Remember when “faster than a speeding bullet” was exciting news? At about 2700 mph, the quickest ammo is moving turtle-slow when compared with the rates needed to study physical phenomena at the level of molecules, atoms or even photons themselves.
Researchers around the globe are studying the fundamental processes of the universe, from how biological cells talk to each other to how light interacts with matter. Discovering how the world works at the furthest outskirts of physical reality promises to improve how people interact with it. Ultimately, better medical, industrial and communications technologies – to name a few – are churned out because of novel investigations of ultrafast phenomena.
Focusing on plasma
Point a high-powered laser at glass, metal or any other material, and you will burn away the substance, “instantaneously” ionizing the material. In the process, you will generate plasma, a gaseous blend of ionized particles. Studying the spectra of plasma provides information about its temperature and density as well as about the strength of the laser’s harmonics. Backlighting plasma with a second laser or x-ray source also provides the opportunity to measure its electron density, which, in turn, enables imaging of related ionization fronts and shock waves.
As with basic photography, getting high-resolution images of even fast-moving plasma interactions requires similarly speedy shutter speeds. Unfortunately, no mechanical shutter comes close to being able to achieve the necessary pace.
Electro-optical gates are used as high-speed shutters because they remove the slow mechanical parts from the process. One type, called an optical Kerr gate, comprises a pair of crossed polarizers sandwiching a nonlinear medium. Struck with a high-intensity laser pulse, the nonlinear medium changes polarity briefly, “opening” long enough to allow the probe light through.
Using an optical Kerr gate is the key to ultrafast imaging of laser-induced plasma, according to researchers at Rutherford Appleton Laboratory and at Imperial College London. The CCD used for imaging is placed behind crossed polarizers; aiming intense gating pulses at the nonlinear Kerr medium alternates its birefringent state, permitting a quick shuttering effect. Courtesy of Dan Symes, Rutherford Appleton Laboratory.
Although the Kerr gating effect has been used in spectroscopy, biomedical imaging and other applications, researchers at Rutherford Appleton Laboratory (RAL) in Chilton, UK, and at Imperial College London now have applied it to plasma imaging.
“A current hot topic in high-intensity laser interactions is the capability to accelerate electrons to gigaelectron (GeV) volt energies in a few millimeters of gas,” said RAL’s Dan Symes. According to Symes, lead author of a paper published in the Jan. 4, 2010, issue of Applied Physics Letters, this acceleration occurs in a wake field behind the laser pulse. “The Kerr gated system would allow us to measure properties of this wake field on [a sub-10-fs] timescale.”
Shown are shadowgraphic images of a 100-µm-thick glass target shot with a high-energy laser both without (a) and with (b) the use of an optical Kerr gate. Reprinted with permission of Applied Physics Letters.
Symes and his colleagues tried several candidate materials to act as the Kerr medium, including fused silica, Schott’s NSF-66 glass, lead-bismuth glass and zinc oxide. Each had widely disparate results. The fused silica did well because its nonlinear response was about 1 fs, Symes said. Although the group coaxed gate times of about 100 fs, it still sees room for improvement.
“The crucial property of the material is that its nonlinear response must be effectively instantaneous,” he said. “In that case, the gating time is limited by the laser pulse duration rather than the material.”
Their next step is to improve the dynamic range and to accurately measure the temporal resolution of the system. Optimizing these properties will result in a system that is orders of magnitude better than conventional gated or streak cameras, Symes said.
Supercool imaging
Many high-resolution molecular spectroscopy studies require extremely low sample temperatures. However, many important molecules cannot be prepared with conventional cooling techniques. This catch-22 can be resolved by placing the molecule under investigation in superfluidic nanometer-scale droplets of helium, which can cool substances to 0.4 K yet have only a weak effect on the physical properties of the atoms and molecules that they host.
Not all materials are equal candidates for use as optical Kerr gates. Zinc oxide works fairly well (a), although it absorbs pulse probes around 400 nm. Lead-bismuth glass (b) produced striations in the image, which excludes it for future consideration. Reprinted with permission of Applied Physics Letters.
Furthermore, according to Oliver Gessner of the Ultrafast X-Ray Science Laboratory at Lawrence Berkeley National Laboratory in Berkeley, Calif., helium nanodroplets have been suggested as possible aides in single-shot imaging applications performed at places such as the Linac Coherent Light Source in Menlo Park, Calif.
“Here, the idea is to slow down Coulomb explosion effects in systems that are being exposed to novel ultraintense x-ray laser sources,” Gessner said. Such damage mechanisms are the result of ultrafast charging processes in matter that is subject to intense laser radiation. Slowing these charge build-up processes by embedding molecules in helium nanodroplets is expected to slow the motion of atoms during the exposure time, thus enabling sharper images of microscopic samples in the same way that it is easier to get a crisp photograph of a tortoise than of a hare at full speed.
To visualize the electronic dynamics of helium nanoparticles, he and his colleagues excited helium nanodroplets with 25-fs pulses of extreme-ultraviolet (EUV) radiation produced in a high-order harmonic-generation scheme using a 785-nm driving laser made by Kapteyn-Murnane Laboratories Inc. of Boulder, Colo. Using this ultrafast EUV light source, they excited helium nanodroplets to about 23.8 eV with a single photon. The excitation energy is above the ionization potential of helium nanodroplets, which is about 23.0 eV, but below the ionization potential of single helium atoms, 24.6 eV.
Following each pump pulse, the researchers sent a probe pulse of 1.58 eV. They recorded photoelectrons emitted from the helium clusters with a Dalstar 1M30P CCD camera made by Dalsa Corp. of Billerica, Mass. The pulse-probe scheme provided snapshots of the electronic dynamics in the nanodroplets with femtosecond resolution starting from the moment of excitation up to hundreds of picoseconds later.
“Femtosecond time-resolved photoelectron imaging of helium nanodroplets enables us to simultaneously follow the electronic and nuclear dynamics of electronically excited states of matter in real time,” Gessner said.
Photoelectron images show electronically excited helium nanodroplets (top row) and helium atoms (bottom row). The timescale across the top indicates the pump-probe delay used during imaging. Reprinted with permission of the American Chemical Society.
The indirect photoemission process initiated by the pump pulses generates ultraslow electrons from the helium clusters. The low-energy photoemission is particularly interesting because it is the result of a convoluted relaxation scheme inside the nanodroplets, Gessner explained. “Nobody knows where the energy actually goes and which internal dynamics are associated with this energy redistribution mechanism.”
Gessner’s group, which reported its findings in the Jan. 28, 2010, issue of the Journal of Physical Chemistry A, wants to push the technique further into the realm of soft x-rays. Ultimately moving toward this higher photon energy regime, Gessner said, would let them visualize well-localized core electrons of atoms with a more sophisticated electronic structure than helium, thus gaining atomic specificity and insight into charge-carrier dynamics in photovoltaic devices and transition states in chemical reactions.
Continuing efforts by the teams led by Symes and by Gessner – as well as ultrafast research being performed at other labs – appear certain to run rings around lethargic ammunition.