Femtosecond Laser Processing of Materials at Tens-of-NM

Ultra-short sub-1 ps <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(&lt; 10^{-12}\mathrm{s})$</tex> pulses focused into a focal spot with a diameter comparable with the wavelength <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$d\sim\lambda\approx 1\ \mu \mathrm{m}$</tex> can create extreme pressure/temperature conditions inside transparent materials within the focal volume. Material at the focal region undergoes highly localised ionisation and is transferred into the plasma state [1] at typical pulse intensity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10^{13}-10^{14}\mathrm{W}/\text{cm}^{2}$</tex>; the light-induced damage threshold of wide bandgap transparent materials is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 10\ \text{TW}/\text{cm}^{2}$</tex>. The dielectric breakdown is driven by a highly nonlinear absorption (multi-photon, avalanche, tunneling) leading to the energy deposition into the skin depth of tens-of-nm of absorptive plasma (converted from a solid-state dielectric). The energy deposited into a small volume creates a high pressure by definition: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{J}/\mathrm{m}^{3}\equiv \mathrm{N}/\mathrm{m}^{2}$</tex>, i.e., Pa in SI units. The transient pressures estimated from energy deposition could reach 1 TPa (10 Megabar) and were consistent with a nano-void (empty volume) formation inside crystalline sapphire [1].