IR Absorption of a Dielectric Matter: Phase Retardation of the Polarization Density

Abstract

In Sect. 1.6, the IR absorption by a condensed matter is discussed in an electrodynamics manner, where only the light “intensity” plays a major role as a result. The physical concept of the vibrational resonance between the dielectric matter and electric field, however, cannot be described appropriately without a concept of “phase.” For example, if a low-frequency electric field oscillation is applied to a matter, the molecules are made oscillated with the same frequency of the oscillation, but the electric energy is not absorbed, i.e., the vibration energy is not transferred to the matter. To understand this mechanism, the phase retardation at the resonance frequency should be discussed. Through this understanding, we will further understand the reason why the optical parameters must be written as “complex.”