Space-time metamaterials: from graphene plasmon amplification to the Fresnel drag of light

Abstract
Conventional metamaterials rely on tuning the electromagnetic parameters in space in order to achieve wave phenomena that surpass those found in nature, such as negative refraction. Recently, time has emerged as a new degree of freedom for metamaterials, as periodic variations of the electromagnetic parameters in time as well as in space allows for new pathways in wavecontrol. In this seminar I will discuss two recent topics that have emerged in this context.

I will start by considering modulations of the refractive index in the form of traveling-wave modulations, and introduce the concept of “luminal metamaterial” when such modulation travels at the same speed as the waves propagate in the medium. The breaking of time reversal symmetry allows for non-reciprocal amplification as waves propagate along the space-time modulated medium. I will present an implementation of a luminal metasurface in double-layer graphene, where space-time modulations of graphene’s conductivity yield broadband nonreciprocal graphene plasmon amplification and compression, while circumventing the intrinsic limitations in the modulation speed of its doping level [1]. Next, I will discuss fundamental differences and similarities between space-time modulations and conventional moving media. I will show how relativistic effects such as the Fresnel drag of light characteristic of moving media can also be present when the electromagnetic parameters are modulated in space and time, even if there is no physical motion of the medium [2].

[1] E Galiffi, PA Huidobro, JB Pendry, “Broadband Nonreciprocal Amplification in Luminal Metamaterials”, Physical Review Letters, 123, 206101 (2019).
[2] P.A. Huidobro, E. Galiffi, S. Guenneau, R.V. Craster, and J.B. Pendry, “Fresnel drag in space-time modulated metamaterials” P roceedings of the National Academy of Sciences, doi.org/10.1073/pnas.1915027116 (2019).