Mathematical modeling of two-photon thermal fields in laser–solid interaction

In this paper, we have developed an analytical model to study the temperature distributions in IR optical materials heated by laser pulses. Our model takes into account the two-photon absorption (TPA). The calculations are based on a three-dimensional model of heat diffusion in solids using the integral transform method. We find out the rigorous analytical expression of the thermal field when one considers both one- and two-photon absorption. The model is valid for any laser–solid system whose interaction can be described by the generalized Beer–Lambert law. Specific results are presented for an application of the model to ZnSe sample. We find out that TPA can produce detectable temperature variation.     

Transparency due to dipole–dipole interaction in photonic crystals doped with nanoparticles

In this paper, we have studied the two-photon absorption process in photonic crystals doped with four-level nanoparticles. It is considered that all the levels of the particles are interacting with the photonic crystal. The decay linewidths of all the four levels are included in the calculations of two-photon absorption process. The effect of the dipole–dipole interaction has also been included in the formulation. Numerical simulations have been performed for the two-photon absorption spectrum on an isotropic photonic crystal. It is found that the system switches between the transparent and the nontransparent states due to the dipole–dipole interaction. The phenomenon of switching depends on the linewidths of these states.     

Fluorescence upconversion properties of a class of improved pyridinium dyes induced by two-photon absorption

We report the fluorescence upconversion properties of a class of improved pyridinium toluene-p-sulfonates having donor–π–acceptor (D–π–A) structure under two-photon excitation at 1064 nm. The experimental results show that both the two-photon excited (TPE) fluorescence lifetime and the two-photon pumped (TPP) energy upconversion efficiency were increased with the enhancement of electron-donating capability of the donor in the molecule. It is also indicated that an overlong alkyl group tends to result in a weakened molecular conjugation, leading to a decreased two-photon absorption (TPA) cross section. By choosing the donor, we can obtain a longest fluorescence lifetime of 837 ps, a highest energy upconversion efficiency of 6.1%, and a maximum TPA cross-section of 8.74×10⁻⁴⁸ cm⁴ s/photon in these dyes.     

双光子吸收材料二苯乙烯衍生物的合成及光谱性质

设计、合成并用红外光谱、1H NMR、元素分析表征了三种用于双光子吸收材料的二苯乙烯衍生物,4,4′-双(二苯氨基-反式-苯乙烯基)联苯(BPSBP),4,4′-双(二乙氨基-反式-苯乙烯基)联苯(BESBP)和4,4′-双(9-咔唑基-反式-苯乙烯基)联苯(BCSBP)。实验结果表明三者最强的单光子吸收出现在350～400 nm之间,且单光子吸收和荧光光谱中表现出明显的溶剂化显色效应,揭示了分子内对称电荷转移的本质,双光子荧光光谱则揭示了单光子和双光子吸收具有相同的发射机理。利用双光子上转换荧光法测试发现,三种双光子吸收材料在800 nm飞秒激光的激发下具有较大的吸收截面,分别为892,617和483 GM,这表明在双光子领域有潜在的应用价值。     

Two-photon excitation of confined biexcitons in CuCl quantum dots

We have studied direct creation processes of confined biexcitons in CuCl quantum dots by polarization-dependent resonant two-photon excitation spectroscopy. The two-photon absorption band for the lowest state of the biexciton (total angular momentum J=0) which appears on the lower energy side of confined exciton band was identified from the analysis of the polarization dependence of the photoluminescence excitation spectrum of the biexciton. Furthermore, the two-photon excitation process for the excited state of the biexciton (J=2) was also found with polarization dependence different from the J=0 biexciton state.     

以芘为中心系列衍生物的单光子和双光子吸收特性理论研究

在含时密度泛函理论的水平上,利用响应场理论方法研究了以芘为中心系列衍生物的单光子和双光子吸收特性. 研究结果表明,在低能量范围内,每个分子有多个电荷转移态. 单光子吸收性质与实验结果符合较好. 随着分子尺寸的增加,分子的最大双光子吸收截面显著增加,其中具有四分枝结构分子的最大双光子吸收截面是单枝结构分子的5.6倍. 同时,分子的双光子吸收截面与分子结构的对称性有关.其中对称结构分子具有较大的双光子吸收截面.     

