Optomechanical Wagon‐Wheel Effects for Bidirectional Sorting of Dielectric Nanoparticles

Abstract

The visual perception of a moving target is not always true. Wheels turning rapidly, for instance, may look like rotating inversely. This phenomenon is known as the wagon‐wheel effect (WWE) and it is caused by the undersampling of visual information. Here, an analogous manifestation of the WWE concept is described in the scenario of light–matter interactions, by showing that the dynamic response of a particle, to an optical trap scanned at different rates, can be diametrically opposed. Further, such behaviors are modulated by the particle dimensions, which can be exploited for particle size selection. The results uncover a distinct paradigm of nontrivial optical manipulation and expand the bidirectional optical sorting range of dielectric particles to the sub‐200 nm scale.

Multifunctional Virus Manipulation with Large‐Scale Arrays of All‐Dielectric Resonant Nanocavities

“…Meanwhile, as light is tightly confined in a circle with a diameter D h ≤ 110 nm (D h is the diameter of the hole), the optical gradient force, which is proportional to the intensity gradient (related to the maximum value in the hole), is hugely enhanced. [53][54][55] Meanwhile, the distribution of the Poynting vector shows that the optical scattering forces acted on a particle placed above nanohole are also pointed toward the nanohole (Figure 2c), double ensuring the caging of viruses into the nanoholes with the optical force. To explore the trapping limit of the nanocavities, we investigate the trapping force on viruses with diameter ranging from 20 to 100 nm and RI of 1.4 (virus) and 1.58 (polystyrene nanoparticle) in Figure 2d.…”

Enhancing near-field optical tweezers by spin-to-orbital angular momentum conversion

“…Optical tweezers are broadly used in several fields of physics and biology to deliver objects at will and to explore and understand mechanics at micro and nanoscale [1][2][3][4][5]. These techniques rely on the predominant restoring forces arising from the strong intensity gradients of a laser beam that is tightly-focused close to the diffraction limit, typically by a high numerical-aperture lens (NA 1.2) [4].…”

Manipulation of a Single Metal Nanowire by an Unpolarized Gaussian Beam

“…Plasmonic metal nanowires are known as promising components in nanooptical systems because of their fascinating ability for concentrating and guiding light in subwavelength volumes. − The accurate manipulation of metal nanowires is highly desired because the plasmonic responses are sensitive to their orientation and position. − An optical-force-based manipulation technique, optical tweezers, has attracted great attention for its noncontact, low-damage, and high-precision properties. − In general, two optical forces acting on the particles are involved: the scattering force, which pushes the particles along the light propagation direction, and the gradient force, which attracts the particles toward the focus. The particles can be trapped near the light focus when the gradient force exceeds the optical scattering force. , Conventional optical tweezers with a tightly focused Gaussian beam have been used to trap and manipulate various particles including semiconductor nanowires, − metal nanoparticles, and biological cells. , However, for metal nanowires, the scattering force dominates over the attractive gradient force because of its strong absorption and scattering features, and stable capture is unattainable in a single Gaussian beam. , Various approaches have been proposed in terms of either enhancing the gradient force or reducing the scattering force. For example, plasmonic tweezer exhibits a significantly enhanced attractive gradient force for particles due to local-field enhancement by plasmon excitation on a metallic surface. − However, the evanescent nature of the surface plasmon largely limits its operation range, and a plasmon-induced strong thermal effect would also affect the manipulation stability and cause damage to the samples.…”

“…1−5 The accurate manipulation of metal nanowires is highly desired because the plasmonic responses are sensitive to their orientation and position. 6−9 An optical-force-based manipulation technique, optical tweezers, 10 has attracted great attention for its noncontact, low-damage, and highprecision properties. 11−15 In general, two optical forces acting on the particles are involved: the scattering force, which pushes the particles along the light propagation direction, and the gradient force, which attracts the particles toward the focus.…”

“…The particles can be trapped near the light focus when the gradient force exceeds the optical scattering force. 10,16 Conventional optical tweezers with a tightly focused Gaussian beam have been used to trap and manipulate various particles including semiconductor nanowires, 17−19 metal nanoparticles, 20 and biological cells. 21,22 However, for metal nanowires, the scattering force dominates over the attractive gradient force because of its strong absorption and scattering features, and stable capture is unattainable in a single Gaussian beam.…”