Dimensional characterization of microfluidic circuits were performed using three-dimensional models constructed from OCT images of such circuits. Were fabricated microchannels on the same BK7 glass plate, under different laser ablation conditions and substrate displacement velocity in relation to laser beam. Were used the following combination of energy, from 30 μJ to 60 μJ and velocity from 588 mm/min to 1176 mm/min, at 1 kHz laser repetition rate and 40 fs of pulse duration (FWHM). For OCT imaging we used an OCP930SR (Thorlabs System Inc) with 930 nm central wavelength, 6 μm of lateral and axial resolution, and image of 500 x 512 pixel corresponding to 2.0 mm x 1.6 mm of lateral and axial scans respectively at 8 frames per second. We also characterized devices like, micropumps, microvalves and microreactors. It was possible register the micropumps and valves in action in real time. Using the OCT images analyses was possible to select the best combination of laser pulse energy and substrate velocity. All the devices were made in raster protocol, where laser beam pass through the same path in a controlled number of times, and with each iteration more material is removed and deeper the channels remain. We found a deformation at the edge of fabricated structures, due to velocity reduction of substrate in relation to laser beam, which causes more laser pulses superposition in these regions, and more material is ablated. The technique was thus evaluated as a potential tool to aid in the inspection of microchannels.
The authors report the fabrication and characterization of passive waveguides in GeO2–PbO and TeO2–ZnO glasses written with a femtosecond laser delivering pulses with 3μJ, 30μJ and 80fs at 4kHz repetition rate. Permanent refractive index change at the focus of the laser beam was obtained and waveguides were formed by two closely spaced laser written lines, where the light guiding occurs between them. The refractive index change at 632 nm is around 10-4 . The value of the propagation losses was around 2.0 dB/cm. The output mode profiles indicate multimodal guiding behavior. Raman measurements show structural modification of the glassy network. The results show that these materials are potential candidates for passive waveguides applications as low-loss optical components.
The backscattered light originated when machining with femtosecond laser pulses can be used to accurately measure the processed surface position through an interferometer, as recently demonstrated by our group, in a setup that uses the same laser beam for ablation and inspection. The present work explores the characteristics of the laser light reflected by the target and its interaction with the resulting plasma to better understand its propagation physics and to improve the dynamic focusing system. The origin of this returning radiation was studied and has been traced, mainly, from the peripheral area of the focal spot (doughnut-like). By means of a Mach-Zehnder setup, the interferometric pattern was measured and analyzed aiming to access the influences of the plasma on the laser beam properties, and therefore on the retrieved information. Finally, the wavefront of the laser that creates and propagates through the plasma was characterized using a Shack-Hartmann sensor.
Boron thin films were produced using femtosecond pulsed laser deposition aiming at the development of a neutron detector. As neutrons have no charge, in order to detect this particle converter materials are applied, promoting nuclear reactions that result in the emission of charged particles, allowing the neutrons presence to be indirectly inferred. Among the possible conversion materials, 10B has a considerable cross section for thermal neutrons and accessible cost. Furthermore, the nuclear reaction produces easily detectable alpha particles, making it a rational option to develop a low cost and portable neutron detector. The boron ablation threshold fluence has been measured by the Diagonal Scan (Dscan) technique that resulted in the minimum laser energy value of 17.7 (6) μJ and fluence of 5.63(19) J/cm2. Boron deposition was performed varying the pulse energy and deposition duration. The growth rate, morphological and physical aspects of the boron pulsed laser deposition were characterized by a Scanning Electron Microscope and an optical profilometer. The films surfaces have a flaky aspect with eventual droplets which had decayed overtime to a more smooth surface. The studied parameters allowed producing a boron coating with the optimal thickness in order to minimize self-absorption effect in the film, thus increasing efficiency.
Taking advantage of the inherent characteristics of femtosecond laser used for machining, we developed an interferometric system able to evaluate and correct the focal position with an accuracy of a few microns, implementing a technique based on low coherence interferometry. This approach measures at the exact spot that the laser is machining, in real time, and is sensitive to any sample that acts as a scatterer to the wavelength in use. The experimental evaluation was divided in two steps: in the first a system based on a superluminescent LED was mounted to check the viability and develop the controlling software; in the second part a setup was mounted employing a femtosecond laser, and several kinds of samples using the active focus control, among which the results obtained with glass sample and a bovine tooth are meticulously described in this paper. The system was able to improve the performance in both samples, keeping them in the confocal region for an extended positioning range, resulting in better engraving by the laser.
