Two-photon Absorption Cross-section Measurements Research Articles

Synthesis and optical properties of linear and branched styrylpyridinium dyes in different environments

A common motif in the design of organic dye compounds is that of an electron donor and an acceptor moiety linked through a conjugated bridge, but more complicated structures can also be built using such motifs as building blocks. Following these principles, we synthesized two styrylpyridinium dyes, a linear one and a branched molecule composed of three units of the mono moiety connected by benzyl group and investigated their photophysical properties in different environments: their linear absorption, one-photon excited fluorescence and two-photon absorption properties. The compound with the symmetrically substituted phenyl core shows a redshift of the absorption and fluorescence bands. Spectrally resolved two-photon absorption cross section measurements carried out by femtosecond Z-scan technique show that the value for the trimer is over twelve times larger than that for the monomer, and not just three times as expected from the molar mass increase. Large two-photon absorption (2PA) values were measured in the near infrared (NIR) region. The prototropic study indicates that the protonation of the amino group blocks a lone pair on the nitrogen atom, making the photoinduced electron transfer process from the amine group to the pyridinium ring ineffective. This results in the reduction of the long-wavelength absorption with a simultaneous increase of the band in the short-wavelength region of the spectrum and a decrease in the fluorescence intensity. The behavior of dyes in the presence of β-cyclodextrin was also studied based on NMR, UV–Vis, and fluorescence spectroscopy. The performed experiments indicated the formation of 1:1 inclusion complex in aqueous solution.

Synthesis of Thieno-Bridged Porphyrins: Changing the Antiaromatic Contribution by the Direction of the Thiophene Ring

Two types of thieno-bridged porphyrins were synthesized by incorporating a thiophene group across their meso and β positions with different directions of the thiophene ring to investigate the aromaticity of these porphyrins with extended π-systems. The 2,3-thieno-bridged porphyrin showed a larger antiaromatic contribution than did the 3,4-thieno-bridged porphyrin. In the former, the antiaromatic contribution is based on a 20-π-electron conjugated circuit. The two thieno-bridged porphyrins were characterized by calculations of nucleus-independent chemical shift and anisotropy of the induced current density as well as by X-ray crystallography, NMR spectroscopy, UV-vis-NIR absorption spectroscopy, electrochemical studies, time-resolved excited-state analysis, and two-photon absorption cross section measurements. Chemical derivatization of the 2,3-thieno-bridged porphyrin was also demonstrated.

Two-photon absorption cross section measurements of various two-photon initiators for ultrashort laser radiation applying the Z-scan technique

In this paper, we describe the Z-scan measurements of the two-photon absorption (TPA) cross section of various two-photon initiators that are suitable for real three-dimensional structuring of photo-polymerizable formulation. The value of the TPA cross section for the initiator P3K was measured to be 256 GM as the maximum value among the synthesized initiators. Procedures for a precise Z-scan measurement including the measurement of Rayleigh length and beam waist radius are presented. The effect of the pulse width on the Z-scan signal is demonstrated. We also used a flow-cell geometry instead of a static cell in order to refresh the materials to avoid the decomposition of the molecules. This resulted in a more realistic TPA cross section which has been proven by suitable reference compounds.

Influence of multiphoton events in measurement of two-photon absorption cross-sections and optical nonlinear parameters under femtosecond pumping

Absolute values of two-photon absorption cross-sections of some laser dyes in methanol solution have been measured by using the transmission method. From the fluorescence yield measurements under multiphoton excitation, the number of involved photons in the process has also been estimated. It is found that the presence of higher photon events may lead to erroneous values of measured two-photon absorption cross-sections. It is shown here that the Z-scan technique is capable of differentiating between the two- and three-photon absorption processes. Nonlinear optical parameters under femtosecond pumping by multiphoton resonant excitation at 800 nm are also given here.

Strongly Interacting Organic Conjugated Dendrimers with Enhanced Two-Photon Absorption

We report a strongly interacting new dendrimer system with an extended spectroscopic unit (coherent domain) beyond the trimer configuration. This report focuses on the mechanism of enhancement of the two-photon absorption (TPA) response of this material as a function of the dendrimer generation number. This enhancement is strongly correlated to the size and geometry of the spectroscopic unit in a strongly interacting macromolecular system. These systems are investigated by a wide variety of time-resolved spectroscopy methods including time-resolved fluorescence, transient absorption, three-pulse photon echo peak shift measurements as well as TPA cross section measurements. The extensive combination of modern spectroscopic methods strongly indicates that the spectroscopic unit (domain) in high generations covers the G1 (nine linear building blocks) and exceeds the trimer size.

