Thermocavitation: a mechanism to pulse fiber lasers.

In this paper, we present a novel mechanism for the generation of laser pulses based on the phenomenon of thermocavitation. Thermocavitation bubbles were generated within a glass cuvette filled with copper nitrate dissolved in water, where the tip of an optical fiber was placed very close to the bubble generation region. Once the bubble is generated, it expands rapidly and the incoming laser light transmitted through the optical fiber is reflected at the vapor-solution interface and reflected back into the fiber, which is coupled to an erbium-doped fiber ring laser. Laser pulses were extracted from the ring cavity and detected by a fast photodetector, which corresponds to a single thermocavitation event, obtaining a pulse repetition rate from 118 Hz to 2 kHz at 1560 nm, with a pulse width ranging from 64 to 57 µs. The repetition rate can be controlled by adjusting the laser power to induce thermocavitation. To our knowledge, this novel mechanism of laser pulses has not been reported in the literature.

3D trapping of microbubbles by the Marangoni force.

In this Letter, we show 3D steady-state trapping and manipulation of vapor bubbles in liquids employing a low-power continuous-wave laser using the Marangoni effect. Light absorption from photodeposited silver nanoparticles on the distal end of a multi-mode optical fiber is used to produce bubbles of different diameters. The thermal effects produced by either the nanoparticles on the fiber tip or the light bulk absorption modulate the surface tension of the bubble wall and creates both longitudinal and transversal forces just like optical forces, effectively creating a 3D potential well. Using numerical simulations, we obtain expressions for the temperature profiles and present analytical expressions for the Marangoni force. In addition, using an array of three fibers with photodeposited nanoparticles is used to demonstrate the transfer of bubbles from one fiber to another by sequentially switching on and off the lasers.

Optothermal generation, trapping, and manipulation of microbubbles.

The most common approach to optically generate and manipulate bubbles in liquids involves temperature gradients induced by CW lasers. In this work, we present a method to accomplish both the generation of microbubbles and their 3D manipulation in ethanol through optothermal forces. These forces are triggered by light absorption from a nanosecond pulsed laser (λ = 532 nm) at silver nanoparticles photodeposited at the distal end of a multimode optical fiber. Light absorbed from each laser pulse quickly heats up the silver-ethanol interface beyond the ethanol critical-point (∼ 243 °C) before the heat diffuses through the liquid. Therefore, the liquid achieves a metastable state and owing to spontaneous nucleation converted to a vapor bubble attached to the optical fiber. The bubble grows with semi-spherical shape producing a counterjet in the final stage of the collapse. This jet reaches the hot nanoparticles vaporizing almost immediately and ejecting a microbubble. This microbubble-generation mechanism takes place with every laser pulse (10 kHz repetition rate) leading to the generation of a microbubbles stream. The microbubbles' velocities decrease as they move away from the optical fiber and eventually coalesce forming a larger bubble. The larger bubble is attracted to the optical fiber by the Marangoni force once it reaches a critical size while being continuously fed with each bubble of the microbubbles stream. The balance of the optothermal forces owing to the laser-pulse drives the 3D manipulation of the main bubble. A complete characterization of the trapping conditions is provided in this paper.

Theoretical and experimental study of acoustic waves generated by thermocavitation and its application in the generation of liquid jets.

Numerical simulations using the Finite-Difference Time-Domain method were used to study the propagation of an acoustic wave within a truncated ellipsoidal cavity. Based in our simulations, a fluidic device was designed and fabricated using a 3D printer in order to focus an acoustic wave more efficiently and expel a liquid jet. The device consists of an ellipsoidal shaped chamber filled with a highly absorbent solution at the operating wavelength (1064 nm) in order to create a vapor bubble using a continuous wavelength laser. The bubble rapidly expands and collapses emitting an acoustic wave that propagates inside the cavity, which was measured by using a needle hydrophone. The bubble collapse, and source of the acoustic wave, occurs in one focus of the cavity and the acoustic wave is focused on the other one, expelling a liquid jet to the exterior. The physical mechanism of the liquid jet generation is momentum transfer from the acoustic wave, which is strongly focused due to the geometry of the cavity. This mechanism is different to the methods that uses pulsed lasers for the same purpose. The maximum speed of the generated liquid microjets was approximately 20 m/s. One potential application of this fluidic device can be found for inkjet printing, coating and, maybe the most attractive, for drug delivery.

