Download High intensity lasers - Institute of Physics

Vi s i o n s i s a s e r i e s o f p a p e r s w h i c h
h i g h l i g h t e x c i t i n g n e w a re a s o f r e s e a r c h
i n p h y s i c s , a n d t h e i r t h e o re t i c a l a n d
technological implications.
Compact solid-state lasers can now
produce pulses with intensities rivalling
those from the large laser systems housed
in central facilities. Physicists are using
FORTHCOMING VISION PAPERS:
Quantum information
Novel uses of nuclear physics
Physics and finance
Spintronics
them to investigate novel phenomena
with exciting potential applications.
High-intensity lasers could be a source of
coherent X-rays for biological imaging,
accelerate subatomic particles, and even
trigger nuclear fusion
ABOUT THE INSTITUTE OF PHYSICS
High intensity lasers
The Institute of Physics is an international
learned society and the professional body
for physicists in Great Britain and Ireland.
The Institute has more than 22,000
individual members.
Department of Higher Education and Research
The Institute of Physics
76 Portland Place
London W1N 3DH
UK
e-mail: [email protected]
w e b s i t e : h t t p : / / w w w. i o p . o r g /
Editor: Nina Hall, Design: Pete Hodkinson.
©
Institute of Physics
FOR FURTHER INFORMATION CONTACT:
VISIONS 1
I N
P H Y S I C S
R E S E A R C H
L
asers, regarded only a
few decades ago as exotic
scientific tools, are now
commonplace – used, for
example, in supermarket barcode
scanners and in CD players.
Today, research into new types of
laser and laser phenomena is still
a hugely active area in physics
laboratories all over the world,
with breakthroughs regularly
reported. One of the most
exciting ongoing developments is
that of moderately-sized lasers
capable of producing light pulses
each with a power of more than a
develop a petawatt (1015W) laser
at the Lawrence Livermore
National Laboratory in the US.
There is now, however, another
route to achieving high intensities
– thanks to a combination of new
solid-state lasers, which emit very
short pulses, and a clever way of
amplifying them. The shorter the
pulse, the higher is its power and
therefore intensity. Crystals of
lasing materials, Ti:sapphire or
the newer CrLiSAF, can spit out
bolts of light as short as 10
femtoseconds. However,
amplifying a short pulse is a
its electric field distorts its
harmonic motion in the light field,
creating higher-order harmonic
frequencies in addition to those of
the laser. Since all the electrons in
the gas are oscillating in unison, a
coherent beam of radiation is
produced with frequencies that
are multiples of the fundamental
laser frequency. These harmonics
go up to 150 times the original
frequency – well into the soft
X-ray range.
Most work has been carried out
on laser interactions with inert
gases such as xenon and, as yet,
Super-intense lasers on a table-top
terawatt, or 1012 watts. The
newest lasers are portable and
can be installed on a laboratory
bench. What is more, they are
relatively inexpensive, costing not
much more than £250,000. Not
surprisingly, the potential of these
instruments is enormous.
problem because the high
brilliance results in damage to the
amplifier. The answer is ingenious
but simple in concept: employ a
pair of diffraction gratings to
stretch the pulse so that its
energy is spread out in time –
thereby reducing the power to
safe levels – then amplify the
pulse, and finally re-compress it
back to its original duration. This
technique is called chirped pulse
amplification (CPA).
High harmonic generation
A Ti:sapphire
crystal pumped
by green laser
beams is used
to produce
very intense
pulses of light
THANKS
GO TO
Until recently, the push to
brighter beams meant building
ever larger, composite lasers that
could deliver very high energy
(kilojoules). These giant systems
are therefore based at national
facilities. Indeed, lasers such as
Vulcan at the CLRC's Rutherford
Appleton Laboratory are still
among the most powerful in the
world. Work is under way to
Lasers with CPA are opening up
many exciting possibilities, some
of them completely unexpected.
When focused with a lens or
mirror, the beam can have
intensities as high as 1019 Wcm-2,
which implies an electric field
considerably greater than that in
an atom. When such a laser beam
interacts with a gas, say helium,
the motion of the atomic
electrons becomes almost
completely dominated by the
oscillating light field. Each
electron is periodically driven
away from the parent ion and
then back towards it. As the
electron approaches the nucleus,
DR JOHN TISCH, PROFESSOR PETER KNIGHT
AND THE
the energy conversion efficiency
is not very high. Nevertheless just
a few months ago, American
researchers showed that by
exploiting some novel optics they
could increase the efficiency onehundredfold. Another approach is
to aim for shorter laser pulses.
