In this study, the controlled formation, trapping, and self-oscillation of vapor microbubbles in ethanol was investigated using low-power continuous wave (CW) lasers. The formation of these microbubbles is achieved by evaporation of ethanol due to heating by light absorption (CW laser emitting at λ = 658 nm) in silver nanoparticles deposited at the distal end of a multimode optical fiber. A second low-power NIR laser (λ = 1,550 nm) coupled to a single-mode optical fiber is then used to trap the microbubbles. It has been shown that the bulk absorption of light at 1,550 nm in ethanol modulates the surface tension of the bubble wall, creating a three-dimensional potential well that efficiently traps the bubbles. Furthermore, it was observed that once the bubble is trapped, random variations in its radius create instabilities in the trap, resulting in microbubble oscillations. The trapped bubble tends to oscillate between two quasi-stationary equilibrium points along the propagation of light. These oscillations are the result of competition between several forces, such as the Marangoni, the upward of buoyancy, and the drag forces. The results presented in this work contribute significantly to the understanding of these phenomena and may have important applications in fields such as microfluidics and bubble manipulation.
We show the 3D quasi-steady-state trapping of ethanol vapor microbubbles and their real-time manipulation in liquids driven by the Marangoni force that is triggered by using a low-power continuous-wave laser. The light absorption phenomenon, caused by the silver nanoparticles photodeposited on the core of optical fibers, is employed for both the generation of vapor microbubbles and triggering the Marangoni effect. The thermal effects, activated by either the silver nanoparticles on the optical fiber´s core or the light bulk absorption, modulate the surface tension of the bubble wall, generating longitudinal and transversal forces, similar to the optical ones. The balance of the optothermal forces drives the quasi-steady-stated 3D manipulation of microbubbles. By numerical simulations, we acquire expressions for the temperature profiles and present analytical expressions for the Marangoni force. Moreover, 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.
A simple and inexpensive method of measuring heart beats was established using Fabry-Perot interferometry (FPI). The sensor consists of a bracelet, the Fabry-Perot cavity is formed with a thin aluminum foil and the tip of a singlemode fiber optic. The fiber and aluminum foil are inserted into bracelet by means of easily assembled mechanical components. Fiber-coupled laser diode (1550 nm wavelength) was used to provide the optical signal through a 50:50 fiber coupler. Furthermore, we used a photodetector to transform this signal into an electrical one and an oscilloscope to analyze this signal. The sensor measures heart rate in the time domain, detecting interference fringes through the sensor and displayed on an oscilloscope. The interference phenomenon takes place in the Fabry-Perot cavity (CFP) which is formed by the thin aluminum plate and the tip of the optical fiber. By analyzing the signal from the oscilloscope, it is possible to measure the heart rate. The analysis presented in this work shows several advantages over traditional electrocardiograms, real-time measurement and simplification in the experimental setup, coupled with its high sensitivity for the analysis of cardiograms.
In this work, an experimental analysis of the Schlieren technique is presented to get the temperature distribution that is generated around the optical fiber. Laser light (λ = 450 nm) travels through this fiber (multimode optical fiber, 105/125 μm) and on its tip contains silver nanoparticles. The thermal gradient arises from the absorption of laser radiation by the silver nanoparticles adhered on the fiber tip. Schlieren Z-type setup was used, which has of a light source, two parabolic mirrors, a knife, and the camera. The analysis of the temperature distribution was carried out by obtaining Schlieren images in air through the digital camera. As a temperature calibration factor, the boiling temperature of ethanol (78 °C) was used, that is, previously, the tip of the optical fiber was immersed in ethanol and the laser power was set to the value before the appearance of the boiling bubbles. Subsequently, the optical fiber was placed in the Schlieren arrangement, and taking the above we find that the tip of the optical fiber reaches ~ 80 °C when it is in the air, this value is approximate to the boiling temperature of ethanol, the possible cause of the temperature variation is given by the transfer of heat in and out of the ethanol.
A simple and inexpensive method to measure vibrations in mechanical structures is presented by means of Fabry-Perot interferometry and Doppler effect. The sensor consists of a transparent hydrogel sphere attached to an single-mode optical fiber from a 50:50 fiber coupler, laser light with a wavelength of 658 nm, and a photodetector which is connected to an oscilloscope. The vibrometric sensor works in the time domain by detecting the number of interference fringes. The tip of the prototype is composed of mechanical couplings that join the fiber with the hydrogel sphere. The sensor allows knowing the vibration frequency at which a mechanical part is oscillating. The analysis presented in this work shows several advantages over conventional methods, such as low cost, real-time measurement, and simplification in experimental setup. The sensor system is capable of capturing vibrations of up to 5000 Hz.