Structure property correlations in alcohols through two-photon absorption cross-section measurements

Two-photon absorption (TPA) cross-sections of neat alcohols are shown to be a femtosecond laser measurable property with useful structure property correlations that are directly correlated to the self-phase modulation of the alcohol in question. The amount of self-phase modulation is shown to be a molecular structure dependent property. The real and the imaginary components of the second-order hyperpolarizability are thus shown to be interrelated. Such TPA measurements prove that it is not possible to predict two-photon absorption properties by simple doubling the wavelengths of one-photon absorption spectra.

Core-Modified Expanded Porphyrins with Large Third-Order Nonlinear Optical Response

The third-order nonlinear optical response through measurement of two-photon absorption cross-sections (TPACS) for aromatic core-modified expanded porphyrin analogues by a femtosecond open aperture Z-scan method is reported. The values reported here are among the largest known so far in the literature for any organic molecules.

Measurement of two-photon absorption cross-section in organic molecular salt with linear and circular polarized radiation

We report on the investigation of two-photon absorption (TPA) in 4′-Diethylamino-N-Methyl-4-Stilbazolium Tosylate (DEST) with 40 picosecond, 1064 nm linear and circular polarization laser radiation. The TPA cross-section σ2 measured is larger for linear polarized radiation (948 GM) than that for the circular polarized radiation (840 GM). The large TPA cross-section of DEST facilitates highly directional and efficient TPA induced upconverted emission in the range of 590–640 nm even though the fluorescence efficiency of DEST is very low (∼0.5%). The net single pass efficiency of upconversion is ∼7%.

Feasibility Study of Direct Measurement of Two-Photon Absorption Cross-Sections

Background The two-photon absorption (2PA) cross-section, σ ( 2 ) , is one of the most fundamental properties in any experiment that uses 2PA. σ ( 2 ) is the direct analogue of the 1 PA cross-section, a, or extinction coefficient, e. Most methods to measure σ ( 2 ) , including 2P fluorescence, [ 1 - 3 ] z-scan, [ 4 ] and loss modulation, [ 5 ] are either indirect or require tightly focussed laser beams. In this paper, we explore the feasibility of measuring σ ( 2 ) directly, using an unfocused laser, via a similar approach to the measurement of e or a in 1 P experiments. 1 PA is described by the Beer-Lambert law with the light intensity decaying exponentially in the sample, whereas for2PA the light decays in an inverse fashion: 1 I - 1 I 0 =σ ( 2 ) Nl (1) where I 0 and I are the initial and final light intensities respectively, N is the number density of absorbers, and l the pathlength. Commercial, amplified Ti:sapphire lasers can routinely provide an intensity I of 5 ×10 2 7 photons cm - 2 s - 1 . According to Eqn (1), a 10 mM solution of a commercial dye (Rhodamine 6G, σ ( 2 ) = 20GM [ 1 ] ) absorbs 5-10% of the unfocussed laser beam over 10cm. Proper extraction of σ ( 2 ) requires precise measurement of the laser beam's spatial and temporal profiles (with, say, a CCD camera and an autocorrelator respectively). Nonetheless, these calculations suggest that direct measurement of σ ( 2 ) is feasible. Choice of solvent is important when performing these experiments. The two criteria for a suitable solvent are solubility (which is easily quantified), and 1 PA. The significance of 1PA can be demonstrated with methanol, which has a small a of 5 × 10 - 2 5 cm 2 molecule - 1 at 800 nm, yet absorbs 16% of the laser power over 10 cm. Competition between solvent 1 PA and dye 2PA results in an absorption decay that is a convolution of first- and second-order kinetic processes. Also, significant 1PA causes thermal lensing. Thermal lensing is the result of absorption inducing a thermal (and therefore a refractive index) gradient, causing the solvent to act as a lens. This is problematic because Eqn (1) requires the laser beam to maintain a constant spatial and temporal profile.

Observations on the measurement of two-photon absorption cross-section

We have measured two-photon absorption (TPA) cross-sections of four organic molecules in solution. The data show that the nonlinear transmission method without consideration of other nonlinear effects results, under certain conditions, in erroneous values for the TPA cross-sections. We also find that the cross-sections measured by excited-state methods, namely two-photon induced fluorescence and a new excited-state method, which is based on transient spectroscopy following two-photon excitation, are in good agreement with the published data. Therefore, caution is warranted when using the transmission method.

Two-photon Absorption Cross-section Measurements in Common Laser Dyes at 1·06 μm

Using a comparative fluorescence method, two-photon absorption cross-sections at 1·06 w m for seven common laser dyes were measured with a moderate power, Q-switched Nd: YAG laser and an intracavity second harmonic generator. The dyes were Rhodamines (560, 590, 610 and 640), DODCI, DCM and Oxazine 725. Using a new method for a direct determination of the Rayleigh length of the focused incident laser beam, all the parameters needed for the experimental determination of the two-photon absorption cross-section are discussed and independently determined by measurements.