Effective Two-photon Absorption Research Articles

The third-order nonlinear optical properties of novel styryl dyes

The third-order nonlinear optical properties of two novel styryl dyes, namely, 1,3-diethyl-5-(4-methoxybenzylidene)-2-thioxodihydropyrimidine-4,6(1 H,5 H)-dione and 5-(3,4-dimethoxybenzylidene)-1,3-diethyl-2-thioxodihydropyrimidine-4,6(1 H,5 H)-dione were studied using the Z-scan technique employing 7 ns laser pulses at 532 nm. The nonlinear refractive index, nonlinear absorption coefficient, magnitude of effective third-order susceptibility and coupling factor were determined. The optical power limiting behavior of the colorants was ascribed to two-photon absorption phenomenon. Nonlinear absorption cross-section increased with increase in π electron density and the effective two-photon absorption cross-section was two orders of magnitude larger than those of commercially available azo compounds. The results suggest that these colorants offer promise as nonlinear optical materials for device applications.

Bisporphyrin connected by tetrathiafulvalene

A new porphyrintetrathiafulvalene composite, where two porphyrins are bridged by te trathiafulvalene (TTF) using acetylene bonds was synthesized. The Q-band of the monomeric porphyrin appears at 590 nm whereas that of the composite is red-shifted to 620 nm and intensified. The Soret band is also red-shifted from 427 nm to 435 nm and much broadened, indicating the expansion of π-conjugation over the porphyrin and tetrathiafulvalene units. The HOMOs and LUMOs were calculated using time-dependent density functional theory. Voltammetric experiments revealed that the first oxidation potential of the TTF moiety in the composite was shifted by +155 mV compa red with TTF in the absence of composite. The effective two-photon absorption (2PA) cross section values were measured by using a nanosecond open aperture Z-scan method. The maximum effective 2PA cross section values were obtained at 760 nm, as 7300 GM in benzonitrile and 5900 GM in toluene. The values obtained in the polar solvent were 1.2 to 1.5 times larger than those in the nonpolar solvent.

Platinum Acetylide Two-Photon Chromophores

To explore the photophysics of platinum acetylide chromophores with strong two-photon absorption cross-sections, we have investigated the synthesis and spectroscopic characterization of a series of platinum acetylide complexes that feature highly pi-conjugated ligands substituted with pi-donor or -acceptor moieties. The molecules (numbered 1-4) considered in the present work are analogs of bis(phenylethynyl)bis(tributylphosphine)platinum(II) complexes. Molecule 1 carries two alkynyl-benzothiazolylfluorene ligands, and molecule 2 has two alkynyl-diphenylaminofluorene ligands bound to the central platinum atom. Compounds 3 and 4 possess two dihexylaminophenyl substituents at their ends and differ by the number of platinum atoms in the oligomer "core" (one vs two in 3 and 4, respectively). The ligands have strong effective two-photon absorption cross-sections, while the heavy metal platinum centers give rise to efficient intersystem crossing to long-lived triplet states. Ultrafast transient absorption and emission spectra demonstrate that one-photon excitation of the chromophores produces an S1 state delocalized across the two conjugated ligands, with weak (excitonic) coupling through the platinum centers. Intersystem crossing occurs rapidly (Kisc approximately 1011 s-1) to produce the T1 state, which is possibly localized on a single conjugated fluorenyl ligand. The triplet state is strongly absorbing (epsilonTT > 5 x 104 M-1 cm-1), and it is very long-lived (tau > 100 micro s). Femtosecond pulses were used to characterize the two-photon absorption properties of the complexes, and all of the chromophores are relatively efficient two-photon absorbers in the visible and near-infrared region of the spectrum (600-800 nm). The complexes exhibit maximum two-photon absorption at a shorter wavelength than 2lambda for the one-photon band, consistent with the dominant two-photon transition arising from a two-photon-allowed gerade-gerade transition. Nanosecond transient absorption experiments carried out on several of the complexes with excitation at 803 nm confirm that the long-lived triplet state can be produced efficiently via a sequence involving two-photon excitation to produce S1, followed by intersystem crossing to produce T1.

Syntheses and properties of acetylene-linked bis- and trisporphyrins toward two-photon photodynamic therapy

Acetylene-bridged bisporphyrins and trisporphyrins having branched bulky bis(carboxylethyl)methyl meso-substituents were synthesized. These compounds showed large effective two-photon absorption cross-section values at 890 nm measured by using a nanosecond Z-scan method. Sodium salt of hydrolyzed trisporphyrins showed broad and red-shifted Q-bands over 900 nm. Two-photon absorption cross-section values of water-soluble dimers in water were similar to, or slightly larger than, those of ester forms evaluated in toluene. Furthermore, the generation of singlet oxygen upon one-photon irradiation for dimers in water was confirmed.

