Nanodiamond optical sorting at the femtonewton scale inside a tapered glass capillary

Abstract

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.