The Plasma Dynamics of Two Color THz Generation

Broadband THz radiation is a well known product of femtosecond filamentation, with particularly strong THz being driven by two color systems. Many papers describe aspects of this THz radiation, but we show that novel plasma dynamics are key to understanding how the THz is generated. We show that an electromagnetic pulse is generated from the Langmuir waves by inverse mode conversion, and this pulse then stimulates and copropagates with an HE1 surface plasmon polariton (which then detaches from the end of the plasma column and propagates forward). In turn higher frequency THz radiation is also generated within the envelope of the ionizing laser pulse. The evolution of this high frequency THz is well described by a simple linear inhomogeneous wave equation, and for a slowly evolving laser pulse the frequency of this THz grows as the square root of the plasma column length.