We present the massive migration of metallic nanoparticles (NPs) due to their interaction with thermal gradients. A single-mode optical fiber is introduced into an ethanol solution with silver NPs of size between 0.5 1 µm- radius. When the laser radiation of λ = 455 nm leaving the optical fiber illuminates the suspended silver NPs, they absorb part of the energy and remit it as heat to the surrounding environment. This causes the formation of a thermal gradient in each particle and the medium. When the NPs are near the temperature gradient generated in the medium, they move to the high or low-temperature zone, the direction depending on the distance between the NPs and the tip of the optical fiber. The phenomenon that describes this behavior is called ∆α photophoresis. The particles present a considerable absorption on the illuminated side, generating a temperature difference relative to the non-illuminated side. The heat is conducted through the medium that surrounds the particles, giving rise to a momentum exchange between the molecules of the medium and the NPs. This exchange causes either an attraction or a repulsion of the NPs relative to the end optical fiber. The displacement of the NPs stems from the influence of the ∆T force, whose direction is equal to the propagation of the beam, and the force ∆α, with the opposite direction on each NP. We also study the appropriate optical and thermal conditions that enhance the migration of the silver NPs.
We present 3D steady-state trapping and manipulation of vapor microbubbles in pure ethanol by optothermal effects induced by low power CW laser. Light absorption from silver nanoparticles photodeposited on the core of a multi-mode optical fiber is used as a heat source to induce microbubbles of several diameters. The optothermal effects generated by the light absorption of light bulk absorption modulates the microbubble wall's surface tension inducing both longitudinal and transversal forces just like optical forces, generating a 3D potential well. By numerical simulations, we find expressions to describe both the temperature profiles and the convective currents. Besides, using an array of three optical fibers with silver nanoparticles photodeposited on their cores we show the transmission of microbubbles from one fiber to another by switching the laser radiation from the CW lasers.
We present the development of a single shot interferometric system based on polarization and diffractive elements recovered from spare computers. The use of salvaged lenses, beam splitter cubes and linear polarizers lower the cost of our system in comparison with those already reported. Our system is also capable of dynamic phase measurements with an accuracy comparable to similar implementations with high end manufactured optics. We report here our results obtained from four simultaneous interference patterns. The presented interferometer can be easily implemented for various applications in single shot polarizing phase shifting interferometry.
The Z-scan measurement technique is a scanning method used to know the non-linear optical behavior due third order process in different materials, some of these behaviors are: as saturable absorption, reversible and non-reversible absorbers, absorption of more than one photon, among others. This technique is based on the displacement of the sample in the direction of propagation (Z) of a laser light beam. In this work the study of the nonlinear properties of three samples was carried out through this technique using a continuous wave laser, with Gaussian profile, and emitting at λ = 1550 nm, the samples used were: ethanol, acetone, and distilled water. Nonlinear absorption and nonlinear refraction were observed for the three samples.
In this work is presented a study of the photophoresis phenomenon induced in a group of silver microparticles suspended in distilled water. To generate this phenomenon, the light of a laser source with a wavelength of 445 nm is guided through a multimode optical fiber which is inserted inside a thin cell, where the microparticles in water are contained. Due to the high absorption by silver microparticles, an uneven uniform heat distribution is generated inside the particles and, consequently, the solvent is heated too. In such manner, the particles will experience a movement directed towards the light source. As time passes, a greater number of silver micro particles are agglomerated at the tip of the optical fiber. To observe the phenomenon in situ, a 20x microscope objective was used together with a CMOS microscope camera.
