Abstract
Powerful coherent emission of broadband few-terahertz radiation can be produced from a laser wake field by linear mode conversion. This occurs when the laser pulse is incident obliquely to the density gradient of inhomogeneous plasmas. The emission spectrum and conversion efficiency predicted by mode conversion theory are in agreement with particle-in-cell simulations. The energy conversion efficiency from laser pulses to this low-frequency emission scales proportional to their frequency ratio by (ω∕ω0)3 and increases with the laser intensity and the plasma density scale length. By adjusting the laser pulse duration and plasma density profiles, one can control the emission frequency, bandwidth, and duration. In two- and three-dimensional geometry, conical wake emission is found in the backward direction when the laser pulse propagates along the density gradient. This can be explained well by the linear mode conversion. To avoid conical emission, a laser pulse incident obliquely to the density gradient can be deployed so that collimated emission becomes dominant in the “specular reflection” direction, suitable for practical applications.
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