Description
Contemporary high-energy lasers are largely restricted to producing light at a small set of discrete wavelengths for which efficient gain media exist, while conversion mechanisms to non-harmonic frequencies remain intrinsically inefficient. One promising route to overcome these constraints exploits the rapid transformation of neutral gas into plasma, a phenomenon called flash ionization, which has been shown to induce non-harmonic frequency shifts in incident laser pulses [1]. Here, we investigate frequency conversion schemes mediated by a spatiotemporally controlled ionization front generated through a laser-driven ionization scheme. The advancing ionization front produces a refractive-index variation that evolves in both space and time, enabling controlled frequency conversion of a separate probe laser. The probe interacting with this moving front undergoes a frequency shift to a tunable output value, which may be either harmonic or non-harmonic relative to its incident frequency. To quantify this process, we develop an analytical model to determine the output frequency dependence on the post-ionization plasma density and the ionization front velocity. The model predicts that independent control over these parameters enables tunable frequency shifting of the probe laser. We validate these predictions using both one- and two-dimensional particle-in-cell (PIC) simulations, finding good agreement between the analytical model and numerical results across a wide range of parameters. In the simulations, a pump-probe configuration is used, where the velocity of the pump envelope is modified by a flying focus geometry [2], producing a controllable-velocity ionization front that modifies the frequency of the probe. In addition to the frequency shift, we quantify the probe energy and pulse duration after the interaction. The simulations further reveal distinct regimes of frequency-shift behavior, reflecting different ionization-front dynamics and interaction conditions. Together, these results provide new insight into ionization dynamics in intense laser–plasma interactions and demonstrate a versatile mechanism for tunable frequency conversion. This approach offers a potential pathway toward efficient, controllable sources of ultraviolet to extreme-ultraviolet radiation, expanding the accessible parameter space of high-energy laser systems.
References
[1] J. T. Mendonça, Theory of photon acceleration (CRC Press, 2000).
[2] M. V. Ambat, J. L. Shaw, J. J. Pigeon, K. G. Miller, T. T. Simpson, D. H. Froula, and J. P. Palastro, Opt. Express 31, 31354 (2023).