GaAs半导体中三光子吸收的研究

本文利用全量子理论推导了半导体中多光子吸收跃迁速率的一般表达式。对各种模型计算了本征GaAs在λ=2.06um下的三光子吸收系数。实验上采用非线性光电导和非线性光透射技术,测量了GaAs中三光子吸收系数。     

高Q Kerr介质腔中非关联双模相干态光场与V型三能级原子相互作用系统中光场的等阶Y压缩效应

本文利用多模压缩态理论,详细研究了高Q Kerr介质腔中非关联双模相干态光场与V型三能级原子相互作用系统中双模光场的一次和二次等阶Y压缩效应,结果表明:1)等阶Y压缩特性强烈地依赖于Kerr介质的三阶非线性极化系数(x)和双模光场中各模的平均光子数n¹、n²;当x<1,n¹=n²<1时,不产生等阶Y压缩效应;而当x=2.5、5、10以及n¹=n²=2、5时,光场可呈现出强烈的一次及二次等阶Y压缩效应;2)在x以及n¹、n²相同的条件下,不同压缩阶次的等阶Y压缩效应的时间演化曲线的压缩周期与压缩阶次成反比,压缩度的幅度则随压缩阶数的升高而迅速下降;3)在平均光子不变(即n¹=n²恒定),但Kerr介质不同(即x变化)的条件下,相同压缩阶数的等阶Y压缩效应的Y压缩度曲线的时间演化周期与Kerr介质的三阶非线性化系数x成反比,压缩度的幅度则随Kerr介质的三阶非线性极化系数x的增大而增强;4)等阶Y压缩效应的持续时间以及等阶Y压缩度的大小等强烈地依赖于Kerr介质的非线性程度和光场强度;一般而言,Kerr介质的非线性程度越高(即x越大),并且光场越强(即n¹=n²取值越大),等阶Y压缩效应持续时间就越短,等阶Y压缩度就越大.     

高Q Kerr介质腔中非关联双模相干态光场与V型三能级原子相互作用系统中光场的不等阶Y压缩效应

本文利用多模压缩态理论,研究了高QKerr介质腔中非关联双模相干态光场与V型三能级原子相互作用系统中双模光场的不等阶Y压缩效应,绘出了第一模“i(i＝1,2)”次方第二模“j(j＝2,3)”次方Y(i-j-Y)压缩度的第一正交分量(Sy_ij¹),第二正交分量(Sy_ij²)的时间演化曲线.结果表明:1)不等阶Y压缩特性强烈地依赖于Kerr介质的三阶非线性极化系数(x)和双模光场中各模的平均光子数n₁,n₂;当x<1,n₁=n₂<1时,不存在不等阶Y压缩效应;而当x＝2.5,5,10以及n₁=n₂=5,10时,光场和系统开始作用后的很短一段时间内呈现不等阶Y压缩效应.2)在x以及n₁,n₂不变的条件下,不等阶Y压缩度时间演化曲线的崩坍-复原时间随压缩阶次的升高而缩短,压缩度随压缩阶数,特别是两模幂次差的升高而迅速下降.3)在平均光子不变(即n₁=n₂恒定),但Kerr介质不同(即x变化)的条件下,不等阶Y压缩度曲线崩坍时间随Kerr介质的三阶非线性极化系数x的增加而缩短,幅度则不受Kerr介质的三阶非线性极化系数变化的影响.4)不等阶Y压缩效应的持续时间等强烈地依赖于Kerr介质的非线性程度,x越大不等阶Y压缩效应持续时间就越短.不等阶Y压缩度演化曲线正向幅度随平均光子数的增大而增大,负向幅度(压缩度)则随平均光子数的增加而减小.     

不同晶系中二阶非线性极化率及SHG信号的研究

本文在对称性理论的基础上,研究了立方晶系、三角晶系和六角晶系中不同晶体结构类型、不同晶面和界面、不同磁化强度M取向下的二阶非线性极化率χ⁽²⁾的表达式以及不同偏振组合下二阶非线性极化强度P⁽²⁾的表达式和反射的SHG信号。     

非线性包层光纤中TE0模的传播特性分析

本文对具有Kerr非线性包层和线性匀芯的光纤在不同光功率下的传播特性进行了理论分析和数值计算.结果表明,在适当光功率下,当模折射率N等于芯折射率n_f时,芯场变得相当均匀,而包层场迅速衰减.对于自聚焦包层,当从N<n_f逐渐变到N>n_f时,芯场剖面越过N=n_f从凸变凹.在拐点,芯场成为均匀的.论文给出了模场分布和光功率与模折射率的关系.指出纤芯出现匀场预期会在光纤器件、光电子器件、光子器件中得到应用.     

