Maxwell noted that light carries angular momentum, and as such it can exert torques on material objects. This was subsequently proved by Beth in 1936. Applications of these opto-mechanical effects were limited initially due to their smallness in magnitude, but later enabled by the invention of laser. Novel and practical approaches for harvesting light for particle rotation have been demonstrated, where the structure is subjected to a positive optical torque along a certain axis21 if the incident angular momentum has a positive projection on the same axis. We report here a counter-intuitive phenomenon of negative optical torque, meaning that incoming photons carrying angular momentum rotate an object in the opposite sense. Surprisingly this can be realized quite straightforwardly in simple planar structures. Field retardation is a necessary condition. The optimal conditions are explored and explained.
Optical vortex beams carry orbital angular momentum and thus exert torque on illuminated objects. A dielectric microtool - a microbarbell - is used in a two-laser optical tweezers to measure the torque of a focused optical vortex. The tool was either freely rotating due to the applied torque or set into oscillations by the counteracting force. Four different trapping configurations provided different ways of sensing the torque and gave consistent results. The value of torque was determined by confronting the experimental results with numerical and analytical models.
We comment on mistakes and inaccuracies of a paper by Chen et al. concerning the optical torque from generic optical fields on dipolar chiral particles, i.e. on those whose scattering is fully described by the first electric, magnetic and magnetoelectric Mie coefficients.
Comment on "Poynting vector, orbital and spin momentum and angular momentum versus optical force and torque on arbitrary particle in generic optical fields"
Structured light mechanically interacts with matter via optical forces and torques. The optical torque is traditionally calculated via the flux of total angular momentum (AM) into a volume enclosing an object. In [Phys. Rev. A 92, 043843 (2015)] a powerful method was suggested to calculate optical torque separately from the flux of the spin and the orbital parts of optical AM, providing useful physical insight. However, the method predicted a new type of dipolar torque dependent on the gradient of the helicity density of the optical beam, inconsistent with prior torque calculations. In this wo…
We present a method to measure the optical torque applied to particles of arbitrary shape held in an optical trap, inferred from the change of angular momentum of light induced by the particle. All torque components can be determined from a single interference pattern recorded by a camera in the back focal plane of a high-NA condenser collecting all forward scattered light. We derive explicit expressions mapping the measured complex field in this plane to the torque components. The required phase is retrieved by an iterative algorithm, using the known position of the optical traps as constrain…
Optical forces in the near fields have important applications in on-chip optical manipulations of small particles and molecules. Here, we report a study of optical force and torque on small particles induced by the optical polarization singularities of a gold cylinder. We show that the scattering of the cylinder generates both electric and magnetic C lines (i.e., lines of polarization singularities) in the near fields, and the C lines can induce complex force and torque on a dielectric/magnetic particle. The force and torque manifest dramatic spatial variations with interesting symmetry proper…
According to quantum theory, measurement and backaction are inextricably linked. In optical position measurements, this backaction is known as radiation pressure shot noise. In analogy, a measurement of the orientation of a mechanical rotor must disturb its angular momentum by radiation torque shot noise. In this work, we observe the shot-noise torque fluctuations arising in a measurement of the angular orientation of an optically levitated nanodumbbell. We feedback cool the dumbbell's rotational motion and investigate its reheating behavior when released from feedback. In high vacuum, the hea…
We report experimental evidence of a mechanism that supports and intensifies induced magnetization at optical frequencies without the intervention of spin-orbit or spin-spin interactions. Energy-resolved spectra of scattered light, recorded at moderate intensities (108 W/cm2) and short timescales (<150 fs) in a series of non-magnetic molecular liquids, reveal the signature of torque dynamics driven jointly by the electric and magnetic field components of light at the molecular level. While past experiments have recorded radiant magnetization from magneto-electric interactions of this type, no …
Optically coupled nanoparticles suffer the action of multiple electromagnetic forces when they are illuminated by light. In general, two kinds of forces are commonly assumed: binding forces that make them attract/repel each other and scattering forces that push the system forwards. Tangential forces and orbital torques can also be induced to align the dimer with the electric field. In this work, new degrees of freedom are found for a dimer of silica nanospheres under illumination with linearly-polarized plane waves. The results have a general validity for arbitrary mesoscale systems: multiple …
Bio-compatible Au nanoparticles exhibit great advantages in the application of biomedical researches, such as bio-sensing, medical diagnosis, and cancer therapy. Bio-molecules can even be manipulated by laser tweezers with the optically trapped Au nanoparticles as handles. In this Letter, optical scattering torque arising from the coupled Au nanoparticles driven by circularly polarized light is theoretically presented. The coupled plasmon resonance modes boost the angular momentum transfer from photons to the Au nanoparticle dimers and trimers through light scattering, which does not bring any…
We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagnetic environment described by the most general linear constitutive relations. The atom is modeled as a two-level system prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and under the Markov approximation, we show that the optical torque has a resonant and non-resonant part, associated respectively with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum, which can both be written explicitly in terms of the system Green fu…
The optical trapping and manipulation of small particles is an important tool for probing fluid properties at the microscale. In particular, microrheology exploits the manipulation and rotation of micron-scale particles to probe local viscosity, especially where these properties may be perturbed as a function of their local environment, for example in the vicinity of cells. To this end, birefringent particles are useful as they can be readily controlled using optically induced forces and torques, and thereby used to probe their local environment. However the magnitude of optical torques that c…
