杏吧原创

Oxfordshire

IMAGINE the combined intensity of every star in our Galaxy focused onto a
spot the size of a pinhead. Dazzling, isn鈥檛 it? You may think that so much light
only exists close to the brightest stars. But for a few picoseconds at a time,
an even brighter light can be found nearer home. Look no farther than
Oxfordshire, home to Vulcan, one of the world鈥檚 biggest and most powerful
lasers.

Vulcan produces light in only a narrow part of the visible and near-visible
spectrum. But researchers at the Rutherford Appleton Laboratory (RAL) in
Oxfordshire have recently gone one better. They have used Vulcan to power an
X-ray laser that may soon be able to peer into living cells and take snapshots
of life鈥檚 most important molecules in action.

To help biologists capture such vital images, any old X-rays will not do.
They must have a wavelength of between 2.2 and 4.4 nanometres, which allows them
to pass unhindered through water, but be absorbed by carbon atoms. This
important part of the X-ray spectrum is called the 鈥渨ater window鈥�.

But why the fuss? After all, scientists have been creating X-rays with these
wavelengths for years. The problem is that most of today鈥檚 X-ray sources are so
dim that molecules must be specially prepared and held completely stationary so
that they do not move during the exposure鈥攁 bit like the subjects of early
Victorian photographs. Attempts to study living processes with conventional
X-rays reveal only a murky world of blurred images or artificially frozen
molecules.

But the pulse from a very bright X-ray laser can freeze the motion of
molecules, rather like a flash gun in conventional photography. With a series of
such images, it will even be possible to make 鈥渕olecular movies鈥� of things like
DNA and proteins at work. And as well as giving 2D images, X-ray lasers will
make holographic images possible, yielding molecular activity in 3D. At RAL, a
physicist, Jie Zhang, and his colleagues from the UK X-ray consortium are
tantalisingly close to this goal.

X-ray lasers amplify light in much the same way as conventional lasers,
although differences between the two systems pose enormous challenges for
Zhang鈥檚 team. At the heart of a laser is a material which can absorb energy and
release it in the form of light. The energy can be supplied in the form of heat
or light from flash tubes or even by other lasers such as Vulcan. On the atomic
scale, this input energy, or pumping energy as it is also called, excites
electrons to leap from lower atomic energy levels to higher ones. When these
electrons drop back to the lower level, they give up the energy in the form of
light, and since the drop is the same for every electron, all the photons
produced have the same wavelength.

There is also another phenomenon at work which ensures that all the photons
are in step, or coherent, to use the language of physicists. Left to their own
devices, the electrons would drop back at random, creating photons that are out
of step. But if a photon hits an excited atom, it triggers the emission of
another photon in step with the first.

This process is known as stimulated emission and both photons can go on to
stimulate other emissions. In fact, in certain special conditions, a chain
reaction occurs in which the number of photons increases exponentially. The
process is called light amplification by the stimulated emission of radiation, a
phrase better known by its acronym鈥攍aser.

Ruined barbecue

Creating these special conditions is relatively straightforward in lasers
that produce visible and near-visible light. But at the shorter and more
energetic X-ray wavelengths it has proved well nigh impossible. Until recently,
that is.

A laser鈥檚 wavelength is determined by the energy difference between the upper
and lower energy levels of the lasing medium. Many substances will lase in the
visible and near-visible regions, but producing more energetic photons requires
atoms in which the electrons can make much larger jumps. Since X-rays in the
water window are highly energetic, that jump must be huge. In fact, the only
substances that can do the job are plasmas in which heavy atoms have been almost
entirely stripped of their electrons. Only then can electrons make the large
jumps necessary to create X-rays.

In conventional lasers, the light is reflected back and forth through the
lasing medium by a pair of mirrors. Each time the pulse passes through the
medium, it is amplified by a small amount and over many passes the amplification
can be vast. Usually, one of the mirrors is semi-silvered, so that a portion
of the light can escape to create the laser beam.

But mirrors that can do this job for X-rays simply do not exist, since the
radiation passes straight through most materials. For X-ray lasers the necessary
amplification must take place in a single pass.

