Abstract

Strong-field ultrafast processes can be investigated in nanolocalized electromagnetic fields with relatively low pulse energy with the help of plasmonic nanostructures[1-3]. This effect can be even further enhanced if one uses mid-IR sources to exploit favorable scalings of the Keldysh gamma parameter and the ponderomotive energy. In this work, the nonlinear photoemission processes were studied by propagating surface plasmons generated by 90 fs long laser pulses at 3.1 μm wavelength in Kretschmann configuration (Fig. 1 (a)) Transition between multi-photon-induced and tunneling emission was demonstrated [4] at an unprecedentedly low intensity (< 1 GW/cm² Fig 2 (a)). In addition, spectrally resolved measurements were carried out by a retarding field analyzer setup, and it was possible to generate electron bunches with 10-50 eV maximum kinetic energy during the acceleration process in the field of surface plasmons even at two orders of magnitude lower incident laser intensity than in the previous measurements at 800 nm wavelength [1] (Fig 2 (b)-(c)). Based on these measurements, we could extract the plasmonic field enhancement factor which was around 30.

© 2015 IEEE