Third-harmonic generation at the interfaces of a cuvette filled with selected organic solvents.

We report on the third-harmonic generation (THG) of tightly focused femtosecond laser pulses at the interfaces of a cuvette filled with organic solvents. Such a system presents four interfaces separating two materials of different refractive indices and third-order nonlinear susceptibilities where the THG takes place because the symmetry around the focus is broken. We selected two cuvettes (silica and B270 crown glass) filled with different organic solvents (acetone, chloroform, and dimethyl sulfoxide) in order to have a variety of interfaces with different linear and nonlinear optical properties. For some of the peaks, the self-focusing modifies the expected cubic power law dependence for THG and as a consequence the four peak profiles may be quite uneven. Although the THG is due to the electronic part of the nonlinear susceptibility, it can suffer from the influence of the self-focusing effect, a Kerr nonlinearity that can have both instantaneous electronic and slow nuclear contributions. This mixture of two distinct third-order nonlinear processes was never considered for such interfaces. All the THG signals could be understood by taking into account the self-focusing effect. Furthermore, the nonlinear refractive indices, n(2), and third-order nonlinear susceptibilities of the solvents, χ((3)), could be determined simultaneously by the THG signals using the cuvette walls as a reference.

Accurate measurement of nonlinear ellipse rotation using a phase-sensitive method.

We report on the accurate measurement of nonlinear ellipse rotation (NER) by means of a phase-sensitive method employing a dual-phase lock-in. The magnitudes and signs of pure refractive electronic nonlinearities of silica and BK7 were determined with this new method using 150 femtosecond (fs) laser pulses at 775 nm. Experimental and theoretical analyses of the NER signal were carried out and the results were compared to those obtained with the Z-scan technique.

Simultaneous thickness and group index measurement with a single arm low-coherence interferometer.

We present a single arm low-coherence interferometer to directly measure the physical thickness and group refractive index of optically transparent samples having flat and parallel surfaces. The optical arrangement, resembling a common-path interferometer, is more compact and stable than the usual dual-arm low-coherence interferometer. It has been used to measure samples of Herasil 102 fused silica, Schott B270 Superwhite crown glass and borosilicate cover glass. The results obtained indicate uncertainties in the third decimal place for index values and thicknesses accurate to within 2 µm.

Advances in rare earth spectroscopy and applications.

Rare earth (RE) elements are prime constituents in a large amount of innovative materials and several technological advances would not be possible without their contribution. In this review, recent progress in the field of rare earth spectroscopy is highlighted, with a special emphasis on clean energy, sensors and telecommunications, providing a broad view on past and recent developments.

Humidity and pressure sensor based on internal reflection.

A low-cost humidity and pressure optical sensor, based on the internal reflection phenomenon, is presented. It takes advantage of the phase difference acquired by s- and p-polarized light undergoing internal reflection to generate an easily detectable minimum in the reflected profile, in a position corresponding to the critical angle. The apparatus presents good sensitivity to relative humidity changes above 70% and a response time below one second. The same device is also capable of measuring changes in pressure and can be used as a vacuum gauge between 1 and 1000 mbar.

Influence of self-focusing of ultrashort laser pulses on optical third-harmonic generation at interfaces.

We report on the third-harmonic generation of femtosecond laser pulses at interfaces. We measured slabs of different types of optical glasses and demonstrated that the asymmetric intensity profile observed for a tightly focused beam can be explained by self-focusing effects.

High-sensitivity optical humidity sensor based on a thin dielectric waveguide.

A low-cost, high-sensitivity humidity sensor based on a low-loss dielectric thin film waveguide (WG) is presented. The guided mode is produced by coupling laser light into the film by optical tunneling through a solid gap deposited on the base of a semi-cylindrical lens. The light reflected from this optical system carries information about the refractive index of the medium neighboring the WG, and is detected by a low-cost CCD linear sensor and analyzed with a microcontroller or personal computer. The technique presents good sensitivity to relative humidity (RH) changes, especially below 10% RH, linear behavior between 20% and 80% RH, and a response time of a few seconds.

Effect of interaction with micelles on the excited-state optical properties of zinc porphyrins and J-aggregates formation.

