Ultrabroadband microwave radiation from near- and mid-infrared laser-produced plasmas in air

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.

Figure 1

Experimental setup for measuring microwave radiation from the laser sparks in air. The horn antenna translates angularly around the plasma at a fixed radius. An angular antenna position of $\theta =0^{\circ }$ corresponds to the laser axis.

Figure 2

Time domain voltage waveforms of the microwave fields radiating from the air plasmas at a constant laser pulse energy of 12 mJ. The fields are received with antennas operating at (a) and (b) 2–18 GHz, (c) and (d) 18–40 GHz, (e) and (f) 40–60 GHz, and (g) and (h) 50–70 GHz for (a), (c), (e), and (g) ${\lambda }_0=3.9\mu \mathrm{m}$ and (b), (d), (f), and (h) ${\lambda }_0=0.8\mu \mathrm{m}$. The horns are located at $\theta ={30}^{\circ }$ off the laser axis. The time axis in each plot is based on an arbitrary reference for time $t=0$, as the trigger signal cable lengths are different for both of the lasers, and the microwave cables used with each of the horns also do not all have the same length.

Figure 3

Polar plots of the angular pattern of the radiation based on $V_{pp}$ for voltage waveforms recorded with the (a) 2–18 GHz, (b) 18–40 GHz, (c) 40–60 GHz, and (d) 50–70 GHz antennas. The orange circles and blue squares correspond to ${\lambda }_0=3.9$ and $0.8\mu \mathrm{m}$, respectively, at a laser pulse energy of 2 mJ. The laser propagates from left to right.

Figure 4

Normalized total power integrated from 2 to 70 GHz at each value of laser pulse energy at both laser wavelengths. The position of the antennas in each set of measurements is (a) $\theta ={30}^{\circ }$, (b) $\theta ={60}^{\circ }$, and (c) $\theta ={90}^{\circ }$. The error bars are calculated by finding the standard deviation of each electric field frequency component, adding these in quadrature since each normalized total power data point represents a numerical integration, and dividing by the number of frequency components in the spectrum.

Figure 5

Calibrated spectra of the free-space microwave electric field radiating from the air plasmas as a function of laser pulse energy. The antennas are positioned at $\theta ={90}^{\circ }$ relative to the laser propagation axis. The two columns of spectra correspond to the different lasers with (a) ${\lambda }_0=3.9\mu \mathrm{m}$ and (b) ${\lambda }_0=0.8\mu \mathrm{m}$. The subplots within (a) and (b) are numbered (i)–(v), corresponding to laser pulse energies of 2, 3, 5, 7.5, and 12 mJ, respectively. The spectra in (b) have been truncated at 40 GHz because the noise rises above the signal. The noise amplitude increases linearly with frequency when the calibration factors are applied. The signal-to-noise ratio in (a) is much larger and therefore the spectra do not have this limitation.

Figure 6

Diagram describing the microwave radiation mechanism. The laser pulse propagates from left to right. The top portion of the figure shows the macroscopic picture of the slowly varying laser intensity and plasma properties that produce a current modulation on microwave timescales. The dashed lines indicate what occurs on a microscopic scale in the vicinity of the laser pulse as it propagates. The vertical arrows extending radially from the plasma point in the direction of the radial electron velocity. While it is initially outward, electrostatic attraction from the plasma draws the electrons back, indicated by the change in the arrows' direction. Collisions with neutrals serve as an additional restoring force that ultimately neutralizes the charge imbalance.

Figure 7

Simulated laser fluence and electron density maps for 5-mJ input pulses focused at $f/15$. (a) Fluence and (b) electron density for ${\lambda }_0=0.8\mu \mathrm{m}$ at 50-fs pulse duration. (d) Fluence and (e) electron density for ${\lambda }_0=3.9\mu \mathrm{m}$ at 100-fs pulse duration. Note the difference in the electron density color bar limits for (b) versus (e). (c) Axial lineouts ($r=0$) of the fluence $\mathcal{F}$ and electron density $n_e$ for both laser wavelengths. (f) Radial lineouts at $z=-5\mathrm{mm}$, which is where the plasma has the greatest radial extent. The thin solid and dashed lines correspond to fluence and electron density for ${\lambda }_0=0.8\mu \mathrm{m}$, respectively, while the heavy solid and dashed lines correspond to ${\lambda }_0=3.9\mu \mathrm{m}$. The longitudinal position $z=0$ is at the geometric focal plane of the focusing optic.