For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10(-15)-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10(-18)-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H(2) and D(2) was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump-probe experiment based on that work: H(2) and D(2) are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.

We report reproducible fabrication of InP-InAsP nanowire light emitting diodes in which electron-hole recombination is restricted to a quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the quantum dot with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically-driven quantum optics experiments. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for single photon applications.Nanowire light emitting diodes (NW LEDs) offer exciting new possibilities for opto-electronic devices. Growth of direct-bandgap NWs on Si 1, 2 will allow optically active elements to be integrated with already highly mature Si technology. For solid-state lighting applications, broad-area LEDs made from NW arrays have higher light-extraction efficiency than traditional planar LEDs 3 , and in the field of quantum optics, NWs offer the possibility to control electron transport at the single-electron level 4 and light emission at the single-photon level 5 .1Since the first demonstration of GaAs NW LEDs in 1992 6 , different geometries and materials have been used to produce NW LEDs operating over a wide range of wavelengths 3, 7-10 . Single-NW LEDs with doping modulation in the axial direction, which is the most interesting geometry for many applications, have been fabricated using GaN-GaInN multi-junctions 3 and a proof-of-principle device has been shown using InP 7 . In this letter we describe the fabrication and characterization of reproducible axial InP NW LED devices, and show that an active InAsP quantum dot region can be incorporated into these devices. The axial geometry allows for controllable injection of electrons and holes into the precisely defined active region, with the additional advantage of high light-extraction efficiency since the optically active region is not embedded in a high refractive index material. Unlike GaInN, InAsP emission can be tuned to infra-red telecommunications wavelengths where there is strong interest in electrically driven single-photon sources 11 .Nanowire p-n junctions were reproducibly grown in the vapor-liquid-solid (VLS) growth mode 12 by use of low-pressure metal-organic vapour-phase epitaxy (MOVPE). 20 nm colloidal Au particles were dispersed on (111)B InP substrates, after which the samples were transferred to a MOVPE system (Aixtron 200), and placed on a RF-heated gas foil rotated graphite disc on a graphite susceptor. The samples were heated to a growth temperature of 420 °C under phosphine (PH 3 ) containing ambient at molar fraction χ PH3 = 8.3×10 -3 , using hydrogen as carrier gas (6 l/min H 2 at 50 mbar). After a 30 s temperature stabilization step, the NW growth was initiated by introducing trimethyl-indium (TMI) into the reactor cell at a molar fraction of χ TMI = 2.2×10 -5 . During the first 20 minutes, hydrogen sulfide (χ H2S = 1.7×10 -6 ) was used for n-type doping, after which the p-type NW part was g...

We report experiments where hydrogen molecules were dissociatively ionized by an attosecond pulse train in the presence of a near-infrared field. Fragment ion yields from distinguishable ionization channels oscillate with a period that is half the optical cycle of the IR field. For molecules aligned parallel to the laser polarization axis, the oscillations are reproduced in two-electron quantum simulations, and can be explained in terms of an interference between ionization pathways that involve different harmonic orders and a laser-induced coupling between the 1s g and 2p u states of the molecular ion. This leads to a situation where the ionization probability is sensitive to the instantaneous polarization of the molecule by the IR electric field and demonstrates that we have probed the IR-induced electron dynamics with attosecond pulses. The prospect of observing and controlling ultrafast electron dynamics in molecular systems is the basis of the current interest to apply attosecond (1 as ¼ 10 À18 s) laser pulses to physical chemistry. Since the first demonstration of attosecond pulses [1,2], pioneering experiments have demonstrated their potential in atoms [3,4], solid state systems [5], and, most recently, molecules [6], where interest has been stimulated by numerical studies which suggest that an electronic (i.e., attosecond or fewfemtosecond) time scale may be important in fundamental chemical processes [7,8]. The inherent multielectron nature of the electron dynamics in many molecular systems is a formidable challenge to theoreticians and experimentalists alike, and requires the development of novel theoretical and experimental techniques.Attosecond pump-probe spectroscopy is based on the generation of attosecond light pulses by high harmonic generation. Presently, attosecond pulses exist as attosecond pulse trains (APTs) [1] and as isolated attosecond pulses [2]. The first application of attosecond pulses to follow rapid electron dynamics in a molecule revealed that the dissociative ionization of hydrogen by a two-color extreme-ultraviolet ðXUVÞ þ IR field results in a localization of the bound electron in the molecular ion that depends with attosecond time resolution on the time delay between the attosecond XUV pulse and the IR laser pulse [6]. This could be observed via an asymmetry of the ejected fragments in the laboratory frame, i.e., after the dissociation was complete [9]. A similar experimental result was also obtained using an APT [10]. In these experiments the attosecond pulses initiated electron dynamics that was subsequently addressed by an IR pulse. A next challenge is to use attosecond pulses as a probe of ultrafast molecular electron dynamics. In this Letter we do so by investigating how a moderately intense IR field influences the electronic states that are accessed in photoionization of hydrogen using an APT.In the experiment, an XUV APT (with two pulses per IR cycle) and a 30 fs FWHM 780 nm (IR) pulse (3 Â 10 13 W=cm 2 ) with identical linear polarization were collinearly propagated and...

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.

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