Electromagnetic Metasurfaces. Christophe Caloz

r comma omega right-parenthesis"/> (e.g. lenses and refractors) and, to a lesser extent, (e.g. angularly asymmetric absorbers) and (e.g. diffractionless refractors). It should be emphasized that this rich diversity of space–time variance and dispersion illustrated in Figure 2.1 is restricted to linear metamaterials. As we will see in Section 4.3, the introduction of nonlinearity further increases the number of degrees of freedom for controlling electromagnetic waves.

Throughout the book, we will adopt a particular naming convention to better describe the type of medium we will be dealing with. A medium with , , and will be referred to as a homoanisotropic medium, where “homo” indicates same excitation-to-response effects, i.e. electric-to-electric or magnetic-to-magnetic. A medium with , , and will be referred to as a heteroanisotropic medium, where “hetero” indicates different excitation-to-response effects, i.e. electric-to-magnetic or magnetic-to-electric. Finally, a medium with nonzero susceptibilities from all four tensors will be generally referred to as a bianisotropic medium.

Figure 2.1 Classification of (bianisotropic) metamaterials in terms of their space–time variance and dispersion. The I- and D-notations refer to the inverse and direct space/time domains.

2.2 Temporal Dispersion

Temporal dispersion is the property according to which the response of a medium, at a given instant of time, depends on the excitation at previous times. This can be mathematically expressed by the time-domain counterparts³ of (2.3), as

      (2.5a)

      (2.5b)

      where only the time dependence of the fields is explicitly mentioned and the spatial dependence is omitted for conciseness. These expressions indicate that the material responses, and , are the temporal convolution of the susceptibilities and excitation functions. They may be alternatively written as

(2.6a)

      (2.6b)

Since the convolution in the time domain corresponds to the multiplication in the frequency domain, the time-domain Fourier transform of (2.6) corresponds to (2.3).

      

      2.2.1 Causality and Kramers–Kronig Relations

      We have mentioned that matter is temporally dispersive and shown that the corresponding medium parameters at a given instant of time depend on the excitation at previous times. Combining these facts with the concept of causality allows one to relate the real and imaginary parts of the material parameters to each other. The corresponding relations are referred to as the Kramers–Kronig relations. Given their crucial importance and given their unclear presentation in the literature, we shall now precisely derive them.

      The fundamental tenet of causality is that an effect cannot precede its cause, so that, in particular, matter cannot respond before being excited. Therefore, assuming that a medium starts to be excited at Скачать книгу