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(0) [Formula: see text] product of the laser intensity I(0) and the laser wavelength λ(0) plays the key role in the energy scaling of strong-field laser-plasma THz generation, the THz output energy, W(THz), does not follow the I(0) [Formula: see text] scaling. Its behavior as a function of I(0) and λ(0) is instead much more complex. Our two- and three-dimensional PIC analysis shows that, for moderate, subrelativistic and weakly relativistic fields, W(THz)(I(0) [Formula: see text] ) can be approximated as (I(0)λ(0)(2))(α), 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(THz)(I(0) [Formula: see text] ) closely follows the scaling dictated by the relativistic electron laser ponderomotive potential [Formula: see text] , converging to W(THz) ∝ [Formula: see text] for very high I(0), 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(e) of hot laser-driven electrons bouncing back and forth between the plasma boundaries displays the same behavior as a function of I(0) and λ(0), altering its scaling from (I(0)λ(0)(2))(α) to that of [Formula: see text] , converging to W(THz) ∝ [Formula: see text] for very high I(0). These findings provide a clear physical picture of THz generation in relativistic and subrelativistic laser plasmas, suggesting the THz yield W(THz) resolved as a function of I(0) and λ(0) as a meaningful measurable that can serve as a probe for the temperature T(e) of hot electrons in a vast class of laser–plasma interactions. Specifically, the α exponent of the best (I(0)λ(0)(2))(α) 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.