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Feature: Ult r acold sour ce s
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The next
coolest thing
Over the past 25 years, laser cooling and trapping
have transformed experimental atomic physics.
Kevin Weatherill and Edgar Vredenbregt describe how
ultracold particle beams could soon do the same for
nanoscience applications
Kevin Weatherill is
an atomic physicist
at Durham University
in the UK, e-mail
k.j.weatherill@
durham.ac.uk.
Edgar Vredenbregt
is a physicist in the
Coherent and
Quantum Technology
group at Eindhoven
University of
Technology in the
Netherlands
Laser cooling and trapping have revolutionized
atomic-physics experiments. Thanks to these techniques, since the late 1980s the timekeeping precision of atomic clocks has increased by two to three
orders of magnitude; new types of matter – such as
ultracold plasmas and Bose–Einstein condensates –
have been produced and studied; and a wide range
of fundamental work has been carried out on the
quantum-statistical properties of matter and solidstate conductance phenomena. The incredible progress in this field is reflected in the two Nobel prizes
awarded for this area of research: one for laser cooling in 1997, and one for Bose–Einstein condensation
in 2001.
The key factor driving this progress has been the
ability to produce extremely cold samples of atoms
and control their behaviour. At room temperature,
atoms whizz around at hundreds of metres per
second, or roughly the speed of a jet plane. Laser
cooling can reduce these speeds to as low as a few
centimetres per second, corresponding to temperatures in the microkelvin (10 –6 K) regime. That is a
staggering figure – about a millionth of the temperature of interstellar space – and it has some important
experimental implications. Common sense suggests
At a Glance: Ultracold sources
Focused-ion beams (FIBs) are routinely used in nanoscience to create tiny
structures and image the surfaces of materials, but the trend towards making
ever-smaller components in the semiconductor industry means that current FIB
technology is nearing its limits
●● The resolution of FIBs could be improved by using ions that have been precooled to a fraction of a degree above absolute zero with laser-cooling and
trapping techniques developed for atomic-physics experiments
●● Several research groups have already developed ultracold ion sources based
on photo-ionizing atoms that have been trapped and cooled in a magnetooptical trap
●● Similar techniques have led to the creation of ultracold electron beams,
which could be used to study structural changes in large, biologically
important molecules
●●
28
that it is much easier to take hold of and measure
something that is moving slowly, and cold atoms
move slowly enough that they hang around for a long
time. Because of this, their properties can be measured more accurately and their coordinates manipulated more easily. In the most fundamental sense,
this is a consequence of Heisenberg’s uncertainty
principle, which asserts that the precision with which
atomic energies can be measured is limited by the
observation time.
Recently, researchers have begun to investigate
ways of using laser cooling and trapping in fields
other than atomic physics. One promising new
area of interest is nanoscale science, where focused
beams of ions are routinely used to create tiny structures, prepare samples for electron microscopy and
study the surfaces of materials (see box on p31). The
semiconductor industry, in particular, is manufacturing ever smaller and more complex structures and,
as components shrink, it becomes harder to investigate them with current focused-ion-beam (FIB)
technology. One reason for this is that conventional
ion sources for FIBs operate at elevated effective
temperatures, as high as several thousand degrees
kelvin. Such high temperatures translate into subP hy sic s Wor ld  Augus t 2012
Feature: Ult r acold sour ce s
Alexa Parker and Adam West
phy sic s wor ld.com
stantial random (thermal) motion of the ions, which
makes it difficult to focus a large ion current to a
small spot. But if this randomness could be reduced
– for example, by reducing the temperature of the
ions to the millikelvin range that is easily achieved
with laser cooling and trapping – the spot size of
ion beams could also be reduced, allowing smaller
structures to be investigated. The ultimate goal of
such research would be to produce laser-like particle
beams that would replace existing technologies used
in nanoscience.
Cool things to do with lasers
Laser cooling works by pushing atoms in a direction
that opposes their motion. For example, an atom
moving into a “headwind” of laser photons will be
pushed backwards each time it absorbs a photon. As
the atom goes through repeated cycles of absorbing photons from one direction, and emitting them
in random directions, it slows down rapidly, experiencing a deceleration about 10 4 times greater than
Earth’s gravity. With counter-propagating beams and
careful choice of laser frequency, the atoms experience a velocity-dependent force that opposes their
motion in any direction. The effect is that the velocP hy sic s Wor ld  Augus t 2012
ity spread of a collection of atoms is greatly reduced,
which is why the process is called “laser cooling”.