This work reports on the photophysical properties of zinc porphyrins meso-tetrakis methylpyridiniumyl (Zn(2+)TMPyP) and meso-tetrakis sulfonatophenyl (Zn(2+)TPPS) in homogeneous aqueous solutions and in the presence of sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide (CTAB) micelles. The excited-state dynamic was investigated with the Z-scan technique, UV-Vis absorption, and fluorescence spectroscopy. Photophysical parameters were obtained by analyzing the experimental data with a conventional five-energy-level diagram. The interaction of the charged side porphyrin groups with oppositely charged surfactants can reduce the electrostatic repulsion between porphyrin molecules leading to aggregation, which affected the porphyrin characteristics such as absorption cross-sections, lifetimes and quantum yields. The interaction between anionic ZnTPPS with cationic CTAB micelles induced the formation of porphyrin J-aggregates, while this effect was not observed in the interaction of ZnTMPyP with SDS micelles. This difference is, probably, due to the difference in electrostatic repulsion between the porphyrin molecules. The insights obtained by these results are important for the understanding of the photophysical behavior of porphyrins, regarding potential applications in pharmacokinetics as encapsulation of photosensitizer for drug delivery systems and in its interaction with cellular membrane.

Infrared cascade and cooperative multicolor upconversion emissions in Y8V2O17:Eu:Yb nanophosphors.

This work reports on the efficient cooperative upconversion and infrared cascade downconversion emissions in a novel Y8V2O17:Eu:Yb nanophosphor. The excitation with UV light produces emission in the 950-1000 nm region, corresponding to the Yb³âº:²F5/2-->²F7/2 transition, as well as visible emissions of the Eu³âº ion. Time-resolved spectroscopy measurements revealed that the mechanism responsible for this transition is the efficient cascade nonresonant energy transfer from VO4³-->Eu-->Yb. When the same nanophosphor is excited with 976 nm radiation, bright reddish upconversion emission of the Eu³âº:5DJ-->7FJ transition is observed as consequence of the Yb + Yb-->Eu cooperative energy transfer mechanism, which was established by analyzing the emission power dependence and the time-resolved spectroscopy of radiative transitions.

Experimental and theoretical investigation of optical nonlinearities in (nitrovinyl)-1H-pyrazole derivative.

This work reports on the optical nonlinearities of a newly synthesized pyrazole derivative, namely (E)-1-(4-chlorophenyl)-4-(2-nitrovinyl)-1H-pyrazole. The Z-scan technique with femtosecond laser pulses was used to determine the two-photon absorption (2PA) cross-section spectrum, which presents a maximum of 67 GM at 690 nm. We have combined hyper-Rayleigh scattering (HRS) experiments and second-order Møller-Plesset perturbation theory (MP2) calculations to study the first hyperpolarizability (ß(HRS)). It was found that the MP2/6-311+G(d) model, taking into account solvent and dispersion effects, provides the ß(HRS) value of 40×10(-30) cm(5)/esu for the compound, in good agreement with the experimental result of 45±2×10(-30) cm(5)/esu.

A simple method to measure critical angles for high-sensitivity differential refractometry.

A total internal reflection-based differencial refractometer, capable of measuring the real and imaginary parts of the complex refractive index in real time, is presented. The device takes advantage of the phase difference acquired by s- and p-polarized light to generate an easily detectable minimum at the reflected profile. The method allows to sensitively measuring transparent and turbid liquid samples.

Investigation of ground- and excited-state photophysical properties of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphyrin with ruthenium outlying complexes.

The present work employs a set of complementary techniques to investigate the influence of outlying Ru(II) groups on the ground- and excited-state photophysical properties of free-base tetrapyridyl porphyrin (H(2)TPyP). Single pulse and pulse train Z-scan techniques used in association with laser flash photolysis, absorbance and fluorescence spectroscopy, and fluorescence decay measurements, allowed us to conclude that the presence of outlying Ru(II) groups causes significant changes on both electronic structure and vibrational properties of porphyrin. Such modifications take place mainly due to the activation of nonradiative decay channels responsible for the emission quenching, as well as by favoring some vibrational modes in the light absorption process. It is also observed that, differently from what happens when the Ru(II) is placed at the center of the macrocycle, the peripheral groups cause an increase of the intersystem crossing processes, probably due to the structural distortion of the ring that implies a worse spin-orbit coupling, responsible for the intersystem crossing mechanism.

Effects of environment on the photophysical characteristics of mesotetrakis methylpyridiniumyl porphyrin (TMPyP).

Porphyrins are an important class of organic molecules, with interesting linear and nonlinear optical properties given mainly by their extended π-conjugation structure. Their photophysical properties can be greatly affected by the surrounding environment, which can be used to tune its final properties. Here we report on an experimental study of the photophysical properties of meso-tetrakis (methylpyridiniumyl) porphyrin (TMPyP) in aqueous and in several organic solvents and its interaction with micelles formed from negatively charged sodium dodecylsulphate (SDS), positively charged cetyl trimethyl ammonium bromide (CTAB) and neutral TRITON X-100. By using the Z-scan technique, flash-photolysis and time-resolved fluorescence techniques, we were able to evaluate the excited state dynamics of the TMPyP, and observed that the tetrapyrrole ring plays important role due to hydrogen bonds formation between nitrogen atom and water, while the side groups determine the porphyrin localization in non-aqueous micelle part.