Optical trapping in the presence of laser-induced thermal effects.

The inclusion of thermal effects in optical manipulation has been explored in diverse experiments, increasing the possibilities for applications in diverse areas. In this Letter, the results of combined optical and thermal manipulation in the vicinity of a highly absorbent hydrogenated amorphous silicon layer, which induces both the generation of convective currents and thermophoresis, are presented. In combination with the optical forces, thermal forces help reduce the optical power required to trap and manipulate micrometric polystyrene beads. Moreover, the inclusion of these effects allows the stacking and manipulation of multiple particles with a single optical trap along with the beam propagation, providing an extra tool for micromanipulation of a variety of samples.

Marangoni force-driven manipulation of photothermally-induced microbubbles.

The generation and manipulation of microbubbles by means of temperature gradients induced by low power laser radiation is presented. A laser beam (λ = 1064 nm) is divided into two equal parts and coupled to two multimode optical fibers. The opposite ends of each fiber are aligned and separated a distance D within an ethanol solution. Previously, silver nanoparticles were photo deposited on the optical fibers ends. Light absorption at the nanoparticles produces a thermal gradient capable of generating a microbubble at the optical fibers end in non-absorbent liquids. The theoretical and experimental studies carried out showed that by switching the thermal gradients, it is possible to generate forces in opposite directions, causing the migration of microbubbles from one fiber optic tip to another. Marangoni force induced by surface tension gradients in the bubble wall is the driving force behind the manipulation of microbubbles. We estimated a maximum Marangoni force of 400nN for a microbubble with a radius of 110 µm.

Controllable direction of liquid jets generated by thermocavitation within a droplet.

A high-velocity fluid stream ejected from an orifice or nozzle is a common mechanism to produce liquid jets in inkjet printers or to produce sprays among other applications. In the present research, we show the generation of liquid jets of controllable direction produced within a sessile water droplet by thermocavitation. The jets are driven by an acoustic shock wave emitted by the collapse of a hemispherical vapor bubble at the liquid-solid/substrate interface. The generated shock wave is reflected at the liquid-air interface due to acoustic impedance mismatch generating multiple reflections inside the droplet. During each reflection, a force is exerted on the interface driving the jets. Depending on the position of the generation of the bubble within the droplet, the mechanical energy of the shock wave is focused on different regions at the liquid-air interface, ejecting cylindrical liquid jets at different angles. The ejected jet angle dependence is explained by a simple ray tracing model of the propagation of the acoustic shock wave inside the droplet.

Saturable and two-photon absorption in zinc nanoparticles photodeposited onto the core of an optical fiber.

In this work, the simultaneous presence of saturable (SA) and two-photon absorption (TPA) in zinc nanoparticles (ZnNPs) photodeposited onto the core of an optical fiber was studied in the nanosecond regime with the P-scan method using a high gain pulsed erbium-doped fiber amplifier. An analysis based on Mie theory was carried out to demonstrate the influence of the absorption coefficient with the particles sizes in the proximity of surface plasmon resonance (SPR). The shift from TPA to SA has been observed as the irradiance is increased. It was found that for irradiances lower than 5 MW/cm², TPA is dominant, whereas for irradiances higher than 5 MW/cm², the SA becomes dominant. Furthermore, the values of the nonlinear absorption coefficient and the imaginary part of third-order nonlinear optical susceptibility were calculated numerically from the transmittance measured. Such TPA makes ZnNPs a candidate for optical limiting applications, and SA makes them a candidate for applications in pulsed fiber laser systems.

Far-field diffraction patterns by a thin nonlinear absorptive nonlocal media.

