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SPIE Professional October 2013

Technology advances at Central Laser Facility

By John L. Collier and Pavel Matousek

With several of the most advanced laser systems in the world, the UK’s Central Laser Facility (CLF) has a rich history of innovation and technology development, empowering the international community of scientists to help solve major economic and societal challenges.

Its specialized instruments have helped scientists map and analyze cloud chemistry, understand photosynthesis, and non-invasively see through skin and other opaque materials to detect diseases, drugs, and explosives.

The shared-use facility based at the Rutherford Appleton Laboratory near Oxford enables technology transfer through its spinout companies and is also helping to train the next generation of top laser scientists. It is operated by the Science and Technology Facilities Council (STFC).

Now its scientists are leading the international HiPER project to develop new concepts in laser fusion energy and pioneering the technique of Optical Parametric Chirped Pulse Amplification (OPCPA) to design the most powerful and most intense lasers in the world.

CLF director John L. Collier and STFC senior fellow Pavel Matousek give an overview of the facility and its research and development programs.

The Central Laser Facility in the UK houses five laser systems that are regularly accessed by scientists from academia and industry. With a total user community estimated at more than 600 scientists, the CLF provides more than 4400 individual user-access days per year, including 2700 for PhD students.

In the last five years, research conducted at the CLF has produced more than 500 peer-reviewed publications, often in extreme areas of sciences.

The CLF's five major laser facilities enable scientists to study computational plasma physics, experiment with micro-fabrication and laser microscopy, manipulate cells with laser tweezers, and perform other experiments.

Vulcan facility

The Vulcan facility, one of the most intense lasers in the world, has broken records on several occasions for producing the highest optical intensity ever on a target. It is based on versatile Nd:glass chirped-pulse-amplification technology delivering beams into two target areas.

In its petawatt mode, it generates 1 PW (500 J/500 fs) beam with a peak intensity of 1021 Wcm–2. The shots can be fired every 20 minutes. The temporal shape of the long pulse beams can be programmed, and frequency doubling crystals can be used to generate the second harmonic.

The Vulcan facility is used in fusion energy, electron- and ion-acceleration research, laboratory astrophysics and plasma physics research.

Vulcan petawatt laser.
Gemini complements Vulcan

The Gemini facility is a high-power laser based on Ti:sapphire chirped-pulse-amplification technology delivering a unique dual beam combination of 2 x 0.5 PW (30 fs, 15 J) on target at a rate of one shot every 20 seconds. It can deliver peak intensities greater than 1021 Wcm–2 with 10–11 amplified spontaneous emission (ASE) contrast.

The capabilities of the Gemini facility balance and complement those of Vulcan.

The Gemini facility.
Artemis beam lines

Artemis is a high-repetition-rate Ti:sapphire laser system providing a unique combination of ultrafast and XUV pulses for studies of ultrafast dynamics in atomic, molecular, and condensed- matter systems. Coherent XUV pulses are produced through high harmonic generation (HHG) and are synchronized to pump or probe pulses from the UV to the far infrared.

One XUV beam line provides widely tuneable femtosecond narrowband XUV pulses with near transform-limited energy resolution, enabling time- and angle-resolved photoemission measurements from condensed matter with XUV probe pulses. A second broadband XUV beamline is used for HHG spectroscopy, including measurements of attosecond dynamics in molecules.

The Artemis facility.
ULTRA used for dynamic processes

ULTRA is one of world’s most advanced systems for multi-dimensional time-resolved spectroscopy, offering a range of ultrafast vibrational techniques. It operates at 10 kHz with a multi-beam specification, synchronously and independently tuneable from the ultraviolet to the deep infrared, providing fs to ms time-resolved IR, ultrafast 2-dimensional infrared spectroscopy, ps to ms time-resolved 2D IR spectroscopy, and ps time-resolved resonance Raman spectroscopy.

ULTRA can operate on samples in the solution and solid phases and is used for the investigation of dynamic processes in areas ranging from fundamental chemistry to structural biology.

OCTOPUS imaging stations

The OCTOPUS comprises a collection of seven advanced imaging stations driven from a central laser hub with more than 20 specialized laser systems. Any number of these can be simultaneously directed to any of the stations for utmost experimental flexibility.

Advanced image processing algorithms are used to image real systems in real environments in real time, often well below the diffraction limit. Applications are mainly in the biomedical sciences, covering areas such as research into in-cell protein structure, cell signaling networks in health and disease, virus infection, and plant biology.

