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 λ_0^2 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 λ_0^2 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 λ_0^2 ) 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 λ_0^2 ) closely follows the scaling dictated by the relativistic electron laser ponderomotive potential ℱ_e , converging to W THz ∝ I_0^1/2λ_0 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 ℱ_e , converging to W THz ∝ I_0^1/2λ_0 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.