Linear Absorption Peak Research Articles

Solvent Polarity Dependent Ultrafast Relaxation Kinetics of ADS800AT Dye.

This work investigated the photoexcitation and relaxation kinetics of the ADS800AT dye dissolved in different solvents using transient absorption spectroscopy (TAS) with a white-light continuum probe. The dye was dissolved in various solvents, including dichloromethane (DCM), 1,2-dichlorobenzene (DCB), ethanol, and methanol, to study their impact on the dye's characteristics. The linear absorption peak varied from 835 to 809nm, depending on the polarity of the solvent, and the pump wavelength for TAS was chosen accordingly. We observed ground-state bleaching and excited-state absorption after exciting the dye with the pump pulse. Global analysis was performed using Glotaran software to fit exponential decay curve models, allowing us to determine the relaxation time of the excited molecule. The relaxation time varied from 198 ps to 508 ps across the different solvents, decreasing as the polarity of the solvent increased. Additionally, we could experimentally correlate the dye molecule's nonlinear properties with the solvent's polarity.

Tunable third‐order NLO properties of acene derivatives with molecular structural modification

AbstractThe substitution of heteroatoms and the expansion of π‐conjugated units have significant effects on the photoelectric properties of polycyclic aromatic hydrocarbons (PAHs). In this study, based on the experimental molecule PBC, 10 acene derivatives containing the pyrrole group were designed by three strategies: (1) changing the connection position of the pyrrole group, (2) exchanging the position of the N atom, and (3) increasing the length of the π‐linker. Density functional theory (DFT) was used to optimize the molecular geometric structure. Time‐dependent density functional theory (TD‐DFT) was used to calculate the relevant parameters of the excited states. The results show that both of the pyrrole group and the N atom in the para‐position, and the addition of the π‐linker can reduce the energy gap, cause redshift of the linear absorption peak, and increase the two photon absorption (TPA) cross‐section. The analyses of the charge density difference (CDD) and the charge‐transfer matrix (CTM) proved that the electron transfer is mainly concentrated in the π‐linker. Moreover, the participation of the multi‐ring skeleton on both sides decreases gradually with the increase of the length of the π‐linker. The length of the π‐linker changes the dominant transition channel and intramolecular charge‐transfer (ICT) characteristics of TPA, thus affecting the transition dipole moments and nonlinear absorption properties. The second hyperpolarizability of the designed molecule PBI5‐p5 is also significantly superior to that of similar materials reported. It is expected that the above molecular design strategies and comprehensive analysis of nonlinear optical (NLO) properties could provide theoretical support for the research on acene derivatives.

Ultrafast third-order nonlinear optical response of CdSe/Al2O3/Ag composite films

The CdSe/Al2O3/Ag composite films were deposited by magnetron sputtering and spin coating, and some samples were annealed at 450 °C. The linear absorption peak of annealed samples showed a clear redshift behavior. The femtosecond Z-scan was adopted to study the ultrafast third-order nonlinear optical response of the CdSe/Al2O3/Ag composite films. In the present study, the nonlinear properties of the composite films were 104 times higher than single CdSe quantum dots, and the properties exhibited by the unannealed samples were better than annealed samples. The increase in laser energy causes an enhancement in saturation absorption and self-focusing properties. The effects of Ag nanoparticles sputtering conditions and CdSe quantum dots content on the optical properties of the samples were also investigated. Unannealed CdSe/Al2O3/Ag composite films with Ag nanoparticles (P = 15 W) and one layer of CdSe quantum dots have the maximum value of β and χ3,which are -4.80 × 10−9m/W and 3.82 × 10−9esu. Composite films can be used in optical sensing, information and communication technologies, and many other optoelectronic applications.

Tunable edge bands and optical properties in black phosphorus nanoribbons under electric field**Project supported by the National Natural Science Foundation of China (Grant No. 10947004) and the Jiangsu Government Scholarship for Overseas Studies.

For several types of the applied electric field configuration on the normal-zigzag black phosphorus nanoribbon (nZZ-BPNR) we investigate the band structure and the linear optical absorption spectrum, especially for the edge states and the corresponding low-energy absorption peaks. The obtained results show that the applied electric field can not only open another band gap at k = 0.5 point, but also can change completely the spacial probabilities of edge states in the two edge bands. The strength of electric field can tune the two band gaps at the Γ point and 0.5 point. Further, one remarkable feature is that the forbidden transition E12 and E21 are allowed. The lowest-excited-energy linear absorption peak E11 originates from the transition between two edge bands at the Γ point. Finally, in comparison with the lowest-excited-energy peaks among various configurations, the second type of electric field configuration can move this peak blue-shift larger than otherconfigurations.

