Studies of nonlinear optical properties of PicoGreen dye using Z-scan technique

In this paper we study the nonlinear optical properties of PicoGreen dye. The investigations involve the single-beam Z-scan technique and measurements were carried out at different incident intensities. Both open and closed aperture Z-scan techniques were performed at 532 nm and it was found that the dye exhibited a reverse saturable absorption with significant nonlinear absorption coefficient and intensity-dependent negative nonlinear refraction coefficient, indicating self-defocusing phenomena. The third-order nonlinear susceptibility and optical limiting threshold were also measured.     

Thermally induced nonlinear optical response and optical power limiting of acid blue 40 dye

We report the investigations of thermally induced third-order nonlinear optical and optical limiting characterizations for various concentrations of acid blue 40 dye in N,N-Dimethyl Formamide, studied by employing z-scan technique under cw He–Ne laser irradiation at 633 nm wavelength. The samples exhibited nonlinear absorption and nonlinear refraction under the experimental conditions. For lower concentration, the samples display both saturable absorption (SA) and reverse saturable absorption (RSA); whereas with increase in concentration, RSA behaviour prevails. The estimated values of the effective coefficients of nonlinear absorption β_eff, nonlinear refraction n₂ and third-order nonlinear susceptibility χ⁽³⁾ were found to be of the order of 10^?2 cm/W, 10^?4 esu and 10^?6 esu respectively. Multiple diffraction rings were observed when the samples were exposed to laser beam due to refractive index change and thermal lensing. The effect of concentration and the laser intensity on the self-diffraction ring patterns was studied experimentally. The acid blue 40 dye also exhibited strong optical limiting properties under cw excitation and reverse saturable absorption is found to be the dominant nonlinear optical process leading to the observed nonlinear behaviour.     

半花菁衍生物LB膜的非线性光学性能研究

本文利用二次谐波产生、紫外－可见吸收光谱、Ⅱ－Ａ曲线等方法，探讨了四种分子结构相近的半花菁染料衍生物ＬＢ膜的非线性光学性能。研究了它们的分子结构与β值大小的关系，分析了引起β值变化的主要原因，证实了极性基团的强弱顺序。并讨论了ＬＢ膜中聚集效应程度不同的原因。通过光学二次谐波信号强度与膜层数的关系，对比了样品的成膜性能。     

Linear and nonlinear optical properties of two-photon absorption dye doped linear copolymer

2-hydroxyethyl methacrylate (abbreviated as HEMA) and acrylonitrile have been used as the monomers to form a trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium p-toluene sulfonate (abbreviated as HMASPS) doped linear copolymer. The spectra of one and two-photon excited fluorescence and two-photon pumped superradiance and two-photon pumped lasing of the polymer all shifted to short wavelengths compared with the solution sample of the dye. The copolymer shows much longer one- and two-photon excited fluorescence lifetimes of nanoseconds. When pumped by the picosecond Nd:YAG laser, the superradiance and lasing can be simultaneously obtained. The maximum two-photon pumped lasing conversion efficiency is 3.3%. The place of the maximum upconversion efficiency of the copolymer does not coincide with that of the maximum nonlinear absorption.     

Two-photon absorption and nonlinear optical properties of a new organic dye PSPI

Trans-4-[p-(pyrrolidinyl)styryl]-N-methylpyridinium iodide (abbreviated as PSPI thereafter) is a two-photon absorption (TPA) dye newly synthesized by our research group. It possesses much larger TPA cross-section and much stronger upconversion fluorescence emission than those of common organic dye (such as rhodamine) when excited with near infrared (IR) radiation. TPA spectrum and upconversion efficiency spectrum of HEASPI solution at various wavelengths were measured. There is 34 nm blue shift for the central wavelength of the TPA induced absorption peak compared with two times of the linear absorption peak. The biggest molecular TPA cross-section σ₂^′ is 2.85×10⁻⁴⁷ cm⁴ s/photon at 930 nm. At 1064 nm, σ₂^′ is 3.12×10⁻⁴⁸ cm⁴ s/photon. The highest efficiency is 3.9% at 1010 nm, whereas 2.9% at 1064 nm. Its optical power limiting properties at 930 nm have also been illustrated.     

