surfaces. Frequency conversion is enhanced by photoinduced electric fields across the Si/SiO₂ interfaces caused by charge-carrier injection from Si into the oxide. The z-scan results and time-dependent measurements allow comparison of the EFISH signal amplitudes and time constants detected in transmission and reflection, demonstrating the need for further investigation.     

Measurement of the second hyperpolarizability of linear conjugated polymer based on attenuated-total-reflection technique

Based on the attenuated-total-reflection technique, a new method has been proposed to determine the second hyperpolarizability gamma(-omega(4);omega(3),omega(2),omega(1)) of the linear conjugated polymer in the off-resonant region by means of quadratic electro-optic effect. An important feature of this method is the absence of a high-power pulse laser, resulting in more convenience and cost effectivity than other techniques.     

Generation of the second optical harmonic in porous-silicon-based structures with a photonic band gap

Efficient generation of the second optical harmonic is observed experimentally in a multilayer periodic structure based on porous silicon. The second-harmonic signal is much stronger than the signal from a uniform porous silicon layer or from the single-crystal silicon substrate. The orientational dependence of the second-harmonic signal is isotropic. The second-harmonic intensity as a function of the reflection angle reaches a maximum in the direction corresponding to the minimum phase detuning in a multilayer periodic structure. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 274–279 (25 February 1999) Deceased.     

Static and dynamic profile of the electric field within the bulk of fused silica glass during and after thermal poling

We use the electric-field-induced second-harmonic (EFISH) technique to characterize, for the first time to our knowledge, the profile of the electric field induced within the bulk of fused silica glass samples both during and after thermal-poling treatment. During thermal poling, the initially homogeneous electric field decays exponentially from the anodic to the cathodic surface. After the poling treatment, a permanent electric field is induced within the bulk of the sample. It also decays exponentially from a positive value on the anodic surface to a negative value on the cathodic surface. The exponential decay constant depends on the thickness of the sample, the poling time, and the temperature. The temperature evolution of this constant makes it possible to retrieve the activation energy of the rapidly moving ionic species. This ensemble of observations indicates that modifications to models previously proposed are necessary.     

Modified phase matching conditions for second harmonic generation in a finite one-dimensional photonic crystal near the bragg condition: Weak and strong diffraction

Enhanced noncollinear second harmonic generation in a finite one-dimensional photonic crystal is analyzed theoretically under conditions of pump field localization near the Bragg reflection. It is shown numerically that phase-matched second-harmonic generation can be implemented in a finite one-dimensional photonic crystal that does not satisfy the conventional phase-matching conditions calculated for effective Bloch modes with narrow spectral lines. The intensity of the generated second-harmonic signal exceeds the second-harmonic intensity attained under the conventional phase-matching conditions by more than an order of magnitude. This phenomenon is explained by interference between Bloch modes having similar amplitudes, wavenumbers, and spectral widths. Since the spatial spectra of waves propagating in a bounded medium have finite widths, the broadened spectral lines of proximate effective Bloch modes resulting from Bragg diffraction of waves tuned to the first transmission resonances near the photonic bandgap edge overlap, merging into a spectral profile with center shifted relative to the original effective Bloch wavevectors. This effect leads to modified phase matching conditions for second harmonic generation in a finite photonic crystal, which are written for the centers of the spectral profiles resulting from modal overlap, rather than for individual effective wavevectors. Substantially different phase matching conditions are obtained for weakly and strongly diffracted beams, whereas conventional phase matching conditions hold only for transmitted signals in the case of weak diffraction.     

Theoretical upper limits and experimental overestimates for molecular hyperpolarizabilities: a symbiosis

A recently developed theory concerning the physical limits on optical hyperpolarizabilities is shown to be useful in the theoretical prediction of contributions of multiphoton fluorescence to experimentally determined hyperpolarizability values. Recent experimental results are compared with the theoretical limit. Without correction for the fluorescence contribution, results for the first hyperpolarizability in the forbidden region can be obtained. Proper correction of the systematic error shifts these values to the theoretical limit. Hence, experimental values above the theoretical upper limit should be suspect, but an appropriate fluorescence suppression technique is available. Reversing the argument, the observation that artificially overestimated values for the first hyperpolarizability are reduced from the forbidden region to exactly this upper limit is found to corroborate the limiting theory.     

轮烯衍生物电子结构及三阶非线性光学性质的理论研究

苯轮烯衍生物具有良好的非线性光学性质. 运用密度泛函理论在不同理论水平和不同基组下计算了轮烯衍生物的静态极化率α和静态第二超极化率γ. 采用含时密度泛函TD-B3LYP方法计算了轮烯分子的紫外吸收光谱. 结果发现: 弥散函数对静态线性极化率α和第二超极化率γ 的计算结果都有显著的影响; 共轭体系的大小和引入的推拉电子基团可以调节轮烯衍生物的第二超极化率. 添加推拉电子基团后不仅能得到更高的非线性光学系数, 也能保证有较好的透光性能, 表明轮稀分子兼具有较高的三阶非线性光学响应和在可见光波段具有良好的透光性的特性. 此外, 采用CAM-B3LYP方法计算了分子1-1和分子2-2的动态(超)极化率. 计算结果表明: 在近红外区, 随着入射光频率的增大, α (ω; ω), γ (-ω; ω, 0, 0) 和γ (-2ω; ω, ω, 0) 都随之增大, 出现近共振增强效应; 在远离共振条件下, α (ω; ω), γ (-ω; ω, 0, 0) 和γ (-2ω; ω, ω, 0) 变化平缓.     

