Number 773, April 12, 2006
by Phil Schewe and Ben Stein
Sharper Focusing of Hard X-Rays
Sharper focusing of hard X-rays has been achieved with a device
developed at Argonne National Lab.
Because of their high energy, X-rays are hard to focus: they can
be reflected from a surface but
only at a glancing angle (less than a tenth of a degree); they can be
refracted but the index of refraction is very close to 1, so that
making efficient lenses becomes a problem; and they can be
diffracted, but the thick, variable pitch grating required for
focusing is tricky to achieve.
The Argonne device is of the
diffraction type, and it consists of a stack of alternating layers
of metal and silicon, made by depositing progressively thicker
layers (see figure at
Physics News Graphics). When
the X-rays fall on such a structure, nearly edge-on, what they see
is a grating pattern (called a linear zone plate) consisting of a
sort of bar-code pattern.
The Argonne device succeeds so well in
focusing X-rays because the position of the zones can be controlled
to within nanometer tolerances through the deposition process, and
the depth of the zones that the X-rays experience can be made
arbitrarily long -- microns long -- by merely cutting a thicker
section of the multilayer wafer. In tests so far, one of these zone
plates, very slightly tilted to the X-rays coming out of a
synchrotron source, has focused 20-kiloelectronvolt X-rays to a line only 30 nanometer
wide, better than previously possible.
According to Argonne
researcher Brian Stephenson (email@example.com, 630-252-3214), an
ideal version of this kind of X-ray lens, which they call a
Multilayer Laue Lens (MLL), should be able to focus X-rays to a spot
of 1 nanometer or less. The likely uses for a better X-ray lens are in
full-field microscopy (making a magnified X-ray image of a sample)
or in scanning probe microscopy (by scanning the beam across a
absorbing photons from a laser, an atom can be excited to any of
various discrete energy levels allowed by quantum mechanics. What
about artificial atoms? A quantum dot, created by the same
lithographic methods used to prepare electronic chips, is nearly a
zero-dimensional zone of semiconducting material; as with electrons
inside atoms, electrons inside the confinement of a quantum dot will
also possess only a restricted menu of allowed energies.
is true for a pair of quantum dots 200 nanometer apart; with just the right
voltage applied, electrons can tunnel from one dot to the other. In
fact, an electron, considered as a spread-out quantum wave
phenomenon, can be considered to reside in both dots at the same
time, a property which makes the quantum-dot "molecule" potentially
useful for carrying out quantum computing operations.
Now, a group of scientists have been able to probe, and to change,
the quantum energy states of a double quantum dot with sound waves,
or more particularly surface acoustic waves excited in the substrate
supporting the dots.
The acoustic waves, less than 1 nanometer in
amplitude, ripple through the surface for distances as long as
hundreds of microns as a sort of nano-earthquake, are created
through the process of piezoelectricity; a small voltage is sent
into a series of tiny electrodes painted onto the surface. This
excites the faint acoustic waves (see figure at
Physics News Graphics).
arrangement, mediated by the delicate interactions between electrons
and phonons, can work in both directions: The quantum dots can be
used to monitor the acoustic waves -- otherwise difficult to detect
because of their tiny
energy -- or the acoustic waves can
be used to interrogate the electronic status of the dots, which
makes possible the aforesaid quantum-information applications.
researchers involved work at the University of Twente and the Delft
University of Technology (Netherlands), NTT Corporation, Tokyo
Institute of Technology, and University of Tokyo (Japan), and Jilin