High-intensity lasers incident on a gas of molecular clusters can violently liberate electrons, turning an inert cloud into a plasma. Once ionized, interactions within the cloud, and between the cloud and the peak of the laser pulse, become nonlinear and difficult to study theoretically. The complex dynamics produce secondary radiation that could be harnessed for beamlines or light sources, but the properties of these secondary products are highly sensitive to the initial laser pulse shape and difficult to characterize predictively.

To enhance secondary production in such a system, a team of researchers paired a feedback loop with a genetic algorithm to shape the temporal profile of a laser pulse incident on a gas of argon clusters. The approach, which was reported in Applied Physics Letters, doubled the energy radiated as X-rays by mixing and matching pulse shapes. It also allowed researchers to optimize secondary production without a tedious and potentially fruitless parameter scan.

The team measured the X-ray yields from a set of random pulse shapes, creating new generations by combining the highest-yield shapes from earlier cohorts. An acousto-optic modulator imprinted each shape onto a multi-terawatt laser pulse, and the process was repeated for nine generations until the pulse converged on an optimal X-ray yield.

The shape of the optimized pulse had a slow leading rise that researchers suggest may expand the plasma and allow it to couple most strongly to the pulse at its peak power. Combining the technique with spatial feedback methods could eventually enable beam sources that can be continually monitored for aberrations and nudged back within specified bounds.

Source: “Temporal feedback control of high-intensity laser pulses to optimize ultrafast heating of atomic clusters,” by M. J. V. Streeter, S. J. D. Dann, J. D. E. Scott, C. D. Baird, C. D. Murphy, S. Eardley, R. A. Smith, S. Rozario, J.-N. Gruse, S. P. D. Mangles, Z. Najmudin, S. Tata, M. Krishnamurthy, S. V. Rahul, D. Harza, P. Pourmoussavi, J. Osterhoff, J. Hah, N. Bourgeois, C. Thornton, C. D. Gregory, C. J. Hooker, O. Chekhlov, S. J. Hawkes, B. Parry, V. A. Marshall, Y. Tang, E. Springate, P. P. Rajeev, A. G. R. Thomas, and D. R. Symes, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5027297.