Two-photon absorption spectra of Salen dye complexes with azo dyes

This work reports the degenerate two-photon absorption spectrum for a series of Salen complexes possessing two azo dye units. The observed spectra peak at 1000 nm, twice the wavelength of the linear absorption peak, and increase monotonically to the blue due to resonant enhancement of the non linearity. The two-photon absorption properties of these molecules were found to be an additive effect of chromophores and independent of the metal ions. The two-photon absorption spectra were described with a sum-over-states model, from which the transition dipole moment and dipole moment change were determined and compared with quantum chemistry calculation.

Phase Transition in a One-dimensional Extended Peierls–Hubbard Model with a Pulse of Oscillating Electric Field: II. Linear Behavior in Neutral-to-Ionic Transition

Dynamics of charge density and lattice displacements after the neutral phase is photoexcited is studied by solving the time-dependent Schrödinger equation for a one-dimensional extended Peierls–Hubbard model with alternating potentials. In contrast to the ionic-to-neutral transition studied previously, the neutral-to-ionic transition proceeds in an uncooperative manner as far as the one-dimensional system is concerned. The final ionicity is a linear function of the increment of the total energy. After the electric field is turned off, the electronic state does not significantly change, roughly keeping the ionicity, even if the transition is not completed, because the ionic domains never proliferate. As a consequence, an electric field with frequency just at the linear absorption peak causes the neutral-to-ionic transition the most efficiently. These findings are consistent with the recent experiments on the mixed-stack organic charge–transfer complex, TTF–CA. We artificially modify or remove the electron–lattice coupling to discuss the origin of such differences between the two transitions.

Phase Transition in a One-dimensional Extended Peierls–Hubbard Model with a Pulse of Oscillating Electric Field: I. Threshold Behavior in Ionic-to-Neutral Transition

Photoinduced dynamics of charge density and lattice displacements is calculated by solving the time-dependent Schrödinger equation for a one-dimensional extended Peierls–Hubbard model with alternating potentials for the mixed-stack organic charge–transfer complex, TTF–CA. A pulse of oscillating electric field is incorporated into the Peierls phase of the transfer integral. The frequency, the amplitude, and the duration of the pulse are varied to study the nonlinear and cooperative character of the photoinduced transition. When the dimerized ionic phase is photoexcited, the threshold behavior is clearly observed by plotting the final ionicity as a function of the increment of the total energy. Above the threshold photoexcitation, the electronic state reaches the neutral one with equidistant molecules after the electric field is turned off. The transition is initiated by nucleation of a metastable neutral domain, for which an electric field with frequency below the linear absorption peak is more effective than that at the peak. When the pulse is strong and short, the charge transfer takes place on the same time scale with the disappearance of dimerization. As the pulse becomes weak and long, the dimerization-induced polarization is disordered to restore the inversion symmetry on average before the charge transfer takes place to bring the system neutral. Thus, a paraelectric ionic phase is transiently realized by a weak electric field. It is shown that infrared light also induces the ionic-to-neutral transition, which is characterized by the threshold behavior.

Two-photon absorption and frequency-upconversion properties of a new organic dye HMASPS

Two-photon absorption (TPA) and frequencyupconversion properties of a new upconversion laser dyetrans-4-[p-(N-hydroxyethyl-N-methyl-amino)styryl]-N-methylpyridinium toluene-p-sulfonate (abbreviated to HMASPS) were reported in this note. The linear absorption, TPA, single-photon induced fluorescence, TPA induced fluorescence and TPA induced upconverted lasing spectra of HMASPS solution in dimethyl formamide (abbreviated to DMF) were measured at room temperature. The red shift for the central wavelength of TPA induced fluorescence peak, which was compared with that of the single-photon induced fluorescence peak, and the blue shift for that of TPA induced upconverted lasing compared with that of TPA induced fluorescence, were explained by using re-absorption effect. TPA peak was at 930 nm. There is an 11 nm blue shift for two-photon energy of TPA peak compared with the linear absorption peak. The molecular TPA cross-section σ2 9′ at 1064 nm was measured to be 6.0×10−48 cm4 · s/photon by using the open aperture Z-scanning system. The highest upconversion efficiency was measured to be 8.4% at 1064 nm.