By combining several laser beams with a magnetic
field, it is possible to simultaneously cool and trap
atoms in a device known as a magneto-optical trap,
or MOT. In the years since the first MOT was created in 1987, this device has become a workhorse of
atomic physics, providing a robust and reliable source
of very cold atoms as a starting point for more complex experiments. And because laser cooling can be
applied to many different atomic species, from alkali
atoms such as rubidium to transition metals such as
chromium and dysprosium, MOTs are also quite versatile. This is important for cold-ion-beam applications, because one might prefer to use ions with a
low mass for imaging purposes and heavy ions for
milling, sputtering or etching nanoscale structures.
One could also introduce specific ions as dopants
into semiconductors.
Once we have a sample of ultracold atoms prepared in a MOT, we can then create cold ions by
using additional laser beams to strip off some of the
atoms’ electrons. If we choose the frequency of the
laser to match the ionization threshold of the atom –
that is, the energy required to liberate one electron –
A very cool machine
This cartoon
schematic of a cold
ion/electron source
shows beams of cold
ions (blue particles)
and electrons (green
particles) being
extracted in opposite
directions from a
cloud of laser-cooled
atoms (red
particles). Red lines
represent the
counter-propagating
laser beams used to
trap and cool the
atoms.
29
Feature: Ult r acold sour ce s
1 Rules of thumb for particle beams
θ
d
This diagram of a cold-particle source illustrates two key measures of beam quality:
emittance and brightness. Emittance is approximately the beam’s size at its source, d,
multiplied by the angle subtended by the beam at its source, θ. Brightness is current
(represented here by the density of red spheres) divided by the square of emittance.
we can produce a mixture of cold ions and electrons
known as an ultracold plasma. The final step in producing charged-particle beams is to apply an electric
field across the plasma, thereby sending the ions and
electrons in opposite directions.
After the beam of ions has been created, it can be
accelerated and focused using electric and magnetic
fields. This feature allows experimentalists to transport ions from their source to a target sample using
a series of “ion optics”, which manipulate particles in
much the same way as conventional optical elements,
such as lenses, manipulate light. As with light, there
is a limit to how tightly an ion beam can be focused
down to a small spot. One factor that limits this spot
size, or resolution, is chromatic aberration. This
effect is well known in conventional optics, where it
occurs because light of different colours is focused
to different positions in space, thereby limiting the
resolution. In ion beams, the source of chromatic
aberration is the energy spread – in other words, the
spread in the velocities of the ions. However, as we
have already seen, laser cooling dramatically reduces
thermal spread, so using a cold-ion source for such
beams should allow for smaller spot sizes.
In addition to chromatic aberration, two additional
properties are of interest when discussing particle
beams. The first of these is emittance, a measure of the
average spread of particle coordinates in position and
momentum “phase space” (figure 1). Emittance can
be roughly estimated as the product of a beam’s size
at its source and the angular divergence of the particles at source, and it tells us how tightly a beam can
be focused by a lens of given numerical aperture in
the absence of any aberrations. In principle, then, one
would expect that if the size of the beam at its source
is reduced, the emittance will also get smaller, and the
beam can therefore be focused to a smaller spot.
However, when focusing is limited by chromatic
aberration, a better measure of beam quality can be
derived from its brightness, which is defined as the
current per unit source area and per unit solid angle
subtended by the beam. If chromatic aberration is
present, the area to which a given ion current can be
focused is proportional to the beam’s energy spread
divided by the square root of the brightness.
In summary, the higher the brightness of an ion
beam and the smaller its energy spread, the more
current can be focused in a given spot size – or, alter-
30
phy sic swor ld.com
natively, the smaller the focal spot will be for a given
current. Brightness, in turn, can be optimized by
extracting a large current from a small source area
within a small solid angle. The catch is that ions are,
of course, charged particles, so they will repel each
other, which makes it difficult to achieve high brightness and small energy spread at the same time.
Nanoscientists have developed several strategies
for optimizing these quantities. Most high-brightness FIBs use liquid-metal ion sources, which have a
source size of a few nanometres. However, this small
source size produces both a large angular spread and
a large energy spread – the latter as a result of Coulomb interactions between the closely spaced ions,
which give rise to a phenomenon known as “disorderinduced heating”. This heating occurs when ions that
are, by chance, close together in the beam, violently
repel each other, thereby increasing the velocity
spread in both the transverse and longitudinal directions in an uncontrollable way. Larger, extended ion
sources may be able to sidestep these problems, but
because they start with a larger beam at the source,
they require a very low angular spread in order to
achieve high brightness.
