Laser Pulse Driven THz Generation via Resonant Transition Radiation in Inhomogeneous Plasmas

TL;DR

This paper investigates how intense laser pulses generate broadband THz radiation in inhomogeneous plasmas, revealing the effects of boundary sharpness and density ramps through simulations and a plasma resonance model.

Contribution

It introduces a combined simulation and theoretical model to analyze THz generation in inhomogeneous plasmas, highlighting the impact of boundary conditions and density ramps.

Findings

Broadband THz emission originates near plasma density variations.

Sharp boundaries produce symmetric radiation independent of plasma density.

Density ramps can enhance THz energy by up to 50 times.

Abstract

An intense, short laser pulse propagating across a plasma boundary ponderomotively drives THz radiation. Full format PIC simulations and theoretical analysis are conducted to investigate the properties of this radiation. Simulation results show the THz emission originates in regions of varying density and covers a broad spectrum with maximum frequency close to the maximum plasma frequency. In the case of a sharp vacuum-plasma boundary, the radiation is generated symmetrically at the plasma entrance and exit, and its properties are independent of plasma density when the density exceeds a characteristic value determined by the product of the plasma frequency and the laser pulse duration. For a diffuse vacuum-plasma boundary, the emission from the plasma entrance and exit is asymmetric: increasing and decreasing density ramps enhance and diminish the radiated energy respectively. Enhancements by factors of 50 are found and simulations show that a 1.66 J, 50 fs driver pulse can generate ~400 \mu J of THz radiation in a 1.2 mm increasing density ramp. We present a model that attributes this effect to a plasma resonance process in the density ramp. The results from the model match those of the simulations for ramp lengths less than 600 \mu m. For longer ramps for which simulations are too time consuming the model shows that the amount of radiation reaches a maximum at a ramp length determined by collisional absorption.