Efficient Nonreciprocal Frequency Conversion with Space-Time Josephson Junction Metasurfaces

This paper presents a novel design of a reflective metasurface featuring cascaded space-time-varying Josephson junctions, with the nonlinear inductance of the junctions modulated in both spatial and temporal domains. These innovative metasurfaces transcend the limitations of traditional linear space-time metasurfaces by incorporating nonlinear Josephson junctions, offering a novel platform for wave engineering at the quantum scale. When illuminated by a normal incident wave at frequency ω0, the metasurface efficiently reflects the wave at a specific angle, while simultaneously achieving pure and efficient frequency conversion to ω0 + ωs or ω0 − ωs depending on the modulation frequency (e.g., upconversion from 3 GHz to 11.5 GHz). Unlike conventional frequency converters, this process exhibits amplification (e.g., 4.45 dB), showcasing the high efficiency of the frequency conversion. Spectral analysis reveals spurious-free frequency conversion, with the output transmitted wave devoid of undesirable mixing products and spurious signals. Additionally, the metasurface demonstrates nonreciprocal frequency conversion, with incident waves from the opposite direction failing to undergo frequency conversion due to the unidirectionality of the space-time modulation. Analytical investigations using Bloch-Floquet solutions elucidate the wave scattering behavior from the metasurface, highlighting its high ratio frequency conversion, low return loss, and nonreciprocal characteristics. This work opens up new avenues for the development of advanced metamaterial-based devices with unprecedented performance capabilities, promising significant advancements in quantum engineering systems.