Highly efficient frequency shifting from temporally modulated Epsilon-Near-Zero surfaces

The dynamical control of material electromagnetic (EM) properties has recently been attracting significant interest. While a static design of the optical properties can provide a complete control of the momentum of a propagating wave, time-dependent materials allow the manipulation of its optical frequency. For instance, suppose a laser pulse incident on the interface between two media where one of them is rapidly changing refractive index in time. The temporal modulation of the boundary between the two media leads to a backward and forward propagating wave with a shifted spectrum [1,2]. This phenomenon, called photon acceleration, has been theoretically investigated in the last century, whilst experimental proof is harder to achieve for homogenous materials or surfaces due to the extremely large and fast changes of the refractive index required.