Topological Antenna: A Non-Classical Beam-Steering Micro-Antenna Based on Spin Injection from a Topological Insulator

TL;DR

This paper addresses the challenge of miniaturizing antennas while preserving efficiency and enabling directed radiation. It introduces a micro-antenna that uses spin injection from a three-dimensional topological insulator into a surface nanomagnet array to excite spin waves that radiate electromagnetic waves, with beam direction controlled by the current flow due to spin-momentum locking. The authors demonstrate a sub-wavelength, highly anisotropic radiator and show beam steering from a single element without a phased array, by switching current directions across antipodal electrodes; they also measure spectra and radiation patterns in an anechoic chamber to validate the concept. The work highlights a new route to TI-based analog devices and compact beam-steering antennas, potentially enabling disruptive mobile and communication technologies.

Abstract

Antennas are the quintessential means to communicate information wirelessly over long distances via electromagnetic waves. Traditional antennas have two shortcomings that have prevented miniaturization: (1) their radiation efficiencies plummet and (2) they radiate isotropically when miniaturized to small fractions of the radiated wavelength. Here, we report a new genre of non-classical antennas that overcome these limitations by employing non-traditional principles and harnessing topological insulators. An alternating charge current of frequency 1-10 GHz injected into a thin film of a three-dimensional topological insulator (3D-TI) injects a spin current of alternating spin polarization into a periodic array of cobalt nanomagnets deposited on the surface of the 3D-TI. This generates spin waves in the nanomagnets, which radiate electromagnetic waves in space, thereby implementing an antenna. The frequency of the electromagnetic wave is the same as that of the current. The antenna dimension is only 0.6-1.8% of the free space wavelength and yet it radiates with an efficiency several orders of magnitude larger than the theoretical limit for conventional antennas. Furthermore, it radiates anisotropically (despite being a "point source") and one can change the anisotropic radiation pattern by changing the direction of the injected alternating charge current, which changes the spin wave patterns within the nanomagnets because of spin-momentum locking in the 3D-TI. This enables beam steering without the use of a phased array. We have overcome several limitations of classical antennas by harnessing the quantum mechanical attributes of a quantum material, namely a 3D-TI.

Figures (9)

• Figure 1: A ferromagnet deposited on a three-dimensional topological insulator. The directional relationships between the charge current, injected spin current and spin polarization of the injected spin current (which is same as that of the top surface) are shown. The magnetization of the ferromagnet will be aligned along the spin polarization. An alternating charge current will result in oscillating spins.
• Figure 2: (a) Schematic of the nano-antenna and two different ways of passing an alternating charge current between antipodal electrode pairs leading to two different directions of charge current flow and hence two different axes of spin polarization in the spin current injected into the nanomagnets. The axes of the oscillating spins are shown for the two cases. The top configuration is referred to as "orientation 1", and the bottom as "orientation 2". (b) Scanning electron micrograph of the fabricated nanomagnet array. There are 15.36 million nanomagnets in the array, making the total antenna area about 0.003 cm$^2$.
• Figure 3: Modality of beam steering. By injecting current between one fixed electrode (connected to a black line) and all the others sequentially using a multiphase clock, we can inject current sequentially in different directions to make the principal lobe of the radiation pattern scan 360$^{\circ}$ and thus implement beam steering.
• Figure 4: Spectra of the electromagnetic emission when the alternating current frequency is 3.4 GHz. The input power from the alternating current source is 31 mW (15 dbm). The spectra are shown for two different placements of the horn antenna with respect to the sample. The horn was placed in the plane of the nanomagnets facing the two different edges. The line joining the horn and the sample is along the direction $x$ and along the direction $y$ in the two cases. In both cases, current was passed between contact pads 1 and 3.
• Figure 5: Spectrum of the scattering parameter S$_{11}$ of the real and the control sample when current is injected in two mutually orthogonal directions labeled as orientations 1 and 2.