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 σ 2 ′ is 2.85×10 −47 cm 4 s/photon at 930 nm. At 1064 nm, σ 2 ′ is 3.12×10 −48 cm 4 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.

Large order–disorder transition in polydihexylsilane films as studied by second-harmonic generation spectroscopy

The one-photon forbidden excited state in an one-dimensional σ-electron-conjugated polymer, polysilane, was investigated by means of second-harmonic (SH) generation spectroscopy. It was found that resonant SH peak attributable to a forbidden excited state was observed on the high-energy side of the linear absorption peak and that the SH intensity exhibits a drastic change accompanied with the backbone order–disorder phase transition in alkylpolysilane. This behavior was discussed on the basis of the electron delocalization in their backbone of one-dimensional conjugated polymers and the resonance condition of non-linear optical processes.

Synthesis and nonlinear optical characterization of nanostructured gold/polymer composites and suspensions

Abstract Nanometric gold particles were synthesized by a liquid/liquid phase-transfer reaction. Composites of these particles were prepared by free-radical polymerization of suspensions of the particles in styr ene and methyl methacrylate monomer. Concentrations of 1.0, 1.3, and 4.4 mg/ml were prepared,with no noticeable agglomeration of particles during processing. These composites and toluene suspensions (0.18 mg/ml) showed a linear absorption peak at 530 nm, which is characteristic of nanosized gold. High-resolution electron microscopy measurement showed that the particle diameters varied from 5 to 10 nm. Degenerate four-wave mixing experiments at 532 nm yielded, on 10-micron films, a maximum value of 1.0 × 10 −10 e.s.u. for the third-order nonlinear optical susceptibility. The toluene suspension had a susceptibility of 7.7 × 10 −12 e.s.u.

Characterization of excited states of centrosymmetric and noncentrosymmetric squaraines by third-harmonic spectral dispersion

We measured the third-harmonic spectrum for each of four squaraine dyes in chloroform. By fitting the experimental dispersion of the third-order susceptibility, γ, to a four-level sum-over-states model, we determine the strength and the location of the lowest-lying two-photon-like transition. In each case, we find that the lowest two-photon-like state appears just above the dominant linear absorption peak in energy and that the transition moment to that state makes a significant contribution to the nonlinearity, as do transition moments to one or more higher-lying two-photon-like states in the ultraviolet. The spectra of two of these dyes contain additional features, evidencing a nonzero dipole moment difference between the ground and the first excited-state dipole moments that we attribute to asymmetry in their structures. From our measurements, we also predict the dispersion of the real and the imaginary optical Kerr susceptibilities in the near infrared for two of these dyes.

Contributions of Low-Lying Excited States to Optical Nonlinearities in Squaraines

ABSTRACTWe measured the third-harmonic spectrum for a series of both centrosymmetric and noncentrosymmetric squaraine dyes in chloroform. By fitting the experimental dispersion of the third order susceptibility, γ, to a four-level sum-over-states model, we determined the strength and location of the lowest lying two-photon-like transition. In each case, we find that the lowest two-photon-like state appears just above the dominant linear absorption peak in energy and that the transition moment to that state makes a significant contribution to the nonlinearity, as do transition moments to one or more higher-lying two-photon-like states in the ultraviolet. The spectra of one centrosymmetric dye contains an additional feature evidencing a nonzero dipole moment difference between the ground and first excited state dipole moments (δμ) that we attribute to a conformational asymmetry. A noncentrosymmetric squaraine dye shows a similar feature, which, as expected, arises from its δμ. Effects of symmetry breaking are described.

Polymer Nanocomposites of Surface-Functionalized Gold

ABSTRACTNanometric gold particles were synthesized by a liquid/liquid phase-transfer reaction. Composites of these particles in polystyrene, poly(methyl methacrylate) and poly(phenylmethylsilane) were prepared at concentrations from 1.0 to 5.0 mg/ml were prepared, with no noticeable agglomeration of particles during processing. These materials showed a linear absorption peak at 530 nm, which is characteristic of nanosized gold. Highresolution electron microscopy measurement showed that the particle diameters were approximately 2.2 ± 1.3 nm. Degenerate four-wave mixing experiments at 532 nm yielded, on 10-micron films, a maximum value of 1.5 × 10−10 e.s.u. for the third-order nonlinear optical susceptibility.