Abstract
Space-time-modulated metastructures characterized by spatiotemporally varying properties have recently attracted great interest and become one of the most fascinating and promising research fields. In the meantime, space-time-coding digital metasurfaces with inherently programmable natures emerge as powerful and versatile platforms for implementing the spatiotemporal modulations, which have been successfully realized and used to manipulate the electromagnetic waves in both the spectral and spatial domains. In this article, we systematically introduce the general concepts and working principles of space-time-coding digital metasurfaces and provide a comprehensive survey of recent advances and representative applications in this field. Specifically, we illustrate the examples of complicated wave manipulations, including harmonic beam control and programmable nonreciprocal effect. The fascinating strategy of space-time-coding opens the door to exciting scenarios for information systems, with abundant applications ranging from wireless communications to imaging and radars. We summarize this review by presenting the perspectives on the existing challenges and future directions in this fast-growing research field.
Highlights
Metastructures known as three-dimensional (3D) metamaterials [1,2,3,4,5,6,7,8] and two-dimensional (2D) metasurfaces [9,10,11] are constructed by periodically or aperiodically arranging subwavelength meta-atoms that can be artificially engineered, which have undergone fast developments in the past 20 years and widely been used to control the electromagnetic (EM) waves in extraordinary ways, leading to numerous fascinating phenomena, novel devices, and exciting applications
Referring to [109] for details, the transmitting process mainly contains three steps: firstly, the field-programmable gate array (FPGA) baseband module generates a bit stream of the original information; subsequently, the bit stream is mapped to different sets of timecoding sequences, which can produce the specific harmonic distributions required by the binary frequency shift keying (BFSK) scheme; and the incident wave is modulated by the time-coding sequences and the TDC metasurface transmits the modulated EM waves carrying the digital information
We have summarized the general concepts and working principles of STC digital metasurfaces and presented their recent progress and representative applications, focusing on the harmonic beam steering/shaping, scattering-signature reduction, programmable nonreciprocal effect, arbitrary multibit programmable phases, nonlinear harmonic manipulations, frequency synthesis, polarization conversion, convolution operation, joint multifrequency syntheses, harmonic information transitions, and new architecture wireless communication systems
Summary
Metastructures known as three-dimensional (3D) metamaterials [1,2,3,4,5,6,7,8] and two-dimensional (2D) metasurfaces [9,10,11] are constructed by periodically or aperiodically arranging subwavelength meta-atoms that can be artificially engineered, which have undergone fast developments in the past 20 years and widely been used to control the electromagnetic (EM) waves in extraordinary ways, leading to numerous fascinating phenomena, novel devices, and exciting applications. In the simplest binary case, by constructing two elements with opposite reflection/transmission phase as digital bits “0” and “1,” digital coding metasurfaces can achieve wave manipulations by altering the coding sequences in a discretized manner, which greatly simplify the design, optimization, and fabrication process [58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74] This digital coding representation introduced in such metasurfaces can be regarded as digital modulation with discrete coding sequences, which can be realized in practice with simple hardware and is naturally suitable for integrating active devices such as diodes. We discuss the perspectives on the existing challenges and future research directions of the STC digital metasurfaces in the conclusion part (Section 4)
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