双模腔场中具有不同耦合常数的两原子多光子辐射谱

研究了与双模腔场具有不同耦合常数的两个二能级原子的多光子辐射谱,给出了双模多光子辐射谱的一般表达式.结果表明,当双模腔场分别处于不同数态时,虽然两原子与双模腔场之间具有不同的耦合常数,但对于任意的N₁和N₂(N_i(i＝1,2,)为模i腔场被每个原子吸收或发射的光子数),辐射谱总是关于共振频率ω₀对称分布;并且,当N₁＝N₂时,对于任意的数态光子数n₁和n₂交换,辐射谱不变.上述特点用解析方法给予了解释.计算了非简并双光子情况下的辐射谱,并得到了一些新结果.双模腔场中单原子及具有相同耦合常数的两原子辐射谱可从本文结果分别做为特例而得到.     

半花菁LB膜的三阶非线性特性

本文利用Z扫描测量技术首次对半花菁LB膜的三阶非线性光学特性进行了研究.实验结果表明,LB多层膜中的半花菁分子具有较高的三阶非线性折射率n²=2.0×10^-8(esu),这主要是半花菁分子中π电子体系的贡献,并且由于532nm在其Q吸收带内,引起三阶非线性的共振增强效应,实验数据与理论拟合结果符合得很好.半花菁LB膜的三阶非线性的响应时间在30ns以内.     

非旋波近似下类克尔介质中“单模光场-受激三能级原子”系统的光子统计演化分析

本文研究了非旋波近似下类克尔介质对“单模光场-受激三能级原子”系统的光子统计演化特性的影响.数值计算的结果表明:类克尔介质的作用直接影响着受激辐射场的平均光子数< n(t) >的Rabi振荡的幅度和频率;而虚光场的影响则表现为受激辐射场的量子噪音,它与激发场强的变化及场与介质的耦合程度密切相关.另外,本文还对< n(t)>进行了频谱分析,结果发现该非线性系统存在着一系列高次谐波;并且,各高次谐波成分和振幅的变化情况完全依赖于激发场强及与介质的耦合程度.     

两原子非简并双光子Jaynes-Cummings模型的辐射谱

本文研究了两原子非简并双光子Jaynes-Cummings模型的辐射谱,结果表明,当双模腔场处于不同数态时,辐射谱呈现如下新特性:当一腔场处于真空态,另一腔场为强场时,辐射谱为对称六峰结构,任两对称峰的间距均与√2n₂g成正比;当双模腔场均为强场时,辐射谱为对称四峰结构,两内峰的间距(4g)与数态光子数n₁及n₂无关;当一腔场处于真空态,另一腔场处于数态|n₂>时,辐射谱一般为对称六峰结构,当n₂=1时,谱变为对称四峰结构。     

奇偶q相干态的高阶反聚束效应

本文研究了奇偶q相干态的高阶反聚束效应,结果表明,当参数q和r²在一定范围内取值时,奇偶q相干态都能呈现出高阶反聚束效应(N≥2),而且2n+1阶的反聚束效应和2n阶的反聚束效应相同。     

The Characterization of Absorptive Nonlinearities

The measurement of the order and magnitude of absorptive nonlinearities is essential to the development and improvement of a wide range of optical materials. This review presents and contrasts some of the most commonly used methods for making such measurements. As background, we introduce the history of, and the essential physics behind, multiphoton absorption (MPA) and phenomena that can interfere with its measurement. We review methods for the direct and indirect measurement of MPA. The direct methods covered include nonlinear transmission, absorption modulation, Z‐scan, and spectrally‐resolved, two‐beam coupling. The indirect methods reviewed, all of which rely on the measurement of multiphoton action cross sections, include nonlinear fluorescence excitation, thermal lensing, MPA‐induced photocurrent, and various techniques for determining the order of effective nonlinear absorption in multiphoton absorption polymerization.     

A model for a stochastic multiphoton absorption process of an anharmonic oscillator

A model of multiphoton absorption of i.r. laser radiation by an anharmonic, molecular, vibrational mode is discussed. The multiphoton absorption is described as a stochastic process for which the reservoir, including rotation and other vibrational modes, modulates the frequency of the active oscillator stochastically. The treatment of the nonlinear oscillator is based on quantization of a classical, driven anharmonic oscillator.     

