Superradiance triggering spectroscopy

The injection of a weak resonant field on a superradiant sample can induce large changes in the delay and spatial pattern of the emission. By measuring the change in direction of the emission lobes as a function of the injection field frequency, we have recorded the spectrum of the $\text{7P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{?}\text{7S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ superradiant transition in cesium. As expected from theory, only a few photons per pulse are needed to record the spectrum. Various applications to very sensitive spectroscopy and to amplifier technology are discussed.