Syntheses and nonlinear absorption properties of conjugated porphyrin supramolecules

Self-assemblies consisting of acetylene-linked bisporphyrins with a 4-nitrophenylethynyl substituent and with a phenylethynyl substituent were synthesized. The Q-band of the phenylethynyl-substituted compound appears at 762 nm whereas that of the 4-nitrophenylethynyl-substituted one was red-shifted to 784 nm and intensified. This may be attributed to the participation of the nitro group in the π-electronic communication system. HOMOs and LUMOs were calculated for these compounds using an INDO/S-CI method. The effective two-photon absorption cross-section values of these self-assemblies were measured by using a nanosecond open aperture Z-scan method. The maximum effective two-photon absorption cross-section values of 4-nitrophenylethynyl- and phenylethynyl-substituted bisporphyrins were obtained as 1.2 × 105 GM and 8.1 × 104 GM , respectively, at 890 nm. A 1.5 enhancement factor was obtained by introducing nitro groups, which was similar to the value previously reported for ferrocene- and fullerene-connected supramolecular porphyrin.

Effect of the concentration of organic dyes on their surface plasmon enhanced two-photon absorption cross section using activated Au nanoparticles

In this article we present the study of the surface plasmon enhanced two-photon absorption of a hydrophilic stilbene derivative (trans-4,4’-diaminostilbene) in aqueous solution at different concentrations. The observed exponential growth of the effective two-photon absorption cross section [σ2′(Au)] is attributed to the electric-field augmentation via surface plasmon resonance between nanoparticles, i.e., hot spots, and the molecular density on Au nanospheres. An unprecedented σ2′(Au)=550 000 GM has been measured. This result opens a new universe of applications in multiphoton imaging, photodynamic therapy, telecommunications, optical limiting, and multidimensional data storage using hybrid systems.

Origin of the Anomalous Two-Photon Absorption in Fluorescent Protein DsRed

The red fluorescent protein DsRed displays a two-photon excitation band around 760 nm which is not accompanied by any feature in the corresponding one-photon spectral region (380 nm). By means of time-dependent density functional theory, we are able to explain such an effect, as arising from an electronic excitation of the DsRed chromophore with ability to couple with a charge-transfer state, through an effective two-photon absorption channel.

APPLICATION OF ADOMIAN'S METHOD TO NONLINEAR ABSORPTION

We use the method of Adomian to investigate the solution of the nonlinear equation giving the evolution of a plane wave in a medium exhibiting linear, two- and three-photon absorption. The second-order solution is in good agreement with numerical simulations. An effective two-photon absorption coefficient is conveniently defined to fit experimental data.

Ab initio studies of two-photon absorption of some stilbenoid chromophores

Two-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to second-order response function. Several important issues in modeling are highlighted which must be addressed for a reliable reproduction of the experimental results. It is evident that the correct order of magnitude of calculated two-photon absorption cross sections can only be obtained if proper account is taken of vibrational broadening of the absorption profiles. A comparison of the theoretical results with the experimental ones indicates that the computed two-photon absorption cross sections are in rough agreement with our previous report, although the observed systematic increase of the cross sections with the electron acceptor strength is not well reproduced. It is suggested that this disagreement may be due not only to the deficiencies of the computations but also to a variety of factors contributing to the experimental value of the effective two-photon absorption cross section, which are not taken into account in the ab initio calculations.

Surface and bulk absorption in CaF2 at 193 and 157 nm

Comprehensive calorimetric absorption measurements were performed for CaF2 crystals at irradiation wavelengths 193 and 157 nm. By using samples with different thickness a separation of surface and bulk absorptance could be achieved and thus, single- and two-photon absorption coefficients could be determined for both wavelengths. For the surface absorptance, a dependence of the polishing grade of the sample was observed at 193 nm. The investigation of earlier reported repetition rate dependencies of the effective two-photon absorption coefficient at 193 nm was extended up to a repetition rate of 2.8 kHz. In contrast to the measurements at 193 nm, no such dependence at all could be found at 157 nm. Both results support already proposed models of the absorption mechanisms in wide band-gap materials.