The end stations include multiphoton confocal, single-photon confocal, single molecule, single-molecule confocal, and super-resolution (PALM and STORM) imaging.

CLF tweezers for healthcare research

The CLF has developed several highly specialized optical traps, or “tweezers,” that are used for holding and manipulating objects such as bacteria, protein crystals, cells, or micron-sized beads.

Their applications include measuring pico-Newton forces within cells either through direct capture or by using laser-controlled nanoprobes in environmental and physical science research.

A new technique for microcrystal manipulation with laser tweezers, for instance, developed by researchers from the Diamond Light Source and the CLF was reported earlier this year and may save precious research days and resources and lead to faster breakthroughs in healthcare.

The novel laser-tweezer process helps to streamline the selection and placement of microscopic protein crystals for crystallographic analysis.

Preparing the crystals can often take a long time, but the new technique allows researchers to select and place the protein crystals on customized sample holders for crystallographic analysis on one of the structural biology beamlines at Diamond Light Source, the UK's national synchrotron science facility.

Studying protein crystals through X-ray crystallography enables researchers to understand the structure and function of a molecule.

Acceleration and ignition experiments

The Vulcan facility has been extensively used to investigate and develop the science underpinning advanced ignition routes towards achieving fusion energy. The achievements include the Fast Ignition concept, as proposed by Ryosuke Kodama and collaborators in 2001.

This technique requires much lower implosion velocities than conventional central hot-spot inertial fusion schemes such as that used at the USA’s National Ignition Facility.

The CLF has also initiated and led the European High Power Laser Energy Research (HiPER) initiative to translate the single-shot proof of principle of inertial confinement fusion into a program of inertial fusion civil energy research, where the process can be repeated many times a second to extract desirable power output at ~1 GW level.

Several breakthroughs in laser-driven plasma- based electron accelerators have also come from CLF, from the first evidence of wave breaking to the acceleration of mono-energetic electrons with energy > 1GeV on Gemini. With the advent of 10 PW lasers, bright mono-energetic electrons with energy >10 GeV promise to become a reality.

Laser-driven proton and ion acceleration experiments are now capable of delivering >50 MeV per nucleon. Work on developing new acceleration mechanisms exhibiting improved beam properties and better energy scaling is currently underway. Potential future applications include proton therapy for cancer treatment.

Intense X-ray sources

The high fields associated with the focused Vulcan and Gemini laser beams have enabled the exploration and evaluation of soft and hard X-ray production physics as sources of secondary radiation. High-harmonic production has been studied on both Vulcan and Gemini using targets ranging from low density gases to solids. A UK team led by Queen’s University of Belfast extended these bright sources into the keV region for the first time using the Vulcan laser.

At high laser intensities and relatively long pulse-duration pulses provided by the Vulcan PW laser facility, scientists reported in 2008 observing that up to 5% of the electron energy in the beam is converted directly into synchrotron X-rays and that the peak brightness of the X-ray beam is 1017 photons/s/mm2/mrad2/0.1% bandwidth witha critical energy of 25 keV, starting to approach the output parameters of second-generation synchrotron sources.

Medical and chemical research

OCTOPUS confocal and super-resolution stations provide information on the interaction of drug and probe molecules with cells and tissues, with applications in medical therapies and diagnostics. Photodynamic therapy is investigated using both ULTRA (chemistry underlying the process) and OCTOPUS (behavior of therapeutic agents in cells and tissues).

OCTOPUS stations are also in use developing techniques to understand, from a systems biology viewpoint, the function of drugs at the molecular level in diseases like cancer, osteoporosis, and neurological disorders. Researchers are also using OCTOPUS to study the structural dynamics of protein folding in Alzheimer’s and motor-neuron disease and to understand how DNA is damaged and repaired.

Researchers at the CLF also invented and support development of Spatially Offset Raman Spectroscopy (SORS), which could probe deep within tissues for noninvasive cancer and bone disease diagnosis.

Companies spin out from CLF

These CLF researchers, along with collaboration with UK industry, government agencies, hospitals, and the defense industry, have played a key role in the formation of about 12 spin out companies.

One company, Cobalt Light Systems, is developing unique products for analyzing chemical materials through translucent layers (bottles, bags, clothes, skin, etc.) using SORS.

Cobalt instruments use SORS technology.