Low angular spread can be achieved using laser
cooling. Essentially, laser cooling allows one to create atomic beams with extremely low transverse
velocity spread by lowering the temperature of the
beam. By irradiating such an atomic beam with an
appropriate laser beam, the atoms can be turned
into ions using photo-ionization, while maintaining the low temperature. This leads to an ion beam
with a very small angular divergence. Care must be
taken, however, to tune the frequency of the ionization laser very close to the ionization threshold of
the atoms, so that very little additional energy is left
over after ionization, because any excess will appear
as velocity spread of the particles. Careful tuning
of the ionization laser minimizes both (transverse)
angular spread and (longitudinal) energy spread of
the resulting beams.
Demonstrations of cold ion beams
At least three different ultracold ion sources have
so far been demonstrated by research groups at the
National Institute of Standards and Technology in
Gaithersburg, Maryland, and at Eindhoven University of Technology in the Netherlands. All three
are based on photo-ionizing atoms that have been
trapped and cooled in a MOT. One employs a light
atom, lithium, that is particularly suited for imaging
applications; another, an intermediate atom, chromium; and the third a heavier atom, rubidium, that
is more suitable for milling. An ultracold caesium ion
beam is also under development at the Laboratoire
Aimé Cotton in Orsay, France.
Between them, these groups have investigated a
number of properties of the extracted ion beams.
One result of these experiments has been to prove
that ultracold sources can indeed produce ion
beams that have an energy spread far below that of
beams produced from conventional liquid-metal ion
sources. The researchers were able to extract ion
currents of several tens of picoamperes, which, while
P hy sic s Wor ld  Augus t 2012
Feature: Ult r acold sour ce s
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Focused ion beams in technology
Focused ion beams (FIBs) are ubiquitous in nanotechnology,
where they are used to create the smallest structures that
technology can currently provide and also for a variety of
other applications. For example, firing FIBs at a surface allows
us either to remove material from the surface by knocking
atoms out with the ion beam, or to add material by injecting
a molecular gas over the sample surface and then “cracking”
the gas molecules with the beam – a process that leaves
stickier components of the gas (such as metals) behind.
The process of removing material is variously known as
milling, sputtering or etching, and it is widely used in the
semiconductor industry for editing and failure analysis of
integrated circuits. Adding material is done using a technique
called ion-beam-assisted chemical vapour deposition, in
which a molecular gas containing the desired material is
decomposed near the surface by a narrowly focused ion
beam. This technique makes it possible to create new
connections on a semiconductor circuit by depositing
conductive material. Ion implantation can also change the
surface chemistry in a localized area or add dopants or
vacancies to materials. An example of this is using FIBs
to create so-called nitrogen vacancies in diamond. These
vacancies act as optically active “quasi-atoms”, and are
interesting for quantum-information research.
FIBs are also routinely used to prepare samples for
imaging. For example, biological materials such as cells can
it might not sound like a lot, is actually more than
sufficient for a FIB with a few-nanometre spot size.
Also, the angular spread of the ion beams was shown
to correspond to a temperature in the microkelvin
range, as expected. This is so low that beams carrying just a few electron-volts of energy can retain their
well-collimated, almost pencil-like character. Operating such beams at low energy makes it possible to
adjust their longitudinal and transverse properties
with pulsed electric fields, for instance to create
novel lenses with negative spherical aberration.
The next step for researchers interested in industrial applications will be to integrate cold ion sources
with existing FIB technology. Jabez McClelland and
co-workers at NIST Gaithersburg made significant progress towards this goal in 2011 when they
mounted an ultracold lithium ion source on a commercial FIB column and used it for ion microscopy,
with negligible sputtering damage (figure 2). At the
required low beam energy, this system already shows
a spatial resolution of a few tens of nanometres, close
to the ~10 nm that can typically be achieved with
commercial FIBs, and there is scope for reducing it
further by increasing the beam energy and/or reducing the energy spread. In addition, the interaction of
lithium ions with surfaces differs from that of traditional ionic species used in ion microscopy such as
helium and gallium, allowing different surface features to be observed.
The main thing that has been lacking in ultracold
ion sources so far is a brightness comparable to that
of liquid-metal ion sources. This lack of brightness
occurs because ultracold ion sources can produce
only limited current densities before disorder-induced
P hy sic s Wor ld  Augus t 2012
+
–
–
e
+
primary
ion beam
+
+
–
e–
e
–
+
+
+
+
–
sample
e–
be precisely sliced using an ion beam so that their internal
structures can be examined using an electron microscope.