One- and two-photon absorption of three-coordinate compounds with different centers (B,Al,N) and a 2,2′-dipyridylnitrogen functional group

A series of three-coordinate octupolar compounds with varied centers (boron, aluminum, and nitrogen), which exhibit very large effective two-photon absorption cross sections have been theoretically studied. The ground state geometries and electronic structures are obtained using the density functional theory with the B3LYP functional and 6-31G(d) basis set, and the results are comparable to the available experimental determinations. Based on the correct geometrical and electronic structures, the one- and two-photon absorptions are predicted by the ZINDO-SOS method. Among these compounds, the boron (B) and aluminum (Al) centers act as acceptors, while the nitrogen center acts as donor according to the net charge changes during the excitation. It is found that (i) the compounds with boron and aluminum centers show two large two-photon absorption peaks, while the molecule with nitrogen center show only one two-photon absorption peak; (ii) the cross sections of the molecules with B or Al as centers are larger than that of the molecule with nitrogen as center; furthermore, the two-photon absorption cross section of the molecule with Al center is larger than that of the molecule with B center, from this point of view, our theoretical prediction provides for the experiment a good new candidate with large two-photon absorption cross section for further research; (iii) lengthening the conjugation bridge by inserting a benzene ring on the organoborane compounds (forming the investigated molecule B-2) enhances the two-photon absorption cross section, and keeping good transparency at the same time.

Understanding the One-Photon Photophysical Properties of a Two-Photon Absorbing Chromophore

A comprehensive one-photon photophysical study has been carried out on AF455, a known two-photon absorbing dye. AF455 is composed of an electron-accepting center with three arms that consist of a π-conjugated group with an electron-donating group at the terminal end of each arm. The objective of this work is to understand the one-photon excitation photophysical properties so that this knowledge will be carried into understanding the two-photon absorption properties. This was done by utilizing steady-state absorption, steady-state and time-resolved emission, femtosecond pump−probe, and nanosecond laser flash photolysis techniques. Through this study it was determined that AF455 undergoes an excited-state intramolecular charge transfer (ICT) upon absorption of a single photon. The extent of ICT stabilization is dependent on the solvent polarity, with increasing ICT stabilization in more polar solvents. The formation of the triplet excited state is small (<8%) and is effected only slightly by a change in the solvent polarity. With an increase in polarity the fluorescence quantum yield decreased and internal conversion was found to become more competitive. On the basis of a two-photon assisted excited-state absorption model we tied in the measured one-photon photophysical properties to understand why the nanosecond effective two-photon absorption cross section is much larger than the femtosecond intrinsic two-photon absorption cross section. We found that AF455 exhibits triplet excited-state absorption in the region of 800 nm that leads to enhancement of the nanosecond effective two-photon absorption cross section.

TWO- AND THREE-PHOTON NONLINEAR ABSORPTION IN As2Se3 CHALCOGENIDE GLASS: THEORY AND EXPERIMENT

The absorption of a monochromatic plane wave in a medium exhibiting linear, two- and three-photon absorption is investigated theoretically. An approximate closed-form expression is derived for the optical intensity. An effective two-photon absorption coefficient is then defined. Using the theoretical results, the absorption coefficients from transmission measurements are determined. Experiment is done in a highly absorbing nonlinear material such as As 2 Se 3 to illustrate our theoretical results. Good agreement is found between experimental data and numerical simulation based on our model. Measurements of nonlinear coefficients characterizing the two- and three-photon absorption are given.

Experimental observation of higher order nonlinear absorption in tellurium based chalcogenide glasses

We show that highly nonlinear two-photon absorption coefficients can be obtained in infrared glasses (tellurium based chalcogenide glasses) at 1.06 μm and in the picosecond range (twice that of As 2Se 3). We observe an intensity-dependant two-photon absorption coefficient. This dependence is attributed to higher order effects occurring in nonlinear absorption process in our materials. The corresponding nonlinear coefficients are deduced from experimental data using a closed-form relation relating the effective two-photon absorption coefficient to the incident intensity. These coefficients become larger as the photon energy increases relative to the band gap.

Optical power limiting and stabilization using a two-photon absorbing neat liquid crystal in isotropic phase

This letter reports on a neat liquid crystal that incorporates a two-photon-absorbing chromophore in its structure. The nonlinear optical properties including two-photon-induced fluorescence spectrum and decay behavior, as well as the effective two-photon absorption coefficient of this liquid crystal in its isotropic phase are presented. Fairly good optical limiting and stabilization performance with this nonlinear material have been achieved by using ∼815-nm and ∼5-ns laser pulses.

