Abstract
An ultrashort laser pulse focused in air creates a plasma that radiates broadband electromagnetic waves. We experimentally compare the generation of microwaves from plasmas produced with two different laser systems that operate in the near- and mid-infrared regimes. Changing the laser wavelength increases the microwave power by 100 times and changing the input pulse energy allows for tuning of the microwave frequency spectrum, which we absolutely calibrate over a range of 2--70 GHz. The variation of the spectrum with laser pulse energy confirms the existence of a distinct mechanism that generates microwave radiation from laser-produced plasmas in gases. We propose that a radial diffusive expansion wave of the plasma electrons drives a longitudinal current along the plasma surface whose amplitude varies with the total residual electron energy imparted by the laser field and this longitudinal current produces the detected radiation.
Highlights
Ultrashort pulse laser-produced plasmas are versatile sources of secondary radiation, that is, electromagnetic radiation arising from currents driven in the plasma by the laser pulse
This paper reports the results of a systematic experimental study of the microwave generation from air plasma in the spectral range of 2 to 70 GHz
We find the cause of the microwave radiation from the air plasma to be physically dissimilar from that in laser-solid interactions, in both cases the source currents can be considered to arise from the high-energy tails of the respective electron energy distributions in the laser-produced plasmas [28,29]
Summary
Ultrashort pulse laser-produced plasmas are versatile sources of secondary radiation, that is, electromagnetic radiation arising from currents driven in the plasma by the laser pulse. Low-frequency secondary radiation at terahertz (THz) frequencies from plasmas generated in air has been investigated extensively with near-infrared (NIR) laser systems typically using two-color pulses [6,7] and has more recently been measured using mid-infrared (MIR) laser drivers [8,9]. These plasmas radiate at even lower frequencies in the microwave, that is, the gigahertz (GHz) regime [10,11,12,13,14].
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