It is also possible to use ion beams to image a surface
directly by detecting ions scattered from the surface or
secondary electrons freed by the impact of ions, as shown
in the diagram above. Alternatively, atomic and molecular
particles ejected from the surface under the impact of ions
can be detected and used to analyse the properties of the
surface, a process known as “time-of-flight secondary ion
mass spectrometry”. In this way, the chemical composition
of surface layers can be determined.
heating becomes an important factor. As a result, new
ways of making cold ion beams that do not rely on
ionizing atoms from a MOT are being investigated,
such as starting from high-flux atomic beams created
using lasers that both cool the beam and compress its
diameter before the photo-ionization stage.
The ionic machine gun
One intriguing possibility that could arise from using
laser-cooled atoms as a source for charged particles
would be the ability to deliver single ions on demand,
in quick succession. Although we are still some way
from achieving this goal, such an “ionic machine
gun” could allow materials and devices to be built
up atom by atom. There are several possible routes
to these “deterministic” (meaning we get a single
ion each and every time we try) single-ion sources.
For example, researchers at the Johannes Gutenberg
University Mainz in Germany have demonstrated
that it is possible to hold a single ion in a trap and
then laser cool it to very low temperatures before
releasing it at a chosen time. The Mainz scheme is
attractive because it could be applied to technologically relevant ions such as group III and V elements
and rare-earth metals. However, methods like this
are always likely to be slow, perhaps producing only a
few ions per second, as the ions are cooled and transported into position within the trap.
An alternative proposal being developed by
researchers (including one of us, KW) at Durham
University in the UK offers faster, single-ion emission and is based on an effect known as the Rydberg
blockade. An atom in a Rydberg state contains an
electron with energy just below the ionization thresh-
An “ionic
machine gun”
could allow
materials and
devices to be
built up atom
by atom
31
Feature: Ult r acold sour ce s
New J. Phys. 13 103035
2 Imaging with cold ions
This image was produced by scanning a pre-cooled beam of lithium
ions across a target surface of tin spheres on carbon, then detecting
the surface (or “secondary”) electrons freed by the impact of the ions.
Light ions such as lithium are particularly well suited for this task
because they produce negligible damage to the surface being imaged.
The key
parameter for
which ultracold
electrons have
an advantage
is coherence
length, which
is the property
that governs
an imaging
beam’s ability
to resolve
spatial order
32
old. Under these circumstances, the electron still
orbits the atom, which is overall neutral, but it spends
a lot of time far away from the nucleus. Because of
this large average separation between the electron
and the nucleus, Rydberg atoms have a large electric dipole moment. One consequence of this is that
a dipole–dipole interaction develops between the
Rydberg atom and nearby atoms. This interaction
shifts the energy levels of the atoms, which means
that their resonant frequency moves away from the
frequency of the laser used to excite atoms into the
Rydberg state.
As a result, atoms that lie within a characteristic
distance of the Rydberg atom (known as the blockade
radius) are prevented, or “blocked”, from joining it in
the Rydberg state. In other words, only one Rydberg
atom is allowed at a time. The electron of this highly
excited atom can then be removed by applying a small
electric field, and we are left with a single ion. The
rate at which ions can be emitted in this case is only
limited by how quickly the electric field can be turned
on and off (in the order of several megahertz).
This same blockade mechanism could also be
applied to create ion beams in which ions are created at more or less fixed distances from each other
in ordered, even crystal-like structures. By ensuring
that the ions are regularly spaced, experimenters
could reduce the effect of statistical charged-particle
interactions on beam brightness – simply by eliminating the chances of any two ions being formed in
very close proximity to each other.
Ultracold electron beams
So far, we have limited our discussion of cold-atom
sources to experiments that produce new types of
ion beams for focused-beam applications. However,
it is also possible to use such sources to produce
electron beams with temperatures as low as a few
kelvin. This is important because electron diffraction and microscopy have long been used to study the
phy sic swor ld.com
structure of materials, and ultracold electrons may
offer new possibilities. The key parameter for which
ultracold electrons have an advantage is coherence
length, which is the property that governs an imaging
beam’s ability to resolve spatial order.
