Homodyne detection of a two-photon resonance assisted by cooperative emission

Focusing on the transient regime, we explore atomic two-photon spectroscopy with self-aligned homodyne interferometry in a $\Lambda$-system large optical depth. The two light sources at origin of interference are single-photon transmission probe, and slow electromagnetically induced transparency. By switching off probe laser abruptly (flash effect), signal is reinforced by cooperativity, showing enhanced sensitivity to frequency detuning. If periodically switched off, amplitude varies remains even for high modulation frequency. This technique has potential applications sensing, such as magnetometry velocimetry, coherent population trapping clocks.