Manipulation and trapping of particles have taken a huge relevance in recent years thanks to many applications with revolutionary contributions to diverse fields. Several experiments have demonstrated that thermal effects can improve the current micromanipulation techniques such as DNA manipulation or assembly of colloidal crystals. In this work, we present the effect of laser-induced thermal effects, such as convection currents and thermophoresis, on the trap stiffness (spring constant) constant of an optical trap of 3-micrometer particles suspended in water. These effects are a consequence of light absorption in a thin layer of hydrogenated amorphous silicon (a-Si:H) deposited at the bottom of the chamber which generates a thermal gradient. Since these effects (and its correspondent forces) are symmetric around the beam focus, trapped particles, experience an increment in the trapping force. Around the beam focus, the drag force associated with convective currents is directed upwards and are compensated by optical scattering force. Depending on the laser power, the trap stiffness increases significantly, so a trapped particle can be dragged along the cell (by displacing the sample and leaving the beam fixed) at velocities around 90 μm/s without escaping the trap, whereas in the absence of the a-Si:H film, the escape velocity of the particle in the trap drops to velocities around 30 μm/s. This presents a simple, yet effective, option for optical manipulation at low powers (<5 mW) and its possible applications in the manipulation of a variety of biological micro samples.
We show that colloidal crystals can be assembled by means of temperature gradients produced by light absorption (λ=1070 nm) in a 21 nm titanium thin film deposited on one of the cell´s walls. Depending on the position of a 100x microscope objective focus within the 20 μm thick cell, three different regimes of crystal formation can be identified: 1) convective currents regime; 2) convective-thermophoresis regime, and 3) thermophoresis regime. We show that defects on the crystal can be modified dynamically by switching on and off the laser beam. In addition, the crystal can be 2D manipulated along the substrate. This technique could lead to the formation of large area colloidal crystals for photonics applications.
Here we report the creation and manipulation of colloidal crystals by inducing temperature gradients in a colloidal suspension of silica microparticles. A colloidal crystal is an ordered array of colloid particles analogous to their atomic or molecular counterparts with proper scaling considerations. The generation and properties of colloidal crystals have been of great interest for diverse science applications such as photonic crystals, chemical sensors among others. We report a technique that utilizes particles of silica of different diameters to form colloidal crystals by temperature gradients produced by light absorption at a metallic thin film deposited on one of the substrates. Moreover, we study the behavior of the particles by having different number of hot zones.
Dispersion is a quite important parameter in optical fiber systems, mainly in pulsed lasers where the temporal width of the pulses are affected by this factor. There are many interferometric arrangements to evaluate this parameter and generally, these systems vary the wavelength to obtain information about the refractive index dependency n(λ), which is contained in the interferogram phase. However, this makes the system quite slow and it requires tunable light sources with a narrow bandwidth. In this work we propose an alternative method to measure dispersion and we present the results of measurements of special optical fibers, such as microstructured, Er doped, Yb doped, and Er/Yb doped fibers, among others. The experimental arrangement consists of a Mach-Zehnder interferometer, where the fiber under test is located in one of its arms while the air is employed as the reference arm. In order to determine the n(λ) dependency, a wide spectrum light source in the wavelength range of interest was used together with an optical spectrum analyzer. The phase information was evaluated from the measured interferogram. The obtained dispersion values were in agreement with those reported by the fabricant and they were used to calibrate the system in order to obtain non reported values.
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