R. E. de Araujo

Evaluation of enamel dental restoration interface by optical coherence tomography

Evaluation of molar dental restorations on enamel is performed using optical coherence tomography (OCT) with 10 microm resolution. Images of approximately 50 microm failure gaps in the restorations are demonstrated and the OCT images are compared with x-ray and optical microscopy pictures. The results demonstrate the potential of the technique for clinical evaluation of dental restorations.

Third-order nonlinear optical properties of bismuth-borate glasses measured by conventional and thermally managed eclipse Z scan

Third-order nonlinearity one order of magnitude larger than silica is measured in bismuth-borate glasses presenting a fast response (<200 fs). The results for the sign and magnitude of the nonlinearity were obtained using a combination of the eclipse Z scan with thermal nonlinearity managed Z scan, whereas the Kerr shutter technique was employed to obtain the electronic time response of the nonlinearity, all performed with 76 MHz repetition rate 150 fs pulses at 800 nm. Conventional Z scans in the picosecond regime at 532 and 1064 nm were also independently performed, yielding the values of the third-order nonlinear susceptibilities at those wavelengths. The results obtained for the femtosecond response, enhanced third-order nonlinearity of this glass (with respect to silica), place this glass system as an important tool in the development of photonics devices. Electro-optical modulators, optical switches, and frequency converters are some of the applications using second-order nonlinear properties of the Bi-glass based on the rectification model.

Thermally managed eclipse Z-scan

We report a new variation of the conventional Z-scan method to characterize the third-order optical nonlinearity of photonic materials. By exploiting the combination of the eclipse Z-scan with a thermal nonlinearity management technique, we demonstrate an improvement in sensitivity and flexibility of the method to simultaneously characterize the thermal and nonthermal nonlinearity of optical materials. The method is demonstrated by measuring the nonlinear refractive index in CS(2), SiO(2) and H(2)O, standard materials, and also in a biomaterial, the amino acid Tryptophan in water solution, using a femtosecond Ti-Sapphire laser operating at 76MHz repetition rate.

One photon nonresonant high-order nonlinear optical properties of silver nanoparticles in aqueous solution

In this work we determine the third, fifth- and seventh-order nonresonant nonlinear optical properties of silver nanoparticles (9 nm average diameter) colloids in aqueous solution under high intensity excitation. The nonlinear optical response and its dependence with the nanoparticles filling factor was measured and theoretically described. We show that for low inclusion concentration, the third order nonlinearity of the colloid can be described by the generalized Maxwell-Garnett model. With the increase of the nanoparticle concentration, changes in the medium nonlinearities was observed leading to high order effects. The fifth- and seventh- order susceptibilities were obtained for highly concentrated silver nanoparticle colloid and the data was supported by a theoretical model. The conventional Z-scan technique was employed, using 80 f s laser pulses at 800 nm, in a regime of high pulse energy (microJ) and low repetition rate (1 kHz).

Investigation of picosecond optical nonlinearity in porphyrin metal complexes derivatives

Abstract Porphyrin metal complex derivatives were experimentally studied using the Z -scan technique and the optical Kerr gate method with picosecond pulses at 532 nm. The sign and magnitude of the nonlinear refraction and nonlinear absorption were determined for a negatively charged meso -tetrakis( p -sulfonatophenyl)porphyrin (TPPS 4 ) and a positively charged meso -tetrakis(4- N -methyl-pyridiniumyl)porphyrin (TMPyP) in their free-base forms and their Fe(III) and Mn(III) complexes. The results were phenomenologically understood and the origin of the nonlinearity was inferred.

Theoretical and experimental investigation of the second hyperpolarizabilities of methyl orange

We describe experimental and theoretical studies of the third-order nonlinear optical coefficients of methyl orange solutions under different pH conditions. A combination of semiempirical and ab initio methods was adopted to investigate the most stable geometrical structures possible for this molecule. The experimental data obtained using the Z-scan technique for the third-order nonlinear optical properties of this compound has allowed the determination of the nonlinear refractive index and nonlinear absorption coefficient under picosecond excitation in the visible (532 nm) spectral region. From those experimental results, the second hyperpolarizability of methyl orange was inferred both for acidic and alkaline solutions. Comparison of these values to the results predicted by semiempirical methods suggests that even at low pH, when the probability of cis-trans isomerization is increased, the trans conformation of the methyl orange molecule should dominate the nonlinear spectra of this compound. The theoretical results were used as an auxiliary tool to identify possible trends on the nonlinear properties changes as a function of the distinct molecular conformations adopted by the methyl orange molecule under different pH conditions.

Eclipsing thermal lens spectroscopy for fluorescence quantum yield measurement.

A modified spatial filtering method that improves the sensitivity of single-beam and mode-mismatched thermal lens spectroscopy (TLS) for fluorescence quantum yield measurement is presented. The method is based on the detection of the external part of a laser beam transmitted by the fluorescent sample (eclipsing detection mode). The experimental results show that the signal/noise (S/N) ratio of the absolute quantum yield of Rh6G can be enhanced up to ~1400% using the eclipsing detection mode on the TLS experimental setup. The method was evaluated by measuring the fluorescence quantum yield of varying concentration of ethanolic solutions of Rhodamine 6G.

Measurements of pKa of organic molecules using third-order nonlinear optics

Abstract We report the use of the Z-scan method, which is a nonlinear optical technique, as a general method to determine a chemical parameter, the p K a , which characterizes the equilibrium constant in acid–base reactions. The measurements were performed with picosecond pulses at 532 nm in aqueous solution of methyl orange, C 14 H 14 N 3 O 3 S − . The experimental results agree well with a phenomenological description of the molecule nonlinearity as well as with the results predicted using the Henderson–Hasselbalch equation.

Nonlinear optical properties of aromatic amino acids in the femtosecond regime

We report experimental and theoretical investigations of the third-order optical nonlinearities of aromatic amino acids (Phenylalanine, Histidine, Tryptophan, and Tyrosine) in aqueous solutions. The Z-scan technique with femtosecond laser pulses at 800 nm was explored for the determination of the nonlinear refractive index, nonlinear absorption coefficient, and the second-order hyperpolarizability of each amino acid. Experimental results were compared with theoretical analysis based on post-Hartree Fock MP2/6-311+G**.

Hartmann-Shack wavefront sensing for nonlinear materials characterization

We present two new techniques exploiting a Hartmann-Shack wavefront sensor to characterize the optical self-focusing effect of nonlinear materials. We demonstrate that the defocus Zernike coefficient (C5) can be used to quantify nonlinear optical properties of materials. In the first technique proposed, the wavefront of a collimated laser beam transmitted through a nonlinear sample is analyzed with different irradiance values. In the second technique,instead of conventional detectors, a Hartmann- Shack sensor is used in a Z-scan setup. The methods are demonstrated by measuring the nonlinear refractive indices of CS2 and Quartz, using femtosecond Ti:sapphire lasers at 76 MHz and 1 KHz repetition rate.