All-optical logic gates based on two-photon absorption in semiconductor optical amplifiers

When the two-photon absorption of a high intensity pump beam takes place in a semiconductor optical amplifier there is an associated fast phase change of a weak probe signal. A scheme to realize fast all-optical XOR logic function using two-photon absorption induced phase change has been analyzed. Rate equations for semiconductor optical amplifiers, for input data signals with high intensity, configured in the form of a Mach–Zehnder interferometer has been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The model shows that both XOR operation and pseudo-random binary sequence generation at 250 Gb/s with good signal to noise ratio is feasible.     

Microfabrication of gold dots in SiO2/TiO2 glass films by two-photon absorption

Photogeneration of Au nanoparticles in SiO₂/TiO₂ glass films was carried out by two-photon absorption with a femtosecond pulse laser. Exquisite microdot-arrays of Au with micrometer spatial resolution were achieved by scanning of the focused laser beam. These structures were constructed in SiO₂/TiO₂ glass films by a sol–gel method. The sol–gel method demonstrated that Au dots microarray are fabricated at any position by two-photon absorption in the glass. The results show the utility of a two-photon absorption technique in the fabrication of complicated patterns with metal particles.     

Nonlinear spectrum effect on the coherent control of molecular systems

This work demonstrates that the detuning of the fs-laser spectrum from the two-photon absorption band of organic materials can be used to reach further control of the two-photon absorption by pulse spectral phase manipulation. We investigate the coherent control of the two-photon absorption in imidazole-thiophene core compounds presenting distinct two-photon absorption spectra. The coherent control, performed using pulse phase shaping and genetic algorithm, exhibited different growth rates for each sample. Such distinct trends were explained by calculating the two-photon absorption probability considering the intrapulse interference mechanism, taking into account the two-photon absorption spectrum of the samples. Our results indicate that tuning the relative position between the nonlinear absorption and the pulse spectrum can be used as a novel strategy to optimize the two-photon absorption in broadband molecular systems.     

Large two-photon absorbance of chitosan–ZnS quantum dots nanocomposite film

ZnS semiconductor nanoparticles with average size of 3.4 nm were prepared in situ in chitosan film. TEM, UV–vis spectra and PL spectra show the ZnS nanoparticles in chitosan template were monodispersed and well passivated. The two-photon absorption coefficient (β) of the chitosan–ZnS quantum dots (QDs) nanocomposite film was obtained to be 2.29×10² cm/Gw from a standard Z-scan setup with femtosecond laser pusles at 790 nm wavelength. Results show the novel biomacromolecule/QDs nanocomposite film has large third-order optical nonlinear absorption, the mechanism responsible for which was discussed.     

反-4,4''-双(N,N-二丁胺基)二苯乙烯分子双光子吸收性质研究

对于实验室合成的反-4,4'-双(N,N-二丁胺基)二苯乙烯分子,实验测量了该分子的单光子和双光子荧光谱,然后从理论上研究了其单光子和双光子吸收特性.研究结果表明,在低能量范围内,分子的单光子吸收主要发生在分子的第一激发态,而分子的双光子吸收主要发生在分子的第二和第四激发态上.该分子在相应系列衍生物中具有最大的双光子吸收截面.分子的相关能对分子的激发态能量影响较大.我们给出了分子基态与电荷转移态的电荷转移过程,并从理论上定性解释了双光子聚合反应的聚合机理.     

Two-photon induced optical-power limiting and upconverted superradiance properties of a new organic dye HEASPI

A new organic dye trans-4-[p-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium iodide (abbreviated as HEASPI hereafter) with large two-photon absorption (TPA) cross section and excellent upconverted superradiance properties was synthesized in our group recently. The TPA cross section was measured to be σ₂′=7.0×10⁻⁴⁸ cm⁴ s/photon by using an open aperture Z-scan system. Linear absorption, single-photon induced fluorescence, two-photon induced fluorescence and two-photon pumped (TPP) upconverted superradiance properties were systematically studied. The highest net upconversion efficiency from the absorbed pump energy to the output upconverted superradiance energy is as high as 19.6% at the pump energy of 2.07 mJ from a mode-locked Nd : YAG picosecond laser. The dye solution also shows a clear optical power limiting effect.     

溶剂对N-{4-[(反式)-2-(4-硝基苯基)乙烯基]苯基}-N,N-二苯氨基分子双光子吸收特性的影响

在从头计算的基础上,利用两态模型对溶剂对N-{4-[(反式)-2-(4-硝基苯基)乙烯基]苯基}-N,N-二苯氨基分子双光子吸收特性的影响进行了理论研究.计算结果表明,溶剂对该分子的双光子吸收截面影响较大,双光子吸收截面随着溶剂极性的增加而增加.     

