An Ultra-Sub-Wavelength Microwave Polarization Switching Antenna for Covert Communication Implemented With Directed Surface Acoustic Waves in an Artificial Multiferroic Magnonic Crystal
Polarization switches are of great technological interest because of their many applications in long distance electromagnetic communication (e.g., polarization division multiplexing). Binary bits can be encoded in the two orthogonal polarizations and transmitted from point to point. Polarization switches, however, are usually much larger than the wavelength of the electromagnetic wave that they transmit. Consequently, most research in this area has focused on the optical regime where the wavelength is relatively short (~1 micron), so that the switch being much larger than the wavelength is not too inconvenient. However, this changes in the microwave regime where the wavelength is much larger (typically > 1 cm). That makes a microwave ultra-sub-wavelength polarization switch very attractive. Here, for the first time to the authors' knowledge, such a switch made of an array of magnetostrictive nanomagnets (~100 nm lateral dimension, ~5 nm thickness) deposited on a piezoelectric substrate to make an "artificial multiferroic magnonic crystal (AMMC)" is reported. A surface acoustic wave (SAW) launched in the substrate with suitable electrodes excites confined spin waves in the nanomagnets via phonon-magnon coupling, which then radiate electromagnetic waves in space via magnon-photon coupling. In some particular direction(s), determined by the AMMC parameters, the polarization of the beam at a given frequency can be rotated through ~90 degrees by switching the direction of SAW propagation in the piezoelectric substrate between two mutually orthogonal directions via activation of two different pairs of SAW launching electrodes. By aligning the transmitter and the receiver along such a direction (known only to authorized users), one can communicate covertly from point to point, without the need for encryption or cryptography.