Metallic nanoparticles are important on several scientific, medical and industrial areas. The control of nanoparticles
characteristics has fundamental importance to increase the efficiency on the processes and applications in which they are
employed. The metallic nanoparticles present specific surface plasmon resonances (SPR). These resonances are related
with the collective oscillations of the electrons presents on the metallic nanoparticle. The SPR is determined by the
potential defined by the nanoparticle size and geometry.
There are several methods of producing gold nanoparticles, including the use of toxic chemical polymers. We already
reported the use of natural polymers, as for example, the agar-agar, to produce metallic nanoparticles under xenon lamp
irradiation. This technique is characterized as a “green” synthesis because the natural polymers are inoffensive to the
environment.
We report a technique to produce metallic nanoparticles and change its geometrical and dimensional characteristics using
a femtosecond laser. The 1 ml initial solution was irradiate using a laser beam with 380 mW, 1 kHz and 40 nm of bandwidth
centered at 800 nm.
The setup uses an Acousto-optic modulator, Dazzler, to change the pulses spectral profiles by introduction of several orders
of phase, resulting in different temporal energy distributions. The use of Dazzler has the objective of change the gold
nanoparticles average size by the changing of temporal energy distributions of the laser pulses incident in the sample. After
the laser irradiation, the gold nanoparticles average diameter were less than 15 nm.
The use of gold nanoparticles (AuNps) as the vehicle for 5-Aminolevulinic acid (ALA) delivery for photodynamic and photothermic plasmonic therapies is a promising approach, especially with the recent demonstration that this photosensitizer immobilization on the particle surface improves reactive oxygen species (ROS) formation, increasing its cytotoxicity. Gold nanorods (AuNRs) present an absorption spectrum shifted to 700 nm, within the tissue transparency window, which allows excitation of the nanoparticles situated deeper in the tissues. Here, we describe a new synthesis method that was applied to control the shape of the gold nanoparticles during its synthesis. To obtain ALA:AuNRs, precursor ALA:AuNps were irradiated by ultrashort laser pulses. The variation of the laser parameters such as pulse energy and duration and irradiation time was assessed. The relevant mechanisms are discussed.
We report the use of the Diagonal Scan (D-Scan) technique to determine the ablation threshold of the AISI 1045 steel, a common engineering material that can be used as a probe for thermal effects, for superpositions ranging from single shot up to more than 10,000 pulses, for three pulses durations (25, 87 and 124 fs). It only took two hours of laboratory time to determine more than 20 ablation thresholds per pulse duration spanning 4 orders of magnitude of superpositions. The large amount of data generated shows a small deviation of the ablation threshold from the expected behavior, which can lead to the use of a model that better describes the dynamics of the ultrashort pulses ablation mechanism in metals.
This work presents a compositional characterization of 1939's Thousand "Réis" and 1945's One "Cruzeiro" Brazilian
coins, forged on aluminum bronze alloy.
The coins were irradiated by a Q-switched Nd:YAG laser with 4 ns pulse width and energy of 25mJ emitting at 1064nm
reaching 3.1010Wcm-2 (assured condition for stoichiometric ablation), forming a plasma in a small fraction of the coin. Plasma emission was collected by an optical fiber system connected to an Echelle spectrometer. The capability of LIBS
to remove small fraction of material was exploited and the coins were analyzed ablating layer by layer from patina to the
bulk.
The experimental conditions to assure reproductivity were determined by evaluation of three plasma paramethers:
ionization temperature using Saha-Boltzmann plot, excitation temperature using Boltzmann plot, plasma density using
Saha-Boltzmann plot and Stark broadening.
The Calibration-Free LIBS technique was applied to both coins and the analytical determination of elemental
composition was employed. In order to confirm the Edict Law elemental composition the results were corroborated by
Neutron Activation Analysis (NAA). In both cases the results determined by CF-LIBS agreed to with the Edict Law and
NAA determination. Besides the major components for the bronze alloy some other impurities were observed.
Finally, in order to determine the coin damage made by the laser, the OCT (Optical Coherence Tomography) technique
was used. After tree pulses of laser 54μg of coin material were removed reaching 120μm in depth.
Measure roughness in some sort of samples can present several problems when it is done in traditional way (with
physical contact). For instance, soft samples will present at least two kinds of problem: (a) the value presented by the
equipment not represents the sample roughness; (b) the equipment can perform serious damages to the sample. Using a
commercial type OCT (Thorlabs Inc.) with 6μm axial resolution (in air) and 6μm lateral resolution, measurements of
roughness standards with Ra nominal values of 0.8, 1.6, 3.2, 6.3, 12.5, 25.0 and 50.0 μm. A homemade software analysis
the OCT images, and automatically calculates the Ra and Rz values. This procedure was performed to validate this
methodology comparing the OCT and roughness standards values.
