Coherently enhanced microwave pulses from midinfrared-driven laser plasmas

Area of Science:

• Physics
• Optics
• Plasma Physics

Background:

• Ultrafast ionization of gases with ultrashort laser pulses generates ultrabroadband radiation.
• Mid-infrared laser pulses offer unique excitation pathways for gas ionization.

Purpose of the Study:

• To characterize the polarization-resolved spectrum of ultrabroadband radiation from gas ionization.
• To investigate the influence of gas pressure on the generated microwave radiation.
• To understand the underlying physical mechanisms of the observed radiation characteristics.

Main Methods:

• Utilizing ultrashort mid-infrared laser pulses to drive ultrafast ionization in a gas medium.
• Employing multiple complementary detection techniques for comprehensive characterization.
• Performing polarization-resolved measurements as a function of gas pressure.

Main Results:

• The generated radiation spans the entire microwave band, extending into the sub-gigahertz range.
• At low gas pressures, a significant enhancement of the low-frequency portion of the spectrum was observed.
• The radiation developed a well-resolved emission cone with a radially dominated energy flux, indicating build-up of coherence.
• Observed intensity, coherence, and polarization are consistent with Cherenkov-type radiation from plasma currents.

Conclusions:

• Ultrafast ionization driven by mid-infrared lasers is a viable source for generating ultrabroadband microwave radiation.
• The low-frequency enhancement and emission cone formation at low gas pressures are explained by Cherenkov-type radiation mechanisms.
• Ponderomotively driven plasma currents play a crucial role in shaping the characteristics of the emitted microwave radiation.