Strong-field ionization of clusters using two-cycle pulses at 1.8~$\mu$m

The interaction of intense laser pulses with nano-scale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters ($10^3$ to $10^5$ atoms) using two-cycle 1.8~$\mu$m laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3keV is observed using laser pulses with a wavelength of 1.8~$\mu$m and an intensity of $1\times 10^{15}$~W/cm$^2$, whereas only electrons below 500eV are observed at 800nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.