Experimental and theoretical study of two-photon absorption in nitrofuran derivatives: Promising compounds for photochemotherapy.

We report experimental and theoretical studies of the two-photon absorption spectrum of two nitrofuran derivatives: nitrofurantoine, (1-(5-nitro-2-furfurilideneamine)-hidantoine) and quinifuryl, 2-(5(')-nitro-2(')-furanyl)ethenyl-4-{N-[4(')-(N,N-diethylamino)-1(')-methylbutyl]carbamoyl} quinoline. Both molecules are representative of a family of 5-nitrofuran-ethenyl-quinoline drugs that have been demonstrated to display high toxicity to various species of transformed cells in the dark. We determine the two-photon absorption cross-section for both compounds, from 560 to 880 nm, which present peak values of 64 GM for quinifuryl and 20 GM for nitrofurantoine (1 GM = 1×10(-50)cm(4).s.photon(-1)). Besides, theoretical calculations employing the linear and quadratic response functions were carried out at the density functional theory level to aid the interpretations of the experimental results. The theoretical results yielded oscillator strengths, two-photon transition probabilities, and transition energies, which are in good agreement with the experimental data. A higher number of allowed electronic transitions was identified for quinifuryl in comparison to nitrofurantoine by the theoretical calculations. Due to the planar structure of both compounds, the differences in the two-photon absorption cross-section values are a consequence of their distinct conjugation lengths.

Excited state dynamics of meso-tetra(sulphonatophenyl) metalloporphyrins.

The excited state dynamics of Zn2+, Fe3+, and Mn3+ meso-tetra(sulfonatophenyl) porphyrin complexes were investigated with a Z-scan technique at 532 nm using 70 ps and 120 fs single pulses and 200 ns pulse trains of a Q-switched and mode locked laser. We determined the characteristic interconversion and intersystem crossing times, quantum yields of the excited S1 state, and S1 --> Sn and T1 --> Tn transition cross-sections. The ground state cross-sections were obtained using UV-vis absorption spectroscopy, and a five-energy-level diagram was used to yield the photophysical parameters mentioned previously.

Two-photon absorption dependence on the temperature for azoaromatic compounds: effect of molecular conformation.

This work reports on the effect of temperature on the two-photon absorption cross section of azoaromatic chromophores. A linear decrease in the two-photon absorption cross section with the temperature was observed for several azochromophores. This process was characterized by introducing a two-photon absorption thermal coefficient (ddelta/dT), whose typical values are approximately 2GM/degrees C for all the azochromophores studied here. Such an effect was attributed to thermal induced molecular conformation changes, described by the sum-over-states model and semiempirical calculations, which affect the molecular dipole moments. The characterization of the phenomenon reported here for other nonlinear materials can help in the design of specific applications using two-photon absorption.

Precise control of superluminal and slow light propagation by transverse phase modulation.

We have used a heterodyne Z-scan technique to produce both superluminal and slow light propagation in media that present either thermal or Kerr nonlinearities. The sample position determines the magnitude and sign of the group velocity and this property was used to control it, with an experimental setup much simpler than those previously reported in similar investigations. The observed effect is attributed to the transverse phase modulation produced by a focused Gaussian beam, and is capable of producing both positive and negative group velocities in the range 1.5 m/s < |upsilon(g)| < c.

Nonlinear absorption dynamics in tetrapyridyl metalloporphyrins.

Nonlinear absorption dynamics of Zn(2+), Cu(2+), and Ni(2+) tetrapyridyl porphyrins in chloroform/methanol solutions were investigated at 532 nm with the Z-scan technique. Additional techniques such as UV-vis absorption spectroscopy and time-resolved fluorescence were used to obtain parameters that are important for the analysis of the population dynamics. A marked difference was observed in the nonlinear absorption and excited-state dynamics of closed (ZnTPyP)- and open-shell metalloporphyrins (CuTPyP and NiTPyP). ZnTPyP presents a reverse saturable absorption whose dynamics can be completely described by means of a simple five-energy-level diagram. On the other hand, CuTPyP and NiTPyP have a different excited-state dynamics, presenting a saturable absorption behavior and faster relaxation rates that were attributed to the presence of unfilled d shells of the central ion.

High-efficiency multipass optical limiter.

We report experimental results on a highly efficient optical limiter that uses a three-mirror, multipass ring configuration. The optical limiter's enhanced performance is based on the accumulative effect that is achieved when the laser beam passes through the sample several consecutive times. Its feasibility was demonstrated for media with different limiting action processes, namely, reverse saturable absorption, nonlinear scattering, and thermal lensing. We employed a frequency-doubled, Q-switched, 10-ns Nd:YAG laser operating at a 5-Hz repetition rate.