Fig. 1: The UMA for independent controls of all fundamental wave properties.
a Schematic of the UMA. Slot-opening meta-atoms are positioned on top of the waveguide to convert the GW into PW with software-defined properties. By applying 1-bit ‘0/1’ space-time-coding sequences to switch the meta-atom between the radiating (‘1’) and non-radiating (‘0’) states, we can control all fundamental properties of the radiated wave. Only the momentum of the target harmonic frequency (m = + 1 harmonic in this case) matches with that of free space. In contrast, other unwanted higher-order harmonics are highly suppressed in both the waveguide and free space without phase-matching. b Configuration of the anisotropic meta-atom, consisting of two ± 45o-inclined slot openings loaded with PIN diodes. c–g Independent control of wave properties for the m = +1 harmonic frequency radiation. c Measured frequency shifting of the extracted free-space PW with different modulation frequencies fM. The momentum control (d), phase control (e), amplitude control (f), and polarization control (g) are achieved by tuning the time gradient \(\partial {t}_{i}(x)/\partial x\), reference time shift ti=1, duty cycle τ, and time gradient and reference time shift for the |u〉, |v〉 polarizations, respectively. h–l Independent control of wave properties for the m = 0 fundamental frequency radiation. h Measured spectrum of the radiated PW for m = 0 fundamental frequency radiation. The momentum control (i), phase control (j), amplitude control (k), and polarization control (l) are achieved by tuning the equivalent spatial period Λ, space shift ∆x of the amplitude envelope, modulation depth M, and spatial period and space shift for the |u〉, |v〉 polarizations, respectively. The input frequency and modulation frequency in (d–l) are f0 = 23.5 GHz and fM = 1.2 MHz, respectively. Here, CW continuous wave, SIW substrate-integrated waveguide, PIN positive-intrinsic-negative.
