Breaking Reciprocity with Space‐Time‐Coding Digital Metasurfaces

• Published in: Advanced materials (Weinheim) 2019-10, Vol.31 (41), p.e1904069-n/a
• Main Authors: Zhang, Lei, Chen, Xiao Qing, Shao, Rui Wen, Dai, Jun Yan, Cheng, Qiang, Castaldi, Giuseppe, Galdi, Vincenzo, Cui, Tie Jun
• Format: Article
• Language: English
• Subjects: Alternative energy sources; Coding; digital metasurfaces; Energy harvesting; Energy management; frequency conversion; Materials science; Metasurfaces; Microwave frequencies; nonreciprocity; programmable; Reciprocity; space‐time‐coding; Wave fronts
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.201904069

Metasurfaces are artificially engineered ultrathin structures that can finely tailor and control electromagnetic wavefronts. There is currently a strong interest in exploring their capability to lift some fundamental limitations dictated by Lorentz reciprocity, which have strong implications in communication, heat management, and energy harvesting. Time‐varying approaches have emerged as attractive alternatives to conventional schemes relying on magnetic or nonlinear materials, but experimental evidence is currently limited to devices such as circulators and antennas. Here, the recently proposed concept of space‐time‐coding digital metasurfaces is leveraged to break reciprocity. Moreover, it is shown that such nonreciprocal effects can be controlled dynamically. This approach relies on inducing suitable spatiotemporal phase gradients in a programmable way via digital modulation of the metasurface‐elements' phase repsonse, which enable anomalous reflections accompanied by frequency conversions. A prototype operating at microwave frequencies is designed and fabricated for proof‐of‐concept validation. Measured results are in good agreement with theory, hence providing the first experimental evidence of nonreciprocal reflection effects enabled by space‐time‐modulated digital metasurfaces. The proposed concept and platform set the stage for “on‐demand” realization of nonreciprocal effects, in programmable or reconfigurable fashions, which may find several promising applications, including frequency conversion, Doppler frequency illusion, optical isolation, and unidirectional transmission. Space‐time‐coding digital metasurfaces are designed to break Lorentz reciprocity, via anomalous reflections accompanied by frequency conversions. A proof‐of‐concept validation is provided via measurements at microwave frequencies. This concept sets the stage for “on‐demand” realization of nonreciprocal effects, in programmable or reconfigurable fashions, with possible applications in frequency conversion, Doppler frequency illusion, optical isolation, and unidirectional transmission.