Coherence length is similar to the more familiar
de Broglie wavelength, or the quantum-mechanical
“size” of a particle. In many imaging applications,
short wavelengths are desirable because the diffraction limit on resolution is proportional to the wavelength. However, when using electron diffraction to
study large objects – such as biologically relevant
molecules – every electron must be able to “see” the
whole structure. For this to happen, the electron’s
de Broglie wavelength must be of the same order (or
larger than) the size of the object. Unfortunately, it
is difficult to create short, coherent pulses containing many electrons because electrons are very light
and therefore strongly affected by inter-particle Coulomb forces. This repulsion increases the spatial and
temporal length of the electron pulse and distorts it
in an uncontrollable way.
Ultracold electron beams, however, have an intrinsically large coherence length by virtue of their low
temperature. This means that, as with ion beams,
it is possible to minimize interactions by using an
extended source without compromising the coherence. Such sources have now been demonstrated
both at Eindhoven University of Technology (by one
of us, EV) and by Robert Scholten and colleagues
at the University of Melbourne in Australia. In both
cases, the beams were created by photo-ionizing
atoms trapped in a MOT. Temperatures of a few
kelvin have been measured for pulses containing
many thousands of electrons. In addition, the spatial charge distributions of the pulses have been
delicately shaped using tailored laser beams. For
instance, cartoon images of atoms and even the “BatSignal” were imprinted on electron distributions (figure 3). In addition to being fun, having such precise
control over the shape suggests that it will be possible
to tailor charge distributions of both ion and electron
beams in order to control the space-charge expansion that otherwise lengthens pulses and reduces
their brightness and coherence. Ultracold operation
has even been demonstrated in combination with
pulse lengths in the order of picoseconds by using
pulsed lasers for the photo-ionization step.
Prospects for the “particle laser”
Laser cooling and trapping appears to be leading
us towards a new class of ultracold charged-particle
sources that can achieve low emittance even for a
substantial source size. The first tangible applications of these sources – which include the lithiumion microscope/FIB created by the NIST group, as
well as the Eindhoven and Melbourne groups’ efforts
to use cold-electron beams for diffraction measurements on biological materials – are under way, and it
is easy to imagine wider possibilities. For one, deterministic ion and atom sources may make it possible
to build up materials and devices atom by atom. The
development of laser-intensified atomic beams can
further boost ion-beam brightness while retaining
P hy sic s Wor ld  Augus t 2012
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Rober t Scholten/Univer sity of Melbourne
3 Delicate shapes
This false-colour image shows a cold-electron bunch that has been shaped to
resemble the iconic “Bat-Signal”. To produce such complex patterns, the laser
beam used to excite the electrons was sent through a device called a spatial
light modulator, which controls the beam’s intensity profile.
the essential advantages of low energy spread and
low angular spread. Ultracold electron sources may
provide a way to study structural changes in large
molecules with atomic-scale spatial resolution across
all relevant timescales using time-resolved electron
microscopy and diffraction.
But there are additional tricks in the book of laser
cooling. Atomic physicists have developed several
advanced ways of cooling atoms that lead to temperatures even lower than those achieved in MOTs.
Methods of producing neatly ordered arrays of atoms
in a so-called optical lattice can also further reduce
disorder-induced heating. Both of these techniques,
if applied to cold-ion and cold-electron sources,
would lead to even colder (and thus brighter) particle
beams. For electrons, the dream is to approach the
ultimately achievable brightness where the particles
are so closely packed in phase space that every available state is occupied and the system becomes degenerate. With trapped atoms, this limit has already been
achieved by creating degenerate Fermi gases, but for
charged particles it is still very far away. With ions,
even higher brightness is possible in principle, and
particle beams with a near-perfect, laser-like wave
character are not unthinkable. The impact of such
degenerate beams would rival that of the optical laser.
In the meantime, the focus is on making ultracold sources robust and user-friendly without the
complex optical infrastructure currently required to
make them. The day when nanoscientists can buy a
commercial analysis instrument with a laser-cooled
source may well be near.
n
More about: Ultracold sources
B Knuffman et al. 2011 Nanoscale focused ion beam from
laser-cooled lithium atoms New J. Phys. 13 103035
M P Reijnders et al. 2009 Low-energy spread ion bunches
from a trapped atomic gas Phys. Rev. Lett. 102 03482
A J McCulloch et al. 2011 Arbitrarily shaped high-coherence
electron bunches from cold atoms Nature Physics 7 785
W Schnitzler et al. 2009 Deterministic ultracold ion source
targeting the Heisenberg limit Phys. Rev. Lett. 102 070501
P hy sic s Wor ld  Augus t 2012
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