Optical cavitation is the formation of vapor cavities in a liquid when a pulsed laser is focused over highly absorbent liquid; however, when a continuous laser is used, the phenomenon is called thermo-cavitation. Recently, thermocavitation has been studied in different materials<sup>1–3</sup>. In this work, we present the analysis of extra-cavity pulses generation by thermo-cavitation induced by a CW laser beam focused into solutions of Hibiscus Sabdariffa in ethanol and Hibiscus Sabdariffa in methanol. Due to the high absorption of the natural dye and the low boiling point of the solvents (< boiling point of the water), heating is produced which gives origin and implosion of bubbles. The process of explosion and implosion of the bubbles acts as an obturator allowing the pulses formation of the light passing through the sample. The characterization of the pulses was performed by moving the sample around the focus of the lens, we observe a modification in the thermo-cavitation time, an analysis of the changes in the frequency rate and the amplitude of pulses was performed. The frequency rate, the amplitude, and the full width half maximum (FWHM) of the pulses were measured. We found that the average frequency decreases, and the average amplitude increases when we move the sample at a distance from the focus. The temporary response of the pulses obtained in both solutions, change as a consequence of the difference between the boiling point of the methanol and ethanol.
Organic semiconductor dyes play a very important role in different criteria and fields of science, technology, and engineering for different applications, especially the absorption of visible light for solar cell devices, photo sensors, fluorescent, photoconductive devices. Basic fuchsine belongs to the triphenylmethane family. It is used on a large scale as a coloring agent for the staining of biological tissues, leather, and textile materials. The Z-scan technique is based on the principles of spatial distortion of the beam and offers simplicity, as well as a very high sensitivity for measuring both the nonlinear refractive index as well as the non-linear absorption coefficient. For the present work we propose the study of 6 star-type dyes derived from basic fushcine with azo groups added, by the Z-scan technique. A 1 × 10<sup>-4</sup> M dilution of each dye in methanol was prepared. Subsequently, the linear characterization was made with the help of the UV-vis technique and finally, the analysis was carried out in the Z-scan equipment. The dyes have a λmax between 500-570 nm, and with n<sub>2</sub> = -1.7 × 10<sup>-9</sup> to 6.63 × 10<sup>-9</sup> and β = 1.1 × 10<sup>-3</sup> to 6.3 × 10<sup>-4</sup>. These results allow us to propose these dyes as good options for their incorporation into solar cells as well as optical absorbers.
To control the light propagation in turbid media, it is necessary to reconstruct the output wavefront. In 2007 Vellekoop et al.<sup>1</sup>, developed an iterative algorithm that divides the input wavefront in N x N channels called segments, after passing through turbid media, the output wavefront is reconstructed by measuring the intensity at a desired point, and then the phase of each channel is updated, the final <i>N</i> x <i>N</i> phase is called optimal phase matrix. The interpolation technique is capable of transforming a <i>N</i> x <i>N</i> matrix into a second 2<i>N</i> x 2<i>N</i> matrix, where, the 50 percent of the resulting matrix elements correspond to the homogeneous distribution of the original matrix values and the remain values are generated by interpolating the neighbors. Our proposal uses the optimal phase matrix obtained by an iterative algorithm, and then the number of segments is increased by interpolation. We analyze the circularity, the signal to noise ratio (SNR), the Full Width at Half Maximum (FWHM) and the correlation for different output wavefronts obtained by the optimal phase matrix and the interpolation optimal phase matrix. Our results show that, Circularity, SNR, and FWHM parameters do not change significantly and the acquisition time of the optimal phase matrix decreases compared with a similar matrix obtained by the iterative algorithm; therefore, our proposed technique that consists in the combination of interpolation and iterative algorithm is useful to study the light transmission in turbid media when a high resolution is needed in the transmission matrix, for example, phase holograms transmission through turbid media.
A trustworthy speckle contrast calculation is fundamental in many applications, such as “laser speckle contrast Imaging” (LSCI), which is a non-invasive technique commonly employed to estimate relative blood speed. In LSCI, the local contrast of a speckle image is calculated using spatial, temporal analysis or a combination of both. In this work, we focus on the spatial analysis. To calculate the local spatial contrast, typically, a sliding window of 5x5 pixels is used to calculate the standard deviation (σ<sub>s</sub>) and the mean intensity (<I><sub>s</sub>) of those 5x5 pixels and the calculated contrast K<sub>S</sub>=σ<sub>s</sub>/(<I><sub>s</sub>) is assigned to the central pixel of the sliding window. In this work, we show that this experimental procedure to calculate the local speckle contrast does not match the corresponding spatial theoretical model and we propose an alternative method that considers correlations of the central pixel with the other ones. We have found a better agreement of the contrast measurement with our numerical calculation.