Observing and taming unusual effects arising from non-trivial light-matter interaction has always triggered scientists to better understand Nature and develop technological tools towards implementing novel applications. Recently, several unusual optomechanical effects have been unveiled when subtle spin-orbit interactions come at play to build up optical forces and torques that are hardly seen in everyday life, such as negative optical radiation pressure, transverse optical forces, or left-handed optical torques. To date, there are only a few experimental proposals to reveal these effects desp…
We establish universal relationships between optical force/torque on a general particle and different parts of linear and angular momentum (AM) of generic monochromatic optical field. It is rigorously proved that the optical force comes about by the transfer of orbital (canonical) optical momentum from light to matter, while the other part of optical momentum, known as spin momentum, does not generate optical force on matter but, instead, stays conserved even when the translational invariance is broken by putting particles into the optical fields. On the other hand, based on a generic multipol…
Optical torque on individual atoms in a Bose-Einstein condensate can rotate the ensemble and generate a current flow [Phys. Rev. A 82, 051402 (2010)]. We exploit the fact to suggest a new mechanism for enhanced and well-controlled rotational motions and show how atoms, interacting with Laguerre-Gaussian beams, experience a torque whose features depend on relative phase of applied fields so that zero and large positive, or even negative, values for the torque can be obtained. Such controllable torque, along with simplicity of tuning the relative phase, can simplify a possible implementation of …
We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description. Our work opens the w…
We calculate analytically and numerically the axial orbital and spin torques of guided light on a two-level atom near an optical nanofiber. We show that the generation of these torques is governed by the angular momentum conservation law in the Minkowski formulation. The orbital torque on the atom near the fiber has a contribution from the average recoil of spontaneously emitted photons. Photon angular momentum and atomic spin angular momentum can be converted into atomic orbital angular momentum. The orbital and spin angular momenta of the guided field are not transferred separately to the or…
Heliconical cholesteric liquid crystals are expected to be more sensitive to torque induced by light field since their structure allows both bend and twist in molecular orientations, differently from the conventional cholesterics in which only twist deformation is involved, requiring much higher fields. We report here a demonstration of tuning the helical pitch in heliconical cholesterics induced by an optical torque. Experimental observations are in agreement with expectations of the classical theory extended to include the effect of the optical field. A dual control of the helical pitch is a…
Research School of Physics, Australian National University（澳大利亚国立大学物理研究学院）；School of Physics and Engineering, ITMO University（ITMO大学物理与工程学院）；Nanophotonic Systems Laboratory, Department of Mechanical and Process Engineering, ETH Zürich（苏黎世联邦理工学院机械与工艺工程系纳米光子系统实验室）
The physical origins of the transverse optical binding force and torque beyond the Rayleigh approximation have not been clearly expressed to date. Here, we present analytical expressions of the force and torque for a dual dipolar dielectric dimer illuminated by a plane wave propagating perpendicularly to the dimer axis. Using this analytical model, we explore the roles of the hybridized electric dipolar, magnetic dipolar, and electric-magnetic dipolar coupling interactions in the total force and torque on the particles. We find significant departures from the predictions of the Rayleigh approx…
Spin and orbital angular momentum of light plays a central role in quantum nanophotonics as well as topological electrodynamics. Here, we show that the thermal radiation from finite-sized bodies comprising of nonreciprocal magneto-optical materials can exert a spin torque even in global thermal equilibrium. Moving beyond the paradigm of near-field heat transfer, we calculate near-field radiative angular momentum transfer between finite-sized nonreciprocal objects by combining Rytov's fluctuational electrodynamics with the theory of optical angular momentum. We prove that a single magneto-optic…
Twisted light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics and microparticle rotation. Here we introduce and experimentally validate a new class of light beams, whose unique property is associated with a temporal OAM variation along a pulse: the self-torque of light. Self-torque is a phenomenon that can arise from matter-field interactions in electrodynamics and general relativity, but to date, there has been no optical analog. In particular, the self-torque of light is an inherent property, …
A universal property of resonant subwavelength scatterers is that their optical cross-sections are proportional to a square wavelength, $λ^2$, regardless of whether they are plasmonic nanoparticles, two-level quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the \emph{incident field}: plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to $λ^2$. In this Article, we identify $λ^2/c$ and $λ^3/c$ as analogous force and torque constants, derived within a more general quadratic scattering-channel framework f…
Light-actuated motors, vehicles, and even space sails have drawn tremendous attention for basic science and applications in space, biomedical, and sensing domains. Optical bound states in the continuum (BIC) are topological singularities of the scattering matrix, known for their unique light trapping capability and enhanced light-matter interaction. We show that BIC modes enable the generation of enhanced and tunable optical forces and torques. A sharp and controllable lineshape is observed in forces and torques spectra when approaching high-Q resonance BIC modes. Wavelength and polarization t…
A recent study of the photonic coupling between metallic nanowires has revealed new degrees of freedom in the system. Unexpected spin torques were induced on dimers when illuminated with linearly polarized plane-waves. As near-field observables, the spectra of torques showed more resolved resonances than typical far-field spectra. Here the study is extended to silicon dimers. Strong forces and torques are exerted by light under both polarizations s and p, contrary to plasmonic systems where the resonant strong forces are found only for p-polarization. The systems made of high-dielectric posses…
Optical forces, helicity, angular momentum and how they are all intertwined
Centro de Física de Materiales (CFM), CSIC-UPV/EHU（材料物理中心，西班牙科学研究中心-巴斯克国立大学）；Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）；Basic Sciences Department, Mondragon Unibertsitatea（蒙德拉贡大学基础科学系）；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯基，巴斯克科学基金会）
The Fresnel-Snell law, which is one of the fundamental laws in optics and gives insights on the behaviour of light at interfaces, is violated if there exists dissipation in the transmitting media. In order to overcome this problem, we extend the angle of refraction from a real number to a complex number. We use this complex-angle approach to analyse the behaviour of light at interfaces between lossy media and lossless media. We reveal that dissipation makes the wavenumber of the light exceed the maximum allowed at lossless interfaces. This is surprising because, in general, dielectric loss onl…
