In the paper "Measurement and control of optical nonlinearities in dispersive dielectric multilayers" published by the AMO Group as part of the attoworld-Team, the scientists report the experimental characterization of an unexpectedly strong nonlinear response in chirped mirrors. At modest peak intensities <2 TW/cm2 - well below the known laser-induced damage threshold of these dielectric structures – the scientists observed a strong reflectivity decrease, local heating, transient spectral modifications, and time-dependent absorption of the incident pulse. Through computational analysis, the researchers found that the incident laser field can be enhanced by an order of magnitude in the dielectric layers of the structure. The field enhancement leads to a wavelength-dependent nonlinear absorption, that shows no signs of cumulative damage before catastrophic failure. The nonlinear absorption is not a simply two-photon process but instead is likely mediated by defects that facilitate two-photon absorption. To mitigate this issue, the team designed and fabricated a dispersive multilayer design that strategically suppresses the field enhancement in the high-index layers, shifting the high-field regions to the larger-bandgap, low-index layers. This strategy significantly increases the maximum peak intensity that the mirror can sustain. However, the finding of an onset of nonlinear absorption even at ‘modest’ fluence and peak intensity has significant implications for numerous past published experimental works employing dispersive mirrors. Additionally, the results will guide future ultrafast experimental work and ultrafast laser design.
Guan Gui et al.:
Optics Express 29, 4947 (2021)