Aristides Marcano

Pump–probe mode-mismatched thermal-lens Z scan

We report on a pump–probe mode-mismatched thermal-lens Z-scan method for the measurement of small absorption coefficients. In this method the pump light beam is focused into the sample to induce a thermal lens, which is tested by a collimated probe-light beam. Comparison between mode-matched and mode-mismatched Z-scan schemes is performed by use of a Fresnel-diffraction approximation model. This method is used to measure the absorption of distilled water and optical glass in the near-infrared and visible regions of the spectrum.

Optimizing and calibrating a mode-mismatched thermal lens experiment for low absorption measurement

We describe a calibrated two-beam mode-mismatched thermal lens experiment aimed at determination of the absorption coefficient and the photothermal parameters of a nearly transparent material. The use of a collimated probe beam in the presence of a focused excitation beam optimizes the thermal lens experiment. The signal becomes independent from the Rayleigh parameters and waist positions of the beams. We apply this method to determine the absolute value of the thermal diffusivity and absorption coefficient of distilled water at 533 nm.

Ultrasensitive thermal lens spectroscopy of water

The accurate knowledge of the water absorption spectrum is of vital importance for many branches of science and technology. Although it was investigated by different techniques, particularly between 300 – 450 nm, there is still significant disagreement between various studies [1–4]. From 3 to 0.3 mm, the water absorption coefficient drops 8 orders of magnitude, reaching a minimum at ∼ 0.4. However, the wavelength and value of the minimum absorption is still unclear.

Spatial Ca2+ Distribution in Contracting Skeletal and Cardiac Muscle Cells

The spatiotemporal distribution of intracellular Ca(2+) release in contracting skeletal and cardiac muscle cells was defined using a snapshot imaging technique. Calcium imaging was performed on intact skeletal and cardiac muscle cells during contractions induced by an action potential (AP). The sarcomere length of the skeletal and cardiac cells was approximately 2 micrometer. Imaging Rhod-2 fluorescence only during a very brief (7 ns) snapshot of excitation light minimized potential image-blurring artifacts due to movement and/or diffusion. In skeletal muscle cells, the AP triggered a large fast Ca(2+) transient that peaked in less than 3 ms. Distinct subsarcomeric Ca(2+) gradients were evident during the first 4 ms of the skeletal Ca(2+) transient. In cardiac muscle, the AP-triggered Ca(2+) transient was much slower and peaked in approximately 100 ms. In contrast to the skeletal case, there were no detectable subsarcomeric Ca(2+) gradients during the cardiac Ca(2+) transient. Theoretical simulations suggest that the subsarcomeric Ca(2+) gradients seen in skeletal muscle were detectable because of the high speed and synchrony of local Ca(2+) release. Slower asynchronous recruitment of local Ca(2+) release units may account for the absence of detectable subsarcomeric Ca(2+) gradients in cardiac muscle. The speed and synchrony of local Ca(2+) gradients are quite different in AP-activated contracting cardiac and skeletal muscle cells at normal resting sarcomere lengths.

Determination of protein hydrogen composition by laser-induced breakdown spectroscopy

We report on the quantitative identification of the hydrogen composition of high molecular weight proteins in a heavy water solution by laser-induced breakdown spectroscopy.

Thermal diffusivity measurement using the mode-mismatched photothermal lens method

We report on a mode-mismatched pump-probe photothermal lens experiment aimed at determination of the thermal diffusivity coefficient of optical samples. In the scheme, the probe beam is collimated and the pump beam is focused. Using a Fresnel diffraction approximation, we show that under these conditions the time dependence of the signal does not depend on the Rayleigh parameters and waist positions of the light beams. This fact allows a more simple and reliable calibration of the experiment in comparison to other schemes. We conduct studies on liquid and solid samples that confirm the predictions of the model. Using the proposed method, we measure the thermal diffusivity coefficient of distilled water, ethanol, methanol, chloroform, nitrobenzene, ethylene glycol, and a solid piece of acrylic plastic. The results are in good agreement with previous measurements.

Elemental analysis of laser induced breakdown spectroscopy aided by an empirical spectral database

Laser induced breakdown spectroscopy (LIBS) is commonly used to identify elemental compositions of various samples. To facilitate this task, we propose the use of an elemental spectral library for single-pulsed, nanosecond LIBS in the spectral range 198-968 nm. This spectroscopic library is generated by measuring optical emissions from plasmas of 40 pure elements. To demonstrate the usefulness of the proposed database, we measure and analyze the LIBS spectra of pure iron and of ethanol and show that we identify these samples with a high degree of certainty.

Automatic Classification of Laser-Induced Breakdown Spectroscopy (LIBS) Data of Protein Biomarker Solutions

We perform multi-class classification of laser-induced breakdown spectroscopy data of four commercial samples of proteins diluted in phosphate-buffered saline solution at different concentrations: bovine serum albumin, osteopontin, leptin, and insulin-like growth factor II. We achieve this by using principal component analysis as a method for dimensionality reduction. In addition, we apply several different classification algorithms (K-nearest neighbor, classification and regression trees, neural networks, support vector machines, adaptive local hyperplane, and linear discriminant classifiers) to perform multi-class classification. We achieve classification accuracies above 98% by using the linear classifier with 21–31 principal components. We obtain the best detection performance for neural networks, support vector machines, and adaptive local hyperplanes for a range of the number of principal components with no significant differences in performance except for that of the linear classifier. With the optimal number of principal components, a simplistic K-nearest classifier still provided acceptable results. Our proposed approach demonstrates that highly accurate automatic classification of complex protein samples from laser-induced breakdown spectroscopy data can be successfully achieved using principal component analysis with a sufficiently large number of extracted features, followed by a wrapper technique to determine the optimal number of principal components.

White Light Photothermal Lens Spectrophotometer for the Determination of Absorption in Scattering Samples

We developed a pump-probe photothermal lens spectrophotometer that uses a broadband arc-lamp and a set of interference filters to provide tunable, nearly monochromatic radiation between 370 and 730 nm as the pump light source. This light is focused onto an absorbing sample, generating a photothermal lens of millimeter dimensions. A highly collimated monochromatic probe light from a low-power He–Ne laser interrogates the generated lens, yielding a photothermal signal proportional to the absorption of light. We measure the absorption spectra of scattering dye solutions using the device. We show that the spectra are not affected by the presence of scattering, confirming that the method only measures the absorption of light that results in generation of heat. By comparing the photothermal spectra with the usual absorption spectra determined using commercial transmission spectrophotometers, we estimate the quantum yield of scattering of the sample. We discuss applications of the device for spectroscopic characterization of samples such as blood and gold nanoparticles that exhibit a complex behavior upon interaction with light.

Performance of multilayer perceptrons for classification of LIBS protein spectra

We investigate performance of neural networks for classification of laser-induced breakdown spectroscopic data of four proteins: Bovine Serum Albumin, Osteopontin, Leptin and Insulin-like Growth Factor II. We utilize principal component analysis algorithm for feature extraction and multilayer perceptrons algorithms with one and two hidden layers. We employ leave-one-out procedure for classifier evaluation. Our experimental results indicate that methods with linear convergence can provide classification accuracy superior to methods with quadratic convergence.