R. I. Tobey

Quasi-phase-matched generation of coherent extreme-ultraviolet light

High-harmonic generation is a well-known method of producing coherent extreme-ultraviolet (EUV) light, with photon energies up to about 0.5 keV (refs 1, 2). This is achieved by focusing a femtosecond laser into a gas, and high harmonics of the fundamental laser frequency are radiated in the forward direction. However, although this process can generate high-energy photons, efficient high-harmonic generation has been demonstrated only for photon energies of the order 50–100 eV (ref. 5). Ionization of the gas prevents the laser and the EUV light from propagating at the same speed, which severely limits the conversion efficiency. Here we report a technique to overcome this problem, and demonstrate quasi-phase-matched frequency conversion of laser light into EUV. Using a modulated hollow-core waveguide to periodically vary the intensity of the laser light driving the conversion, we efficiently generate EUV light even in the presence of substantial ionization. The use of a modulated fibre shifts the energy spectrum of the high-harmonic light to significantly higher photon energies than would otherwise be possible. We expect that this technique could form the basis of coherent EUV sources for advanced lithography and high-resolution imaging applications. In future work, it might also be possible to generate isolated attosecond pulses.

Simplified setup for high-resolution spectroscopy that uses ultrashort pulses

Using a broadband femtosecond laser and a simple optical setup, we demonstrate narrow-bandwidth-tunable excitation of vibrational modes in CCl4 and CHBr3 liquids. The resolution obtained is 80 times higher than the laser bandwidth. A pair of time-shifted, linearly chirped pulses is created by use of a high-order dispersion-compensated prism-interferometer setup. We use this pulse pair to selectively excite Raman-active transitions. Our setup represents a significant simplification with improved resolution, of previous approaches to the use of ultrashort pulses for chemically selective spectroscopy.

Transient grating measurement of surface acoustic waves in thin metal films with extreme ultraviolet radiation

The authors demonstrate the use of coherent extreme ultraviolet (EUV) light in laser induced transient grating spectroscopy. In this work, the authors study the dispersion of pulsed laser induced surface acoustic waves in a thin uniform nickel film by monitoring the diffraction of 30nm EUV light from the surface. Transient deformation of the sample surface leads to absolute diffracted intensities approaching 10−3 for 1A displacement, an ∼700-fold increase in sensitivity compared with optical probing. EUV probing provides a convenient geometry for observing short wavelength acoustic propagation.

Nanoscale photothermal and photoacoustic transients probed with extreme ultraviolet radiation

We demonstrate the use of extreme ultraviolet (EUV) light for time-resolved measurements of the photothermal and photoacoustic response of materials. By using wavelengths that are 20× shorter than visible light, we demonstrate the potential for dramatically increased sensitivity and signal levels, in some cases by almost two orders of magnitude compared with visible light. This approach will enable probing of surface acoustic dynamics on nano-scale spatial dimensions of 50nm and below, using a small scale coherent EUV sources employing high-harmonic generation.

Time-resolved UPS: a new experimental technique for the study of surface chemical reactions on femtosecond time-scales

Abstract Recent progress in the generation of ultrashort XUV-pulses by means of high harmonic generation provides a means to monitor the dynamics of surface chemical reactions using photoemission spectroscopy. In this paper we describe details of an experimental setup for time-resolved photoemission spectroscopy using high harmonic generation. We also present results where the different steps involved in the laser-induced change of an adsorbate–surface bond were monitored with sub-100 femtosecond time-resolution.

Transient 1D holographic detection of surface corrugation with extreme ultraviolet radiation

Using a novel time-resolved holographic geometry and extreme ultraviolet (EUV) radiation, we probe surface displacement dynamics on ultrafast timescales. The use of EUV light allows for surface sensitive probing with sub-Angstrom displacement resolution and sub-100 fs time resolution.

Detection of high frequency acoustic transients using coherent EUV light

Ultrafast Extreme Ultraviolet (EUV) radiation is used to probe transient surface phenomenon in three experimental geometries. Optical irradiation of the sample surface generates thermal and acoustic transients that are subsequently probed with a time-delayed EUV pulse. In all experimental geometries we show excellent signal-to-noise ratios (>10:1) and increased sensitivity to surface deformations (<.02nm) directly attributable to the reduced wavelength of the probing light.

Detection of ultrahigh frequency acoustic transients using coherent EUV light

Ultrahigh-frequency narrow-band surface acoustic waves are detected using diffraction of extreme ultraviolet (EUV) light from a laser-generate transient grating. Initial experiments have probed surface periodicities as small as 750 nm, with corresponding acoustic frequencies of 4.7 GHz.

Ultrashort-pulse EUV and soft x-ray sources based on high-harmonic generation : principles and applications

High harmonic generation (HHG) is a useful source of coherent light in the extreme ultraviolet (EUV) region of the spectrum. However, both the conversion efficiency and the highest achievable photon energy have in the past been limited in the past by the inability to phase-match the frequency conversion process. In this paper, we summarize recent results on the development of new techniques for phase-matching the high-harmonic conversion process. We also summarize finding from three series of experiments that make use of the coherent EUV light generated using HHG: 1) probing of acoustic dynamics in materials; 2) monitoring of chemical dynamics at surfaces using photoelectron spectroscopy; and 3) time-resolved plasma imaging.

Detection of thermal and acoustic transients in materials using heterodyned euv radiation

Nonlinear optical photoacoustic techniques have been extended from the visible to the EUV using high harmonics for the first time. This will enable measurements of thermal and acoustic transients in materials with sub-100 nm spatial resolution