Cobalt Light Systems has developed products and technologies for noninvasive screening of bottles for liquid explosives for aviation security, quality control of pharmaceutical products, and scanning of incoming raw materials noninvasively in pharmaceutical manufacturing.

The CLF’s most recent spinout, Scitech Precision, supplies micro-targets for high-power laser experiments developed originally at CLF’s target fabrication facilities.

IR research on chemical reactions and processes

The ULTRA infrared station is used to investigate chemical reactions in solution on the picosecond and femtosecond timescales. A success reported in 2011 was the elucidation of reactions of CN radicals; comparisons with the gas phase revealed subtle, yet important differences due to interactions with the solvent.

A novel technique for mapping energy transfers within molecules, giving an instantaneous snapshot of electron interactions and types of couplings, was developed on Artemis by Ian Mercer. The technique of angle-resolved coherent (ARC) wave-mixing uses broadband light from a hollow fiber source in a new twist on four-wave mixing.

ARC measurements of a photosynthetic protein from purple algae showed peaks due to energy transfer in and between different pigments in the protein. The work confirmed evidence of strong coherent coupling in the protein and revealed the time-ordering of transition energies.

OPCPA developed in UK

The CLF pioneered the technique of Optical Parametric Chirped Pulse Amplification (OPCPA) for the generation of extreme peak powers and intensities in excess of 1 PW. The concept gets around the issue of limited temporal compressibility of typically spectrally narrow high-power laser beams.

This opens a new route to very high peak-power laser systems (>100 PW). The concept was quickly adopted as the pre-amplifier of choice for high-energy petawatt laser systems around the world.

At CLF, OPCPA is used as the front end for the Vulcan Petawatt beam line and forms the basis of a project to upgrade the Vulcan High Power Laser facility to ~10-20 PW peak power, providing unique access to focused intensities of >1023 Wcm–2.

When completed, this will represent the most powerful and most intense laser in the world. It will be capable of investigating some of the new areas of extreme physics including high-energy density plasma physics promoted to a completely new and uncharted territory where the effects of QED in a plasma environment start to play a significant, if not a dominant, role.

Development of DiPOLE

Development of the next generation of efficient laser sources is key to realizing many of the conceptual applications developed on high-power lasers. Due to the inherent inefficiencies of the existing flashlamp-pumped laser technology, high-energy laser systems (>10 J) are broadly limited to pulse periods of tens of seconds.

In contrast, laser diodes offer high electrical efficiency, a low divergence output giving efficient pump radiation coupling, and an output spectrum which can be tuned to the absorption region of the laser medium. The resulting overall efficiency of such a source is typically greater than 10%, compared to 0.1% for traditional flashlamp-driven systems.

This opens up new opportunities combining high peak power with high average power. In this area, the CLF is developing a system, Diode Pumped Laser for Optical Experiments (DiPOLE) with architecture inherently scalable in single beams to energies beyond 1 kJ at repetition rates above 10 Hz. Such technology promises to open a host of new applications such as security screening of containers, healthcare imaging, and oncology.

John Collier joined the Central Laser Facility at Rutherford Appleton Laboratory in 1995 and has been director since 2009. He has a PhD in laser physics from University of Swansea where he is presently an honorary professor.

Pavel Matousek is an STFC senior fellow at the Central Laser Facility, where he has worked for more than 20 years, primarily in the areas of advanced vibrational spectroscopy, nonlinear optics and high-power lasers. He is a founding director and chief scientific officer of Cobalt Light Systems and a visiting professor at University College London. He has MSc and PhD degrees in physics from Czech Technical University in Prague.

DOI: 10.1117/2.4201310.07

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About the CLF

The Central Laser Facility, which houses several of the most advanced laser systems in the world, is operated by the Science and Technology Facilities Council (STFC) at Rutherford Appleton Laboratory near Oxford, UK.

The national facility was formed in 1975 to provide laser facilities for the benefit of the UK's scientific community, with the user program commencing in 1977.

photo of laserThe CLF employs about 120 staff and its facilities are regularly accessed by scientists from all over the world.

Its scientists lead the High Power laser for Energy Research (HiPER) project and are key players in the European Extreme Light Infrastructure (ELI) program and LaserLab-Europe.

Scientific teams at CLF support both experimental and theoretical research. For instance the plasma physics experiments carried out on Vulcan and Gemini receive strong support from the in-house Plasma Physics group which provides computational and theoretical support to ensure the effective utilization of its facility resources.