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Tuesday, 28 February 2012

Felicity: High-harmonic generation in H2O

We demonstrate high-harmonic generation in H2O using 800 and 1300nm laser pulses up to a maximum intensity of 5×1014W/cm2. Under optimal phase-matching conditions, photon energies up to ~60 and ~87 eV are produced by using 800 and 1300nm light, respectively. The harmonic spectra in H2O, when compared with Xe with a similar ionization potential, exhibit significant extension of the cutoff region, indicating suppression of ionization arising from molecular orbital symmetry.



Tuesday, 21 February 2012

Simon: Controlling the XUV Transparency of Helium Using Two-Pathway Quantum Interference

Atoms irradiated with combined femtosecond laser and extreme ultraviolet (XUV) fields ionize through
multiphoton processes, even when the energy of the XUV photon is below the ionization potential.
However, in the presence of two different XUV photons and an intense laser field, it is possible to induce
full electromagnetic transparency. Taking helium as an example, the laser field modifies its electronic
structure, while the presence of two different XUV photons and the laser field leads to two distinct
ionization pathways that can interfere destructively. This work demonstrates a new approach for coherent
control in a regime of highly excited states and strong optical fields.

sildes here:

Tuesday, 7 February 2012

Richard: Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength1, 2, 3, 4, 5, 6, 7. The introduction of X-ray free-electron lasers8, 9, 10makes it possible to pump new atomic X-ray lasers11, 12, 13 with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon14at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.

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