Enhanced coherent transition radiation from midinfrared-laser-driven microplasmas.

We present a particle-in-cell (PIC) analysis of terahertz (THz) radiation by ultrafast plasma currents driven by relativistic-intensity laser pulses. We show that, while the I [Formula: see text] product of the laser intensity I and the laser wavelength λ plays the key role in the energy scaling of strong-field laser-plasma THz generation, the THz output energy, W, does not follow the I [Formula: see text] scaling. Its behavior as a function of I and λ is instead much more complex. Our two- and three-dimensional PIC analysis shows that, for moderate, subrelativistic and weakly relativistic fields, W(I [Formula: see text]) can be approximated as (Iλ), with a suitable exponent α, as a clear signature of vacuum electron acceleration as a predominant physical mechanism whereby the energy of the laser driver is transferred to THz radiation. For strongly relativistic laser fields, on the other hand, W(I [Formula: see text]) closely follows the scaling dictated by the relativistic electron laser ponderomotive potential [Formula: see text], converging to W ∝ [Formula: see text] for very high I, thus indicating the decisive role of relativistic ponderomotive charge acceleration as a mechanism behind laser-to-THz energy conversion. Analysis of the electron distribution function shows that the temperature T of hot laser-driven electrons bouncing back and forth between the plasma boundaries displays the same behavior as a function of I and λ, altering its scaling from (Iλ) to that of [Formula: see text], converging to W ∝ [Formula: see text] for very high I. These findings provide a clear physical picture of THz generation in relativistic and subrelativistic laser plasmas, suggesting the THz yield W resolved as a function of I and λ as a meaningful measurable that can serve as a probe for the temperature T of hot electrons in a vast class of laser-plasma interactions. Specifically, the α exponent of the best (Iλ) fit of the THz yield suggests a meaningful probe that can help identify the dominant physical mechanisms whereby the energy of the laser field is converted to the energy of plasma electrons.