Stable optical lateral forces from inhomogeneities of the spin angular momentum

Abstract

Transverse spin momentum related to the spin angular momentum (SAM) of light has been theoretically studied recently and predicted to generate an intriguing optical lateral force (OLF). Despite extensive studies, there is no direct experimental evidence of a stable OLF resulting from the dominant SAM rather than the ubiquitous spin-orbit interaction in a single light beam. Here, we theoretically unveil the nontrivial physics of SAM-correlated OLF, showing that the SAM is a dominant factor for the OLF on a nonabsorbing particle, while an additional force from the canonical (orbital) momentum is exhibited on an absorbing particle due to the spin-orbit interaction. Experimental results demonstrate the bidirectional movement of 5-μm-diameter particles on both sides of the beam with opposite spin momenta. The amplitude and sign of this force strongly depend on the polarization. Our optofluidic platform advances the exploitation of exotic forces in systems with a dominant SAM, facilitating the exploration of fascinating light-matter interactions.

1 nm‐Resolution Sorting of Sub‐10 nm Nanoparticles Using a Dielectric Metasurface with Toroidal Responses

“…In the dipole theory, the optical force on a small nonmagnetoelectric dipole can be expressed aswhere α e is the electric polarizability, [ 18,46,47 ] n m is the refractive index of the embedding medium, σ ext is the particle extinction cross section, P is the Poynting vector, and c is the light speed in vacuum. S e is the spin angular momentum, which can expressed as , where ε is the permittivity of the medium, and ω is the angular frequency of light.…”

“…where α e is the electric polarizability, [18,46,47] n m is the refractive index of the embedding medium, σ ext is the particle extinction cross section, P is the Poynting vector, and c is the light speed in vacuum. S e is the spin angular momentum, which can expressed as…”

Optical Forces on Multipoles Induced by the Belinfante Spin Momentum

“…In the realm of micro-and nanoscience, there is a vast variety of particles of great interest, which are not sufficiently small to be approximated as a dipole. [9,[19][20][21]24,25] Both plasmonics [26] and the emerged field of Mie-tronics [27] focus on structures that sustain multipolar Mie modes, for their rich optical properties and important applications. As a matter of fact, the BSM force has thus far been a subject of great interest in the realm of optical manipulation, but its theory is still limited to the dipole regime, while there has been increasing work dealing with nondipolar objects.…”

“…As a matter of fact, the BSM force has thus far been a subject of great interest in the realm of optical manipulation, but its theory is still limited to the dipole regime, while there has been increasing work dealing with nondipolar objects. For example, in a very recent paper, [ 25 ] the authors study the lateral BSM force, which acts on large particles that cannot be treated as dipoles. Because of the absence of a multipole theory in this context, they assessed the BSM force by the Maxwell stress tensor method which however, will be inaccurate once other types of optical force are present.…”

Superfast and sub-wavelength orbital rotation of plasmonic particles in focused Gaussian beams

“…It has found applications in many fields such as biology, atomic physics and colloid science [2][3][4] . With the development of both experiment techniques and optical theories (such as nanophotonics), optical manipulation continuously arouses research interests with pulling force 5,6 , lateral force [7][8][9] and negative/inverse torque 10,11 to provide a diversity of controlled motion of particles [12][13][14][15] . These controlled motion at the micro/nanoscale promises applications in micromachines [15][16][17][18] , highprecision measurement [19][20][21] , advanced nanofabrication 22,23 and optofluidic trapping/sorting [24][25][26] .…”