Nonlinear two-photon absorption properties induced by femtosecond laser with the films of two novel anthracene derivatives

Two novel anthracene derivatives containing 4-vinylpyridine (FPEA) and 2-vinylpyridine (TPEA) poly(methyl methacrylate) films are prepared on quartz glass substrates. Their nonlinear absorption properties are investigated by using a 120-fs, 800-nm Ti:sapphire femtosecond pulsed laser operating at a 1-kHz repetition rate. The unique nonlinear absorption properties of these new compounds are observed by utilizing a Z-scan system. These two-photon absorption (TPA) properties are proven by the two-photon fluorescence excited at 800 nm. The FPEA and TPEA films have nonlinear TPA coefficients of 0.164 and 0.148 cm/GW and the TPA cross sections of 3.345 × 10-48 and 3.081 × 10-48 cm4 ·s/photon, respectively. The influence of the chemical structures on the nonlinear TPA properties of the compounds is also discussed. The highly nonlinear TPA activities of the films implied that the new anthracene derivatives are suitable materials with promising applications in super-high-density three-dimensional data storage and nano- or microstructure fabrication.     

两个新合成化合物EPVPC和OPVPC非线性光学特性的理论研究

在杂化密度泛函理论（DFT/B3LYP）的水平上，研究了最新实验室合成的两个化合物分子9-乙烷基-3-{2-[4-2-吡啶-4-乙烯基苯]-乙烯基}-9氢咔唑（EPVPC）和9-十八烷基-3-{2-[4-2-吡啶-4-乙烯基苯]-乙烯基}-9氢咔唑（OPVPC）的非线性光学特性。理论结果与现有的实验测量结果符合得的较好。利用扩展的少态模型方法计算了分子的双光子吸收截面，结果表明三态模型可以很好地描述它们的最大双光子吸收截面。数值模拟显示这两个化合物都具有较大的双光子吸收截面，并且，在低频范围内，OPVPC分子比EPVPC分子显示出较强的双光子吸收特性。     

双光子吸收对光子晶体定向耦合全光开关的影响

 利用时域有限差分方法,在非线性条件下,考虑双光子吸收效应的同时,建立了分析2维光子晶体定向耦合波导光开关的模型。数值计算结果表明,对于长度较短的波导耦合器件,在强控制光和非线性条件下,双光子吸收效应会对波导的耦合作用产生一定的影响,从而改变入射光的透射特性,使光开关控制的动态过程发生变化。故而在设计全光开关器件时,对强控制光条件下的非线性双光子吸收效应有必要作为影响因素考虑进去。     

硒化镉载流子的超快动力学研究

硒化镉是一种可用于X射线全光分幅相机和全光条纹相机的重要探测材料。用基于相位物体的泵浦探测方式,研究了硒化镉在1030nm波长,飞秒脉冲下的载流子超快动力学和非线性光学特性。得到了双光子吸收系数、载流子吸收截面、载流子复合时间等参数。实验表明,硒化镉载流子的动力学和非线性特性是由束缚电子和载流子共同决定的。束缚电子的克尔效应和双光子激发都是瞬态的,而载流子复合持续了较长时间。这些参数和载流子图像的的获得,为X射线超快探测器件的设计和改进提供了参考。     

Z-scan study of nonlinear absorption of gold nano-particles prepared by ion implantation in various types of silicate glasses

Metal nano-clusters composite glasses synthesized by ion implantation have been shown as promising nonlinear photonic material. In this paper, we report on the nonlinear absorption measurements of gold nano-particles implanted in four structurally different types of silicate glasses. All targets containing gold nano-particles in a layer 500 nm under the surface of the glass have been prepared by ion implantation with subsequent annealing. The targets were characterized by UV–VIS absorption spectroscopy, transmission electron microscopy (TEM) and by the Z-scan technique. The resulting nano-particles differed in size, range of particle size and shape as well as depth distribution characteristic for glasses with different chemical compositions. With the Z-scan technique, it can be shown that the nano-particles produced in silicate glasses exhibit substantial two-photon absorption (TPA). The TPA coefficient differed depending on size, shape, and depth distribution of the metal nano-clusters and the structure and composition of the glass substrates. The highest TPA coefficient (16.25 cm/GW) was found for the glass BK7 in which the largest non-spherical nano-particles have been observed in the thinnest layer.     

