Detection and analysis of low-frequency electromagnetic emissions from ns laser induced breakdown of air

We present the results on the electromagnetic (em) radiation emitted in the 70 MHz - 1 GHz frequency range from the laser induced breakdown of atmospheric air. Laser pulses (7 ns) from second harmonic of an Nd:YAG laser (532 nm) were used to breakdown atmospheric air to form plasma. During the plasma evolution and expansion, dipole moment is induced in the homonuclear molecules of nitrogen and oxygen (the main components of atmospheric air), which naturally have no permanent electron dipole moments. The RF spectra originating from the longitudinal oscillation of these induced dipoles was detected using the RH-799 broadband Diamond antenna. A spectrum analyzer (Agilent PSA E444A, 3 Hz to 50 GHz) was used to monitor and record the RF spectrum from plasma. By tuning the length of the antenna, lines corresponding to the different resonant frequency were observed at different laser energies. The total emitted RF energy was found to be increasing with the input laser energy up to certain input laser energy, beyond which emission properties were modified drastically. This was observed due to the presence of multiple breakdown sources due to the self-focusing of the ns laser pulses, modifying the collisions between the plasma electrons and eventually modifying the induced dipole moment in the detection range. The emitted radiation showed a specific polarization property associated with the input em radiation.