We report the direct observation of broadband negative optical-phase accumulation in an ultrathin carbon-nanotube (CNT) film, resulting in an apparent superluminal advance of a picosecond electric field pulse. Measurements of terahertz pulse transmission through the CNT film, relative to an open aperture, revealed an earlier arrival of the THz pulse by 40 fs, the spectrum of which was quantitatively reproduced by a Drude model with only a single adjustable parameter—the carrier momentum relaxation time. Negative phase shift observed across a broad spectral range of the THz pulse was actively controlled under optical excitation, as shown by a change in the signal magnitude using a modest 500 mW laser beam illumination of the CNT film. Because the observed effect depends solely on the ratio of this relaxation rate to the radiation frequency, any conductive layer with comparable scattering dynamics will exhibit the same negative phase shift, arising from a negative group velocity without violating causality. Demonstrating negative phase accumulation and its optical tunability in a nanoscale conductive carbon film provides a route to THz pulse phase engineering, offering a promising basis for ultra-high-speed phase and intensity modulators in next-generation photonic and optoelectronic systems.
Gorbun et al. (Wed,) studied this question.