While optical forces could be used to select and sieve nanoparticles based on their optical properties, their mechanical
action is usually too weak to overcome the fast Brownian diffusion of nanoparticles dispersed in a liquid phase. Among
various theoretical proposals and experimental realizations of optical sorting techniques, glass capillaries with micrometer
or submicrometer diameters offer a promising approach to achieve efficient optical sorting of nanoparticles. By confining
dispersed nanoparticles into the light path over few-millimeter-long distances, it provides enough time/distance for weak
optomechanical interactions to affect the motion and the concentration of nanoparticles at the macroscopic scale. In this
work, we report on our recent experimental results.
Tapered glass capillaries can be fabricated having cross-sections only few square-micrometers large. Efficient light focusing and guiding in thin, millimeter-long capillaries filled with a liquid solution provides competitive advantages for the optical sorting of nanoparticles. In this work, optical transport of fluorescent nanodiamonds is demonstrated both in water and DMSO. Size-dependent sorting of a large amount of nanodiamonds is also achieved using a backward liquid flow. An analytical model is used to calculate the femtonewton-scale forces acting on the particles and evaluate the sorting efficiency of tapered capillaries.
While optical forces may provide very efficient techniques to manipulate and sort nanomaterials according to their optical properties, experimental realizations remain challenging. In most cases, the key issue lies in the imbalance between the fast Brownian diffusion of nanoscale objects and the weak, localized effect of optical forces. We propose here a new approach based on tapered glass capillaries with only few-μm2-large cross-section areas. The transparent pipe-like structure of tapered glass capillaries allows for simultaneously confining and guiding both the light and the liquid solution in a narrow, few-mm-long optofluidic channel. In this work, a tapered glass capillary is filled with a liquid dispersion of fluorescent nanodiamonds, cleaved, and sealed. Light from a green laser source is then coupled to one end of the capillary. Optical transport of nanodiamonds is observed by fluorescence microscopy. Velocities of nanodiamonds reaching few tens of micrometers per second are measured at the waist of the tapered capillary. In the presence of a liquid flow inside the optofluidic channel, size-dependent sorting of a large ensemble of nanodiamonds is demonstrated. Based on an analytical model, we evaluate the influence of the nanodiamonds’ size on both the optical and the hydrodynamic drag forces acting on the nanoparticles. Our results show that tapered glass capillaries provide a suitable optofluidic platform to achieve efficient optical sorting of nanoparticles by exploiting optical forces as weak as few femtonewtons.
Optical forces may provide an elegant solution to achieve optical sorting of nanoparticles according to their optical properties. Yet, efficient nanoparticle optical sorting would require all the nanoparticles to be gathered and kept inside the light path for a sufficient time. In order to overcome nanoparticle diffusion, we investigate the use of tapered glass capillaries as optofluidic waveguides. We demonstrate size-dependent optical transport of fluorescent nanodiamonds inside a tapered capillary. Particle velocities reaching few tens of micrometers per second were achieved, and size estimations are performed based on the nanoparticles’ velocities. Nanoparticle sorting is also demonstrated by balancing the optical transport of the nanodiamonds with a liquid flow in the opposite direction.
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