The present work shows the oscillation of a microbubble using temperature gradients. This gradient is caused by the absorption of laser light by silver nanoparticles (AgNPs) immobilized on the tip of a single-mode optical fiber FO (9/125 μm). The immobilization of these nanoparticles was performed using the technique known as photodeposition. Subsequently, the tip with the nanoparticles was immersed in ethanol. We used a infrared (λ=1550 nm) laser with fiber optic output which was controlled (modulate) with a waveform generator. When the laser pulse is at its high level, a radial temperature gradient is generated and the liquid near the tip of the optical fiber evaporates creating a microbubble. This microbubble remains attached to the face of the optical fiber due to the Marangoni force (FM) that brings it to the point of highest temperature. When the laser pulse changes to its low level, the temperature gradient disappears and the Marangoni force becomes zero. This causes the buoyancy force (FB) to become predominant driving the microbubble to the surface. However, for a new laser pulse the cycle repeats itself, keeping the microbubble oscillating within a region. As the laser modulation frequency increases the oscillation distance of the microbubble decreases.
We present both the 3D trapping and manipulation of microbubbles by temperature gradients, induced by low power CW laser in absorbing liquid (ethanol). Two optical fibers were used: a multimode one for bubble generation and a single-mode one for both trapping and manipulating. One distal end of the multimode fiber was coupled to a Qswitched pulsed laser (λ=532 nm and pulse width τp=5 ns). The light propagates in the fiber and gets absorbed at silver nanoparticles, previously photodeposed at the other distal end, heating up the surrounding liquid and generating the microbubbles. On the other hand, a CW laser (λ = 1550 nm) was coupled to one distal end of the single-mode fiber, the other distal end was immersed in ethanol, inducing thermocapillary force, also called Marangoni force, that is the cornerstone in the trapping and manipulating of bubbles. The bubble generated on the multimode fiber travels towards the single-mode fiber by a careful switching of the temperature gradients. In addition to the Marangoni force, the microbubble immersed in ethanol suffers both drag force and buoyancy force. So, the equilibrium among these forces drives the 3D trapping and manipulation of the microbubble. To our best knowledge, this is the first time that 3D trapping and manipulation using low CW power es presented.
We present the generation and 3D manipulation of microbubbles by thermal gradients, induced by low power nanosecond pulsed laser in non-absorbent liquids. Light absorption at photodeposited silver nanoparticles on the optical fiber tip heat up the surrounding liquid, which leads to optothermal effects. With each laser pulse a microbubble is detached from the optical fiber end, creating a microbubbles-stream. The microbubbles move away from the optical fiber end driven by non-spherical cavitation until they coalesce creating a main-bubble which is attracted towards the optical fiber end by Marangoni force. In addition, the main-bubbles are under the influence of buoyancy and gravity forces, which act upwards and downwards, respectively. The balance of these forces allows the 3D manipulation of the main-bubble. The main-bubble position can be controlled by careful control of the pulse energy. To our knowledge this is the first time that 3D manipulation of microbubbles using pulsed lasers is demonstrated.
Generation and 3D manipulation of microbubbles by means of temperature gradients induced by low power laser radiation is presented. Photodeposited silver nanoparticles on the distal end of two optical fibers act as thermal sources after light absorption. The temperature rises above liquid evaporation temperature generating a microbubble at the optical fibers end in non-absorbent liquids. Alternatively, switching the thermal gradients between the fibers, 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
In this research an interferometric system was developed that generates four simultaneous interferograms with independent phase shifts using modulated polarization. The proposed system consists of three coupled interferometers: the first system is a polarized Mach-Zehnder interferometer, which generates the pattern, the second and the third interferometer system, function as replicators of the first pattern, so the four patterns are generated. To show the novelty of the developed system, the calculation of optical path difference (OPD) for phase samples are shown.
The modulus of the gradient of the color planes (MGC) is implemented to transform multichannel information to a grayscale image. This digital technique is used in two applications: (a) focus measurements during autofocusing (AF) process and (b) extending the depth of field (EDoF) by means of multifocus image fusion. In the first case, the MGC procedure is based on an edge detection technique and is implemented in over 15 focus metrics that are typically handled in digital microscopy. The MGC approach is tested on color images of histological sections for the selection of in-focus images. An appealing attribute of all the AF metrics working in the MGC space is their monotonic behavior even up to a magnification of 100×. An advantage of the MGC method is its computational simplicity and inherent parallelism. In the second application, a multifocus image fusion algorithm based on the MGC approach has been implemented on graphics processing units (GPUs). The resulting fused images are evaluated using a nonreference image quality metric. The proposed fusion method reveals a high-quality image independently of faulty illumination during the image acquisition. Finally, the three-dimensional visualization of the in-focus image is shown.
