Anomalous spectral redshift of intense terahertz pulses propagating in weakly ionized gases

Intense laser pulses propagating in weakly ionized gases commonly undergo spectral blueshift due to the self-induced plasma generation. Here, we uncover that intense terahertz (THz) pulses behave differently and experience distinctive spectral redshift during propagation in weakly ionized gases. This anomalous spectral evolution, unlike the Kerr nonlinearity-induced scenario, relies strongly on the electron–neutral collisions. Our theoretical modeling, supported by numerical simulations, attributes the underlying physical cause to the high electron–neutral collision frequencies comparable to the THz frequency. Under the action of strong THz fields, the collision frequency rises faster than the electron density, leading to a temporally increasing refractive index and, consequently, a spectrally redshifted THz pulse. Both unidirectional-pulse-propagation calculations and particle-in-cell simulations are performed to validate this phenomenon and study its dependence on the THz pulse-gas parameters. Our results demonstrate the unique nonlinear propagation dynamics of intense THz pulses in media compared with the conventional near-infrared laser cases and could have important implications for the emerging strong-field THz wave–matter interactions.