Laser-plasma accelerators of only a centimetre’s length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multi-gigaelectronvolt energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.

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The epitaxial growth of silicon films by chemical vapor deposition ͑CVD͒ is strongly affected by temperature and hydrogen ͑H͒ termination. We report measurements of p-polarized optical second-harmonic ͑SH͒ spectra generated in reflection from clean 2ϫ1-reconstructed and H-terminated epitaxial Si͑001͒ surfaces with no intentional doping by Ti:sapphire femtosecond laser pulses for SH photon energies 3.0р2បр3.5 eV near the bulk E 1 resonance. Temperatures were varied from 200 to 900 K and H coverages from 0 to 1.5 monolayers ͑ML͒. Increases in temperature at fixed H-coverage redshift and broaden the E 1 resonance, as observed in linear bulk spectroscopy. Increases in H coverage from 0 to 1 ML at fixed temperature strongly quench, redshift, and distort the lineshape of the E 1 resonance even though reflection high-energy electron diffraction shows that the surface maintains the dimerized 2ϫ1 reconstruction. The latter spectroscopic variations cannot be explained by vertical strain relaxation in the selvedge region, nor by bulk electric-field-induced SH ͑EFISH͒ effects. We instead attribute these variations to a monohydride-induced surface chemical modification, which we parametrize as a surface EFISH effect because submonolayer H strongly alters surface electric fields by redistributing charge from surface dimers into the bulk. The effects of vertical strain relaxation are weakly evident as a blueshift of the E 1 resonance accompanying dihydride termination ͑1.0-1.5 ML͒, which breaks the surface dimer bond. This modification is parametrized as a separate field-independent alteration to the surface dipole susceptibility surface (2) . Finally, guided by these SH spectroscopic studies, we demonstrate dynamic real-time ͑100-ms resolution͒ SH monitoring of H coverage ͑5% accuracy͒ during temperature programmed hydrogen desorption and CVD epitaxial growth of silicon from disilane. ͓S0163-1829͑97͒07144-0͔

We study femtosecond-laser-pulse-induced electron emission from W(100), Al(110), and Ag(111) in the subdamage regime (1-44 mJ/cm 2 fluence) by simultaneously measuring the incident-light reflectivity, total electron yield, and electron-energy distribution curves of the emitted electrons. The total-yield results are compared with a space-charge-limited extension of the Richardson-Dushman equation for short-time-scale thermionic emission and with particle-in-a-cell computer simulations of femtosecond-pulsed-induced thermionic emission. Quantitative agreement between the experimental results and two calculated temperaturedependent yields is obtained and shows that the yield varies linearly with temperature beginning at a threshold electron temperature of -0.25 eV The particle-in-a-cell simulations also reproduce the experimental electronenergy distribution curves. Taken together, the experimental results, the theoretical calculations, and the results of the simulations indicate that thermionic emission from nonequilibrium electron heating provides the dominant source of the emitted electrons. Furthermore, the results demonstrate that a quantitative theory of space-charge-limited femtosecond-pulse-induced electron emission is possible.

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The mechanism of DC-Electric-Field-Induced Second-Harmonic (EFISH) generation at weakly nonlinear buried Si(001)-SiO 2 interfaces is studied experimentally in planar Si(001)-SiO 2 -Cr MOS structures by optical secondharmonic generation (SHG) spectroscopy with a tunable Ti:sapphire femtosecond laser. The spectral dependence of the EFISH contribution near the direct two-photon E 1 transition of silicon is extracted. A systematic phenomenological model of the EFISH phenomenon, including a detailed description of the space charge region (SCR) at the semiconductor-dielectric interface in accumulation, depletion, and inversion regimes, has been developed. The influence of surface quantization effects, interface states, charge traps in the oxide layer, doping concentration and oxide thickness on nonlocal screening * of the DC-electric field and on breaking of inversion symmetry in the SCR is considered. The model describes EFISH generation in the SCR using a Green function formalism which takes into account all retardation and absorption effects of the fundamental and second harmonic (SH) waves, optical interference between field-dependent and field-independent contributions to the SH field and multiple reflection interference in the SiO 2 layer. Good agreement between the phenomenological model and our recent and new EFISH spectroscopic results is demonstrated. Finally, low-frequency electromodulated EFISH is demonstrated as a useful differential spectroscopic technique for studies of the Si-SiO 2 interface in silicon-based MOS structures. PACS numbers: 42.65.Ky, 73.65.Qv, 68.35.-p Optical Second Harmonic Generation (SHG) has been one of the most intensively studied phenomena in surface and interface optics [1-3] for the last decade. The interest in SHG stems from its unique sensitivity to the structural and electronic properties of surfaces and interfaces of centrosymmetric media. This unusually high surface/interface-sensitivity comes about because, in the electric dipole approximation, SHG is forbidden in the bulk of materials with inversion symmetry [4,5], but allowed at interfaces, where inversion symmetry is broken by the discontinuity of crystalline structure. Related nonlinear sources of SHG are localized in a thin (several nanometers thick) surface or interface layer. In semiconductors, inversion symmetry is also broken by the DC-electric Field (DCF) in the subsurface Space Charge Region (SCR), which is created by initial band bending and/or external bias application. The lack of inversion symmetry in the SCR results in DC-Electric-Field Induced Second-Harmonic (EFISH) generation, which manifests itself through electromodulation of the SHG intensity. Thus, all important properties of surfaces, buried interfaces and subsurface layers -their charge [6-8], electronic surface state density [9-11], roughness (morphology) [12,13], adsorption (adatom and admolecule surface density) [14-17], initial band bending [18-20], etc. -can, in principle, be determined by means of the SHG probe. The technological importance of ...