Controllable location-dependent frequency conversion based on space-time transformation optics

Electromagnetic wave frequency conversion is a fundamental technique in telecommunications, especially the conversion from a single-frequency source to a multi-frequency output or a continuous spectra spread throughout space or time. By linking the differential intervals between the physical and virtual space with the ratio of the desired wavelength and the original wavelength, we can derive the mapping between the virtual and physical space-time within the space-time transformation optics (TO). With the functional magneto-electric coupling medium induced from the mapping, we demonstrate a class of converters that can generate arbitrary wavelength and location-dependent frequency distribution on a one-dimensional transmission line. Moreover, we have shown that multiple converters can be deliberately arranged both in the spatial and temporal dimension to achieve long-lasting time-varying frequency and space-time lens that can serve as a compressor and stretcher in chirp pulse amplification. Our concept for wave manipulation based on space-time TO may prepare the ground for a general solution to frequency conversion in various fields.