The collinear generation of radiation with a multicomponent spectrum is obtained upon scattering of picosecond pulses in gaseous hydrogen coherently excited by the pulsed biharmonic pump in resonance with the Raman-active rotational S 0 (1) transition. Probe pulses at 355 nm were almost completely transformed to the radiation with the multicomponent spectrum of width ~5000 cm -1 and the components spaced by 587 cm -1 . The solution of a system of equations for the four-photon Raman — parametric interaction is obtained in the plane-wave dispersionless approximation in the spectral and time representations. The equations describe the transformation of probe pulses in gas with the induced coherence into an arbitrary number of interacting spectral components. The consideration performed in the second order of the dispersion theory provides good agreement with experiments on the transformation into different spectral components depending on the probe-pulse delay.
Observation of pulsed superradiance from two-level systems under direct excitation is challenging because the much stronger excitation pulse overlaps with the emitted radiation in time, space, and spectrum. Previous demonstrations have therefore mostly relied on excitation via auxiliary levels or multiphoton excitation at different wavelengths, limiting applicability. Here we present an interferometric pump-probe technique that enables observation of picosecond superradiance from a moderately-dense-but-optically-thin ensemble of two-level atoms resonantly excited under substantial Doppler and collisional broadening. Sequential pump and probe pulses tuned to the 555.6-nm E1 transition of 174 Yb allowed us to reconstruct the temporal waveform of the superradiant field from time-averaged transverse fluorescence. The fluorescence displayed interference fringes as a function of the pump-probe delay, whose envelope corresponds to the superradiant polarization P ( t ); taking its time derivative yields the radiated electric field. Pulse widths extracted from the reconstructed field follow a power-law scaling of
1/N00.44 , where N 0 is the number of atoms in the interaction volume. The measured temporal dynamics and scaling behavior are in good agreement with simulations based on the coupled Maxwell-Schrödinger equations. This technique provides direct access to ultrafast superradiant dynamics without requiring auxiliary transitions, multiphoton excitation, or ultrafast detectors.