The energy of an electromagnetic wave is converted as the wave passes through a temporal boundary. Thus, effective temporal control of the medium is critical for frequency conversion. Here, we propose rapidly time-variant metasurfaces as a frequency-converting platform and experimentally demonstrate their efficacy at terahertz frequencies. The proposed metasurface is designed for the sudden merging of two distinct metallic meta-atoms into a single one upon ultrafast optical excitation. This sudden merging creates a spectrally designed temporal boundary on the metasurface, by which the frequency conversion can be achieved and engineered. Interestingly, the time delay between the abrupt temporal boundary and the input terahertz pulse is found to be strongly related to the phase of the converted wave as well as its amplitude. As the proposed scheme does not rely on the nonlinearity, it may be particularly advantageous for the frequency conversion of waves with weak intensities.

电磁波的能量在通过时间边界时会被转换。因此，有效的介质时间控制对于频率转换至关重要。在这里，我们提出快速时变的超表面作为频率转换平台，并通过实验证明其在太赫兹频率下的功效。提出的超颖表面设计用于在超快光学激发下将两个不同的金属超原子突然合并为一个。这种突然的合并在超颖表面上创建了一个频谱设计的时间边界，通过该边界可以实现和设计频率转换。有趣的是，发现突变的时间边界与输入的太赫兹脉冲之间的时间延迟与转换后的波的相位及其幅度密切相关。