Beam Patterning With Directed Surface Acoustic Waves in Extreme Subwavelength Magneto-Elastic Nano-Antennas

Conventional electromagnetic (EM) antennas cannot be aggressively miniaturized since their gain and radiation efficiency plummet when their sizes become much smaller than the radiated wavelength. Recently, we demonstrated a new genre of unconventional extreme subwavelength nano-antennas that are several orders of magnitude smaller than the wavelength they radiate, and yet they radiate efficiently, beating the conventional Harrington limits on the gain and radiation efficiency by many orders of magnitude. This is made possible by their unique unconventional mechanism of activation. These nano-antennas are implemented with 2-D periodic arrays of ~100-nm-sized nanomagnets deposited on piezoelectric substrates. A surface acoustic wave (SAW) launched in the substrate excites resonant spin waves in the nanomagnets at discrete (GHz) frequencies via phonon-magnon coupling, which radiates EM waves very efficiently at those frequencies via magnon-photon coupling. Normally, one would expect such ultrasmall antennas to behave as point sources that radiate isotropically. Surprisingly, they do not because of the intrinsic anisotropy in the nanomagnet array. The radiation patterns in the plane of the nanomagnets and the two transverse planes are anisotropic. By changing the direction of SAW propagation in the plane of the nanomagnets, one can change the radiation patterns in all three planes, which heralds a new method of beam steering or active electronic scanning.