Dielectric nanoparticles, and silicon nanoparticles in particular [1,2], are becoming increasingly promising for various applications in photonics, nonlinear optics, optomechanics, and medicine. Plenty of applications exploit the benefits of low-loss Mie resonances, exhibited in the optical range by silicon nanoparticles with sizes of the order of 200 nm. The frequencies of the resonant Mie modes are determined by the size and shape of the particles. However, many of the fabrication techniques result in a polydisperse mixture of different sizes and shapes, and prompt for a post-processing to provide a uniform output. Having an entirely optical tool for such separation [3,4] is highly desirable for sterile, hazardous or highly dynamic microfluidic environments.
Following our recent publication , in this contribution we present the calculated optical forces acting on silicon nanoparticles in aqueous environment, analyse their potential for optical sorting in a number of schemes, and discuss the experimental implementation of the proposed methods in our setup.
The optical forces acting on silicon nanoparticles are shown to reveal their substantial dependence on the particle size. This dependence results in different velocities of the light-driven drift of the nanoparticles, depending on their size and the frequency of the incident light. We propose to employ these features to realise optical sorting, according to the following scenarios. First, we use two counter-propagating beams of different wavelength, which move particles of different sizes in opposite directions; by varying the intensity ratio between the two beams, different subsets of the particle sizes can be separated. A similar approach has been implemented for plasmonic particles . Second, we suggest to impose two counter-propagating beams upon a uniform flow of a disperse mixture, which results in the particles of different sizes being pushed along different directions in space, so that an efficient angular separation is possible within certain size ranges. Third, we propose an efficient angular separation in an all-optical way, by directing the two beams at an angle. This scenario offers an efficient angular separation without any imposed flow.
In this work, we consider two laser beams with wavelengths of 532 and 638 nm. For this particular case, angular sorting scheme provides a unique size-angle dependence, yielding up to 70° span of deflections, in the size ranges of 120–160 nm, 190–220 nm, and a few smaller sets. We demonstrate that the proposed angular sorting techniques are robust against the Brownian motion, requiring a run of about 100 μm to achieve a 10-nm distinction in size, while using moderate (0.1 W) power. Finally, we consider the forces acting on silicon nanoparticles in the evanescent wave illumination and show that the proposed methods can be applied for a broad size dimensions using p-polarised light.
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