Such a coherent X-ray source
has many advantages. The
system is portable and emits a
tightly collimated beam of very
short pulses generated at high
repetition rates – and because the
input laser is tunable, so is the
output X-ray beam. Laser physicists
already envisage many applications
such as the holographic imaging
of molecular structure in living
cells in the desirable 'water
window' region of the spectrum
(where water is transparent to Xrays but carbon is not), and timeresolved chemistry experiments.
The interaction of high-intensity
laser beams with other forms of
matter, as well as gases, has also
revealed some remarkable and
useful phenomena. Reactions
with solids are energetic enough
to ionise atoms and generate very
LASER CONSORTIUM
AT
hot plasmas. These are sources of
very bright, incoherent X-rays of
energies up to 1 MeV.
Clusters’ last stand
However, even more
extraordinary is current work
carried out by John Tisch and
colleagues at Imperial College,
London on the interaction with
clusters of rare gas atoms. Such
atomic conglomerates, which may
contain from four up to a million
atoms, are held together by van
der Waals forces. Physicists
became interested in clusters
because they represent a bridge
between a gas and a solid, in
other words, they have the
properties of both single atoms
and bulk matter.
What the researchers
discovered was a great surprise.
When the bright laser light is
shone on clusters of 200 or
more atoms, the reaction is
even more energetic than in
the case of the solid.
Virtually all the laser energy is
absorbed by the clusters causing
them to 'melt' into tiny balls of
extremely hot plasma. The
interaction is enhanced partly as a
result of the heat being trapped in
the clusters by the surrounding
vacuum (unlike in the solid where
it can conduct away) and partly
because a resonance condition is
reached which increases the
electric field inside the miniplasmas. This leads to an elevated
ionisation rate and increased
collisions between ions. The hot
plasma of very highly charged
ions (up to 40+) then blows apart
in all directions. The ions then
slowly recombine, emitting X-rays
at a reasonably efficient level
(some of the X-ray emission is
believed to be fast). Again, X-rays
produced thus would be useful for
a number of applications such as
high-resolution radiography in
medicine, X-ray lithography, and
as a diagnostic tool for plasmas
(increasingly used in industrial
manufacturing processes).
There are more speculative
possibilities for the cluster
configuration. The intense laser
radiation, combined with the
efficient coupling into a gas of
selected atomic clusters, could be
used to produce exotic plasmas of
the kind that are of interest to
astrophysicists, or even to trigger
nuclear fusion between clusters
of deuterium and/or tritium atoms.
Researchers in the UK are
thinking of testing out this idea
with experiments on deuterium
clusters. They are also
investigating whether high
harmonic radiation can be
produced as efficiently from
clusters as from ordinary inert
gases. UK, French and American
researchers are very much at
the forefront of such research in
high intensity lasers. We can look
forward to many more rewarding
developments over the next
few years.
Simulation
of an
exploding
cluster
A spectrum of high harmonics
showing the excellent beam quality
A C C E L E R AT I N G PA R T I C L E S W I T H L A S E R S
Plasmas induced by high-intensity lasers
principle is to convert the oscillating electric
stronger than in a typical linear particle
could also be used to accelerate subatomic
field of a picosecond laser pulse into an
accelerator. Electrons can then 'surf' on the
particles such as electrons, thus providing
oscillating electric field in the plasma – a
wave, accelerated by the strong electric field.
a compact device for particle physics
plasma wave. As the laser pulse moves
One new scheme is called the self-modulated
experiments. Although a practical accelerator
through the plasma, the free electrons
wake-field approach, in which the start of the
is still a long way from realisation, active
oscillate, causing periodic variations in the
laser pulse generates a plasma. This interacts
progress is being made.
charge density resulting from separation of
with the rest of the pulse in a way that causes
Researchers are working on a number of
electrons and ions in the plasma. The
periodic variations in plasma density along
schemes that take advantage of the powerful
travelling plasma wave can be extremely
the length of the pulse. This scheme is
electric fields generated in such plasmas. The
large, generating fields up to 1000 times
currently being tested with Vulcan.