We report here the development and construction of a two-flashlamp pumping cavity for a Cr:LiSAF rod, to be operated
as a multipass amplifier in a Chirped Pulse Amplifier system. The pumping cavity was designed to minimize the
thermal load on the gain medium by the utilization of intracavity filters, aiming operation with high gain and the highest
possible repetition rate. Operating as a laser, 30 Hz repetition rate and 20 W average power were obtained for the first
time at a maximum gain per pass of 1.5. Changing the pumping characteristics, the laser provided 16 W at 8 Hz
repetition rate, at a maximum gain of 3.6. A four-pass multipass amplifier geometry was designed for the pumping
cavity, that was integrated and synchronized to a Ti:Sapphire Chirped Pulse Amplifier system. The amplification
properties of the gain medium were determined, in one, two and four passes, along with the gain dependence on the
repetition rate. The amplifier final configuration provided amplification by a factor 150 to 20 ps stretched pulses,
resulting in final compressed pulses with 60 fs and 0.5 TW of peak power at 5 Hz repetition rate.
The usual method to determine the ablation threshold of solid samples by ultrashort laser pulses is done by focusing the
laser beam on the samples surface by a known lens, requires the knowledge of all the geometrical parameters (lens focus,
beam propagation parameters, beam quality, sample position), and a series of measurements for different pulse energies.
We present here a simpler method for determining ultrashort laser pulses ablation threshold for solid samples. The
method uses a focusing lens, and requires only the knowledge of the pulse power, employing a diagonal translation of
the sample through the laser beam waist, resulting in a pattern etched on the sample surface. The ablation threshold value
is obtained measuring only one dimension of this pattern and a straightforward mathematical relation, There is no need
to know any other geometrical parameter of the laser beam or of the lens used. The technique was employed to
determine the ablation threshold of pure and Cr doped LiSAF samples for 20 picoseconds pulses, and a dependence with
the Cr concentration was observed.
Development of cholesterol biosensors is of great importance in clinical analysis because the concentration of
cholesterol in blood is a fundamental parameter for the prevention and diagnosis of a number of clinical disorders such
as heart disease, hypertension and arteriosclerosis. In general, determination of cholesterol is based on
spectrophotometry; but this method involves complicated procedures and the cost is high because expensive enzyme
must be used in each assay.
We report here the observation, for the first time, of the enhancement of Europium-Tetracycline complex
emission in cholesterol solutions. This enhancement was initially observed with the addition of the enzyme cholesterol
oxidase, which produces H2O2, the agent driver of the Europium tetracycline complex, to the solution. However, it was
found that the enzyme is not needed to enhance the luminescence. A calibration curve was determined, resulting in an
easy-handling immobilization method with a cheap stable material. This method shows that the complex can be used as a
sensor to determine cholesterol in biological systems with good selectivity, fast response, miniature size, and
reproducible results.
We report here the development, construction and characterization of a flashlamp pumped Cr:LiSAF rod pumping cavity designed to minimize the thermal load on the crystal. The cavity is a close coupled one with 2 Xe lamps and absorptive filters between the lamps and the Cr:LiSAF rod, and is refrigerated with cooled water. A compact and stable (g1×g2=0.57) resonator was designed for lasers tests and gain medium characterization, and we expected to obtain operation at 20 Hz repetition rate. Nevertheless, the thermal load minimizing design was so successful that allowed laser operation up to 30 Hz with an average power of 20 W. When operating with a 10% transmission output coupler this laser exhibited an overall laser efficiency of 0.6% under 100 J electrical pumping, and a slope efficiency of 0.8%. Under these conditions, a maximum gain per pass of 1.5 was obtained, suitable for regenerative amplifiers. To increase the gain, the intracavity filters were substituted by glass plates, resulting in a gain per pass of 3.6, adequate for multipass amplifiers. In this configuration, and operating as a laser resonator, it showed a maximum overall efficiency of 2.81% under 88 J electrical pumping with a 25% transmission output coupler, and maximum output power of 18 W at 8 Hz. A study of the thermal load on the crystal was conducted by observation of the upper laser level lifetime, and we concluded that there are no noticeable accumulated thermal effects on the Cr:LiSAF emission.
In this work we report the creation of color centers in LiF and YLF crystals by high intensity, ultrashort laser pulses. We used pure and Tm3+ and Oxygen doped samples, all irradiated with a Ti:Sapphire CPA laser system and also with electron beam, at room temperature. In both kinds of irradiations the production of photochromic damages and color centers that have absorption bands in UV and visible range was observed. A comparison between the two kinds of irradiation was done and the involved processes are described in this paper. F2+ stable centers were produced by the ultrashort laser pulses irradiation in contrast to the well-known, short lived centers produced by electron beams, and a mechanism was proposed to explain the observed stability.
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