water window created by differential absorption of oxygen and carbon. This allows
a natural contrast between a biological
specimen and the water that surrounds it.
Davide Bleiner, an assistant professor
in applied physics at the University of
Bern, heads up a group looking at such
applications. He is working with fellow
Bern professor and longtime EUV researcher Jürg Balmer, who developed
the compact source used by the group.
These devices fire laser pulses onto a
solid target. The initial ones ablate the target and create a plasma, and they are
followed by a main driving pulse. This
arrives at an angle, producing a wave
that travels through the plasma at a speed
calibrated to improve EUV production
Currently, the shortest wavelength possible with such an approach is 8. 8 nm,
obtained using lanthanum. Cutting the
wavelength will require going up in the
periodic table and increasing the driver
pulse energy. That could prove prohibitive
in a laboratory-scale installation for high-atomic-number elements. Some standard
techniques to cut wavelength can’t be used
because of their impact on imaging and
“We don’t want to do harmonics be-
cause that would provide poor EUV pho-
ton fluxes and broaden the spectrum, such
Fortunately, designs exist that use diffraction gratings on concave
substrates. These reduce the optical system to one reflective surface
that simultaneously disperses and focuses spectra. In a suitable vacuum
enclosure, they work to wavelengths of slightly < 30 nm, even when
using near-normal incidence angles. Spectral diagnosis of laser output
wavelengths is relatively fast and easy with these instruments. Appreciable wavelength regions in the vacuum-UV (VUV) can be measured
simultaneously when equipped with microchannel plate intensifiers or
direct-detection CCD detectors. These designs also are useful for scanning monochromator applications and for “dialing in” a particular
wavelength for subsequent focus optics and sample illumination.
Spectrometers for work at wavelengths of < 30 nm also may use
diffraction gratings on concave substrates. Angle-of-incidence changes
from normal to grazing boost reflective efficiency (see Figure 1, the
chart of modeled data, which underscores the requirement for use of
spectrometers in the soft x-ray and EUV, particularly when multiple optics are used). By using a 2° or 3° grazing angle of incidence, reflective efficiency can be > 80 percent at wavelengths as short as 8 nm.
Some soft-x-ray and EUV spectrograph gratings have been designed
and produced for dedicated regions. They can render spectral meas-
Figure 2. Czerny-Turner design uses multiple reflective surfaces and vacuum
enclosure for wavelengths of >110 nm.
urements of laser output in a fixed region relatively simple. More versatile, Rowland circle grazing incidence spectrometers are analytical
tools for greater wavelength regions and can work as scanning monochromators or spectrographs, depending upon selected detection
equipment; however, neither is ideal for dialing in an EUV wavelength
and illuminating a sample.
To accomplish this, more complex instruments with concave spherical, toroidal or off-axis parabolic collimation and focusing mirrors, in
combination with plane diffraction gratings, often are used. These can
become quite large, complex and, eventually, “beam line” instruments.
Because optical aberrations contribute significantly to instrument performance at grazing incidence angles, care must be taken to adopt
suitable designs and to plan for the best possible optics. Done well,
these select a discrete EUV laser wavelength with good spectral resolu-
Figure 3. Seya-Namioka design uses concave diffraction grating to simultaneously disperse and focus. Useful for wavelengths of > 30 nm.
tion and focus to a sample; e.g., in a photoelectron spectrometer.
Another important design consideration occurs when pulse broadening is of concern. A diffraction grating, rotated to tune for a particular
wavelength, can introduce appreciable broadening of narrow laser
pulses. To minimize the broadening introduced by rotation of a single
diffraction grating, spectrometers may be built as doubles. Two instruments work together to select wavelengths, while their mirrored optical
paths negate broadening effects. In recent years, developments include
double off-plane grazing incidence instruments that further minimize
residual broadening effects.
Spectrometers currently are available for much of the soft x-ray,
extreme UV and VUV regions, although not always in the form in
which we are accustomed to seeing. Selecting a spectrometer requires
prioritizing analytical goals, wavelength range, spectral resolution, dispersion and possibly optical aperture requirements in light of experimental needs. Shorter wavelengths require ever more care in selection
of optical schemes to maintain efficiency and performance.
The optical designs pictured in Figures 2 through 4 demonstrate the
progression from the multisurface normal-incident Czerny-Turner – often
used for visible spectroscopy – to the single-surface grazing-angle
spectrometers – useful in the soft x-ray and EUV regions.
Figure 4. Rowland circle grazing incidence design works at 2° or 3° angle
of incidence and is useful for wavelengths of >1 nm.