William Payne pulls on his white coat and investigates lasers in the laboratory
Invented in a laboratory, lasers have found some of their most innovative applications in research. That continues today. In physics, chemistry, medicine, climate science, agriculture and astronomy, lasers are constantly finding new applications at the very forefront of research.
Nor is their influence restricted to the hard sciences. Increasingly they are also being applied in areas of the arts. The study of history, archaeology and art history are all beginning to employ lasers. And their use is revealing new truths about familiar objects in our culture, such as the art of the Flemish old masters, and Stonehenge.
Pinpointing Parkinson’s neurodegenerative cascade
Tunable free electron lasers are prompting great interest in the research community. Scientists at Brookhaven Institute are using free electron lasers to develop compact linear accelerators for use in nuclear medicine that are far smaller and cheaper than conventional linear accelerators.
The US Navy’s Free Electron Laser at the Thomas Jefferson National Accelerator Facility is the most powerful tunable laser in the world. At the end of October, the US Navy’s FEL laser broke another record, shining at more than 14kW in the infrared. The US Navy is planning to use its laser for materials, physics and medical research applications. And it also wants to develop ship-based and submarine-based laser weapons.
Researchers at Duke University in North Carolina and the Institute of Biomedical Technologies in Segrate, Italy, are using Duke’s tunable free electron laser to identify the chemistry that underlies the degenerative condition, Parkinson’s disease.
Combined with a photoelectron emission microscope, the researchers have found evidence that a pigment that occurs in human hair may also play a crucial role in tipping the brain into Parkinson’s disease. A dark brown pigment, called neuromelanin, steadily accumulates in the human brain from the age of about five onwards. This pigment was suspected of involvement in Parkinson’s, as brain cells that die in Parkinson’s are heavily pigmented.
The Duke investigations have shown that neuromelanin functions as an anti-oxidant agent, and the brain almost certainly produces it to protect itself from oxidation damage and too high iron levels in the blood. The FEL laser study showed, however, that the structure of neuromelanin is vulnerable to a ‘tipping point’: too much iron and the outer shell of the pigment is stripped away, revealing an inner core that is itself oxidative. The scientists suggest that the structure of neuromelanin, revealed by the FEL analysis, is the key to the start of the neurodegenerative cascade that culminates in Parkinson’s disease.
Ultrafast lasers model microstrokes
Small strokes are poorly understood events. They occur in microscopic blood vessels, causing either blockages or haemorrhaging. Yet researchers believe that an accumulation of small strokes can underlie conditions such as cognitive deterioration and senile dementia.
A team led by Chris Schaffer at the University of California San Diego (UCSD) has applied ultrafast lasers to develop an animal model for small stroke studies.
The UCSD team uses the output of a regenerative amplifier (Coherent Legend-USP) to create ruptures or occlusions in blood vessels at depths up to 500 microns within the rat brain cortex. They simultaneously use an ultrafast oscillator (Coherent Mira-HP) to image the in vivo response using two-photon fluorescence microscopy, at sub-cellular resolution in some cases. Because of the high (80MHz) repetition rate of this oscillator, they can generate these fluorescence images at several frames per second. And because both processes involved multi-photon absorption, which occurs exclusively at the beam focus, these processes have full three-dimensional resolution.
According to Schaffer, the team has been surprised by the dynamic response of the surrounding vessels to these events: ‘We’ve seen some quite surprising observations, such as blood vessels reversing their direction of flow as the vessel network attempts to route blood around the localised disruption.’
The table top particle accelerator
The biggest beasts in the physics world are particle accelerators. These are vital for understanding the fundamental nature of particles, and underpin research into nuclear physics and quantum matter.
But modern particle accelerators are hugely expensive and, well, just huge. The US national particle accelerator at Fermilab in Illinois is four miles in length, while the joint European accelerator at CERN in Switzerland is 17 miles in length. Both cost hundreds of millions of pounds to construct. The next generation, such as the Superconducting Super Collider (SSC) planned for Texas, will be 84km in circumference and cost billions.
The enormous length and size of these accelerators – SSC will be three times larger than Manhattan; CERN is the circumference of a mountain – is the result of the great energies required in modern nuclear research. Photons are accelerated through miles of piping by thousands of superconducting magnets. This makes particle accelerators so expensive that only states of the size and wealth of the United States and the European Union can afford them.
Researchers at Imperial College in London have developed an alternative approach, using lasers, that could bring the ability to conduct fundamental particle physics research to the laboratories of most university physics departments.
Rutherford Appleton Laboratory (RAL) uses a high-power, short-pulse laser system in the laboratory.
The team from Imperial College London together with the Rutherford Appleton Laboratory (RAL) and scientists from the University of Strathclyde and the University of California Los Angeles (UCLA), employed a high-power, short-pulse laser system, demonstrating that beams of electrons could be accelerated directly from a plasma state to energies up to 100MeV, over a distance of only 1mm.
The team used RAL’s short-pulse, high power laser system, ASTRA. The power in the ASTRA 20 terawatt laser is many times the power generation capacity of the UK but the pulse length is only a tiny fraction of a second, about 40 femtoseconds.
