Dielectric properties at high frequencies

Learning Outcomes & Key Concepts

1. Dielectric Basics

   • Definition: Insulators with bound electrons (no free carriers), polarizable under electric fields.

   • Key Property: High resistivity (10^8–10^18 Ω·cm) vs. conductors (e.g., Cu: 10−6 Ω·cm).

   • Band Structure: Large bandgap (Eg​≫kT) prevents electron conduction.

2. Polarization Mechanisms

   • Electronic: Electron cloud distortion (high frequencies, e.g., UV/visible light).

   • Ionic: Ion displacement (IR frequencies).

   • Orientation (Dipole): Alignment of permanent dipoles (microwave frequencies, e.g., water).

   • Frequency Dependence:

     • εr​ (permittivity) varies with frequency due to polarization lag (Debye relaxation).

   

1. Low Frequencies (Radio/Microwave):

   • All polarization types (orientation, ionic, electronic) contribute.

   • Debye relaxation occurs in polar liquids (e.g., water) as dipoles align with the field.

2. Intermediate Frequencies (Infrared - IR):

   • Ionic polarization stops (ions can’t keep up with rapid field changes).

   • Only electronic polarization remains active.

3. High Frequencies (Optical/UV):

   • Only electronic polarization persists (electrons respond fastest).

   • The dielectric constant drops to match the refractive index squared (n²).