We present the multiple trapping of microspheres using structured diffractive lenses in a setup of optical tweezers. Diffractive lenses are experiencing a wide development due to their successful implementation in areas such as ophthalmology or biophysics. The implementation of multifocal diffractive lenses in an experimental arrangement of optical tweezers affords the generation of multiple traps in a stable and controlled manner. We carried out the multiple trapping of polystyrene microspheres of radius ∼ 2 − 2.4 μm immersed in distilled water. An infrared laser and a multifocal diffractive lens enabled a set of axial foci. The structured lens is generated in a spatial light modulator (SLM) calibrated for a 2π phase shift at λ = 1064 nm and the tailored beam is focused by a high numerical aperture objective. Each focal point generated by the lens acts as a separate optical trap, whereby the confinement of an individual microsphere is possible. The versatility provided by structured diffractive lenses allows increasing the number of optical traps by modifying the lens profile. In addition, such strategy increases the flexibility in the experimental arrangement of optical tweezers.
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 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.
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.
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