Insulating EB with lower doping level shows much stronger IRAV absorption than ES, whose optical response is dominated by the negative real permittivity due to high Fermi velocity and conductivity . The appearance of IRAV peaks is similarly observed in VO2 transition and PANI-CSA ES with even higher DC conductivity, although the trend may vary or even become opposite, as in the case of polythiophenes and polyacetylenes where the dopant-induced vibration has a much higher mobility .