Attosecond Technology

Amplification of Impulsively Excited Molecular Rotational Coherence
Molecular phase modulation (MPM) uses the rapid variation of refractive index in an ensemble of coherently vibrating or rotating molecules to spectrally modify radiation, allowing broadband radiation to be generated. Typically, coherent molecular motion is prepared using a rapidly changing pump field, or fields, which drive the dynamics. A key challenge is to control the phase of the molecular dynamics with respect to additional ultrafast optical sources. In a recent Physical Review Letter, researchers at the University of Oxford proposed and demonstrated a solution to this problem. The scheme involves preparation of high-coherence molecular dynamics which are phase-stable with respect to ultrashort pulses.
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Phys. Rev. Lett. 104, 193902 (2010) - Link to article

FROG-CRAB trace measured at Imperial College London.

First isolated attosecond pulses measured in the UK

One of the fastest processes in nature is the motion of an electron in an atom. This occurs on the attosecond timescale (1 attosecond is 10^-18s). In order to "track" electron motion on this timescale, one must controllably produce pulses of light with similar or even shorter duration, and take "snapshots" of the process as it evolves over time - much like a conventional camera but with a shutter speed more than a million billion times faster. The production and measurement of such pulses is notoriously difficult and has only recently become feasible in a few laboratories worldwide. Our recent measurements at Imperial College London are the first in the United Kingdom, adding another powerful attosecond technique to our toolbox of attosecond technology and bringing the UK to the forefront of attosecond metrology.

The scheme used is similar to the pioneering development by Ferenc Krausz and colleagues at the Max Planck Institute, Garching, Germany (work started at the Technical University in Vienna). Laser pulses (central wavelength 800nm) from a commercial femtosecond kHz source are spectrally broadened to cover the visible-infrared spectrum using a differentially-pumped hollow-core fibre developed here at Imperial [Applied Physics B 85,4 (2006): 525-529]. These pulses are compressed to sub-10 femtoseconds (1 femtosecond is 10-15s) duration, near the absolute limit possible at this wavelength. These "few-cycle" pulses are waveform-stabilised through a process known as carrier-envelope phase stabilisation, ensuring that the electric field waveform of each pulse is nearly identical -- a vital requirement for the characterisation and stable generation of isolated attsecond pulses. In order to realize the even shorter (attosecond) bursts of light desired we exploit a process known as high harmonic generation in neon gas, which frequency-multiplies the light into the xuv spectrum. Alone, this technique produces a regular train of pulses, one pulse for each half-cycle of the generating laser waveform. However, through careful spectral filtering of the pulses, using the combination of a zirconium foil and a molybdenum-silicon multilayered mirror we isolate a single attosecond pulse with ~92eV photon energy (wavelength about 13nm).

The production of attosecond pulses is only the first step -- any application necessitates a full characterisation of these pulses. For this we have custom built an attosecond streak camera, used to obtain a FROG-CRAB trace. The isolated attosecond pulse is inherently synchronised with the infrared laser pulse used to generate it. Both pulses are focussed onto another gas jet of neon atoms. A neon atom absorbs the energetic attosecond pulse, knocking free a photo-electron, which carries away the excess energy. Simultaneously, the electric field of the infrared laser pulse pushes the photoelectron towards or away from a time-of-flight detector, thereby streaking the momentum of the photoelectron. This momentum shift depends on the vector potential of the laser pulse at the instant in time it overlaps with the attosecond pulse. By precisely scanning the delay between both pulses the waveform of the laser pulse is mapped out, and more importantly, the full characteristics (phase and amplitude) of the attosecond pulse can be retrieved. Initial analysis indicates an attosecond pulse duration of approximately 280 attoseconds, close to the limit anticipated for the arrangement used.

With a source of isolated attosecond pulses available the next step is to use them in time-resolved studies of atoms, molecules and surfaces. For example, we have just completed the construction of a new surface-science target chamber with early experiments set to look at nano-plasmonic field-enhancements.

Half-cycle cut-offs

We made the first observation of “half-cycle cut-offs” (or HCOs) during high harmonic generation (HHG). HCOs are the highest frequency bursts of radiation emitted each half-cycle of the laser pulse during HHG. They have been theoretically predicted but had not previously been measured. We showed that HCOs are extremely sensitive to the sub-cycle (i.e. attosecond) details of the electric field evolution of the few-cycle driving laser pulse. In particular, the centre frequency of the HCOs was found to be a function of the CEP of the laser pulse, an essential parameter in the characterisation of few-cycle pulses, but notoriously difficult to measure. We used this to develop a completely new measurement technique of the CEP that offers many advantages over the handful of existing measurement techniques. Most notably it can work for much longer pulses (up to 5 cycles) and on a single-shot basis, This result was published in Nature Physics in January 20072, making the front cover of that issue.

Nature Physics 3, p52, Jan 2007

Link to article - Link to News and Views

Fastest measurement of molecular dynamics

The motion of hydrogen and deuterium is captured in the first few femtoseconds after ionisation by an intense laser field, by observation of the high harmonic spectrum emitted on recollision of the ionised electron. This technique allows these first few moments of motion of the molecule to be captured with a resolution of <100 attoseconds.

Science, 312, p424, April 2006

Link to article - Link to Perspectives - Link to BBC website coverage