Dmitri A. Oulianov

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-dimensional Z scan for arbitrary beam shape and sample thickness

A two-dimensional Z-scan technique has been developed using a two-dimensional charge-coupled device (CCD) detector to study the nonlinear optical properties and beam profile evolution within optical limiting devices. Using the split step beam propagation method, the far field pattern at each Z position can be calculated precisely for any arbitrary beam shape and sample thickness. A two-dimensional far field pattern is recorded by the CCD detector, and the evolution of the beam distribution inside the nonlinear optical medium can be obtained directly. This technique has been applied to study the nonlinear optical parameters and laser beam profiles in nonlinear liquids. We have used thin and thick samples and Gaussian, top-hat and split Gaussian beam distributions, and the experimental results agreed very well with the calculation. This technique also offers a simple and accurate means for optimizing the design of optical limiting devices.

Optical limiting and picosecond relaxation of carbocyanines upper electronic states

Abstract Nonlinear absorption, anomalous fluorescence and relaxation of high-lying electronic states in six carbocyanine dyes, including cryptocyanine (DCI), DDI, DTDCI, DTTCI, DOTCI and HDIDCI, in solution and in polymer, were studied by means of picosecond transient absorption spectroscopy and nonlinear transmission experiments. Absorption cross-sections of the S 1 →S n transition, and decay rates of the second singlet excited state, S 2 , were measured. All dyes showed strong reverse saturable absorption in the 450–600 nm region with large excited to ground state absorption cross-section ratios. For DTDCI this ratio, at 470 nm, was measured to be 350, which is the largest ever reported. All molecules have shown strong optical limiting effect. However, in all molecules except DCI a saturation of the optical limiting process was observed as expected, owing to relatively long, up to 17.5 ps, lifetime of the S 2 state. The S 2 state fluorescence quantum yields were also measured.

Kinetics and mechanism of donor–acceptor azulenic optical limiting molecules

Abstract The transient absorption spectra and kinetics of ten new donor–acceptor azulenic molecules, after excitation of the charge-transfer band, have been studied by means of ultrafast transient spectroscopy. All of the molecules show intense and broad reverse saturable absorption bands in the visible region, which appear within the excitation pulse ( τ =1 ps), and decay with picosecond lifetimes. The early time steps of the photochemical mechanism of these azulenes are proposed. Owing to the high absorption cross-section, fast kinetics and broad spectral response, these molecules may serve as the active media for optical limiting devices.

Polar-Solvent Effect on the Photocycloisomerization of Symmetrical Bis[anthracenes]: A Transient Ultrafast Kinetic Study

Bis[anthracenes] are the few among the fluorescing nonconjugated bichromophores that possess photoreactive properties. The 9,9′-[methylenebis(oxy)]bis[anthracenes] 1 (AOCH2OA) exhibit the highest known intramolecular photocycloaddition quantum yield from the S1 state and, moreover, display a higher yield in polar solvents, an unexpected result for symmetrical systems. No excimer fluorescence was detected in solution at room temperature. The 10,10′-dimethoxy derivative 1b was studied by picosecond (ps) laser spectroscopy. In nonpolar solvents (methylcyclohexane), SnS1 was the only transient absorption detected, whereas, in polar solvents (MeCN), the growth and the decay of a second transient were recorded, and the second transient was attributed to a zwitterion A+.−A−.. The kinetics data were derived, and it was concluded that electron transfer and ion recombination should be at the origin of the observed rate enhancement of cycloadduct formation in polar solvents.

Investigation of benzoporphyrin and azulenic compounds by two-dimensional z-scan technique

A two-dimensional Z-scan technique employing a CCD camera was used to study the nonlinear optical parameters of materials. Using the known beam distribution at the lens plane, measured by the CCD camera, and the split step beam propagation method, we simulate the evolution of the beam profile within the sample. This technique may be applied to any arbitrary beam distribution and sample thickness. We applied our two- dimensional Z-scan technique for the investigation of zinc tetraphenyltetrabenzoporphyrin and three azulene-containing donor-acceptor compounds. It was found that some samples exhibit strong nonlinear refraction while others show strong nonlinear absorption. The use of these materials in optical limiting devices is discussed.

Ultrafast time-resolved EXAFS spectroscopy using an energy dispersive spectrometer

We describe the experimental system used for ultrafast time resolved EXAFS spectroscopy. It consists of a laser driven x- ray diode which generates the ultrashort hard x-ray pulses used in pump/probe experiments and a dispersive spectrometer which makes possible the simultaneous recording of a broad spectrum. The data presented, using this dispersive spectrometer, shows that we can record more than 500 eV simultaneously with a resolution of 8 eV, or 3 eV resolution for 250 eV energy spectrum.

Nonlinear properties and kinetics of some donor-acceptor optical-limiting molecules

The nonlinear optical properties of four novel azulene-type donor-acceptor compounds have been measured by means of the 2D Z-scan technique. These compounds exhibit considerable nonlinear refraction and one shows strong nonlinear saturated absorption at 527 nm. The transient spectra and kinetics, after excitation of the charge-transfer absorption band, have been studied by means of a picosecond transient absorption spectroscopy, allowing us to measure absorbance changes as small as (Delta) A equals 0.005 and time resolution of 1.5 ps. Several compounds exhibited quite strong reversed saturable absorption in a wide region of the visible spectrum. The reversed saturable absorption induced an increase in the effective absorption of the material at high power, which suggests that, it may be useful as an optical limiting medium.

Nonlinear 3D optical storage and comments on two-photon cross section measurements

We describe the application of two-photon processes and fluorescence to 3D optical data storage. We also report the results of two-photon absorption (TPA) cross section measurement of several organic molecules in solution. The data show that the nonlinear transmission method without consideration of other nonlinear effects results 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 absorption following two-photon excitation, are in good agreement with the accepted values. Therefore one needs to be cognizant of these facts when using the transmission method.