We study the time structure, the frequency composition, and the shot to shot fluctuations of the radiation emitted by a free-electron laser starting from shot noise in the electron beam longitudinal distribution, taking into account slippage and finite bunch length effects. We find a very different behavior when the bunch length, ${\mathit{scrl}}_{\mathit{b}}$ is much longer thatn the cooperation length, ${\mathit{scrl}}_{\mathit{c}}$, or of the order of a few ${\mathit{scrl}}_{\mathit{c}}$. The field evolution is dominated by slippage effects in both cases, and shows the presence of superradiant spikes.

Dicke superradiance has been observed in many systems and is based on constructive interferences between many scattered waves. The counterpart of this enhanced dynamics, subradiance, is a destructive interference effect leading to the partial trapping of light in the system. In contrast to the robust superradiance, subradiant states are fragile, and spurious decoherence phenomena hitherto obstructed the observation of such metastable states. We show that a dilute cloud of cold atoms is an ideal system to look for subradiance in free space and study various mechanisms to control this subradiance.

We demonstrate analytically and numerically superradiant spiking behavior in the leading and trailing regions of a radiation pulse propagating within a long electron pulse in a single-pass, high-gain free-electron laser (FED). A single superradiant spike is observed when the radiation pulse is shorter than a cooperation length ${\mathit{L}}_{\mathit{c}}$. We show this work may be relevant to the understanding of the spiking behavior in the FEL oscillator, and to possible spiking mechanisms in a perturbed steady-state amplifier.