Diffraction gratings are primarily used to diffract waves. The effect is similar to a prism splitting white sunlight into a rainbow of colors, except that diffraction gratings can be applied to many different parts of the electromagnetic spectrum where prisms don’t work. Gratings can even diffract matter waves. Gratings are often used to generate a spectrum of an object of interest (e.g. a star, a molecule), which tells you the intensity of radiation emitted from that object as a function of wavelength. This kind of data can provide a wealth of information about the makeup and properties of the emitting object.
The SNL has a long history of being a worldwide leader in the fabrication of highly specialized diffraction gratings in advanced applications, mostly related to space science.
X-ray Diffraction Gratings
Both fabrication and theory of x-ray diffraction gratings are a challenge. The x-ray band is typically considered to cover wavelengths between 0.01 and 10 nanometers, i.e. three orders of magnitude in wavelength. Many different approaches are used to try to maximize grating efficiency for sub-regions or even specific wavelengths in the x-ray band.
For applications in x-ray astronomy the requirements are even more stringent. X rays are absorbed by the earth’s atmosphere, and therefore x-ray telescopes have to be deployed in space. This demands the lowest possible mass for every component of an astronomy x-ray telescope. In addition, if the telescope contains a grating spectrometer, x-ray astronomers are generally interested in a fairly broad band of wavelengths, which means that the spectrometer needs to be efficient over the whole band of interest.
The SNL recently has concentrated on the development of efficient broad band gratings for the so-called soft x-ray band, ranging from ~ 0.5-10 nm in wavelength. Most x rays are easily absorbed by matter, which makes it very difficult to manipulate them without high losses, and this is especially true for soft x rays.
Soft x-ray diffraction gratings fall into two main categories: Transmission gratings and reflection gratings. Transmission grating spectrometers have the advantage of being insensitive to grating misalignments and non-flatness. X-ray transmission gratings are also very thin and therefore light-weight, and they become highly transparent at higher energies, which allows harder x rays to be collected at the imaging focus by, for example, a microcalorimeter imaging spectrometer. Reflection gratings have the advantage that they can be blazed for higher orders, which increases dispersion, and that diffraction efficiency can be high due to the use of small angles of grazing incidence. The drawbacks of grazing-incidence reflection gratings are that they are very sensitive to alignment and figure errors, and that they need to be long in the direction of propagation in order to intercept a given mirror aperture. Both drawbacks lead to relatively long and thick grating substrates and significantly more mass per mirror area than transmission gratings.
Critical-angle transmission (CAT) gratings represent a recent breakthrough development at MIT’s Space Nanotechnology Lab. They are blazed transmission gratings that combine the advantages of both transmission and reflection gratings.
Before the invention of CAT gratings the SNL pioneered fabrication of the most efficient in-plane and off-plane x-ray reflection gratings.
The SNL blazed the trail for small-period x-ray transmission gratings in the early nineties via interference lithography, resulting in the High-Energy Transmission Grating Spectrometer (HETGS) on board of the Chandra X-ray Observatory.