The appearance of an x-ray interference pattern in a Fabry-Perot interferometer
has been achieved, for the first time, by a group of physicists at the
University of Hamburg (Yuri Shvyd'ko, 49-40-8998-2200).
This might lead to a new generation of x-ray optical devices, such as
high-resolution x-ray spectral filters, or x-ray clocks, and, more important
still, a new way of calibrating length measurements at the atomic scale.
X-rays are a potent type of electromagnetic radiation, with a much higher
energy and smaller wavelength than visible light. But because x-rays
are so potent and because they see various materials as having essentially
the same indices of refraction, x-rays are much harder to reflect at
a surface. Indeed, x-ray telescopes in orbit use only grazing-incidence
(reflected through an angle of a milliradian or less) mirrors to focus
x-rays on a detector.
In the last few years, though, the scientists in Hamburg have succeeded
in reflecting x-ray light directly backwards with special sapphire (Al2O3)
mirrors; the price for this high-angle reflectivity (other than the
difficulty of preparing faultless crystalline mirrors) is that the reflection
occurs only for an extremely narrow spectral range (x-ray color), precluding
the mirrors' use in telescopes, where x-radiation over a broad range
is important. In the Hamburg device, an x-ray version of a Fabry-Perot
Interferometer (FPI), the reflecting waves will resonate if the cavity
between two exquisitely polished mirrors is a multiple of the radiation
half-wavelength. Light entering the cavity bounces back and forth between
the mirrors producing multiple sub-waves emerging from the cavity. Their
interference shows up as a modulation in the radiation that exits the
cavity, both on time and wavelength scales. The Fabry-Perot interference
pattern provides a means of measuring of the x-ray wavelength, and this
provides an opportunity for creating a new, higher-precision, length
standard. Currently the most accurate way to measure x-ray wavelength
is to produce a Bragg scattering pattern by sending x rays into a silicon
crystal, whose lattice spacing (the distance between atoms) is known
with a relative uncertainty of about 6 x 10-8.
There is, however, a nuclear process related to the Mossbauer effect
which produces x-rays (better known as Mossbauer radiation) with an
extraordinarily narrow spectral line. The most familiar is the Mossbauer
radiation originating from the decay of the first excited state of 57-Fe
nuclei. The radiation wavelength of about 0.086 nm is perfectly suited
for atomic scale measurements. Its stability, about 10-15,
is comparable to the best cesium fountain clocks. If Mossbauer x rays
could be used to calibrate an FPI device capable of operating in both
x-ray and visible ranges, then this could facilitate a stable, reproducible,
wavelength (and hence length) standard far better than is possible (about
3 x 10-11) with, say, helium-neon lasers.
An important step toward this goal has now been attained in the experiments
of the Hamburg group conducted at synchrotron radiation facilities including
the Advanced Photon Source at Argonne (near Chicago) and HASYLAB at
DESY (near Hamburg). The x-rays, from the synchrotron-radiation sources,
were chosen to be as similar to Mossbauer rays as possible. For the
first time, interference patterns in a Fabry-Perot interferometer have
been observed for x-rays. From the attenuation time of the multiple
sub-waves emerging from the cavity, the spectral sharpness of the Fabry-Perot
interference fringes was estimated to be less than a micro-electron-volt.
This is more than 100 times better than the best x-ray crystal monochromators
can do. (Shvyd'ko
et al., Physical Review Letters, 10 January 2003; also see figure
and related Physical
Review Focus article,
17 July 2000).