Recently two emerging areas of research, attosecond and nanoscale physics, have started to come together. Attosecond physics deals with phenomena occurring when ultrashort laser pulses, with duration on the femto-and sub-femtosecond time scales, interact with atoms, molecules or solids. The laser-induced electron dynamics occurs natively on a timescale down to a few hundred or even tens of attoseconds (1 attosecond=1 as=10 −18 s), which is comparable with the optical field. For comparison, the revolution of an electron on a 1s orbital of a hydrogen atom is ∼ 152 as. On the other hand, the second branch involves the manipulation and engineering of mesoscopic systems, such as solids, metals and dielectrics, with nanometric precision. Although nano-engineering is a vast and well-established research field on its own, the merger with intense laser physics is relatively recent. In this report on progress we present a comprehensive experimental and theoretical overview of physics that takes place when short and intense laser pulses interact with nanosystems, such as metallic and dielectric nanostructures. In particular we elucidate how the spatially inhomogeneous laser induced fields at a nanometer scale modify the laser-driven electron dynamics. Consequently, this has important impact on pivotal processes such as above-threshold ionization and high-order harmonic generation. The deep understanding of the coupled dynamics between these spatially inhomogeneous fields and matter configures a promising way to new avenues of research and applications. Thanks to the maturity that attosecond physics has reached, together with the tremendous advance in material engineering and manipulation techniques, the age of atto-nano physics has begun, but it is in the initial stage. We present thus some of the open questions, challenges and prospects for experimental confirmation of theoretical predictions, as well as experiments aimed at characterizing the induced fields and the unique electron dynamics initiated by them with high temporal and spatial resolution.
We have observed the production of multi-keV electrons through the irradiation of Xe clusters by an intense, near infrared, femtosecond laser pulse. We find the electron kinetic energy distribution consists of two features, a "warm" peak of between 0.1 and 1 keV and a "hot" peak of energy between 2 and 3 keV. These measurements are consistent with a picture of rapid electron collisional heating in the cluster and exhibit good agreement with numerical modeling of the electron energy distribution.[S0031-9007(96)01420-2] PACS numbers: 36.40.Vz, 33.80.Rv, 36.40.Gk Though the nature of intense, short pulse laser interactions with single atoms and solid targets has been the subject of extensive experimental and theoretical investigation over the last 15 years [1], the dynamics of intense laser interactions with large molecules and atomic clusters has scarcely been studied during this time. The production of highly charged ions from individual atoms through multiphoton [2] and tunnel ionization [3] in a strong field has been thoroughly researched, as have the energy distributions of the electrons produced during these interactions [above threshold ionization (ATI)] [4][5][6]. Concurrently, the production of hot, high density plasmas by the intense irradiation of a solid by a short pulse laser has also been the subject of detailed studies [7,8]. Experiments on individual atoms have confirmed that the majority of electrons released by single atoms in a laser field of intensity ,10 16 W ͞cm 2 typically exhibits kinetic energies of ,100 eV [6]. Interactions with solids, on the other hand, have been shown to be much more efficient at coupling laser energy into electron energy. The electron temperature in these experiments is, however, usually clamped at a few hundred eV due to the conduction of the laser energy into surrounding cold, bulk material [9].Only recently has the nature of intense laser interactions with van der Waals bonded atomic clusters of 20-100 Å been addressed in experiments. These experiments have suggested that the laser-cluster interaction is much more energetic than that of isolated atoms, producing bright xray emission (100-5000 eV photons) when a low density gas containing clusters is illuminated [10][11][12]. The interactions also appear to be quite different than those of laser solid target interactions since a cluster, though like a solid, having high local density and therefore a high collision frequency, is unlike a solid because it is an isolated system, much smaller than a laser wavelength. Consequently, the laser interacts uniformly with all the atoms, much more like the interaction of a laser with a low density gas. Recent experiments by Ditmire et al. have indicated that the electrons in a cluster undergo rapid collisional heating for the short time ͑,1 ps͒ before the cluster disassembles in the laser [11]. These measurements indicated indirect evi-dence for keV electron production in the cluster through time resolved x-ray spectroscopic data. In fact, irradiation of Xe clusters at intensi...
We have studied the high-intensity femtosecond photoionization of inertially confined noble-gas clusters. The explosion of the resulting highly ionized, high-temperature microplasma ejects ions with substantial kinetic energy. We have observed Xe ions with kinetic energy up to 1 MeV and charge states as high as 40 1. This ion energy is over three orders of magnitude higher than has previously been observed in the Coulomb explosion of molecules or clusters of any kind and indicates that there is a fundamental shift in the nature of intense laser-matter interactions between molecules and large clusters.
We have developed and carried out detailed characterization of a cryogenically cooled ͑34-300 K͒, high-pressure ͑55 kTorr͒ solenoid driven pulsed valve that has been used to produce dense jets of atomic clusters for high intensity laser interaction studies. Measurements including Rayleigh scattering and short pulse interferometry show that clusters of controlled size, from a few to Ͼ10 4 atoms/cluster can be produced from a broad range of light and heavy gases, at average atomic densities up to 4ϫ10 19 atoms/cc. Continuous temperature and pressure control of the valve allows us to vary mean cluster size while keeping the average atomic density constant, and we find that many aspects of the valves behavior are consistent with ideal gas laws. However, we also show that effects including the build up of flow on milliseconds time scales, the cooling of gas flowing into the valve, and condensation of gas inside the valve body at temperatures well above the liquefaction point need to be carefully characterized in order to decouple the operation of the jet from the laser interaction physics.
We report on the generation of harmonic radiation (in the 70-90 nm range) from clusters of ∼10 3 Xe atoms formed in a gas jet. We find that the harmonic yield from the clusters exhibits an anomalous cubic scaling with backing pressure to the gas jet. This scaling is consistent with a cluster dipole moment resulting from collective oscillations of electrons around the central ions of the cluster. Using a nanosecond ultraviolet prepulse to dissociate the clusters, we have also attempted to compare harmonic yields from clusters with those produced from monatomic Xe, under otherwise identical conditions. Our results suggest that yields from clusters might exceed those from monomers by up to a factor of five.
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