An optical method to obtain a colloidal solution starting from a mixture of silver nanopowder and ethanol is presented. The particles of the silver nanopowder do not exhibit a specific shape, however in the colloidal solution are spherical. This method is carry out when the mixture is irradiated with a pulsed laser at 532 nm via optical fiber. Due to a stronger absorption of the laser light by silver nanoparticles arise both photofragmentation and photomelting processes. The photomelting process starts when the laser energy is 5 mJ/cm2, inducing an enlargement of nanoparticles whereas the photofragmentation occurs when the laser energy is 25 mJ/cm2 causing a reduction on their sizes (the higher energy is, the smaller nanoparticles are). Results show that it is possible to obtain a colloidal silver solution and to control the particle size by adjusting the laser energy. Experiments were performed at 5 and 25 mJ/cm2, and the results are presented.
In microscopy, the depth of field (DOF) is limited by the physical characteristics of imaging systems. Imaging a scene with the all the field of view in focus can be an impossible task to achieve. In this paper, metal samples are inspected on multiple focal planes by moving the microscope stage along the z − axis and for each z plane, an image is digitalized. Through digital image processing, an image with all the focused regions is generated from a set of multi focus images. The proposed fusion algorithm gives a single sharp image. The merger scheme is simple, fast and virtually free of artifacts or false color. Experimental fusion results are shown.
The radiation pressure forces for Au nanoparticles in the Rayleigh regime under the influence of a coherent source of infrared light from 0.7-1.5 μm of a Gaussian beam with fundamental mode corresponding to the TEM00 mode are studied. An intensity distribution of the source in terms of the spot size and power are considered to analyze the gradient, scattering and absorption forces on a sphere located arbitrarily on a Gaussian beam. The results have shown, through an analysis stability, the optical manipulation is better suited for longer wavelengths, small particles, and a beam waist significantly reduced.
By using the Hot Filament Chemical Vapor Deposition (HFCVD) technique tungsten thin films were deposited on
amorphous quartz substrates. To achieve this, a tungsten filament was heated at 1300 °C during 30 minutes maintaining
a constant pressure inside the chamber at 460 mTorr and substrate at 700 °C. Transition from tungsten oxide deposits to
tungsten thin films, by varying the substrate temperature, were characterized by means of Scanning Electron Microscope
(SEM), Atomic Force Microscope (AFM), X-Ray Diffraction and, micro-Raman spectroscopy. The SEM micrographs
reveal that the tungsten films have no more than 200 nm in thickness while XRD show evidence of the films crystallize
in the á-tungsten modification. On the other hand, AFM shows that the tungsten thin films exhibit a uniform and smooth
surface composed with semi-spherical shapes whose diameters are below than 50 nm. Furthermore, to the naked eye, the
as-deposited tungsten films exhibit a high mirror-like appearance.
A study of optically induced Zn/ZnO nanoparticles selective deposition using a coherent light source on single-mode
fiber optic end is presented. In the numerical studies, Zn/ZnO spherical nanoparticles are considered dissolved in
isopropyl alcohol with different diameters under the influence of a Gaussian beam with fundamental mode and linear
polarization. The results of this study show that the gradient force is not sufficient to move Zn nanoparticles toward
optical fiber end face, but it is sufficient to move ZnO nanoparticles of a certain diameter. In the experimental studies,
Zn/ZnO nanoparticles were mixed with isopropyl alcohol and subsequently deposited on the fiber end face using an
infrared laser. The results obtained by atomic force and optical microscopy show a good uniform distribution of
nanostructures deposited on the core of the fiber end face.
Experimental studies of titania nanotubes (TiO2) and tungsten oxide nanospheres (WO3) as devices of saturable
absorption for a fiber laser in ring configuration to optical short-pulse generation are presented. A deposition technique,
based on optical pressure radiation generated from a coherent source at 1550 nm is used to deposit the nanostructured
materials. Since this nanomaterials can be deposited directly on the optical fiber, this proposal results very interesting for
applications of saturable absorbers. Experimental results, by using nanotubes TiO2 and nanospheres deposited on the
fiber as a saturable absorption device, show that the TiO2 nanotubes exhibit better saturable absorption properties than
WO3 nanospheres, generating pulses with a wavelength of 1550 nm, frequency of 10 MHz, temporally width of 4.5 ps
and an output power of 1 mW.
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