Optical sorting

Abstract: An extended interference pattern close to the surface may result in either a transmissive or an evanescent surface field for large-area manipulation of trapped particles. The affinity of differing particle sizes to a moving standing-wave light pattern allows us to hold and deliver them in a bidirectional manner and demonstrate experimentally particle sorting in the submicrometer region. This is performed without the need of fluid flow (static sorting). Theoretical predictions support the experimental observations that certain sizes of colloidal particles thermally hop more easily between neighboring traps. A generic method is also presented for particle position detection in an extended periodic light pattern and applied to characterization of optical traps and particle behavior.

Abstract: Optical manipulations utilizing the mechanical effect of light have been indispensable in various disciplines. Among those various manipulations, optical pulling has emerged recently as an attractive notion and captivated the popular imagination, not only because it constitutes a rich family of counterintuitive phenomena compared with traditional manipulations but also due to the profound physics underneath and potential applications. Beginning with a general introduction to optical forces, related theories, and methods, we review the progresses achieved in optical pulling forces using different mechanisms and configurations. Similar pulling forces in other forms of waves, including acoustic, water, and quantum matter waves, are also integrated. More importantly, we also include the progresses in counterintuitive left-handed optical torque and lateral optical force as the extensions of the pulling force. As a new manipulation degree of freedom, optical pulling force and related effects have potential applications in remote mass transportation, optical rotating, and optical sorting. They may also stimulate the investigations of counterintuitive phenomena in other forms of waves.

Abstract: We present a generic technique allowing size-based all-optical sorting of gold nanoparticles. Optical forces acting on metallic nanoparticles are substantially enhanced when they are illuminated at a wavelength near the plasmon resonance, as determined by the particle’s geometry. Exploiting these resonances, we realize sorting in a system of two counter-propagating evanescent waves, each at different wavelengths that selectively guide nanoparticles of different sizes in opposite directions. We validate this concept by demonstrating bidirectional sorting of gold nanoparticles of either 150 or 130 nm in diameter from those of 100 nm in diameter within a mixture.

Abstract: Near-field optical micromanipulation permits new possibilities for controlled motion of trapped objects. In this work, we report an original geometry for optically deflecting and sorting micro-objects employing a total internal reflection microscope system. A small beam of laser light is delivered off-axis through a total internal reflection objective which creates an elongated evanescent illumination of light at a glass/water interface. Asymmetrical gradient and scattering forces from this light field are seen to deflect and sort polystyrene microparticles within a fluid flow. The speed of the deflected objects is dependent upon their intrinsic properties. We present a finite element method to calculate the optical forces for the evanescent waves. The numerical simulations are in good qualitative agreement with the experimental observations and elucidate features of the particle trajectory. In the size range of 1 µm to 5 µm in diameter, polystyrene spheres were found to be guided on average 2.9 ± 0.7 faster than silica spheres. The velocity increased by 3.00.5 µms−1 per µm increase in diameter for polystyrene spheres and 0.7 ± 0.2 µms−1 per µm for silica. We employ this size dependence for performing passive optical sorting within a microfluidic chip and is demonstrated in the accompanying video.