Directional scattering and multipolar contributions to optical forces on silicon nanoparticles in focused laser beams

Länk, Nils Odebo; Johansson, Peter; Käll, Mikael

doi:10.1364/oe.26.029074

Cited by 26 publications

(24 citation statements)

References 46 publications

(60 reference statements)

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“…A strong suppression of forward scattering was already experimentally observed in the microwave regime [16] at this condition, in agreement with the near-zero-forward intensity condition for Rayleigh particles [5,7]. A strong suppression of forward scattering is an important issue in light transport and scattering in nanostructured complex media [39][40][41][42][43][44][45][46][47][48], being also relevant in the discussion of the, so-called, anapole modes [19,49,50], as well as in the context of optical forces [51][52][53][54][55]. In most of the above-mentioned works, the equivalence between the GSKC and the near zero-forward condition was taken from granted even though it was only demonstrated in the Rayleigh limit, where the particle scattering cross sections are very small.…”

supporting

confidence: 77%

Optimal backward light scattering by dipolar particles

Olmos‐Trigo

Abujetas

Sanz-Fernández

et al. 2020

Phys. Rev. Research

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The near-zero forward intensity condition for light scattering by a dielectric dipolar sphere is usually associated with the generalized second Kerker condition, at which equal amplitude electric and magnetic dipolar responses are phase-shifted by π . As we show, this condition does lead to optimal backward light scattering for a given scattering cross section. However, it is clearly insufficient to give rise to the nearly zero optical forward scattering, in striking contrast to the actual view of the problem. In fact, we show that the generalized second Kerker condition leads to an energy radiation pattern that ranges all possible optical scattering diagrams depending on the total scattering cross section. Interestingly, we demonstrate that optimization of backward intensity, near the electric and magnetic dipolar resonances, leads to the counterintuitive result of a far-field energy radiation pattern with nearly zero backscattering.The conditions of perfect zero light scattering from magnetic spheres were brought to the physical scene by Kerker, Wang and Giles [1] by assuming magnetodielectric spheres with a particular combination of relative electric permittivity and magnetic permeability μ. They proved that when = μ, the backscattered radiation from the sphere is identical to zero. In contrast, for very small particles, when = (4−μ)/(2μ+1), the forward scattering was shown to be reduced to zero, except for the peculiar case of = μ = −2 [2].The study of these two optical responses, often referred to as Kerker conditions and, in particular, the inconsistency between the zero-forward condition and the optical theorem, have attracted a great interest [2][3][4][5][6]. The apparent paradox was solved by Alù and Engheta [5] by introducing the concept of near-zero-forward condition for magnetic Rayleigh particles. Kerker conditions, originally discussed for hypothetical magnetodielectric particles, were later shown to apply to subwavelength dielectric (μ = 1) particles of high refractive index (HRI) materials [7,8]. Remarkably, the scattering properties of these HRI nano-particles are given by their dipolar electric and magnetic responses [9][10][11][12][13][14].For HRI subwavelength isotropic spheres, the scattering can be fully described by the first electric, a 1 , and magnetic, b 1 , Mie coefficients [15],* where the phase-shifts α 1 and β 1 are real in the absence of absorption. Under plane wave illumination, the intensity in the backscattering direction can be exactly zero whenever a 1 = b 1 ⇔ α 1 = β 1 , independently of any other particle's material property [7]. At this condition, the particle's optical response consists of equal amplitude crossed electric-and magneticinduced dipoles oscillating in-phase (first Kerker condition), leading to destructive interference between scattered fields in backscattering. This prediction was experimentally demonstrated first for millimeter-scale ceramic spheres in the microwave regime [16] and shortly after for nanometer-scale HRI Si [17] and GaAs [18] nanospheres. Since ...

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supporting

confidence: 77%

Optimal backward light scattering by dipolar particles

Olmos‐Trigo

Abujetas

Sanz-Fernández

et al. 2020

Phys. Rev. Research

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“…This explains why this interference effect cannot affect the intensity‐gradient force in the case of circularly polarized Gaussian beams, as observed in ref. []. Additionally, because its curl ∇ × F r (Ki) ∝ ∇cos2 θ × ∇ I does not vanish in general, F r (Ki) represents a nonconservative mechanical property of the intensity gradients.…”

Section: Resultsmentioning

confidence: 99%

“…in studying the asymmetry of light scattering, and it has now pervaded various branches of nanophotonics . On the other hand, with the establishment of dipolar optomechanical model, the Kerker interference rapidly arouse attentions from the wide field of optical manipulation . The unique mechanical effect of the Kerker interference has breathed a new life into this area, refreshing our understanding on optical forces.…”

Section: Introductionmentioning

confidence: 99%

“…Despite its critical role in determining the radiation pressure and lateral force, the Kerker interference seems unable to affect the essence of the intensity‐gradient force . As the cornerstone of modern optical tweezers, the intensity‐gradient force is conventionally considered conservative, with a uniform directional property, pointing either toward (attractive) or away from (repulsive) the intensity maxima .…”

Section: Introductionmentioning

confidence: 99%

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Kerker‐Type Intensity‐Gradient Force of Light

Nieto‐Vesperinas

Qiu

et al. 2020

Laser & Photonics Reviews

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The intensity gradient of light represents the most important property for optical tweezers to manipulate small particles, which is known to produce a conservative optical force that is either attractive or repulsive. Here, it is shown that Kerker interference, the interplay between electric and magnetic dipoles induced in nanoparticles, permits the intensity gradient to exert a nonconservative optical force, in the case of a standard optical trap created with linearly or elliptically polarized Gaussian beams. The Kerker‐type intensity‐gradient force has an “anisotropic” directionality, and it tends to repel particles away from the beam axis. Such repulsive effects can greatly sensitize the particle trapping behavior of optical tweezers to the particle size. Utilizing these peculiar properties, all‐optical sorting of Si nanoparticles is theoretically demonstrated, with tunable size‐selection criterion and accuracy.

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“…(Black) Electronic dipole contribution (σ e = n 2 ωα e /c ), (red) magnetic dipole contribution(σ m = n 2 ωα m /c), (green) electric dipole plus magnetic dipole contributions and (blue) total extinction cross section configuration, corresponds to the first Kerker condition of the silicon particle α m = α e . Kerker conditions have been previously proved to be important in the particular case of pulling forces [36] 2.4 Description of the experiment proposed…”

Section: The Particlementioning

confidence: 99%

A proposal to measure Belinfante’s curl of the spin optical force based on the Kerker conditions

2021

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The linear momentum of electromagnetic radiation is proportional to the Poynting vector. However, when light interacts with a dipole, the radiation pressure generated is no longer proportional to the Poynting vector: the so-called Belinfante’s momentum or equivalently, the curl of the spin density of the light field, must be considered. In this paper, we propose an optical configuration, based on two evanescent counter-propagating waves, perpendicularly polarized, capable of detecting Belinfante’s mechanical action. The two beams interact with a high-refractive-index particle like silicon. The direction of the radiation pressure exerted on the particle, proportional only to the curl of the spin density, depends on the electric and magnetic response of the particle and changes sign at the radiation wavelengths corresponding with the Kerker conditions.

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Directional scattering and multipolar contributions to optical forces on silicon nanoparticles in focused laser beams

Cited by 26 publications

References 46 publications

Optimal backward light scattering by dipolar particles

Optimal backward light scattering by dipolar particles

Kerker‐Type Intensity‐Gradient Force of Light

A proposal to measure Belinfante’s curl of the spin optical force based on the Kerker conditions

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