Real-Time Observation of Frustrated Ultrafast Recovery from Ionization in Nanostructured SiO2 Using Laser-Driven Accelerators

Ionizing radiation interactions in matter can trigger a cascade of processes that underpin long-lived damage in the medium. To date, however, a lack of suitable methodologies has precluded our ability to understand the role that material nanostructure plays in this cascade. Here, we use transient photoabsorption to track the lifetime of free electrons (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>τ</a:mi><a:mi>c</a:mi></a:msub></a:math>) in bulk and nanostructured <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>SiO</c:mi></c:mrow><c:mrow><c:mn>2</c:mn></c:mrow></c:msub></c:mrow></c:math> (aerogel) irradiated by picosecond-scale (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msup><e:mrow><e:mn>10</e:mn></e:mrow><e:mrow><e:mo>−</e:mo><e:mn>12</e:mn></e:mrow></e:msup><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi mathvariant="normal">s</e:mi></e:mrow></e:math>) bursts of x rays and protons from a laser-driven accelerator. Optical streaking reveals a sharp increase in <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:msub><h:mi>τ</h:mi><h:mi>c</h:mi></h:msub></h:math> from <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mrow><j:mo><</j:mo><j:mn>1</j:mn><j:mtext> </j:mtext><j:mtext> </j:mtext><j:mi>ps</j:mi></j:mrow></j:math> to <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mo>></l:mo><l:mn>50</l:mn><l:mtext> </l:mtext><l:mtext> </l:mtext><l:mi>ps</l:mi></l:math> over a narrow average density (<n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:msub><n:mi>ρ</n:mi><n:mi>av</n:mi></n:msub></n:math>) range spanning the expected phonon-fracton crossover in aerogels. Numerical modeling suggests that this discontinuity can be understood by a quenching of rapid, phonon-assisted recovery in irradiated nanostructured <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:msub><p:mi>SiO</p:mi><p:mn>2</p:mn></p:msub></p:math>. This is shown to lead to an extended period of enhanced energy density in the excited electron population. Overall, these results open a direct route to tracking how low-level processes in complex systems can underpin macroscopically observed phenomena and, importantly, the conditions that permit them to emerge.