Pump-probe interferometry of picosecond superradiance under direct resonant excitation

Abstract

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 \(1/{N}_{0}^{0.44}\) , 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.