LASER PULSE DRIVEN TERAHERTZ (THz) GENERATION IN INHOMOGENEOUS PLASMAS
Antonsen, Thomas M.
MetadataShow full item record
Intense, short laser pulses propagating through inhomogeneous plasmas can ponderomotively drive terahertz (THz) radiation. Theoretical analysis and full format PIC simulations are conducted to investigate two mechanisms of laser pulse driven terahertz generation: (i) a resonant transition radiation (RTR) mechanism occurring as a pulse crosses a plasma boundary and (ii) a slow wave phase matching mechanism (SWPM) that occurs in corrugated plasma channels. These studies are the first to investigate ponderomotively driven THz self-consistently in the interesting situations in which the interaction occurs over a scale many wavelengths long. For the resonant transition radiation mechanism, both theory and simulation results show the conical 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 a factor of 50 are found, and simulations show that a 1.66 J, 50 fs driver pulse can generate ~400 μJ 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 μm. 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. For the slow wave phase matching mechanism, excitation of terahertz radiation by the interaction of an ultra-short laser pulse and the fields of a miniature, corrugated plasma waveguide is considered. Plasma structures of this type have been realized experimentally, and they can support electromagnetic (EM) channel modes with properties that allow for radiation generation. In particular, the modes have subluminal field components, thus allowing phase matching between the generated THz modes and the ponderomotive potential of the laser pulse. Theoretical analysis and full format PIC simulations are conducted. We find THz generated by this slow wave phase matching mechanism is characterized by lateral emission and a coherent, narrow band, tunable spectrum with relatively high power and conversion efficiency. We investigated two different types of channels, and a range of realistic laser pulses and plasma profile parameters were considered with the goal of increasing the conversion of optical energy to THz radiation. We find high laser intensities strongly modify the THz spectrum by exciting higher order channel modes. Enhancement of a specific channel mode can be realized by using an optimum pulse duration and plasma density. As an example, simulation results show a fixed driver pulse (0.55 J) with spot size of 15 μm and pulse duration of 15 fs excites approximately 37.8 mJ of THz radiation in a 1.5 cm corrugated plasma waveguide with on axis average density of 1.4 × 10^18 cm^−3, conversion efficiency exceeding 8% can be achieved in this case.