Nonadiabatic Nano-optical Tunneling of Photoelectrons in Plasmonic Near-Fields.

Nonadiabatic nano-optical electron tunneling in the transition region between multiphoton-induced emission and adiabatic tunnel emission is explored in the near-field of plasmonic nanostructures. For Keldysh γ values between ∼1.3 and ∼2.2, measured photoemission spectra show strong-field recollision driven by the nanoscale near-field. At the same time, the photoemission yield shows an intensity scaling with a constant nonlinearity, which is characteristic for multiphoton-induced emission. Our observations in this transition region were well reproduced with the numerical solution of Schrödinger's equation, mimicking the nanoscale geometry of the field. This way, we determined the boundaries and nature of nonadiabatic tunneling photoemission, building on a key advantage of a nanoplasmonic system, namely, that high-field-driven recollision events and their signature in the photoemission spectrum can be observed more efficiently due to significant nanoplasmonic field enhancement factors.

Femtosecond LIPSS on indium-tin-oxide thin films at IR wavelengths.

We investigated laser-induced periodic surface structures (LIPSS) generated on indium-tin-oxide thin films with femtosecond laser pulses in the infrared region. Using pulses between 1.6 and 2.4 µm central wavelengths, we observed robust LIPSS morphologies with a periodicity close to λ/10. Supporting finite-difference time-domain calculations suggests that the surface forms are rooted in the field localization in the surface pits leading to a periodically increased absorption of the laser pulse energy that creates the observed periodic structures.