Plasmonic Metasurface Resonators to Enhance Terahertz Magnetic Fields for High‐Frequency Electron Paramagnetic Resonance

Abstract

Nanoscale magnetic systems play a decisive role in areas ranging from biology to spintronics. Although, in principle, THz electron paramagnetic resonance (EPR) provides high‐resolution access to their properties, lack of sensitivity has precluded realizing this potential. To resolve this issue, the principle of plasmonic enhancement of electromagnetic fields that is used in electric dipole spectroscopies with great success is exploited, and a new type of resonators for the enhancement of THz magnetic fields in a microscopic volume is proposed. A resonator composed of an array of diabolo antennas with a back‐reflecting mirror is designed and fabricated. Simulations and THz EPR measurements demonstrate a 30‐fold signal increase for thin film samples. This enhancement factor increases to a theoretical value of 7500 for samples confined to the active region of the antennas. These findings open the door to the elucidation of fundamental processes in nanoscale samples, including junctions in spintronic devices or biological membranes.

Magnetic Resonance Modulation in Ferrocene-alkyne Conjugated Polymer for Electromagnetic Wave Absorption

“… The principle of minimum energy allows to determine the mode and pattern of extra-nuclear electron arrangement of iron atoms in each sample, i.e., the number of extra-nuclear single electrons of iron in the samples increases with increasing alkyne content (from 0 to 1 and then to 2), and the spin quantum number ( S ) goes from 0 (PFc-1) to 1 (PFc-2) and then to 3/2 (PFc-3). − This indicates that the magnetic moments produced by the samples are increasing as the alkyne content increases, which is in accordance with the trend of the magnetization intensity measured by vibrating sample magnetometer (VSM) (Figure h). Therefore, we can speculate that the reason for the exchange resonance may be that the unpaired electrons of PFc-3, which is superparamagnetic under the action of specific frequency of the external magnetic field, rapidly generate magnetic moments, and their energy level structure also changes, i.e., the electrons undergo excited leaps, which results in some electrons in the lower energy level absorbing the energy of the electromagnetic wave and leaps to the higher energy level (Figure d). − …”

Challenges for exploiting nanomagnet properties on surfaces

“…There are specific setups for single crystals and thin films. Recent reports on new EPR resonator design that concentrate the field strength in a 2D area showed great performance for thin-film and monolayer samples 52 Further advances in this technology will indeed be very interesting for the study of the magnetic properties of molecular monolayers or thin layers. The resonators used for biological samples 53 or for single crystals can be used to collect data on flat surfaces.…”

Terahertz magnetic response of plasmonic metasurface resonators: origin and orientation dependence

“…Recent advances in THz EPR resonators have opened up new possibilities in overcoming these inherent challenges associated with the need for excitation of magnetic transitions, which are weaker compared to electric ones. In particular, we have recently realized novel resonators based on plasmonic metasurfaces 23 . Generally, plasmonic metasurfaces, comprised of artificially structured nanoantennas, enable unprecedented control over electromagnetic waves, offering new avenues in manipulating light at the subwavelength scale 24 .…”

“…With the aim of confining the magnetic field to the surface to enhance the magnetic response of 2D materials, in a previous work we have realized a plasmonic metasurface resonator (PMR) consisting of an array of gold diabolo-like antennas fabricated on a quartz substrate with a metallic back-reflector (Fig. 1 a) 23 . A strong magnetic field intensity enhancement at around 290 GHz with quality (Q-) factor of 40 was demonstrated both by simulations and experiments (Figs.…”

“…The near-field intensities were determined 10 nm above the antennas top plane and rescaled by the source intensity to obtain the field enhancement. More details in reference 23 . …”