IMPERIAL COLLEGE, LONDON
FOR THEIR CONSIDERABLE HELP WITH THIS
VISION
PAPER
L
asers, regarded only a
few decades ago as exotic
scientific tools, are now
commonplace – used, for
example, in supermarket barcode
scanners and in CD players.
Today, research into new types of
laser and laser phenomena is still
a hugely active area in physics
laboratories all over the world,
with breakthroughs regularly
reported. One of the most
exciting ongoing developments is
that of moderately-sized lasers
capable of producing light pulses
each with a power of more than a
develop a petawatt (1015W) laser
at the Lawrence Livermore
National Laboratory in the US.
There is now, however, another
route to achieving high intensities
– thanks to a combination of new
solid-state lasers, which emit very
short pulses, and a clever way of
amplifying them. The shorter the
pulse, the higher is its power and
therefore intensity. Crystals of
lasing materials, Ti:sapphire or
the newer CrLiSAF, can spit out
bolts of light as short as 10
femtoseconds. However,
amplifying a short pulse is a
its electric field distorts its
harmonic motion in the light field,
creating higher-order harmonic
frequencies in addition to those of
the laser. Since all the electrons in
the gas are oscillating in unison, a
coherent beam of radiation is
produced with frequencies that
are multiples of the fundamental
laser frequency. These harmonics
go up to 150 times the original
frequency – well into the soft
X-ray range.
Most work has been carried out
on laser interactions with inert
gases such as xenon and, as yet,
Super-intense lasers on a table-top
terawatt, or 1012 watts. The
newest lasers are portable and
can be installed on a laboratory
bench. What is more, they are
relatively inexpensive, costing not
much more than £250,000. Not
surprisingly, the potential of these
instruments is enormous.
problem because the high
brilliance results in damage to the
amplifier. The answer is ingenious
but simple in concept: employ a
pair of diffraction gratings to
stretch the pulse so that its
energy is spread out in time –
thereby reducing the power to
safe levels – then amplify the
pulse, and finally re-compress it
back to its original duration. This
technique is called chirped pulse
amplification (CPA).
High harmonic generation
A Ti:sapphire
crystal pumped
by green laser
beams is used
to produce
very intense
pulses of light
THANKS
GO TO
Until recently, the push to
brighter beams meant building
ever larger, composite lasers that
could deliver very high energy
(kilojoules). These giant systems
are therefore based at national
facilities. Indeed, lasers such as
Vulcan at the CLRC's Rutherford
Appleton Laboratory are still
among the most powerful in the
world. Work is under way to
Lasers with CPA are opening up
many exciting possibilities, some
of them completely unexpected.
When focused with a lens or
mirror, the beam can have
intensities as high as 1019 Wcm-2,
which implies an electric field
considerably greater than that in
an atom. When such a laser beam
interacts with a gas, say helium,
the motion of the atomic
electrons becomes almost
completely dominated by the
oscillating light field. Each
electron is periodically driven
away from the parent ion and
then back towards it. As the
electron approaches the nucleus,
DR JOHN TISCH, PROFESSOR PETER KNIGHT
AND THE
the energy conversion efficiency
is not very high. Nevertheless just
a few months ago, American
researchers showed that by
exploiting some novel optics they
could increase the efficiency onehundredfold. Another approach is
to aim for shorter laser pulses.
Such a coherent X-ray source
has many advantages. The
system is portable and emits a
tightly collimated beam of very
short pulses generated at high
repetition rates – and because the
input laser is tunable, so is the
output X-ray beam. Laser physicists
already envisage many applications
such as the holographic imaging
of molecular structure in living
cells in the desirable 'water
window' region of the spectrum
(where water is transparent to Xrays but carbon is not), and timeresolved chemistry experiments.
The interaction of high-intensity
laser beams with other forms of
matter, as well as gases, has also
revealed some remarkable and
useful phenomena. Reactions
with solids are energetic enough
to ionise atoms and generate very
LASER CONSORTIUM
AT
hot plasmas. These are sources of
very bright, incoherent X-rays of
energies up to 1 MeV.
Clusters’ last stand
However, even more
extraordinary is current work
carried out by John Tisch and
colleagues at Imperial College,
London on the interaction with
clusters of rare gas atoms. Such
atomic conglomerates, which may
contain from four up to a million
atoms, are held together by van
der Waals forces. Physicists
became interested in clusters
because they represent a bridge
between a gas and a solid, in
other words, they have the
properties of both single atoms
and bulk matter.
