The new millennium witnessed the addition of the word "attosecond" (1 as = 10-18 s) to the vocabulary of physics. Light pulses with attosecond duration are generated either by nonlinear frequency conversion of an ultra-short infrared pump pulse or Fourier synthesis of broad bandwidth radiation, whereas the shortest laser pulses are ~1fs as seen in the time scale to the left. Attosecond light pulses are an important scientific advance because the duration corresponds to the electron motion time-scale in the atomic ground state (atomic unit of time = 24 as). The goal of attosecond science is to observe and control ultra-fast electron motion on the natural atomic timescale.
In order to study events that take place on an attosecond timescale, an attosecond probe is required. One source of an attosecond pulse of light is the process of high harmonic generation (HHG). In the HHG process, infrared light undergoes a non-linear process and is converted to coherent XUV light in the form of attosecond pulses. For this process to occur, an atom must be exposed to extraordinarily strong electric fields, comparable to the binding field strength of electrons to the atom (roughly 10 billion volts per meter). The HHG process is described by a three-step model: photoionization via quantum tunneling, electron wavepacket propagation in the electric field, and recombination when the electron is driven back to the atom by the external field. Each step contributes an amplitude and a phase to the emitted field, and the phase of the attosecond HHG emission from the atoms contains information regarding the underlying electron dynamics.
Many experimental and theoretical investigations have been conducted over the past 30 years that have resulted in a comprehensive understanding of a single atom interacting with an intense laser field. Similar efforts have been invested in the more applicable subject of intense laser-solid interactions but the degree of understanding and control is significantly less due to the complexity associated with this problem. Nano-scale, Van der Waal bonded atomic clusters generated in high-pressure gas jets provide an interesting alternative target for the study of laser-matter interactions. Using clusters, one has an experimental local target that is of solid density (n ~ 1022 cc-1) but exists in an environment of low mean background density (n ~ 1014-17 cc-1). This means that the energy coupling is similar to that in a solid, but without the difficulty of transmitting the light into a bulk target. In addition, it has been theorized that the conversion of the laser field into high harmonics is greater with clusters than with single atoms (typical efficiency ~ 10-7). The clusters are considerably smaller than the laser wavelength, contain a significant number of active electrons at effectively the same position in the laser field, and each of these electron oscillators may coherently contribute to a global cluster dipole, potentially increasing HHG efficiency. This makes clusters of several thousand atoms an exciting nonlinear medium for HHG. Currently we are pursuing research into the fundamental physics associated with laser-cluster interactions and HHG in clusters.