In this work we present numerical results of the far field intensity distributions obtained for a Gaussian beam after crossing a thin nonlinear nonlocal material that exhibit nonlinear refraction and absorption. The distributions are obtained for different positions along the Z axis and different signs of the nonlinear absorption. The results demonstrate that the far field intensity patterns obtained for strong nonlocal media are more affected by the presence of the nonlinear absorption than weak nonlocal media.

Simple correction factor for laser speckle imaging of flow dynamics.

One of the major constraints facing laser speckle imaging for blood-flow measurement is reliable measurement of the correlation time (τ(C)) of the back-scattered light and, hence, the blood's speed in blood vessels. In this Letter, we present a new model expression for integrated speckle contrast, which accounts not only for temporal integration but spatial integration, too, due to the finite size of the pixel of the CCD camera; as a result, we find that a correction factor should be introduced to the measured speckle contrast to properly determine τ(C); otherwise, the measured blood's speed is overestimated. Experimental results support our theoretical model.

Selective photodeposition of zinc nanoparticles on the core of a single-mode optical fiber.

An experimental and theoretical study about selective photodeposition of metallic zinc nanoparticles onto an optical fiber end is presented. It is well known that metallic nanoparticles possess a high absorption coefficient and therefore trapping and manipulation is more challenging than dielectric particles. Here, we demonstrate a novel trapping mechanism that involves laser-induced convection flow (due to heat transfer from the zinc particles) that partially compensates both absorption and scattering forces in the vicinity of the fiber end. The gradient force is too small and plays no role on the deposition process. The interplay of these forces produces selective deposition of particles whose size is directly controlled by the laser power. In addition, a novel trapping mechanism termed convective-optical trapping is demonstrated.

Single-Pixel Near-Infrared 3D Image Reconstruction in Outdoor Conditions.

In the last decade, the vision systems have improved their capabilities to capture 3D images in bad weather scenarios. Currently, there exist several techniques for image acquisition in foggy or rainy scenarios that use infrared (IR) sensors. Due to the reduced light scattering at the IR spectra it is possible to discriminate the objects in a scene compared with the images obtained in the visible spectrum. Therefore, in this work, we proposed 3D image generation in foggy conditions using the single-pixel imaging (SPI) active illumination approach in combination with the Time-of-Flight technique (ToF) at 1550 nm wavelength. For the generation of 3D images, we make use of space-filling projection with compressed sensing (CS-SRCNN) and depth information based on ToF. To evaluate the performance, the vision system included a designed test chamber to simulate different fog and background illumination environments and calculate the parameters related to image quality.

Time-resolved analysis of cavitation induced by CW lasers in absorbing liquids.

We present novel results on thermocavitation using a CW medium-power near infrared laser (lambda=975 nm) focused into a saturated copper nitrate saline solution. Due to the large absorption coefficient at the laser wavelength, the solution can be heated to its superheat limit (T(sh) approximately 270-300 degrees C). Superheated water undergoes explosive phase transition around T(sh) producing approximately half-hemispheric bubbles (gamma approximately 0.5) in close contact with the substrate. We report the temporal dynamic of the cavitation bubble, which is much shorter than previously reported under similar conditions. It was found that the bubble radius and pressure wave amplitude emitted on bubble collapse decreases exponentially with the power laser. Thermocavitation can be a useful tool for the generation of ultrasonic waves and controlled ablation for use in high-resolution lithography.

Polarization-controlled contrasted images using dye-doped nematic liquid crystals.

We explore the polarization dependence of the nonlinear response of a planar nematic liquid crystal cell doped with 1% wt of methyl red dye. The results obtained show that the refractive index change can be switched from a positive value to a negative one as the polarization of the beam changes from parallel to perpendicular with respect to the rubbing direction. This property is exploited in a phase contrast system, where a dynamic phase filter is photoinduced in a liquid crystal cell placed in the system's Fourier plane. Real-time contrast inversion in the resulting images is demonstrated.

