Electromagnetic wave behavior in a conducting medium is a thought-provoking subject for a graduate-level electrodynamics course. Here, we focus on electromagnetic waves incident upon a conductor and highlight how the same dispersion relation, spanning 20 orders of magnitude in frequency, transforms the conductor from a perfectly reflecting to a perfectly transparent medium according to the classical free-electron model. We show that the spectral responses of the conductor can be divided into three radically different regimes. This article presents a graphic illustration of wave reflection, transmission, and penetration properties for copper in these regimes, along with physical interpretations and a brief discussion on the limitations of the free-electron model.
Relativistic electronics research in recent years has produced powerful millimeter waves on the MW level, while also extending the frequency range into the terahertz (THz) region and beyond. These developments have opened up new horizons in applications. The current study is motivated by the associated need for high-power absorbers not readily available at such frequencies. Our focus is on effective absorber schemes which can handle high power while also possessing a structural simplicity for easy implementation. In and above the THz region, the electrical conductivity can no longer be treated as a real constant. We begin with a derivation of the field penetration depth applicable to all frequencies. Requirements to meet the intended criteria are then determined from the wave penetration and reflection properties. Design examples in the 1–1000 GHz range are illustrated, which consist of a thin lossy conducting layer on the surface of a pyramidal shaped metal base. It is shown in theory that such structures can function effectively in the millimeter and THz regions.
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