Optical trapping has proven to be an efficient method to control particles, including biological cells, single biological macromolecules, colloidal microparticles, and nanoparticles. Multiple types of particles have been successfully trapped, leading to various applications of optical tweezers ranging from biomedical through physics to material sciences. However, precise manipulation of particles with complex composition or of sizes down to nanometer-scales can be difficult with conventional optical tweezers, and an alternative manipulation tool is desired. Incorporating optical nanofibers into an optical tweezers setup allows us to overcome some of these limitations. Optical nanofibers have the added advantages of being easily connected to a fibered experimental setup, being simple to fabricate, and providing strong electric field confinement and intense magnitude of evanescent fields at the nanofiber's surface. Many different particles have been trapped, rotated, transported, and assembled with such a system. Here, we discuss some recent observations of microparticle manipulation, such as Janus particle manipulation, negative torque, and transverse spin effects.
Janus particles (JPs) are composite particles of two or more parts with distinct chemical and physical characteristics. As the current methods of manipulation rely on chemical reactions or thermal gradients, they are restricted by the carrier fluid’s content and characteristics. In addition, the high absorbance of the mettalo-dielectric JPs causes strong repulsion from the optical traps. This poses practical challenges in manipulating them effectively. To tackle such limitations, we propose manipulating JPs by the optical forces in the evanescent field of a nanofiber. The field locks the JP to the nanofiber, restricting motion in the radial direction while allowing propulsion along the propagation direction of the laser beam. This work theoretically examines a JP (here, silica microspheres concentric half-dome caps of titanium and gold) in the evanescent field of a nanofiber. The various effects of the force and torques are discussed.
Spin angular momentum (SAM) of light only has a longitudinal component for paraxial waves, but the transverse component of the SAM can be significant in a strongly confined light field. Here, we experimentally demonstrate the spinning of an anisotropic particle – bipolar liquid crystal (LC) - by trapping it using a circularly polarized optical tweezers and observe the dependence of the particle’s spinning rate on different laser powers and different states of input polarization. Furthermore, we detect the transverse spin of light by measuring the rotation frequency of a trapped liquid crystal particle, placed in the vicinity of the evanescent field near an optical nanofiber surface using optical tweezers. This result demonstrates that the transverse spin of light can twist anisotropic particles.
We develop single fiber based optical tweezers employing photophoretic forces to trap absorbing particles exploiting the rotational motion displayed by the particles to be stably trapped radially. This implies that optical beams with a large off-axis intensity would prove to be more efficacious in trapping. Thus, we generate a pure Gaussian as well as a superposition of Gaussian and Hermite-Gaussian beam modes from a single optical fiber which is dual mode at our operating wavelength, and show that the latter is more efficacious in trapping by about 1.8 times than the former. Finally, we show that multimode fiber traps - which have even larger off-axis intensity distribution - are the most effective in trapping and manipulating absorbing particles.
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