Efficient in vitro photodynamic inactivation using repetitive light energy density on Candida albicans and Trichophyton mentagrophytes.

BACKGROUND: We compared the effectiveness of a single irradiation vs repetitive irradiation of light, for in vitro photodynamic inactivation (PDI) of Candida albicans and Trichophyton mentagrophytes, by using methylene blue (MB) and rose bengal (RB) as photosensitizers (PS). METHODS: MB from 5 to 60 µM and RB from 0.5 to 10 µM, with energy densities from 10 to 60 J/cm2, were tested in C. albicans. We further optimize the PDI by reducing the light energy density and PS concentration for the single irradiation experiments by using repetitive doses (two and three times). MB was tested in C. albicans and T. mentagrophytes, and RB was tested in C. albicans. RESULTS: MB-PDI and RB-PDI in C. albicans significantly reduced the number of colony-forming units per milliliter (CFU/mL) when compared to the control groups. Using a single irradiation, over 99% growth inhibition of C. albicans was obtained with MB at 20 µM-60 J/cm2, and with RB at 1 µM-30 J/cm2 and 5 µM-10 J/cm2. With repetitive doses, similar results were obtained by reducing several times the light energy density and the PS concentration for C. albicans and T. mentagrophytes. CONCLUSIONS: The results showed that RB was more effective than MB for C. albicans inactivation. In addition, it is possible to significantly reduce the amount of PS and light energy density requirements by using repetitive irradiations in both genera tested. It makes the technique less invasive and could reduce the side effects in people extremely sensitive to the PS or the light.

Hollow spheres as individual movable micromirrors in optical tweezers.

We introduce the use of hollow micron-sized spheres with a finite-thickness glass shell as individual micromirrors operating by total internal reflection (TIR) when illuminated off-axis. We also demonstrated that this kind of spheres can be optically trapped and manipulated in two dimensions using a Gaussian beam in a conventional optical tweezers setup, which allows the precise positioning of the micromirrors at specific locations within a sample cell. This mirrors constitutes a new micro-tool in the context of the so called lab-on-a-chip.

Trapping and manipulation of microparticles using laser-induced convection currents and photophoresis.

In this work we demonstrate optical trapping and manipulation of microparticles suspended in water due to laser-induced convection currents. Convection currents are generated due to laser light absorption in an hydrogenated amorphous silicon (a:Si-H) thin film. The particles are dragged towards the beam's center by the convection currents (Stokes drag force) allowing trapping with powers as low as 0.8 mW. However, for powers >3 mW trapped particles form a ring around the beam due to two competing forces: Stokes drag and thermo-photophoretic forces. Additionally, we show that dynamic beam shaping can be used to trap and manipulate multiple particles by photophotophoresis without the need of lithographically created resistive heaters.

Effects of speckle/pixel size ratio on temporal and spatial speckle-contrast analysis of dynamic scattering systems: Implications for measurements of blood-flow dynamics.

Laser Speckle Contrast Imaging (LSCI) is an optical technique used to generate blood flow maps with high spatial and temporal resolution. It is well known that in LSCI, the speckle size must exceed the Nyquist criterion to maximize the speckle's pattern contrast. In this work, we study experimentally the effect of speckle-pixel size ratio not only in dynamic speckle contrast, but also on the calculation of the relative flow speed for temporal and spatial analysis. Our data suggest that the temporal LSCI algorithm is more accurate at assessing the relative changes in flow speed than the spatial algorithm.

Integration of image exposure time into a modified laser speckle imaging method.

Speckle-based methods have been developed to characterize tissue blood flow and perfusion. One such method, called modified laser speckle imaging (mLSI), enables computation of blood flow maps with relatively high spatial resolution. Although it is known that the sensitivity and noise in LSI measurements depend on image exposure time, a fundamental disadvantage of mLSI is that it does not take into account this parameter. In this work, we integrate the exposure time into the mLSI method and provide experimental support of our approach with measurements from an in vitro flow phantom.