Synthesis and nonlinear optical properties of new symmetrically substituted stilbenes

Three new chromophores and trans-4-( N-(ethyl 4″-nitrobenzoate)- N-ethyl amino)-4′-(dimethyl amino) stilbene (DMANHAS) have been synthesized and whose chemical structures have been characterized by 1H NMR, IR, and elemental analyses. Linear absorption, single-photon-induced fluorescence and two-photon-induced fluorescence are experimentally studied. Trans-4-( N-2-hydroxyethyl- N-ethyl amino)-4′-(dimethyl amino)stilbene (DMAHAS) and trans-4-( N-2-hydroxyethyl- N-ethyl amino)-4′-(diethyl amino)stilbene (DEAHAS) have effective two-photon absorption cross-sections of σ 2=0.91×10 −46 cm 4 s/photon and σ 2=1.19×10 −46 cm 4 s/photon at 532 nm by using an open aperture Z-scan technique, respectively. When pumped with 800 nm laser irradiation, DMAHAS and DEAHAS indicate strong two-photon-induced blue fluorescence of 436 and 440 nm, respectively, while trans-4-( N-(ethyl 4″-nitrobenzoate)- N-ethyl amino)-4′-(diethyl amino) stilbene (DEANHAS) and DMANHAS exhibit no fluorescence.

Self-trapped exciton luminescence and repetition rate dependence of two-photon absorption in CaF2 at 193 nm

Abstract Repetition rate-dependent measurements of nonlinear absorptance and self-trapped exciton luminescence ( π -luminescence) of high-purity CaF 2 crystals at 193 nm irradiation were performed in the range between 25 and 300 Hz. The CaF 2 samples showed different (effective) two-photon absorption coefficients β eff , which are attributed to different impurity or defect concentrations inside the crystals. For samples with low β eff , no dependence on repetition rate could be observed, while samples with high values reveal a strong increase of β eff for increasing repetition rates. In contrast, the intensity of self-trapped exciton luminescence is nearly the same for all samples and constant for all repetition rates. Different two-photon absorption mechanisms were derived from these results and a simple kinetic model for the absorption behaviour was developed, including the formation of transient absorbing states. In addition, laser induced bleaching of Ce 3+ emission during irradiation with a total dose up to 900 mJ/cm 2 could be observed in the luminescence spectra.

Synthesis of C60-diphenylaminofluorene dyad with large 2PA cross-sections and efficient intramolecular two-photon energy transfer.

The first, highly two-photon active C60 derivative comprised of a A-sp3-D conjugate structure was synthesized showing effective two-photon absorption cross-sections (sigma 2' = 196 x 10(-48) cm4 sec-1 molecule-1) in the nanosecond regime among the best values for diphenylaminofluorene-based AFX chromophores.

Superradiance and lasing properties of new two-photon absorption dye DEASPS

Pumped by picosecond pulses from a Nd:YAG laser, a new lasing dye, trans-4-[4′-(N,N-diethylamino)styryl]-N-methyl pyridinium methyl sulfate (abbreviated to DEASPS), shows both intense superradiance and strong lasing properties in benzyl alcohol solution. By using streak camera systems, the superradiance and lasing can be distinguished both spectrally and temporally. It has been found that the peak wavelength of lasing is at 620 nm with a red-shift of about 12 nm to the superradiance wavelength. The lasing pulse shows an oscillatory effect that it is not found in the superradiance pulse. The fluorescence lifetime is 529 ± 40 ps and the effective molecular two-photon absorption is (1.25 ± 0.1) × 10−48 cm4 ·s·photon −1, measured using a nonlinear transmittance method. This dye shows effective optical limiting of the pumping wavelength.

Comparative studies of absorptance behaviour of alkaline-earth fluorides at 193 nm and 157 nm

Absorptance losses in MgF2, CaF2 and BaF2 during 193-nm (DUV) and 157-nm (VUV) irradiation are investigated by employing a high-resolution laser calorimetric technique which allows the determination of both single- and two-photon absorptance at energy densities up to 110 mJ/cm2. A strong wavelength dependence of the DUV and VUV absorption characteristics is observed: while effective two-photon absorption takes place at 193 nm, either no similar effect at all (in the case of BaF2) or only a very minor effect (CaF2) is observed at 157 nm. A first explanation for this absorption behaviour is given, implying the energetic band structure of CaF2. In addition it is shown that, due to the strong nonlinear dependency, above a critical energy density the absorptance at 193 nm can exceed the absorptance at 157 nm. Furthermore, different single- and two-photon absorption coefficients are determined for different CaF2 samples at 193 nm, indicating a two-step absorption mechanism. In addition, laser-induced aging is found in a MgF2 sample at 193 nm, but not at 157 nm.