In situ measurement of the effective nonlinear absorption order in multiphoton photoresists

Knowledge of the order of the effective nonlinear absorption in multiphoton photoresists is a key element in the development of improved materials for multiphoton absorption polymerization (MAP). The direct measurement of this nonlinearity has proven challenging. A new technique called 2‐beam initiation threshold (2‐BIT) is presented that allows for the unambiguous, in situ measurement of the order of the effective nonlinear absorption using a simple optical arrangement that can be employed with virtually any MAP setup. This technique is benchmarked using three common commercial photoinitiators that have been used previously in MAP and one common dye that acts as a photoinitiator. The linear absorption spectrum is demonstrated to be a poor predictor of the effective order of nonlinear absorption at a given wavelength. Surprisingly, for two of these initiators the effective nonlinear absorption process is dominated by 3‐photon absorption in the 800 nm wavelength range, suggesting that 2‐BIT is a valuable means of identifying initiators that can improve the resolution of MAP.

     

Multiphoton transitions in direct-gap semiconductors

Multiphoton transitions in direct-gap semiconductors are studied considering a “non-perturbative” approach in which the final state is represented by a Volkov wavefunction. Analytical solutions are obtained for multiphoton transition rates. The relation between multiphoton absorption and tunneling is discussed.     

Phonon-Assisted Photoluminescence in a Spherical Nanocrystal

Using the matrix density in the representation of path integrals for an electron, the multiphoton nonlinear absorption light coefficient in the second order of interaction energy with polar optical phonons is derived. This coefficient describes any electron interaction mechanism with phonons. From the interaction mechanism, the main role is played by dimensional resonance when the electron continuously absorbs energy from the field as a result of synchronizing its oscillation with the field. This dimensional resonance is possible when the frequency characterizing the laser field is a multiple of the phonon frequency. Whether a photon is absorbed or emitted, the initial level from where the transition occurs defines the temperature dependence. The absorption spectrum has the form of stripes whose intensity depends on the resonance character. The most pronounced absorption is at the triple resonance, where values of radiation and oscillatory and optical phonon frequencies are equal. __________ Published in Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 654–659, September–October, 2005.     

Transform-Limited Pulses Are Not Optimal for Resonant Multiphoton Transitions

Maximizing nonlinear light-matter interactions is a primary motive for compressing laser pulses to achieve ultrashort transform limited pulses. Here we show how, by appropriately shaping the pulses, resonant multiphoton transitions can be enhanced significantly beyond the level achieved by maximizing the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in which, by selectively removing certain spectral bands, the peak intensity of the pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we increase the absorption rate by a factor of 7.     

Doping transition metal ions as a method for enhancement of ablation rate in femtosecond laser irradiation of silicate glass

We present a doping method to improve the femtosecond laser ablation rate and promote ablation selectivity. Doping transition metal ions, Co2＋ or Cu2＋, in silicate glass apparently change absorption spectroscopy and induce resonant absorption at wavelengths of 600 and 800 nm, respectively. Comparing with femtosecond laser processing of the same glass without doping, we find that the threshold fiuenee decreases and the ablation rate increases in resonant absorption in doped silicate glass. Resonant absorption effectively increases multiphoton ionization for seed-free electron generation, which in turn enhances avalanche ionization.     

3D grafting via three-photon induced photolysis of aromatic azides

Utilization of multiphoton absorption has led to important advances in microscopy and photofabrication. Herein, our recent results on precise immobilization of molecules within 3D polymeric matrices by means of multiphoton grafting are presented. Assessment of nonlinear absorption properties of difunctional aromatic azide, selected for this purpose was performed by the Z-scan technique. It indicates that a three-photon absorption process is responsible for photolysis of this compound. Successful photografting for production of 3D patterns with lateral resolution down to 4???m was achieved. Our results demonstrate the potential of the developed three-photon grafting method for precise 3D site-specific functionalization of different materials.     

Methods of measuring the nonlinear-optical coefficients of the refractive index of nonlinear media

A cousistent analysis of a method of measuring the coefficient n ₂ of the Kerr nonlinearity and the two-photon absorption coefficient in presented. The method is based on the study of the dependence of the nonlinear transmittance with respect to the far-field axial intensity of the laser beam on the position of the layer of the nonlinear medium on the z (longitudinal) coordinate of this beam (z scanning). The possibility for the measurement of n ₂ and the two-photon (multiphoton) absorption coefficient based on the study of the far-field variations in the spatial profile of the beam having passed through the nonlinear medium is also analyzed. The calculations prove the efficiency of the new method for measuring n ₂ and the two-photon (multiphoton) absorption coefficient.