An Anapole Probe for Measuring Light’s Magnetic Topological Spin Structure

Professor Xiaocong Yuan’s research group at the Nanophotonics Research Center, Institute of Microscale Optoelectronics, Shenzhen University, has devised a unique anapole probe to monitor the magnetic field-dominated photonic spin structure in light.

The suggested approach may help develop spin photonics and characterize the magnetic characteristics of optical spin. The team’s report can be read in Nature - Light: Science & Applications.

Topologically nontrivial spin textures exhibit emergent electrodynamics and can be propelled over macroscopic distances by spin currents. An example of a topologically nontrivial spin texture is a skyrmion.

What are Skyrmions?

A skyrmion can be compared to a subatomic hurricane, a knot of twisted field lines, or a swirling quasi-particle. They rank among the most complex physics ideas to comprehend.

In a magnetic skyrmion, the tangled magnetic field lines form a nanometer-scale pattern that is impossible to tear apart without. They are wrapped around one another like key rings hooking into other key rings. The skyrmion can regenerate from lines at each given position as the formation drifts from one location to another in a magnetic field. These unique properties and nanoscale size make them excellent prospects for creating next-generation memory storage devices and unconventional computing.

Photonic equivalents of magnetic skyrmions have recently been demonstrated in a 2D form in evanescent waves or 3D propagating structured light.

Spin structures' deep-subwavelength characteristics offer new optical metrology techniques, such as high-precision displacement detection and magnetic domain monitoring.

Realizing appropriate detection methods of topologically nontrivial spin textures is a significant problem for these applications and general study of nanoscale magnetic structures.

The topological spin structures of evanescent waves controlled by magnetic fields can be measured using a novel magnetic anapole probe, as Prof.Yuan’s team proposed in this study.

Experimental Details

Manipulating the interaction between multipolar modes is necessary to create the perfect magnetic probe with a significant magnetic field response but no electric reaction. The realization of a pure magnetic dipole scatter has historically been hampered by the overlap of the spectral features between the magnetic and electric dipole and modes for a pure high refractive index nanoparticle, such as a Si-nanoparticle.

The destructive interference observed between the toroidal dipole and the electric dipole modes inside a silicon nanoparticle, known as the anapole mode, was recently shown to inhibit the electric dipole scattering. This feature can function as a magnetic scatter in the distant field.

However, as the Si-nanoparticle’s size increases, the toroidal dipole mode appears. In the visible wavelength region, this would cause the magnetic dipole mode to redshift, making it challenging to overlap the magnetic dipole mode with the anapole mode.

In light of this, Prof.Yuan’s group devised an Ag-core inside a Si-shell core-shell nanoparticle. The anapole mode’s resonance frequency can be changed using this design to coincide with the magnetic dipole mode within the required wavelength range. The geometric properties of the nanoparticle were tuned to create this optimal anapole probe.

Once the anapole probe was constructed, it was utilized to evaluate the near-field distributions of waveguide modes created by several tightly focused beams. This was to demonstrate the effectiveness of the suggested technique.

When mapped on a detector screen, the waveguide mode excited by an azimuthally polarized beam showed a stark intensity pattern. With numerous concentric rings, a cylindrically symmetric standing wave was imaged. The azimuthally polarized beam's transverse electric (TE) waves resulted. These TE waves, which were excited from all azimuthal directions, interfered with one another. Near the geometric center, the intensity is detected to be zero.

With the in-plane magnetic fields measured, the spin characteristics of the TE waveguide mode can be described in a way comparable to the transverse magnetic (TM)-type surface Plasmon mode.

3D finite-difference time-domain simulations of the scattering spectra for the Ag-core and Si-shell nanospheres were performed using commercial software.

Discussion and Outlook

An anapole probe was designed to quantify the magnetic component of diverse photonic spin topological textures. The probe is a mixed nanosphere of an Ag-core and an outer Si-shell.

At the chosen wavelength of operation, the magnetic dipole resonance was the dominant feature, along with removing the anapole mode's electric-field scattering. 

For the first time, measurements were made of the photonic spin topological textures produced by TE-type evanescent waves, including individual Skyrmion and Skyrmion/Meron lattices with various symmetries.

The proposed approach will help researchers better understand the physical processes connected to electromagnetic fields' magnetic components while also developing spin photonics.

Reference

Meng, F., Yang, A., Du, K. et al. (2022) Measuring the magnetic topological spin structure of light using an anapole probe. Light Sci Appl 11, 287. https://www.nature.com/articles/s41377-022-00970-x

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