Mid IR single-cycle light bullet self-reconstruction after an air gap under single-pulse femtosecond filamentation in LiF

We have studied the penetration of the extremely compressed wave packet—“light bullet” (LB)—through an air gap upon femtosecond single-pulse mid-IR filamentation in LiF. Applying the laser coloration method and performing numerical simulations, we have found that the single-cycle LB, which is formed before the air gap up to 0.5 mm wide, completely recovers after passing a certain distance inside LiF after the gap. This distance demonstrates nonlinear dependence on the gap width and LB pathway before the gap. We conclude that the observed self-reconstruction of the LB is caused by the high localization of the light field in the air gap due to strongly convergent wave front of the bullet, while the effect of the surrounding low-energy background of the pulse can be neglected.