Phys. Rev. X 3, 041009 (2013)

Lasing of optically excited molecular nitrogen ions, which occurs in so-called air lasers, has been studied by an international team of scientists. Haisu Zhang and co-workers have discovered that the stimulated emission of such a system contains periodic modulations in the time domain that are directly related to the coherent rotational wave packets of the molecular nitrogen ions. Indeed, the rotational wave packets of the molecules can be reconstructed in a single-shot manner from the frequency-resolved laser spectrum. The team says that this finding is important for two reasons: it provides new insights into the operation of such air lasers, which are currently not well understood, and it suggests that an air laser is a promising tool for remote characterization and reconstruction of rotational wave packets produced in the atmosphere during the free propagation of intense femtosecond laser pulses.