Effect of urea and thiourea on nonlinear optical hippuric acid crystals

Pure, urea and thiourea doped hippuric acid (HA) single crystals have been grown in acetone using slow evaporation technique at a constant temperature, with the vision to improve the properties of the crystals. The crystal systems and the unit cell parameters of the grown crystals were identified from single crystal X-ray diffraction. The crystalline nature of the grown crystals was confirmed by powder X-ray diffraction and the diffraction peaks were indexed. The variations in composition due to the addition of dopants were identified by CHNS analysis. FT-IR studies reveal the presence of different vibrational bands. The optical characteristics were assessed by UV–vis analysis and it indicates the transmission in the visible region. TGA and DSC studies indicate the thermal behavior of pure and doped crystals. The Second Harmonic Generation (SHG) has been tested using Kurtz Powder Technique for the pure and doped crystals. It is found that the thiourea doped hippuric acid crystals have SHG efficiency of 2.08 times higher than that of potassium dihydrogen phosphate (KDP) single crystal. The dielectric studies were carried out, and the variations of dielectric constant and dielectric loss with temperature have been studied.     

The nonlinear optical response of a fluorine-containing azoic dye

The nonlinear optical nonlinearities of a fluorine-containing azoic dye in tetrahydrofuran have been investigated by using Z-scan technique with picosecond and nanosecond lasers. The experimental results reveal that the azoic dye has large optical nonlinearity under the excitations of picosecond and nanosecond 532 nm. At the picosecond 532 nm the solution presents negative nonlinear refraction due to the electronic effect, while the larger nonlinear refraction under nanosecond laser excitation is induced by thermal effect. Moreover, the different nonlinear absorption behavior under picosecond and nanosecond excitations is analyzed.     

Investigation of nonlinear optical properties of organic dye by Z-scan technique using He-Ne laser

The third-order nonlinear optical properties of Basic Green 1 dye were measured by the Z-scan technique and measurements were carried out at different concentrations and several incident intensities. The results showed that the Basic Green 1 dye exhibited large nonlinear refractive coefficient () and nonlinear absorption coefficient () at the wavelength 632.8 nm of He-Ne. The negative sign of the nonlinear refractive index n₂ indicates that this material exhibits self-defocusing optical nonlinearity. These results show that Basic Green 1 dye has potential applications in nonlinear optics.     

Two-dimensional dye crystals with controllable optical properties

The structure, molecular ordering and optical properties of single crystals of cyanine dyes grown by adsorption from a water subphase to a positively charged lipid monolayer are discussed. These crystals are one monolayer thick, of uniform dimensions between 10 and 100μm (depending on nucleation conditions) and of rectangular shape. Single crystals were studied by transmission electron diffraction and by polarized absorption and emission spectroscopy.

We show that the crystals consist of two rows of densely stacked molecules with two different orientations of the long molecular axes. This leads to two perpendicularly polarized absorption bands. The measured splitting is in accordance with results of extended dipole calculations. The latter were performed for crystals of three slightly different molecules where the angles between the long axes varied between 70° and 100°. The aliphatic tails form a lattice which is epitaxially related to the centered rectangular one of the dyes. It is incommensurate, and the tails are tilted with tilt azimuth either parallel to the a or to the b axis (depending on the type of dye) of the dye lattice.

The procedure allows formation of mixed crystals with structural parameters, depending on composition, between those of the pure compounds. Hence also the optical spectra can be tuned via composition maintaining sharp band edges and emission bands.     

Nonlinear optical properties of organic dye dimer-monomer systems

An analysis is provided for the discussion of nonlinear optical properties of aggregated dye systems. The intensity-dependent transmission of fluorescein solutions excited by nanosecond dye laser pulses is shown to fit a monomer-dimer equilibrium shifted towards monomer formation by excitation. The implications for spatial light modulation and other nonlinear optical applications are discussed.     

Influence of solitons on the conductance properties of double-stranded deoxyribonucleic acid

A numerical study is presented to investigate the role of solitons in the electronic states of double-stranded DNA (dsDNA) molecule in the metal/DNA/metal system. Based on tight-binding Hamiltonian model and within the framework of a generalized Green’s function technique, we consider a ladder model for poly(dG)-poly(dC) DNA molecule containing M cells with four sites (two base pair sites and two backbone sites) in each cell. In the presence of a sublattice of solitons, our results show that the homogeneous soliton distributions induce the electronic states in the band gap of DNA molecule. In addition, the room temperature current-voltage characteristic of the system shows a linear and ohmic-like behaviour.     