Creating these plasmas is difficult but it is only the beginning of the
problems. The laser will work only if the stimulated emission leads to a self
sustaining chain reaction. This process of amplification occurs only above a
certain threshold point that depends on a number of factors, such as the
temperature and density of the plasma as well as the number of photons passing
through it. Below this threshold the chain reaction simply fizzles out, like a
barbecue in the rain.

But simply achieving amplification is not enough. An X-ray laser can only be
useful if the photons are amplified a million fold, says Zhang. And this only
happens when the energy extracted from the lasing medium reaches a
maximum鈥攁 condition known as saturation. 鈥淪aturation is one of the most
important things for X-ray lasers,鈥� says Zhang. 鈥淚f an X-ray laser doesn鈥檛 reach
saturation it is not really usable. The energy is too low.鈥�

Saturation was the major stumbling block for all earlier attempts to create
an X-ray laser. The first were made more than ten years ago at the Lawrence
Livermore Laboratory in California. This is the home of Nova, the world鈥檚
biggest laser, which can pack tens of thousands of joules of energy into a
single punch.

The original experiments used Nova to zap a thin heavy-metal foil to create a
plasma of highly stripped ions which would then generate X-rays. But the density
of the plasma varied enormously over very short distances. These density
gradients acted like a lens to steer incoming laser light out of the plasm
before saturation could occur.

Until last year, the best that physicists at Nova could achieve was saturated
operation at 15.5 nanometres, way above the water window. At the time,
scientists had no hope of improving on this disappointing performance. They
calculated that it might be possible to pump enough energy into the plasma to
create saturation, but only with a beam far more powerful than Nova or any other
facility could produce.

Then came a new technique which had been under development by physicists at
Livermore since 1993. They used two pulses, the first to create the plasma and
the second to stimulate emission. With this technique they discovered that many
new elements would lase. 鈥淥nce we developed the pre-pulse technique, we quickly
showed that titanium worked, chrome worked, and iron worked鈥� everything
between silicon and germanium,鈥� says Joe Nilsen, a physicist at Livermore who
led the work. Nevertheless, the all-important saturation still eluded them.

Zhang鈥檚 team at RAL developed this technique still further. Zhang believed
that the interval between the pulses played a critical role. He reasoned that it
allowed the plasma to cool down and to expand. These effects are useful because
a cooler plasma absorbs energy more efficiently while the expansion allows the
density to become more uniform over longer distances. So he increased the
interval to several nanoseconds, much longer than the separation used by other
groups.

And instead of zapping his target material with two high power pulses, he
worked out that a gentle pre-pulse of about 10 joules could create the plasma
and be followed by a stronger pulse of 100 joules to suddenly heat the plasma.
Highly amplified X-ray lasing should then begin.

Earlier last year, the RAL team tried out this idea for the first time. The
results were dramatic. On the first attempt, they smashed the existing
wavelength record with a beam of X-rays with a wavelength of 14 nanometres.
Beams of 12.6 and 12 nanometres quickly followed All this with a beam energy of
only 100 joules鈥攁 power that is well within the range of many medium sized
laboratory lasers. Researchers had once believed that only the world鈥檚 largest
lasers would be able to power X-ray lasers. 鈥淚 think there will be tremendous
progress, because Zhang鈥檚 experiment has proved that the saturated X-ray laser
is really becoming accessible to smaller facilities,鈥� says Nilsen.

Christmas gift

X-ray lasers are already finding applications, even before the development of
those that fall within the water window. 鈥淭aking the X-ray laser and developing
standard optical techniques such as interferometry and holography opens up a
whole new world,鈥� says Nilsen. For example, laser scientists at Livermore are
using beams with a wavelength of between 10 and 20 nanometres to penetrate the
plasmas created during fusion reactions. Longer wavelengths simply cannot
penetrate as deeply. The team is eagerly awaiting experiments with even shorter
wavelengths that will be able to peer into these plasmas with greater clarity
and depth. Fusion reactions are desperately complex, and researchers want to
understand how they work in the generation of fusion power and the detonation of
nuclear weapons.

But the real goal is the water window. Since the first attempt, the RAL team
has tried heavier elements in an attempt to create more energetic rays. Last
December, they used samarium to create X-rays with a wavelength of 7 nanometres.
And with tantalum, they have calculated that they should achieve the 4.4
nanometres goal, possibly by the end of this year. 鈥淲e are all very excited, but
the most exciting thing is the application of the technology,鈥� says Zhang.