What the researchers
discovered was a great surprise.
When the bright laser light is
shone on clusters of 200 or
more atoms, the reaction is
even more energetic than in
the case of the solid.
Virtually all the laser energy is
absorbed by the clusters causing
them to 'melt' into tiny balls of
extremely hot plasma. The
interaction is enhanced partly as a
result of the heat being trapped in
the clusters by the surrounding
vacuum (unlike in the solid where
it can conduct away) and partly
because a resonance condition is
reached which increases the
electric field inside the miniplasmas. This leads to an elevated
ionisation rate and increased
collisions between ions. The hot
plasma of very highly charged
ions (up to 40+) then blows apart
in all directions. The ions then
slowly recombine, emitting X-rays
at a reasonably efficient level
(some of the X-ray emission is
believed to be fast). Again, X-rays
produced thus would be useful for
a number of applications such as
high-resolution radiography in
medicine, X-ray lithography, and
as a diagnostic tool for plasmas
(increasingly used in industrial
manufacturing processes).
There are more speculative
possibilities for the cluster
configuration. The intense laser
radiation, combined with the
efficient coupling into a gas of
selected atomic clusters, could be
used to produce exotic plasmas of
the kind that are of interest to
astrophysicists, or even to trigger
nuclear fusion between clusters
of deuterium and/or tritium atoms.
Researchers in the UK are
thinking of testing out this idea
with experiments on deuterium
clusters. They are also
investigating whether high
harmonic radiation can be
produced as efficiently from
clusters as from ordinary inert
gases. UK, French and American
researchers are very much at
the forefront of such research in
high intensity lasers. We can look
forward to many more rewarding
developments over the next
few years.
Simulation
of an
exploding
cluster
A spectrum of high harmonics
showing the excellent beam quality
A C C E L E R AT I N G PA R T I C L E S W I T H L A S E R S
Plasmas induced by high-intensity lasers
principle is to convert the oscillating electric
stronger than in a typical linear particle
could also be used to accelerate subatomic
field of a picosecond laser pulse into an
accelerator. Electrons can then 'surf' on the
particles such as electrons, thus providing
oscillating electric field in the plasma – a
wave, accelerated by the strong electric field.
a compact device for particle physics
plasma wave. As the laser pulse moves
One new scheme is called the self-modulated
experiments. Although a practical accelerator
through the plasma, the free electrons
wake-field approach, in which the start of the
is still a long way from realisation, active
oscillate, causing periodic variations in the
laser pulse generates a plasma. This interacts
progress is being made.
charge density resulting from separation of
with the rest of the pulse in a way that causes
Researchers are working on a number of
electrons and ions in the plasma. The
periodic variations in plasma density along
schemes that take advantage of the powerful
travelling plasma wave can be extremely
the length of the pulse. This scheme is
electric fields generated in such plasmas. The
large, generating fields up to 1000 times
currently being tested with Vulcan.
IMPERIAL COLLEGE, LONDON
FOR THEIR CONSIDERABLE HELP WITH THIS
VISION
PAPER
Vi s i o n s i s a s e r i e s o f p a p e r s w h i c h
h i g h l i g h t e x c i t i n g n e w a re a s o f r e s e a r c h
i n p h y s i c s , a n d t h e i r t h e o re t i c a l a n d
technological implications.
Compact solid-state lasers can now
produce pulses with intensities rivalling
those from the large laser systems housed
in central facilities. Physicists are using
FORTHCOMING VISION PAPERS:
Quantum information
Novel uses of nuclear physics
Physics and finance
Spintronics
them to investigate novel phenomena
with exciting potential applications.
High-intensity lasers could be a source of
coherent X-rays for biological imaging,
accelerate subatomic particles, and even
trigger nuclear fusion
ABOUT THE INSTITUTE OF PHYSICS
High intensity lasers
The Institute of Physics is an international
learned society and the professional body
for physicists in Great Britain and Ireland.
The Institute has more than 22,000
individual members.
Department of Higher Education and Research
The Institute of Physics
76 Portland Place
London W1N 3DH
UK
e-mail: [email protected]
w e b s i t e : h t t p : / / w w w. i o p . o r g /
Editor: Nina Hall, Design: Pete Hodkinson.
©
Institute of Physics
FOR FURTHER INFORMATION CONTACT:
VISIONS 1
I N
P H Y S I C S
R E S E A R C H