三苯胺衍生物分子非线性光学性质的理论研究

在密度泛函理论水平上,利用响应函数方法,研究了1-{(1E)-2-[4-(二苯胺基)苯基]乙烯基}-4-[4-N,N-二甲胺]苯(PVMB)和1-[(1E)-2-(4-(1E)-2-{4-[4-N,N-二甲胺]苯基}乙烯基)苯基]苯胺}苯基)乙烯基]-4-[4-N,N-二甲胺]苯(DPVMB)两分子的双光子吸收特性.计算结果表明,这两个化合物都具有较好的双光子吸收特性,且具有两分支结构的DPVMB分子比具有单支结构的PVMB分子有更强的双光子吸收强度.计算数值结果和实验结果符合地较好.     

Synthesis and luminescence properties of dye-doped deoxyribonucleic acid films

Dye-doped deoxyribonucleic acids (DNA)–tetradecyltrimethylammonium (TTA) films have been prepared. Rhodamine 6G, known as laser dyes, can be spontaneously doped by immersing the DNA–TTA film in rhodamine 6G-acetonitrile solutions. It is surmised that rhodamine 6G monomers and dimers diffuse within the hydrophobic TTA sites, and then monomers presumably intercalate between adjacent base pairs of DNA. Optical absorption spectra reveal that rhodamine 6G molecules in the sample undergo an unusual transformation from the dimer state to the monomer state with the elapse of time. Rhidamine 6G molecules doped in DNA–TTA show enhanced photostability and concentration quenching than those in PMMA. The environment, conformation and chemical stability of rhodamine 6G are different between DNA–TTA and PMMA, and are presumably modified by the intercalation.     

Effect of Mn doping on the structural and optical properties of sol-gel derived ZnO nanoparticles

Un-doped and Mn-doped ZnO nanoparticles were successfully synthesized in an ethanolic solution by using a sol-gel method. Material properties of the samples dependence on preparation conditions and Mn concentrations were investigated while other parameters were controlled to ensure reproducibility. It was observed that the structural properties, particle size, band gap, photoluminescence intensity and wavelength of maximum intensity were influenced by the amount of Mn ions present in the precursor. The XRD spectra for ZnO nanoparticles show the entire peaks corresponding to the various planes of wurtzite ZnO, indicating a single phase. The diffraction peaks of doped samples are slightly shifted to lower angles with an increase in the Mn ion concentration, signifying the expansion of the lattice constants and increase in the band gap of ZnO. All the samples show the absorption in the visible region. The absorbance spectra show that the excitonic absorption peak shifts towards the lower wavelength side with the Mn-doped ZnO nanoparticles. The PL spectra of undoped ZnO consist of UV emission at 388 nm and broad visible emission at 560 nm with varying relative peak intensities. The doping of ZnO with Mn quenches significantly the green emission while UV luminescence is slightly affected.     

Z-scan and optical limiting properties of Hibiscus Sabdariffa dye

The intensity-dependent refractive index n ₂ and the nonlinear susceptibility χ ⁽³⁾ of Hibiscus Sabdariffa dye solutions in the nanosecond regime at 532 nm are reported. More presicely, the variation of n ₂, β, and real and imaginary parts of χ ⁽³⁾ versus the natural dye extract concentration has been carried out by z-scan and optical limiting techniques. The third-order nonlinearity of the Hibiscus Sabdariffa dye solutions was found to be dominated by nonlinear refraction, which leads to strong optical limiting of laser.     

掺杂石墨烯系统电场调控的非线性太赫兹光学特性研究

石墨烯是单原子厚的二维狄拉克相对论费米子系统,其优秀的光电学性质得到了广泛的关注和研究. 本论文利用量子理论研究掺杂石墨烯系统外电场和光场共同作用下的非平衡载流子的非线性太赫兹光学性质. 研究发现,掺杂石墨烯带内光吸收表现出强的非线性太赫兹光学特性. 随着外加偏压电场的增大,石墨烯非线性光学响应增强;随着外界太赫兹光频率的减小,非线特性增强. 研究表明通过改变电场强度,可以有效调节石墨烯系统太赫兹非线性光学特性. 研究结果为探索和发展以石墨烯为基础的新型纳米太赫兹光电器件的研究和实际应用提供了理论依据. 关键词： 石墨烯 太赫兹 非线性 光电流