Near-field focusing (NFF) holds significant importance for applications such as communication, wireless power transfer, and sensing. Reconfigurable metasurfaces enable real-time NFF, but current methods relying on 1-bit or finite quantization limit performance and efficiency. Here, a space-time-modulated (STM) metasurface is proposed to overcome these limitations, enabling high-quality NFF performance with low cost and reduced spatial requirements. By introducing a new phase term at harmonic frequencies (f_c + kf_o), where f_c = 24.5 GHz is the carrier, and f_o = 100 kHz is the modulation frequency and k is the harmonic order (k = ±1, ±2, ±3…). The STM method provides nearly continuous phase compensation at the +1^st harmonic, resulting in superior NFF performance compared to the fundamental frequency. Furthermore, as the number of meta-atoms reduces, the STM metasurface maintains a stable focusing performance, whereas traditional 1-bit transmission metasurface exhibits elevated sidelobe levels. Consequently, the STM approach provides enhanced NFF performance with fewer meta-atoms, resulting in fewer active components, reduced hardware complexity, and smaller spatial footprints. An 8 × 8 prototype is fabricated and experimentally validated to assess its NFF characteristics. The findings of this research substantiate the proposed technology and lay the groundwork for future applications in compact, high-performance imaging and detection systems.

中文翻译：

---

时空调制超表面支持具有增强相位调制的高性能近场聚焦

近场聚焦 （NFF） 对于通信、无线电力传输和传感等应用具有重要意义。可重构超表面支持实时 NFF，但当前依赖 1 位或有限量化的方法限制了性能和效率。在这里，提出了一种时空调制（STM）超表面来克服这些限制，以低成本和降低空间要求实现高质量的 NFF 性能。通过在谐波频率 （f_c + kf_o） 处引入新的相位项，其中 f_c = 24.5 GHz 是载波，f_o = 100 kHz 是调制频率，k 是谐波阶数 （k = ±1， ±2， ±3...）。STM 方法在 +1 ^次谐波下提供近乎连续的相位补偿，与基频相比，NFF 性能更胜一筹。此外，随着超原子数量的减少，STM 超表面保持稳定的聚焦性能，而传统的 1 位透射超表面则表现出更高的旁瓣水平。因此，STM 方法以更少的元原子提供了增强的 NFF 性能，从而减少了有源组件，降低了硬件复杂性，并缩小了空间足迹。制造了一个 8 × 8 原型并进行了实验验证，以评估其 NFF 特性。这项研究的结果证实了所提出的技术，并为未来在紧凑、高性能成像和检测系统中的应用奠定了基础。