Skin Layer as a Tool for Probing Strongly Absorbing Media

A new look at the formation of the skin layer is suggested. It is based on an analysis of forming the radiation field as a phase transition of the field in the near zone to the field in the far zone. In the near zone, the field is in the quasi-static state, and in the far zone, it is in the form of the travelling wave. The boundary of this transition, called the causal surface, is sufficiently localized and acts as a certain causal surface for the free travelling wave field. The field in the near zone is attenuated with distance by a power law; moreover, this is the case not only for nonabsorbing media, but also for strongly absorbing media. In the far zone, the field attenuation differs. In absorbing media, the field is attenuated fast following an exponential law and gives rise to a skin layer. Thus, the skin layer is formed on the causal surface rather than in the immediate proximity of the source. Manipulation with the position of the skin layer allows absorbing media to be sensed. This conclusion is important for applications, for example, for the development of ultradeep geolocation systems and new non-invasive treatment methods in medicine.