有机染料HEASPS在不同波长时的激发态吸收和双光子吸收性质研究

提出了有机染料反式 4 [4′ (N 羟乙基 N 乙基胺基 )苯乙烯基 ] N 甲基吡啶对甲苯磺酸盐 (简称HEASPS)非线性吸收的三态模型。认为在较短波长 (<1 0 0 0nm)时 ,双光子吸收和激发态吸收同时存在。成功地解释了该染料双光子吸收峰相对线性吸收峰的两倍的明显蓝移 ,以及最高上转换效率波长相对最强吸收波长的明显红移。提出了一种新的计算方法 ,利用不同波长的非线性透过率和不同波长的上转换效率 ,求出了该染料的双光子吸收截面和激发态吸收系数。     

Picosecond nonlinear optical properties of cuprous oxide with different nano-morphologies

Cuprous oxide nanoclusters, microcubes and microparticles were successfully synthesized by a simple co-precipitation method. Phase purity and crystallinity of the samples were studied by using X-ray powder diffraction. Transmission electron microscopy (TEM) images show different morphologies like nanoclusters, microcubes and microparticles. For linear and nonlinear optical measurements, the as-synthesized Cu₂O with different morphologies were dispersed in isopropanol solution. The absorption spectrum recorded in the visible regions shows peaks that depend on the morphology of the particles and the peak shifts towards red region as one goes from nanoclusters to microparticles. Simple open-aperture Z-scan technique is used to measure nonlinear optical properties of cuprous oxide at 532 nm, 30 ps excitation at 10 Hz repetition rate. Cuprous oxide nanoclusters show reverse saturable absorption (RSA) behaviour, the microcubes and microparticles at a similar concentration exhibit saturable absorption (SA) type of behaviour at lower peak intensities and exhibit RSA within SA at higher peak intensities. The results show that the transition from SA to RSA can be ascribed to the two-photon absorption (TPA) process.     

有机双光子吸收材料的研究进展

本文综述了具有优异双光子吸收/双光子荧光性质的有机功能材料的研究工作和成果。近年来,有机材料凭借其出众的性质、丰富的种类、多重功能性以及快速的非线性光学响应引起国际科学和技术界的极大兴趣。文章介绍了测量材料的非线性光学性质和超快响应的实验方法,它们已被广泛应用于有机材料的研究工作,并且取得了重大的进展。本文描述了对偶极、四极、多枝结构、大环结构以及聚合物等有机材料的非线性光学性质和超快光学响应的研究结果。基于有机材料的迷人性质,具有优异非线性光学性质和超快响应的有机材料将会在很多领域显示出巨大的应用潜力。     

All-optical set-reset flip-flop based on the passive microring-resonator bistability

In this paper, we propose a design for an all-optical R-S flip-flop with separate set and reset inputs, which is based on the optical bistability due to Kerr and two-photon absorption (TPA) effects in the microring resonators. Based on the steady state analysis of the light propagation in nonlinear microrings, variations of hysteresis parameters versus the system variables, such as transmission coefficient, nonlinear refractive index and detuning angle, are analyzed. The switching temporal behavior of the proposed flip-flop is obtained by the microring transient analysis. It is shown that the contrast ratio parameter and the switching power of the proposed all-optical flip-flop can be controlled by system parameters.     

Z-scan analytical theory for material with saturable absorption and two-photon absorption

By using the Adomian decomposition method, we present the transmitted optical intensity of a nonlinear medium with the concurrence of saturable absorption (SA) and two-photon absorption (TPA) processes. We obtain the analytical expression of the open-aperture Z-scan and investigate the interplay between SA and TPA in the Z-scan trace. Through Z-scan measurements at different levels of laser intensities, both the saturable intensity and TPA coefficient could be obtained quickly yet unambiguously.     

A variational approach of nonlinear dissipative pulse propagation

A variational technique to deal with nonlinear dissipative pulse propagation is established. By means of a generalization of the Kantorovitch method, suitable for non-conservative systems, we are able to cope with an extended nonlinear Schr?dinger equation (NLSE) which describes pulse propagation under the influence of nonlinear loss and/or gain, in particular, in the presence of two-photon absorption (TPA). Based on the characteristics of the exact solution of the NLSE in the absence of TPA, we investigate the effects of frequency dispersion of the nonlinear susceptibility associated to the two-photon resonance, obtaining the necessary conditions for a solitary wave solution, even in the presence of a self-steepening term. Received: 4 August 1997 / Received in final form: 25 November 1997 / Accepted: 14 January 1998     

Two-photon absorption of high-power picosecond pulses in PbWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, PbMoO<Subscript>4</Subscript>, and CaMoO<Subscript>4</Subscript> crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm². The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO₄ and ZnWO₄ crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO₄ to β = 0.14 cm/GW for CaMoO₄, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.     

Efficient two-photon absorption of CdSe-CdS/ZnS core-multishell quantum dots under the excitation of near-infrared femtosecond pulsed laser

We report that two-photon absorption (TPA) properties of semiconductor CdSe-core CdS/ZnS-multishell quantum dots (QDs) in toluene under excitation of femtosecond laser at 800 nm. The results show efficient TPA process and large TPA cross section of three types of size QDs, which is 1900, 5710, and 16060 GM (1 GM = 10⁻⁵⁰ cm⁴ s photon⁻¹), respectively. TPA cross section dramatically increases with increased core size, showing a strong size-dependence effect. Furthermore, two-photon excitation (TPE) fluorescence intensity not only depends on TPA capacity, but also relies on improved quantum yield resulting from passivation of QD surface by different coated monolayers (MLs). These facts in combination with the narrow fluorescence bandwidth make these QDs as promising probes for multicolor two-photon microscopy.     

