Electromagnetic emission at the Langmuir frequency from laser interaction with an unmagnetized plasma

We present the first experimental observation of electromagnetic emission at the Langmuir frequency, ωₚ, resulting from laser-plasma interaction in an underdense and unmagnetised plasma. The emission is spatially resolved and analyzed in polarization. It originates from an electrostatic wave excited by the Raman instability. Despite its frequency being very close to the plasma's cutoff frequency, ωₚₑ, this radiation can escape the plasma by following the decreasing slope of the plasma density profile. Radiation at 2ωₚ, is also observed. This experimental platform offers a powerful tool to explore the processes of wave coupling at the origin of some radio solar bursts.