Coherent preparation by laser light of quantum states of atoms and molecules can lead to quantum interference in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to electromagnetically induced transparency and related effects, which have placed gas-phase systems at the center of recent advances in the development of media with radically new optical properties. This article reviews these advances and the new possibilities they offer for nonlinear optics and quantum information science. As a basis for the theory of electromagnetically induced transparency the authors consider the atomic dynamics and the optical response of the medium to a continuous-wave laser. They then discuss pulse propagation and the adiabatic evolution of field-coupled states and show how coherently prepared media can be used to improve frequency conversion in nonlinear optical mixing experiments. The extension of these concepts to very weak optical fields in the few-photon limit is then examined. The review concludes with a discussion of future prospects and potential new applications. CONTENTS

We study numerically the generation of high-order harmonics by two-center molecules for arbitrary angles between the molecular axis and the laser polarization axis. For fixed angle, the harmonic spectrum exhibits a minimum at a frequency which is independent of the laser parameters. The amplitude of each harmonic is strongly angle dependent, and a pronounced minimum is found at the same angle where a sudden jump in the harmonic phase occurs. By calculating the spatial dependence of the harmonic amplitudes and phases, we are able to explain these effects in terms of interfering contributions from various regions within the molecule. DOI: 10.1103/PhysRevLett.88.183903 PACS numbers: 42.65.Ky, 33.80.Rv The interaction of strong laser pulses with atoms or molecules leads to the generation of high-order harmonics [1][2][3][4]. This process is one of the most studied aspects of strong-field physics because it serves as a source of coherent radiation at high frequencies. Nevertheless, most of the experimental and theoretical work has been devoted to atoms, and, in contrast, high-harmonic generation by molecules still leaves open many questions.The production of high harmonics in atoms is well explained by a recollision mechanism [5]: The strong electric field ionizes the atom or molecule and thus creates a free electron. The electron is then accelerated by the oscillating field and may be driven back to the core if the laser pulse is linearly polarized. A high-energy photon is emitted if the recollision with the core leads to recombination. Other than radiation, recollisions can also result in elastic backscattering [6] or double ionization [5]. In the case of atoms, a maximum kinetic energy of 3.17U p for the returning electron is predicted, leading to a cutoff at a photon energy of I p 1 3.17U p . Here, U p E 2 0 ͑͞4v 2 ͒ is the ponderomotive potential in the laser field of amplitude E 0 and frequency v, and I p is the ionization potential. By its simplicity, the recollision picture is extremely useful for purposes of interpretation and will be utilized to explain our results below.Previous theoretical work on two-center molecules has pointed towards interesting effects which are specific to molecules: Early work has demonstrated the importance of charge-resonance effects [7]. Further, an electron can be emitted at one center and recombine at the other center, giving rise to new cutoff laws [8,9]. Also, it was demonstrated that non -Born-Oppenheimer dynamics becomes relevant in the case of oriented isotopically asymmetric molecules [10]. Particularly related to the present work are earlier model calculations which showed that the harmonic yield depends on the orientation of the molecule with respect to the electric field [8,11]. Reference [8] concentrated mainly on internuclear separations above the equilibrium distance. For molecules at their equilibrium distance, Refs. [8,11] compared only between parallel and perpendicular alignment of the molecules relative to the field. Furthermore, the orientation dependence...

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...

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