Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams or beams with helical phase structure. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 1936, when he reported a tiny torque developed in a quartz waveplate due to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating. We have observed rotation rates in excess of 350 Hz.

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Optical alignment and spinning of laser-trapped microscopic particles

Cavity magnonics

Space–Time Metasurfaces for Power Combining of Waves

We propose and analyze surface-plasmon-driven electron spin currents in a thin metallic film. The electron gas in the metal follows the transversally rotating electric fields of the surface plasmons (SPs), which leads to a static magnetization gradient. We consider herein SPs in a thin-film insulator-metal-insulator structure and solve the spin diffusion equation in the presence of a magnetization gradient. The results reveal that the SPs at the metal interfaces generate spin currents in the metallic film. For thinner film, the SPs become strongly hybridized, which increases the magnetization gradient and enhances the spin current. We also discuss how the spin current depends on SP wavelength and the spin-diffusion length of the metal. The polarization of the spin current can be controlled by tuning the wavelength of the SPs and/or the spin diffusion length.

https://iopscience.iop.org/article/10.1088/1367-2630/ab764c/pdf

Effects of surface plasmons on spin currents in a thin film system

Chirality as generalized spin–orbit interaction in spintronics

We propose a mechanism of angular momentum conversion from optical transverse spin in surface plasmon polaritons (SPPs) to conduction electron spin. Free electrons in the metal follow the transversally spinning electric field of the SPP, and the resulting orbital motions create inhomogeneous static magnetization in the metal. By solving the spin diffusion equation in the SPP, we find that the magnetization field generates an electron spin current. We show that there exists a resonant condition where the spin current is resonantly enhanced, and the polarization of the spin current is flipped. Our theory reveals an alternative functionality of SPPs as a spin current source.

Electron spin transport driven by surface plasmon polaritons

We investigate electron spin currents induced optically via plasmonic modes in the Kretschmann configuration. By utilizing the scattering matrix formalism, we take the plasmonic mode coupled to an external laser drive into consideration and calculate induced magnetization in the metal. The spatial distribution of the plasmonic mode is inherited by the induced magnetization, which acts as an inhomogeneous effective magnetic field and causes the Stern-Gerlach effect to drive electron spin currents in the metal. We solve the spin diffusion equation with a source term to analyze the spin current as a function of the spin diffusion length of the metal, the frequency, and the incident angle of the external drive.

Optically induced electron spin currents in the Kretschmann configuration

Electromagnetic waves in a system with a space- and time-dependent boundary experience both diffraction and Doppler-like frequency conversion In order to analyze such situations, conventional methods call for either the eigenmodes or the dyadic Green's function in space- and time-dependent media Here, we propose a dynamical differential method which does not require either of them Our method utilizes a dynamical coordinate transformation in order to simplify the calculation of the optical response of the space- and time-dependent system We reveal that the diffraction symmetry is broken in the presence of traveling-wave-type spatiotemporal modulation

Calculating spatiotemporally modulated surfaces: A dynamical differential formalism

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 only changes the intensity profiles of the light, so this excess wavenumber cannot be produced in the bulk even if there exists dielectric loss. Additionally, anomalous circular polarisation emerges with dissipation. The direction of the anomalous circular polarisation is transverse, whereas without dissipation the direction of circular polarisation has to be longitudinal. We also discuss how the excess wavenumber can increase optical force and how the anomalous circular polarisation can generate optical transverse torque. This novel state of light produced by dissipation will pave the way for a new generation of optical trapping and manipulation.

https://iopscience.iop.org/article/10.1088/2040-8986/ab1a84/pdf

Daigo Oue

Papers

Effects of surface plasmons on spin currents in a thin film system

Electron spin transport driven by surface plasmon polaritons

Optically induced electron spin currents in the Kretschmann configuration

Calculating spatiotemporally modulated surfaces: A dynamical differential formalism

Dissipation effect on optical force and torque near interfaces