制备条件对Langmuir－Blodgett膜中分子聚集态的影响

通过对不同制膜条件下样品的π－Ａ曲线及紫外－可见吸收光谱的测量，研究ＬＢ单分子层膜中半花菁衍生物（ｈｅｍｉｃｙａｎｉｎｅｄｅｒｉｖａｔｉｖｅ简称ＨＤ）分子的聚集状态及其形成机制。纯半花菁衍生物单分子层膜中分子以Ｈ聚集体的形式存在导致吸收峰的兰移及有效分子二阶非线性极化率β（基频波长λ＝１．０６４μｍ）的减小。实验表明聚集体的形成主要发生在压膜过程。提高压膜速度或降低压膜终极压力（拉膜压力）可显著减小分子的聚集程度，从而提高单分子层膜的光学非线性极化率。     

双振子模型手性分子介质的二次谐波理论

推导了在二次谐波条件下耦合双振子模型手性分子的一阶超极化率，给出了薄膜手性介质的二阶极化率，分析了分子的微观参量对二次谐波产生效率的影响.理论计算所得的偏振旋转角谱和已有的实验结果相符. 关键词： 手性分子 分子极化率 介质极化率 二次谐波     

Trends in the electronic and vibrational contributions to the dipole moment,polarizabilities, and first and second hyperpolarizabilities of the hydrides of Li,Na and K

The dipole moments μ, polarizabilities α, and first and second hyperpolarizabilities, β and γ of LiH, NaH and KH (MeH) have been computed using Hartree-Fock, MP2 and CCSD(T) theories. The static electronic and vibrational contributions to these properties are presented. The vibrational properties have been analysed into contributions due to zero-point vibrational averaging and pure vibrational terms. An alternative dissection of the vibrational properties into nuclear relaxation and curvature terms has also been considered. KH has been selected as a model system to study how the number of electrons, which are correlated (2, 10 and 20) affect both electronic and vibrational properties. The 10 electron approximation gives results that are practically the same as those computed by taking into account all 20 electrons of KH. The double-harmonic approximation has been shown to give satisfactory results for the pure vibrational contributions to the polarizability and the first hyperpolarizability, while this approximation is useful for demonstrating, qualitatively, the significance of the pure vibrational contributions to the second hyperpolarizability. In many cases the vibrational contributions are rather small percentages of the corresponding electronic contribution. However, several exceptions to the above observation have been noted. In all the cases considered the vibrational properties should be computed if reasonably accurate property values are sought. Electron correlation is important for both electronic and vibrational contributions to the electrical properties of the hydrides considered. The results are in satisfactory agreement with most of the best theoretical and experimental data concerning bond lengths, vibrational frequencies and electrical properties.     

Reflected fourth-harmonic radiation from a centrosymmetric crystal

We present measurements of fourth-harmonic generation in reflection from the interface between two centrosymmetric media [Si(001)- SiO(2)] , using femtosecond pulses well below damage threshold. Analyses of signal amplitudes, rotational anisotropy, and sensitivity to surface roughening reveal that the surface dipole fourth-harmonic contribution dominates the bulk quadrupole contribution much more strongly than for second-harmonic generation.     

Absolute Raman intensity measurements and determination of the vibrational second hyperpolarizability of adamantyl endcapped polyynes

Linear sp‐carbon polyynes are ideal molecules for the direct investigation of the physical properties of one‐dimensional conjugated systems. Traditionally, the main obstacle to the study of these systems has been their instability under normal laboratory conditions. Thanks to improved chemical syntheses, a broadened range of polyynes is now available for study, including the hydrocarbon series endcapped with adamantyl groups, Ad[n]. In this work, the quantitative Raman spectroscopic analysis is reported for each member of this homologous series. The Raman scattering intensities corresponding to the major features of the polyyne segment (i.e., the effective conjugation coordinate or Я lines) are then used to evaluate the nonlinear optical (NLO) behavior via determination of the vibrational contribution to the second hyperpolarizability (γ_vib). This study shows that γ_vib values as a function of length are of the same order of magnitude as those arising from the electronic contribution to the molecular second hyperpolarizability, γ_ele, as reported for triisopropylsilyl polyynes, TIPS[n], using the differential optical Kerr effect technique. The nonlinear response for Ad[n] polyynes, supported by the analogous response for TIPS[n] polyynes, confirms that linear sp‐carbon molecules have large second hyperpolarizabilities that show a power‐law increase in γ‐values versus length that is larger than other known π‐conjugated systems, such as polyenes. Furthermore, the NLO response of polyynes versus length approaches the theoretical limit described by Kuzyk more closely than other conjugated organic oligomers. Copyright © 2012 John Wiley & Sons, Ltd.     

