Speaker
Abstract
Intense, short laser pulses propagating through inhomogeneous plasmas generate terahertz (THz) radiation. We consider the excitation of THz radiation by the interaction between an ultra-short laser pulse and a miniature plasma waveguide. Full format PIC simulations and theoretical analysis are conducted to investigate the slow wave phase matching process enabled by corrugated plasma channels.
Summary
We investigate a mechanism by which an intense, ultrashort laser pulse can ponderomotively drive THz radiation in underdense corrugated plasma waveguides [1, 2]. Such axially periodic plasma waveguides [3] support electromagnetic (EM) channel modes with subluminal phase velocities, thus allows the phasing matching between the generated THz modes and the ponderomotive potential associated with laser pulse, making significant THz generation possible. We find the generated THz is characterized by lateral emission and a coherent, tunable narrow band spectrum.
The THz mechanism is simulated using the full format PIC code TurboWAVE. A range of realistic laser pulse and plasma profile parameters are considered with the goal of increasing the conversion efficiency. At low intensities, the conversion efficiency increases linearly with the intensity consistent with a ponderomotive current drive. However, for high intensities, higher order THz modes are excited by nonlinear currents, which enhances the efficiency scaling. Pulse duration does not affect the THz mode frequency but will determine the amplitude of the driver, thus the enhancement of a specific channel mode can be achieved by using an optimum pulse duration. THz generation resulting from this mechanism generally increases by reducing plasma density to allow better coupling of the radiation in the channel to the outside. It also increases with the channel length and in order to generate significant THz, the laser pulse energy must be efficiently depleted into the corrugated channel. We’ve investigated different channel profiles and pulse parameters to optimize the conversion efficiency of optical energy to THz radiation.
As an example, a fixed driver pulse (1.66 J) with spot size of 15 $\mu$m and pulse duration of 50 fs excites approximately 83.7 $\mu$J of THz radiation in a 500-$\mu$m-long corrugated waveguide with on axis average density of $1.4\times 10^{18}~cm^{-3}$.
[1] T. M. Antonsen, J. P. Palastro, and H. M. Milchberg, Phys. Plasmas 14, 033107 (2007).
[2] A. J. Pearson, J. Palastro, and T. M. Antonsen, Phys. Rev. E 83, 056403 (2011).
[3] B. D. Layer et. al., Phys. Rev. Lett. 99, 035001 (2007).
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