Terahertz generation in plasmas using two-color laser pulses

Area of Science:

• Plasma Physics
• Nonlinear Optics
• Terahertz Science

Background:

• Intense, short laser pulses interacting with plasma can generate various forms of radiation.
• Terahertz (THz) radiation, with frequencies typically between 0.1 and 10 THz, has applications in imaging, spectroscopy, and communications.

Purpose of the Study:

• To analyze the generation of terahertz radiation from the nonlinear mixing of fundamental and frequency-doubled laser pulses in plasma.
• To understand the role of plasma fluid equations, including relativistic and ponderomotive effects, and electron collisions in this THz generation mechanism.

Main Methods:

• Solving plasma fluid equations to third order in laser fields.
• Incorporating relativistic and ponderomotive force terms.
• Analyzing the time dependence of the third-order susceptibility based on laser pulse durations.

Main Results:

• Terahertz generation is characterized by a third-order susceptibility dependent on laser pulse durations (picosecond to subpicosecond).
• The terahertz frequency is inversely proportional to the laser pulse duration (approximately 1/tau(L)).
• Relativistic and ponderomotive contributions to susceptibility nearly cancel without collisions, highlighting the critical role of electron collisions.

Conclusions:

• The terahertz field amplitude scales with the square root of the product of fundamental and second harmonic laser intensities (proportional to sqrt(I(1))*sqrt(I(2))).
• Maximum terahertz emission occurs when laser and terahertz polarizations are aligned.
• Emitted terahertz fields can reach tens of kV/cm with durations comparable to laser pulses, and emission direction is sensitive to experimental parameters.