Optical Anapole Metamaterial

The first document ever published about static toroidal dipole in the context of nuclear physics was in the late 1950s’. Since then subsequent studies have established that dynamic toroidal dipoles are an independent entity in the family of electro dynamic multipole expansion. This therefore means that the toroidal dipole is naturally different from the electric and magnetic multipoles that are associated with nonvanishing induced charge density and nonzero induced transverse current density, respectively. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a non-radiating charge current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Unfortunately, the resulting excitations have proven difficult to couple to external radiation without resorting to complex excitation geometries.

Recently, a team of researchers led by Professor Din Ping Tsai from the Department of Physics, at National Taiwan University demonstrated a quasi-planar plasmonic metamaterial exhibiting induced transverse toroidal response and associated resonant anapole response in the optical part of the spectrum, driven by normal incident light. In particular, they reported on a toroidal metamaterial comprised of a multilayered structure containing a planar array of vertical split-ring resonators (VSRRs) suspended in a dielectric medium and covered with a perforated gold film. Their work is currently published in the research journal, ACS Nano.

In brief, the research method employed commenced with sample fabrication where a ZEP520A layer was spin coated onto a fused silica substrate and then was baked on a hot plate. Subsequently, an E-spacer layer was spin coated onto the ZEP520A layer. The researchers then fabricated two prongs of vertical split-ring resonators using e-beam exposure and lift-off process. Once the samples were fabricated, measurements were undertaken using techniques such as the Fourier-transform infrared spectrometer and near-infrared polarizer. Lastly, simulations, that enabled the researchers obtain the optical reflection as well as the transmission spectra for the samples, were carried out.

The authors observed that all the significant properties of the anapole metamaterial could be tuned through a single well-defined geometrical parameter, which could be controlled to a great precision during fabrication. Consequently, the latter was seen to benefit the design of the optical metamaterials with the desired multipole response, while the others were suppressed on purpose. Moreover, they realized that their technique offered fine control over the radiation loss, and near-field enhancement in nano-photonics.

In summary, the study presented successful design, analysis and fabrication of the planar plasmonic metamaterial, which supports transverse toroidal dipole and anapole excitations that dominate the metamaterial’s resonant response in the optical part of the spectrum. In general, the results obtained here prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional transmission and reflection experiments. Altogether, their work has potential immediate applications in sensing, nonlinear optics and opto-mechanics.