At 4.4 nanometres, the tantalum laser promises to be a gift for biologists.
X-rays are already used extensively in materials analysis because they are
scattered or absorbed by the electrons in atomic structures. Add to this the
ability to see through water and you have a biological microscope par
excellence. Currently, the only X-ray sources bright enough to be useful
for microscopy are synchrotrons鈥攇iant particle accelerators that produce
pulses of X-rays. But even the most intense synchrotons are orders of magnitude
dimmer than an X-ray laser will be.

Molecular movies

The short and extremely bright X-ray laser pulses will be useful for two
reasons. Bigger molecules tend to diffract X-rays only weakly so only the
brightest X-ray sources offer any hope of investigating large proteins, DNA or
viruses. Much of the shape and behaviour of such crucial molecules remains a
mystery.

The shortness of the pulse will also freeze the motion of the molecules. This
should be a major improvement on the blurred images nowadays created during the
several minutes of exposure that can be required in a synchrotron. Over such a
long period, the specimen can move and resolution is lost鈥攍ike an action
photograph taken with too long an exposure.

All that will change with the exposures possible with X-ray lasers. 鈥淥ne shot
of 40 or 50 picoseconds will freeze any process inside a living cell,鈥� says
Zhang. Earlier this year, his team set the record for brevity for X-ray lasers
at 30 picoseconds. But he thinks that less sophisticated machines with only
nanosecond exposures may still yield useful results. Of course, the intense
radiation will always damage or even destroy whatever is in its path. But
because an X-ray laser delivers so much light so quickly, the sample simply
doesn鈥檛 have time to degrade. And by making many exposures on a sequence of
samples at different stages of a reaction, researchers will be able to make a
鈥渕ovie鈥� of molecular motions, each frame preserved in perfect clarity. Zhang
hopes that this kind of technique could herald the first 鈥渆ye witness鈥� account
of processes such as DNA replication.

The first step will be to test the technique on a well-known structure.
鈥淭here are some tissues that are easy to prepare鈥攎uscle for instance. Its
periodic structure would make it a good test case,鈥� says John Squire, a
researcher at Imperial College in London. The limiting factor with lasers has
always been getting the wavelength short enough for adequate resolution of
complex biological structures. X-ray lasers should make all the difference.

Skeletal muscle has a well-understood and partially crystalline structure
that can be studied easily using X-ray diffraction. But a full 鈥渕ovie鈥� of muscle
filament movement will help researchers gain a better understanding of how
healthy muscle works and could even provide valuable insights into what can go
wrong in muscular diseases. 鈥淭he technique could be very valuable,鈥� says
Squire.

After such a test Zhang intends to be more ambitious. Many processes in
living cells depend on transient interactions between proteins or between
proteins and nucleic acids. And the sheer size of some complex proteins makes
imaging them with conventional X-ray sources difficult or even impossible. Here,
X-ray laser images could open up avenues of discovery barred to existing
technologies.

With these techniques it may be possible to tease apart some of the greatest
puzzles facing molecular biologists. How do hormones and chemicals in the brain
called neurotransmitters trigger the chemical changes that pass across the
surface of cells? How do molecules pass along nerve fibres from one part of the
body to another? In the brains of Alzheimer鈥檚 patients, amyloid proteins build
up inside the brain damaging its function. Why does this process occur? And some
researchers believe Creutzfeldt-Jakob disease is caused when prion proteins
change from one configuration to another. How does this happen?

Protein structures are also used by many pharmaceuticals companies to design
compounds as potential new drugs. In the past, new drugs were often selected by
a process of trial and error. Today, researchers are learning to design them by
studying their structure and how they behave. So called structure-based drug
design is already leading to commercial products. But the technique needs a
detailed understanding of molecular structure, shape and behaviour. In future,
this may only be possible using X-ray lasers.

Of course, there is much work ahead鈥擷-ray lasers by themselves may not
answer all these questions. But come the end of the year, the life sciences
community may have a powerful new addition to their armoury of analytical
tools.