X Rays with a Berry Twist
X rays pass easily through many materials, which makes these
photons useful for imaging but also hard to control, limiting the
development of x-ray optics. Inspired by how visible light can be
manipulated in photonic crystals, researchers are studying ways to
engineer materials at the atomic scale to achieve better control on
x-ray photons. In a paper in Physical Review Letters, Yoshiki Kohmura at the RIKEN SPring-8
Center, Japan, and colleagues, report on the use of a lattice
deformation method that allows controlling the translation of an x-ray
beam.
In 2006, researchers predicted that a tiny bend imposed on a crystal
lattice could result in a sideways translation of an x-ray beam that was
106
times larger than the crystal deformation. The extreme shifting is
caused by the Berry-phase effect: a geometric phase shift occurring when
the x-ray beam hits the crystal at an angle slightly lower or higher
than that required for Bragg scattering. This was demonstrated by a team
at the SPring-8
synchrotron, who observed millimeter beam shifts of x rays passing
through a silicon crystal whose curvature was monotonically deformed by
only 80 nanometers (see 14 June 2010 Viewpoint).
Now, researchers at the same lab have achieved a larger x-ray beam
translation in a crystal with an undulating, rather than uniform,
deformation. Kohmura et al.
grew germanium quantum dots on a silicon crystal; the lattice mismatch
between the two materials causes an undulating lattice deformation.
Depending on where the incident x rays hit the deformation (giving
different angles of incidence), the beams were shifted in two different
directions. This effect may be used to measure crystal strain fields in
heterostructures, or to realize beam splitters that separate incoming x
rays into two parallel beams, for applications to interferometry and
pump-probe studies. – David Voss
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