Physicists discovered how anisotropy and anapole states impact the Purcell factor in silicon metasurfaces.

The results have been published in the journal Laser and Photonics Reviews. A metamaterial is an artificially engineered structure composed of numerous elements known as meta-atoms. The arrangement of these meta-atoms is precisely defined and dictates the physical properties of the system. Due to its artificial nature, it exhibits physical characteristics that are not typical of natural materials.

For instance, metamaterials can possess a negative refractive index, negative dielectric or magnetic permeability, superlensing effect (the ability of specific optical systems to form images with a precision exceeding the diffraction limit), invisibility, and much more. Their applications are quite broad and are in demand across various fields, such as medicine, photonics, quantum technologies, and even construction.

In their new work, physicists have modeled a metasurface where simultaneous implementation of bianisotropy and anapole states is possible. In materials exhibiting bianisotropy, the electric and magnetic polarizabilities are interconnected and influence one another. Conversely, an anapole state is characterized by complete cancellation of scattering by a meta-atom, yet the electric field within and near the particle significantly increases in this case.

The researchers demonstrated that if the metasurface exhibits these properties at the same wavelength, a substantial enhancement of the Purcell effect is observed. This effect involves the alteration of spontaneous emission rates of a point light source located in a resonator cavity.

“Metasurfaces that operate on other effects to achieve high Purcell factor values do exist. For example, anapoles concentrate electric energy and enhance the electric Purcell factor. However, we have achieved such high values for both electric and magnetic effects for the first time,” said Alexander Shalin, the leading researcher at the laboratory of controlled optical nanostructures at MIPT.

The proposed structure consisted of meta-atoms in the form of silicon nanodisks with a partial rectangular slit. These formed an infinite two-dimensional structure with a lattice constant of 425 nm. Such a metasurface allows for the simultaneous realization of bianisotropy and anapole states at the same wavelength. In the anapole state, the electric field is concentrated within the meta-atom and its slit. Bianisotropy enables the "pumping" of part of the energy into the magnetic component of the field within the particle through the electromagnetic coupling of polarizabilities. As a result, the described system exhibits a high concentration of both electric and magnetic fields within the slit of the resonator.

“The main challenge was to realize two different effects at the same frequency and to tune their correct interaction. We have accomplished this for the first time in the world, although, of course, the primary result is the increase in the electric and magnetic Purcell factors,” shared Alexander Shalin.

The optical response modeling results showed that the described metasurface possesses both bianisotropy and anapole states within the wavelength range of 750-805 nm. The researchers calculated that this metamaterial has a high electric Purcell factor of approximately 450 and a magnetic Purcell factor of about 1000.

The simultaneous application of both phenomena in silicon metasurfaces significantly expands the possibilities for controlling the emission of quantum sources. These new properties pave the way for the development of next-generation optical devices for quantum computers, optoinformational circuits, and a wide array of various nanophotonic applications.

“We would like to implement our metasurface experimentally to verify everything, and then consider specific applications of such meta-atoms and structures in practical tasks,” said Alexander Shalin.

The research was partially supported by a grant from the Russian Science Foundation.