Time reflection and refraction in synthetic frequency dimension

The duality of space and time in Maxwell's equations has prompted interest in time boundaries and the accompanying temporal analog of spatial reflection and refraction. However, achieving observable time-boundary effects at optical frequencies in real materials is challenging. In this Letter, we demonstrate that time reflection and refraction can be observed in a two-band model centered around a nonzero reference energy. Our model can be physically implemented in the synthetic frequency dimension as a system of two coupled dynamically modulated ring resonators. We find that modulation at microwave frequencies is sufficient to observe time-boundary effects for optical waves in synthetic frequency dimension. Our Letter shows that implementing multiband models in synthetic dimensions opens a new avenue for further exploration of time boundaries.