Like playing a game of scissors-paper-rock, a team of scientists led by Thornton
E. (Ernie) Glover of Berkeley Lab's Advanced Light Source (ALS), Linda Young
of Argonne National Laboratory, and Ali Belkacem of Berkeley
Lab's Chemical Sciences Division has used laser light to control x-ray beams
- by first changing the material medium through which the x-rays pass.
Controlling x-rays with ultrashort slices of light is a step toward controlling
how matter behaves, shaping x-rays with other x-rays, and eventually directing
the paths chemical reactions can take. Working at the Advanced Light Source's
femtosecond beamline 6.0.2, a team of scientists shows how it can be done.As
a new generation of powerful light sources comes online, intense x-ray beams
may be able to control matter directly and allow one beam of x-rays to control
Using the ALS's femtosecond (quadrillionth of a second) spectroscopy
beamline 6.0.2, Glover and his colleagues sent ultrashort pulses of laser light
and higher-frequency x-rays together through a gas cell filled with pressurized
neon. Excited by the laser pulses, the gas, which normally absorbs x-rays, became
transparent to the x-ray pulses during their quick passage.
“We were inspired by the interesting new science demonstrated in quantum
optics experiments that use visible light to control visible light,” says
Glover. “One spectacular example is slowing light to a near standstill
in some media. The ability to, in effect, stop light in a medium has potential
applications for quantum information storage and processing.”
Glover says another example of optical control is using visible light to induce
transparency in a medium. “We embarked on our own research in the hope
that it would lead to new and interesting ways to use x-rays as well as visible
Light's behavior in a medium like air or glass or water is determined
by the interaction of its electromagnetic field with the medium's electrons.
In a quantum-mechanical phenomenon called coherent superposition, a “pump”
pulse of light couples two different material states so that when a subsequent
“probe” pulse boosts an electron to either of the excited states,
the electron ends up in both states simultaneously.
Although this had been done with visible light, no one had successfully controlled
a probe x-ray pulse this way before the work of Glover and his colleagues. Higher-energy
x rays interact with electrons in different atomic shells and create excited
states that decay a thousand times faster than those created by visible light
- thus interrupting the attempt to form a coherent superposition and using
it as a control mechanism.
“The superposition state has to last for a useful length of time,”
Glover explains. “But x-rays interact strongly with an atom's inner
core electrons, and x-ray excitation of core electrons leaves holes behind which
are filled by other, more weakly bound electrons so quickly that the superposed
state lasts for only a femtosecond or so.”
Glover says one approach to solving this short-lifetime problem is “by
increasing the number of photons” - using very intense optical pulses
to more strongly couple the material states. For a given laser pulse energy,
the intensity increases as the pulse length decreases.
But to see the combined light-matter system, the x-ray pulse has to be at least
as short or shorter than the laser pulse, and both pulses have to move through
the medium together. These conditions are met by using synchronized pulses,
measuring about 200 femtoseconds, of both optical light and x-rays.
A very special beamline
“Beamline 6.0.2 was the first and still one of only three places in the
world where these experiments could be done,” Glover says. The experiment's
intense laser pulses created brief coherent superposition states in the dense
neon gas inside the cell, which rendered the pressurized neon in the gas cell
transparent to the x rays.
“Quantum mechanicaly speaking, there is destructive interference between
two absorption pathways and this reduces the absorption,” says Glover.
“That is, it makes the medium transparent.” For the first time,
optical pulses had been used to control how x-rays interact with matter.
The experimenters quickly put this ephemeral neon window to practical service,
using it to measure the duration of the femtosecond-scale x-ray pulse to high
accuracy more simply than has been possible before, with the added ability of
shaping x ray pulses on a femtosecond time scale.
“To our knowledge there are no other viable approaches to shaping x-ray
pulses with femtosecond precision,” Glover says. “By demonstrating
a way to shape x-rays on the femtosecond timescale, we've opened the door
to ‘quantum control' experiments - now possible only with
long-wavelength light - in the x-ray regime.”
X-rays have element specificity - they can be tuned to talk to particular
kinds of atoms in a molecule much more effectively than visible light can. “A
number of advances seem possible,” says Glover. “Shaping pulses
on this timescale will be important in experiments that seek to control chemical
reactions, phase transitions, and other phenomena.”
Further afield may be the potential of using light to control the phase of
x-ray pulses. It's difficult to fabricate perfect mirrors and zone plates
for focusing x-rays, but that problem could be overcome by using lenses of gas
controlled by light. Phase control over x-ray pulses could lead to new ways
to make images of complex structures like protein crystals.
X-ray pulses whose shape, length, and intensity are precisely controlled might
evenbe able to label individual atoms and follow them through a complex series
of chemical reactions like photosynthesis. Says Glover, “We may be able
to exert control over how matter evolves and what paths chemical reactions take.”
In the next generation of light sources, using free-electron lasers to produce
ultrabright, ultrafast, high-repetition rate x-rays, such potential uses of
the ability to control x-rays with light would open a dazzling panorama of understanding
and control over the natural world.