Speckle contrast analysis had been used for multiples purposes, for instance, laser speckle contrast imaging (LSCI) has been used to estimate the relative blood flow speed in a non-invasive way. The speckle contrast can be calculated using a spatial or temporal algorithm or a combination of both. Our work focuses into the contrast temporal algorithm. A contrast image calculated with the temporal contrast algorithm requires a sequence of L equal-sized frames. The contrast images are performed pixel by pixel, however, the experimental contrast calculation does not match with the current temporal theoretical model especially when the exposure time T is smaller than the correlation time τ<sub>c</sub>. In this work, we propose to correlate neighboring pixels along the temporal axis to improve the contrast calculation. The contrast measurements using our proposal provide a better agreement than the current models.
Visualization of deep blood vessels in speckle images is an important task as it is used to analyze the dynamics of the blood flow and the health status of biological tissue. Laser speckle imaging is a wide-field optical technique to measure relative blood flow speed based on the local speckle contrast analysis. However, it has been reported that this technique is limited to certain deep blood vessels (about ρ=300 μm) because of the high scattering of the sample; beyond this depth, the quality of the vessel’s image decreases. The use of a representation based on homogeneity values, computed from the co-occurrence matrix, is proposed as it provides an improved vessel definition and its corresponding diameter. Moreover, a methodology is proposed for automatic blood vessel location based on the kurtosis analysis. Results were obtained from the different skin phantoms, showing that it is possible to identify the vessel region for different morphologies, even up to 900 μm in depth.
Optical tweezers constitute an increasingly used tool for the study of biomechanical properties of cells. Here we report experiments for the projection induction of NIH3T3 fibroblast cells, using a single-trap optical tweezers. The system is based on a 1064-nm, 50mW infrared gaussian laser beam, a 100x microscope objective with 1.25 numerical aperture and a temperature-controlled warming plate to maintain cell viability. Eighteen cells were exposed to the focussed laser beam in different cell zones and another 18 cells were observed without laser stimulation as a control population. The results show that the probability of lamelipodia growth increases on exposed cells by a factor 1.5.
This work aims to design a filter to attenuate high- and medium- frequency noise in optical test images without changing
the edges and original characteristics of the test image, generated by traditional filters (spatial or frequential). The noise
produced by the LCD pixels (used as a diffraction grating in the Ronchi test) was analyzed. The diffraction is modulated
by the spherical wavefront of the mirror under test, generating at least two frequency band noise levels. To reduce this
bi-frequential noise, we propose to use an array of filters with the following structure: a low-pass frequential filter LPFF,
a band- pass frequential filter BPFF and a circular mask spatial filter CMSF; thus obtaining the composed filter
CF=LPFF-(BPFF)(CMSF). Various sizes of filters were used to compare their signal-to-noise ratio against simple filters
(low-pass and band-stop).
The aim of this work is to propose the use of printed acetate sheets as quasi-sinusoidal and quasi-triangular diffraction
gratings, as low-cost alternative gratings for application in non-invasive optical tests. Gratings were generated with
Matlab® software and made with various models of laser and inkjet printers. A study of the profile gratings that depend
on the symmetry in the sample was included, gratings were placed in the entrance pupil of a positive lens (illuminated by
a collimated plane wave) to observe their Fourier transforms. It was found that diffraction patterns of various types of
quasi-sinusoidal and quasi-triangular profiles were very close to that of sinusoidal gratings. Gradual change in the size of
printed ink spots was observed in more detail through a magnification of 40x. Additionally, an atomic force microscope
was used to measure the average superficial roughness of the impressions as to observe the behavior of the ink on the
The Ronchi test with a Liquid Crystal Display (LCD) phase grating is used for testing convergent optical systems. The
rulings are computer-generated and displayed on the LCD. We prove that it is possible to make a variable electronically
phase grating by using an LCD. By displaying various phase-shifted rulings and capturing the corresponding
ronchigrams, the phase is obtained with the conventional phase-shifting algorithms. Experimental results are shown.
The aim of this work is to propose the use of printed acetate sheets as quasi-sinusoidal diffraction gratings, as low-cost
alternative gratings for application in non-invasive optical tests. Gratings were generated with Matlab® software and
made with various models of laser printers. A study of the discretization effects that depend on the symmetry in the
sample was included, gratings were placed in the entrance pupil of a positive lens (illuminated by a collimated plane
wave) to observe their Fourier transforms. It was found that diffraction patterns of various types of semi-sinusoidal
profiles were very close to that of sinusoidal gratings. Gradual change in the size of printed ink spots was observed in
more detail through a magnification of 40x. Additionally, an atomic force microscope was used to measure the
roughness average of the impressions as to observe the behavior of the ink on the acetate.