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.
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.
Semiconductor nanocrystals (quantum dots, QDs) represent a milestone in the field of luminescent nanoparticles owing to their unique optical properties. Silica encapsulation of colloidal QDs in optimized synthetic conditions provides an excellent method to reduce their cytotoxicity maintaining, at the same time, their optical properties.1 The ability to optically confine and spatially control these biocompatible nanostructures in liquid media boosts their investigation for bioimaging both as an ensemble as well as at a single particle-level.
In this study we explore the optical trapping of silica-encapsulated QDs in a near infrared counter-propagating experimental configuration.2 Optically trapped QDs exhibit two photon-absorption mediated luminescence without additional excitation sources.3,4 We find that the luminescence, collected through one objective, evidences photo-bleaching and wavelength blue-shifts depending on the dispersive medium composition and power density in the laser focus.
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