Simultaneous imaging of multiple focal planes using a two-photon scanning microscope

Despite all the advances in nonlinear microscopy, all existing instruments are constrained to obtain images of one focal plane at a time. In this Letter we demonstrate a two-photon absorption fluorescence scanning microscope capable of imaging two focal planes simultaneously. This is accomplished by temporally demultiplexing the signal coming from two focal volumes at different sample depths. The scheme can be extended to three or more focal planes.     

Two-photon absorption coefficient dichroism in Ⅱ-Ⅵ semiconductor crystals

Considering two beams propagate in semiconductor crystal, this paper discusses the polarization dependence of pump beam-induced intensity attenuation of probe beam due to two-photon absorption (TPA). Numerical calculation and experimental measurement demonstrate that TPA coefficient is polarization dependent. For homogeneous materials, probe beam attenuation arises from the imaginary part of diagonal and off-diagonal components of third-order nonlinear susceptibilities.     

Generation of a dark nonlinear focus by spatio-temporal coherent control

The transition probability of a two-photon absorption (TPA) process in atomic Cesium, excited by phase-controlled temporally focused ultrashort pulses is shown to be spatially modulated in a controlled manner. In particular, we demonstrate the generation of a dark nonlinear focus. By controlling the excitation pulse shape along the propagation coordinate we create a region in space where the TPA rate vanishes which is flanked by bright regions.     

具有不同共轭长度的D-π-A-π-D型低聚物的非线性光学性质

通过实验和理论方法研究了共轭长度依赖的芴酮基线型共轭低聚物的非线性光学性质. 首先,红外光谱和稳态吸收光谱测量表明：共轭单元的增加会增强碳-碳双键伸缩振动峰,并导致稳态吸收峰的红移. 同时,双光子荧光强度会随着激发光能量的增长而逐渐增强,在引入芴-亚乙烯基单元后,双光子荧光效率有了明显的升高. 进而利用完全态求和方法模拟低聚物的双光子吸收截面,得到的理论值与通过飞秒开孔Z-扫描技术获得的实验结果吻合得较好. 研究结果表明共轭体系的延展确实会对低聚物双光子吸收特性的提高起到重要的作用.     

氧化石墨烯-卟啉复合材料光限幅性能的理论研究

利用时域有限差分法数值求解速率-场强方程,研究了一系列共价连接的氧化石墨烯-卟啉复合材料在纳秒时域内的光限幅性质和双光子吸收. 计算结果表明,氧化石墨烯-卟啉复合分子与单独的卟啉分子相比展现出增强的光限幅效应,并且有着更大的双光子吸收截面.与之前得到的含重金属的卟啉分子具有更强非线性光学性质的结论不同,不含金属元素的卟啉分子与氧化石墨烯结合后展现出了更强的非线性光学性质. 理论计算的结果与实验测量符合较好. 此外,着重研究了介质的厚度和脉冲宽度对分子双光子吸收截面的影响. 结果表明,随着介质厚度的增加或者脉宽的变宽分子的动态双光子吸收截面增大.     

Ultrafast nonlinear optical response of carbon nanotubes functionalized with water soluble porphyrin

A new nanocomposite is obtained by functionalizing carbon nanotubes (CNTs) with a water soluble metalloprophyrin using a simple chemical technique and characterized by optical absorption, IR, and Raman spectroscopy. Results from spectroscopic studies indicate the noncovalent nature of interaction between CNTs and porphyrin. The ultrafast nonlinear response is characterized by measuring the nonlinear absorption coefficient and refractive index by z-scan technique in the femtosecond pulse regime. The nanocomposite is found to exhibit two-photon absorption (TPA) with a reasonably large nonlinear optical coefficient, whereas pure CNTs is known to exhibit saturable absorption. Design of such water soluble nanocomposites offers scope for obtaining materials with enhanced ultrafast optical nonlinearity.     

Theoretical investigation on nonlinear absorption of multilevel organic molecular system in ns, ps and fs regime

The nonlinear absorption properties of organic materials in nanosecond (ns), picoseond (ps) and femtosecond (fs) regime were theoretically investigated by rate equations based on multilevel organic molecular system, respectively. The simulation results show that the RSA is mainly due to the absorption contribution of the triplet first excited state in the ns regime and that of the singlet first excited state in the ps regime, both of which are a third-order nonlinear phenomena. The RSA in fs regime is mainly due to the excited state absorption (ESA) induced by two-photon absorption (TPA), which is a fifth-order nonlinear phenomenon.