Metasurfaces, in microwave and optical domains

The term metasurface refers to electromagnetic or optical structures separating two media, whose thickness is small compared to the working wavelength, typically of the order of a tenth, and whose properties go beyond the conventional interfaces of metals or dielectrics. This concept of metasurface follows on from that of metamaterials. It differs from metamaterials in that the geometrical constraint of the structure's thickness, which is small compared to its wavelength, means that we no longer speak of equivalent optical indexes as in the case of metamaterials, but rather emphasize the transmission, reflection, absorption and polarization transformation properties of these structures. It also differs in the wealth of properties that researchers have attributed to these metasurfaces.

Historically, these metasurfaces were preceded by frequency-selective surfaces in the 1940s, following the invention of radar in the 1930s [E 1 166] , which in turn stemmed from the invention of radio. It could even be said that Marconi's antenna at Poldhu in Cornwall in 1901 was a metasurface made of metal wires. As their name suggests, these frequency-selective surfaces behaved like frequency filters, with electromagnetic properties dependent on the frequency of the incident wave and the wave's polarization. As we shall see, metasurfaces have enriched this concept by extending it to include properties such as polarization transformation, anomalous refraction and holography. As a result, frequency-selective surfaces can be considered metasurfaces ahead of their time.

In the microwave field, the development of metasurfaces is closely linked to the development of radar and telecommunications. In the radar field, they are used to attenuate or modify the radar echo of an object, or of certain elements of that object. In telecommunications, they can be used to focus or deflect one or more beams, to transform the polarization of a wave, etc.

In the optical field, metasurfaces are of more recent invention, typically 2010. Indeed, the constraints on their realization in this field are such that their development has followed the progress of nanotechnologies in terahertz, then in infrared, and finally in the visible [E 6 420]...