Anomalous collisionally induced transparency

Whether and how intense laser pulses can pierce optically opaque media remains a fundamental question of interest in laser-matter interactions. Both existing mechanisms enabling laser propagation in overdense plasmas, i.e., relativistic self-induced transparency and electromagnetically induced transparency, require strongly relativistic laser intensities. Here we report on the anomalous transparency of nonrelativistic intense laser pulses through overdense plasmas mixed with sufficiently high-density neutral atoms. Theoretical modeling and particle-in-cell simulations reveal a distinctly different mechanism, collisionally induced transparency (CIT), arising from the suppression of collective directional self-induced currents by the frequent electron-neutral collisions. Parametric study of the laser transparency window illustrates that the long-wave infrared and terahertz pulses are more prone to the CIT effect than the conventional near-infrared pulses. A delayed two-color laser scheme is proposed to validate and boost the CIT phenomenon. As an exotic nonlinear effect, the CIT demonstrated here opens up a different avenue for manipulating the propagation dynamics of long-wavelength strong-field electromagnetic waves and their interactions with weakly ionized media.