Acetylene-Substituted Two-Photon Absorbing Molecules With Rigid Elongated Pi-Conjugation: Synthesis,Spectroscopic Properties and Two-Photon Fluorescence Cell Imaging Applications

Two asymmetrical molecules with substituted acetylene as central rigid elongated conjugation are reported as potential chromophores for two-photon microscopic imaging. These molecules consist of a typical D–π–A structure, have different donors (D), the same π-conjugated center (π) and the same acceptor (A). Structural characterization and spectroscopic properties, including single-photon (linear) absorption, quantum yields, single-photon fluorescence, and two-photon absorption spectra, were studied in solvents with different polarity. These acetylene-substituted molecules were found to have high two-photon absorption cross-sections (for example, 690 GM for molecule 1 in toluene), which were determined by a two-photon induced fluorescence method using a femtosecond Ti: sapphire laser as excitation source. Single- and two-photon cellular imaging experiments demonstrate that the substituted acetylene derivatives could be one kind of promising two-photon fluorescence probes for cellular imaging.     

Theory of coherent,degenerate two-photon absorption and emission

The theory of the coherent, two-photon resonant interaction of a monochromatic field with N atoms is given. It is seen that the dynamics of the atom-field system can be completely determined when the field is “strong”. Two specific examples are given: (i) two-photon absorption by atoms in ground state, and (ii) stimulated two-photon emission by fully excited atoms, assuming a coherent field in both cases. In case (ii), the field shows photon-antibunching after the decay of half of the atoms. The merits of our approach are shown by comparing with other treatments. Our results can also be applied to certain degenerate four-wave mixing processes which are described by a similar Hamiltonian.     

Two-photon induced anisotropy in PMMA film doped with Disperse Red 13

In this paper we present optical anisotropy induced by 150-fs laser pulses operating at 775 nm in poly(methyl methacrylate) (PMMA) films doped with Disperse Red 13 (DR13). Two-photon absorption of DR13 induces reversible birefringence in the sample, a feature associated with the molecular orientation caused by isomerization of the azochromophores. No irreversible anisotropy, associated with photobleaching of the chromophore was observed. The slope of 2 obtained for the induced birefringence as a function of the excitation irradiance corroborates the two-photon nature of the 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 logic NAND gate based on two-photon absorption in semiconductor optical amplifiers

We propose a new scheme to realize all optical logic NAND operating at high speeds up to 250 Gb/s utilizing the ultrafast phase response during two-photon absorption (TPA) process in semiconductor optical amplifiers (SOA). NAND gate is important because other Boolean logic elements and circuits can be realized using NAND gates as basic building blocks. Rate equations for semiconductor optical amplifiers (for input data signals with high intensity) configured in the form of a Mach-Zehnder interferometer have been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The performance of this scheme is analyzed by calculating the quality factor of the resulting data streams. The results show that both AND and NAND operations at 250 Gb/s with good signal to noise ratio are feasible.     

Synthesis and two-photon optical characterization of D-π-D type Schiff bases

The title compounds, several Schiff bases with D-π-D type have been successfully synthesized from reaction of 4-(N-dialkylamino) benzaldehyde and hydrazine or p-phenylenediamine in ethanol. These compounds were characterized by IR, ¹H NMR, elemental analysis and electronspray mass spectrometry. One-photon fluorescence and two-photon fluorescence properties have been investigated. The two-photon absorption (TPA) cross section (σ) of these compounds was obtained by using the open-aperture Z-scan technique. Based on theoretical calculations and factual test, the influence that a series of substituted groups and the conjugated frame imposed on TPA cross section were systematically discussed.     

一种新合成的八极矩发色团分子单光子和双光子吸收性质的质子化效应

利用第一性原理基础上发展的一种计算方法研究质子化效应对具有1, 3, 5-三嗪核和吡咯末端基团八极矩分子三嗪的单光子和双光子吸收性质的影响. 结果表明,当三嗪被质子化时,该分子的单、双光子吸收性质发生了很大的变化. 一是出现了更多的电荷转移态,并且吸收带发生了红移;二是双光子吸收截面得到了很大的增强. 质子化效应有利于提高分子内电荷转移的强度,定性地解释了实验工作. 并提出了一种基于化合物三嗪质子化效应的双光子吸收转换开关.