Bright–Dark Terahertz Acoustic Bullets

The possibility of forming a bright–dark terahertz acoustic bullet at the second-harmonic generation in a paramagnetic crystal placed in an external magnetic field is investigated. Such a bullet is a narrow beam of a double-frequency elastic strain field of, along which a dark nanosecond-duration spot, where the strain field is absent, propagates at the speed of sound. The important role of temporal dispersion caused by spin–phonon coupling and spatial dispersion caused by the discreteness of the crystal lattice in the formation of the bullet is emphasized. It is shown that the formation of the bullet in the absence of phase- and group-velocity matching is possible in the presence of upper bounds on the time duration of the dark spot and on the transverse size of the acoustic beam.