Second-harmonic generation from complementary split-ring resonators

We present experiments on second-harmonic generation from arrays of magnetic split-ring resonators and arrays of complementary split-ring resonators. In both cases, the fundamental resonance is excited by the incident femtosecond laser pulses under normal incidence, leading to comparably strong second-harmonic signals. These findings are discussed in terms of Babinet's principle and in terms of a recently developed microscopic classical theory that leads to good agreement regarding the relative and the absolute nonlinear signal strengths. The hydrodynamic convective contribution is found to be the dominant source of second-harmonic generation--in contrast to a previous assignment [Science 313, 502 (2006)].     

Efficient second-harmonic generation by scattering from porous gallium phosphide

We experimentally study second-harmonic generation by femtosecond Cr: forsterite-laser radiation scattered on the surface of porous gallium phosphide with characteristic pore sizes and distances between the pores comparable with the second-harmonic wavelength. The intensity of the second-harmonic signal from samples with initial crystallographic surface orientations (110) and (111) is more than an order of magnitude higher than the intensity of the second harmonic generated in reflection from single-crystal gallium phosphide. The efficiency of second-harmonic generation by macroporous gallium phosphide substantially increases as the pump wave-length becomes shorter. The influence of light localization and scattering effects on the enhancement of second-harmonic generation and polarization properties of the second-harmonic is discussed.     

Breit-Pauli and direct perturbation theory calculations of relativistic helium polarizability.

Large Gaussian-type geminal wave function expansions and direct perturbation theory (DPT) of relativistic effects have been applied to calculate the relativistic contribution to the static dipole polarizability of the helium atom. It has been demonstrated that DPT is superior for this purpose to traditional Breit-Pauli calculations. The resulting value of the molar polarizability of 4He is 0.517254(1) cm3 x mol(-1), including a literature estimate of QED effects. As a by-product, a very accurate value of the nonrelativistic helium second hyperpolarizability, gamma = 43.104227(1) atomic units (without the mass-polarization correction), has been obtained.     

A compact automatic wavemeter for use with tunable infrared diode lasers

We have studied the magnetic-dipole and electric-quadrupole contributions to second-harmonic generation (SHG) of centro-symmetric media. It can be shown that the phenomenological parameters , , can be arranged to form an effective -tensor, which describes dipole-forbidden SHG as the effect of a surface layer upon excitation with a single plane electromagnetic wave. An experimental technique is proposed allowing a determination of the usually very small term due to magnetic-dipole interaction using coherent compensation of its contribution by a coverage with an appropriate dye monolayer.     

一种评价固体板材表面性质的非线性超声兰姆波方法

在基频与二倍频兰姆波相速度匹配但群速度失配的条件下, 通过选择适当的兰姆波二次谐波时域信号的测量起止时间, 可完全扣除换能器对二次谐波积分振幅测量所带来的影响。本文提出采用兰姆波二次谐波的积分振幅作为评价参量, 以实现对板材表面性质的准确评价。当板材表面性质发生改变时, 原本在理想表面条件下成立的基频与二倍频兰姆波相速度匹配的条件不再严格满足, 这将显著地影响到兰姆波的二次谐波发生效率, 相应的二次谐波积分振幅随表面性质的改变也将发生非常敏感的单调变化。实验结果表明,利用扣除换能器影响之后所测得的兰姆波二次谐波的积分振幅,可对板材表面性质的变化情况进行准确评价。     

Photonic band-gap enhanced second-harmonic generation in a planar lithium niobate waveguide

Enhanced second-harmonic generation (SHG) conversion efficiency was theoretically predicted in waveguide geometry with coupling to a one-dimensional grating photonic band gap (PBG). We report a series of experiments using samples made with lithium niobate. A waveguide was fabricated near the surface by applying the proton-exchange technique. The characteristics of waveguide modes were determined by several techniques: prism coupling, diffraction, and Cherenkov radiation. The WKB method was used to analyze the results. Ultraviolet laser lithography was applied to make PBG gratings on the sample. We further investigated Cherenkov second-harmonic generation (CSHG), i.e., SHG radiated into the substrate, under the condition of a band-edge PBG resonance in the waveguides. The SHG inside planar waveguides was also experimentally investigated. We fabricated waveguides with multiple pump modes and found that the second mode was more efficient in enhancing the second harmonic signal. This result is explained by our model. Several samples were investigated in detail; the highest conversion efficiency of CSHG with a PBG was enhanced around 50 times above the CSHG signal without a PBG. A numerical model was constructed with parameters calculated from our sample characterization data to interpret the experimental results.     

Multiphoton ionization of atoms by a two-color laser pulse

The probability of multiphoton ionization of atoms in a laser radiation field containing an additional second harmonic is calculated by the imaginary time method [9, 10]. The conditions are found when the second-harmonic contribution to the ionization of atoms dominates over that of the first harmonic. It is shown that the average momentum of photoelectrons ejected from atoms depends on the phase shift between the first and second harmonics and their mutual polarization. The obtained asymptotic expressions can be used for the qualitative explanation of experiments on generation of terahertz radiation from the optical breakdown region in gas in the focus of a femtosecond laser.