Antonio A. Rodríguez-Rosales

Impact of dyes on the nonlinear optical response of liquid crystals implementing the Z-scan technique

The study of the nonlinear refractive index response γ of several organic dyes and their impact on the nonlinear optical (NLO) properties of nematic liquid crystals (LC) was performed via Z-scan measurements. For his purpose, a low power CW He-Ne laser system (λ ≈ 633 nm) was implemented. Studies were carried out at the low absorption spectroscopic region of the implemented samples (dyes, liquid crystals and mixtures at different ratios of these materials). Samples were prepared at 1% weight of the used solvent (THF) and were sandwiched in glass cells with a gap thickness of ~100 μm. The implemented dyes have shown the largest optical nonlinearities and represent the main contributors to the cubic NLO-properties of the LC:Dye mixtures. In our particular studies, 5CB liquid crystal doped with DR1 azo-dye, resulted in the simultaneous positive and negative exhibition of nonlinear refractive indexes γ, depending on the polarization state of the excitation laser beam. Experimental conditions and results are described in detail.

Review of applications of medical lasers

Since the invention of lasers almost thirty years ago, physicians and researchers have worked in an increasing number of applications in medicine. The physical characteristics of lasers such as monochromaticity, coherence, and directionality, identifies them as unique light sources. A review of the main applications of medical lasers is presented. We present applications with photothermal effects with gas lasers, photoablation with solid lasers, photochemical effects with tuned lasers and recently, pulsed laser in photo-biology and tomography. A summary of applications is listed and a partial list of references is given.

Time of flight dependent linearity in diffuse imaging: how effective is it to evaluate the spatial resolution by measuring the edge response function?

We describe the behavior of linearity in diffuse imaging by evaluating the differences between time-resolved images produced by photons arriving at the detector at different times. Two approaches are considered: Monte Carlo simulations and experimental results. The images of two complete opaque bars embedded in a transparent or in a turbid medium with a slab geometry are analyzed; the optical properties of the turbid medium sample are close to those of breast tissue. A simple linearity test was designed involving a direct comparison between the intensity profile produced by two bars scanned at the same time and the intensity profile obtained by adding two profiles of each bar scanned one at a time. It is shown that the linearity improves substantially when short time of flight photons are used in the imaging process, but even then the nonlinear behavior prevails. As the edge response function (ERF) has been used widely for testing the spatial resolution in imaging systems, the main implication of a time dependent linearity is the weakness of the linearity assumption when evaluating the spatial resolution through the ERF in diffuse imaging systems, and the need to evaluate the spatial resolution by other methods.

Design and construction of a compact Z-scan portable system

The design and construction of a compact instrument that automatically measures Kerr-based third order nonlinearities (both nonlinear refractive index: γ, and nonlinear absorption: β) in materials, is presented. The instrument includes control of the polarization state of the input laser beam and was calibrated with well known reference samples. The mechanical translation system and the polarizer rotation-stage are controlled via a home-made electronic circuit, whereas the data acquisition from three photodiodes is performed by a National Instruments 12-bits DAQ. The entire system is fully controlled by means of an application program encoded in LabView. The importance of the developed experimental device is its reliability, compactness, easy implementation and transport, table-top installation, low cost and high accuracy.

Simultaneous exhibition of positive and negative nonlinear response of dye-doped liquid crystal with polarization dependence of the Z-scan technique

The Z-scan technique was use to study the negative and positive nonlinear refractive index responses of methyl-red doped 5 CB liquid crystals samples as a function of a He-Ne laser beam polarization.

Ultrashort laser pulse characterization by three optical methods: autocorrelation, optical interference, and spectral analysis

In this work, we present results on the characterization of ultrashort laser pulses in the range of tens of femtoseconds by three techniques: autocorrelation, spectral analysis and optical interference. Pulses are generated by a Ti-Sapphire (Ti:Sa) laser pumped by a solid state laser. The temporal width of the pulse (FWHM) was measured at different wavelengths from 730 to 820 nm. At the same wavelength, we obtained different values depending on the characterization technique used. We discuss those results and the theoretical models used in each case. For autocorrelation and spectral analysis, we assume an almost-Gaussian pulse to calculate the pulse width. The mathematical model employed allowed us to estimate deviations from this approximation. The experimental results obtained by interferometry allowed us to control the spatial and temporal distance between pulses. The spectral properties of almost-Gaussian functions are considered and applied to characterize to a second-order approximation in the expansion of the coefficients the pulses. Specifically, adding small amounts of odd-order Hermite-Gauss to a Gaussian induces a second-order increase in the time-bandwidth product, while the increase in the time-bandwidth product from adding even-order Hermite-Gaussian is higher-order and hence smaller. We compare the almost-Gaussian functions with femtosecond temporal width pulses data obtained for the Ti:Sa laser.

Time-resolved spectroscopic techniques in laser medicine

Spectroscopic lasers techniques are very useful for the detection and treatment of cancer and removing atherosclerotic plaque. Photobiology and photochemical studies, with the new generation of lasers high resolution time-resolved optical tomography is mentioned. A brief review of some of these applications is discussed and a partial list of recent references is given.

Ultrashort laser pulses: a comparison between a mathematical model and experimental data

The spectral properties of almost-Gaussian functions are considered and applied to the characterization to the second-order approximation in the expansion of the coefficients of almost-perfect optical pulses. Specifically, adding small amounts of odd-order Hermite-Gaussians to a Gaussian induces a second-order increase in the time- bandwidth product, while the increase in the time-bandwidth product from adding even-order Hermite-Gaussian is higher- order and hence smaller. We indicate the class of small perturbations of Gaussian functions which change neither the temporal profile of the intensity nor the intensity of the spectral profile. We compare the almost-Gaussian functions with femtosecond temporal width pulses data given by a Ti:Sapphire laser.

Softening of the gutta-percha by laser irradiation and its thermal impact in the canal obturation

An Argon Laser was employed as a thermal source for the softening of the guttapercha, which is used in the root canal obturation; the preliminary results showed this technique can be superior when compared with other techniques. The tests of sealing with methylene blue with an average penetration of 0.76 mm is obtained on the walls of the channel, with almost a zero risk on the surrounding tissues by effect of the temperature on the surface of the dental pieces.