For particular plasma densities and laser focusing conditions, the plasma waves produced during the interaction could grow so large that they ‘break’ and inject short bunches of electrons into the adjacent wave. Just like a surfer picking up energy from an ocean wave, the electrons in the laser pick up energy from waves in the plasma.
Laser electron accelerators may offer a cheaper and smaller alternative says Professor Karl Krushelnick of the Department of Physics at Imperial College, who led the joint project. ‘Ultimately our work could lead to the development of an accelerator that scientists could put in a university basement,’ he says. ‘Such a small-scale local facility would give many scientists the ability to run experiments that currently they can only do at national or international centres.
‘Who knows, one day you might even do high-energy physics in a university laboratory. It would be strange but it’s not impossible to imagine.’
Winding the clock back on old masters
All the new applications mentioned so far have been in the sciences. Lasers were invented by physicists. And it is in physics and chemistry that their principal research applications can be found. Applications in the life sciences, medicine and the climate sciences are more recent.
But now, research lasers can now be found even in the arts. In art history, researchers are developing new techniques using lasers to study artists’ original use of colour. And in history, British archaeologists have used lasers to identify and study hidden Bronze Age sculptures embedded into the Stonehenge monument in Wiltshire, while other British archaeologists have pioneered the use of LiDAR laser to study ancient field systems and ecologies in unrivalled detail.
Over hundreds of years, paintings lose their original colours. Layers of varnish and the ageing process over centuries lead to much darker appearances than the artist almost certainly intended. Identifying the original pigments is crucial, both for a better understanding of the artist’s original vision and painting techniques – central to the study of art history – and to improve methods of art restoration and conservation.
A typical old master’s painting consists of a canvas, a layer of animal glue, a primer, a number of layers of paint as the artist built up tone and depth, and finally many layers of varnish added over the years to protect the painting.
In recent years, some attempts at art restoration have resulted in paintings that appear stripped back to the primary underlayer. This includes restoration work on the Sistine Chapel and many Renaissance masterpieces in the London National Gallery and US art galleries.
At present, the best techniques for studying pigments in paintings are mass spectroscopy and chromatography. However, these depend on the careful separation of each layer of paint, which can be as little as tens of micrometres thick, by hand, a difficult and time-consuming task.
Scientists working at the FOM Institute for Atomic and Molecular Physics, part of the University of Amsterdam, have developed a new technique that does not require each individual layer of paint to be isolated. The team, led by Dr Nicolas Wyplosz, has developed a technique based on laser desorption mass spectrometry (LDMS). Laser light is fired at a small cross-sectional area of paint sample. Molecules from the paint sample are vaporised by the laser, and atoms released to the surface. These particles can then be identified by mass spectrometry. Work at the Institute with LDMS has already led to a new understanding of the molecular basis of the ageing process, as well as the identification for the first time of the plant constituents that were used in a number of Renaissance paints.
Discovering the art of Stonehenge
Among other areas of research where photonics is beginning to have an impact is in the rather unusual field of archaeology, where measurement devices are becoming as common among archaeologists as estate agents and builders. The British are among the pioneers in applying lasers in this particular field.
At Stonehenge, Wessex Archaeology, the body responsible for archaeology in the south west of England, has used lasers to carry out the first comprehensive survey of Bronze Age art at the site. What it has revealed is changing our understanding of the purpose and function of Stonehenge in ancient times.
In 1953, a number of carvings were discovered on the side of one of the stones, or sarsens. These appeared to be of axeheads. Although stereoscopic photographs of some of the carvings were taken by a team from the University of London in the 1970s, Stonehenge appears to hold a very large number of these Bronze Age carvings, which have never been fully documented or studied before.
Wessex Archaeology and Glasgow-based archaeological photonics specialist Archaeoptics Ltd used a Minolta VI-900 scanner capable of capturing 300,000 points in three seconds. At Stonehenge, they acquired nine million 3D points on the stones in 30 minutes. They then took two days to create highly accurate 3D models from these points. The raw data captured by the scanner are in the form of ‘point clouds’, unconnected three-dimensional points. To be more useful for visualisation and analysis, these were converted into ‘solid’ surfaces formed from millions of triangles. The models were then manipulated in a software package called Demon developed by Archaeoptics. Various lighting techniques were developed at Wessex Archaeology to further enhance the images.
Study of the sculptures shows that during the Bronze Age, about 1,800 BC, around 500 years after Stonehenge was built, the site became the focus for a cult of, or memorial to, the dead. The axe carvings can clearly be linked with similar carvings found at burial sites in Somerset, Argyllshire and other Bronze Age sites throughout Britain. The original purpose of the site, to mark the winter solstice, may well have been forgotten although commemoration of the winter solstice is associated throughout European mythology with the cult of the dead and ultimate resurrection.
One thing distinguishes the application of lasers in every area of research: everywhere they are used, they reveal new insights in areas that have puzzled or challenged researchers for decades.
Lasers not only open up new areas of research, they are also creating more compact, lower-cost alternatives to conventional approaches and technologies. In research, as in many other fields, lasers are proving to be a truly disruptive technology.