High-order harmonic generation from a solid-surface plasma by relativistic-intensity sub-100-fs mid-infrared pulses

Area of Science:

• Plasma Physics
• Nonlinear Optics
• Attosecond Science

Background:

• High-order harmonic generation (HHG) in plasmas driven by intense lasers is crucial for attosecond pulse generation and advanced laser-matter interactions.
• Achieving the relativistic regime for HHG typically requires laser intensities around 10^19 W/cm^2, often using near-infrared lasers.
• The relativistic regime of HHG offers unique applications in X-ray generation, particle acceleration, and nonlinear optics.

Purpose of the Study:

• To investigate the feasibility of relativistic HHG at lower laser intensities using mid-infrared laser pulses.
• To explore the impact of longer driver wavelengths on the intensity requirements for relativistic HHG.
• To demonstrate the generation of relativistic HHG signatures with lower-intensity mid-infrared lasers.

Main Methods:

• Utilizing high-peak-power, 80-fs, 3.9-μm mid-infrared laser pulses focused onto a solid surface.
• Achieving laser field intensities in the range of 10^17 W/cm^2, significantly lower than typical relativistic HHG requirements.
• Analyzing the generated high-order harmonics for characteristic relativistic HHG properties.

Main Results:

• Relativistic HHG was successfully observed at laser field intensities of 10^17 W/cm^2 in the mid-infrared.
• The generated harmonics exhibited signature properties of relativistic HHG, including specific beam directionality and extended spectral plateaus.
• A high yield of high-order harmonics was sustained for both transverse electric (p) and transverse magnetic (s) polarized driver fields.

Conclusions:

• Mid-infrared lasers enable relativistic HHG at substantially lower intensities due to the wavelength scaling of relativistic thresholds.
• This finding lowers the experimental requirements for achieving relativistic HHG, broadening access to attosecond science and laser-matter interaction studies.
• The results pave the way for new applications in X-ray generation and laser-driven particle acceleration using more accessible laser systems.