X-ray optics

The x-ray optics is concerned with the propagation of X-ray radiation and its interaction with matter. It is not only in the wavelength range of the actual X-ray radiation (0.01 to 10 nm ) are applied, but also at wavelengths up to 100 nm (VUV ) radiation.

In the X-ray optics, a distinction between soft and hard X-rays. A soft X-ray of the area is referred to, in which the wavelength of the radiation is greater than the spacing of the atoms in the solid ( 0.1 nm to 0.5 nm). Here, the solid is considered as a homogeneous medium. Hard X-rays, however, is the area in which the wavelength is shorter than the distance between the atoms in the solid state, ie in the range 0.01 to about 0.5 nm Here comes the atomic structure of the solid advantage.

Differences in optics with visible light

X-ray, the refractive index of matter is about 1, the real part of the refractive index, which determines the phase velocity, is even smaller than 1 The reason for this refractive index is that the oscillation frequency of electromagnetic radiation in the X-ray region is greater than the oscillation frequency of the outer electron the illuminated atoms, perform the electric field in the x-ray radiation forced oscillations above its resonance frequency. This property can be used to define the X-ray radiation.

In the X-ray range vacuum is therefore the optically denser medium. The result: a simple interface reflects only good when it is illuminated at grazing incidence, so that total reflection at the interface occurs - similar to a beam of light (for light the optically denser medium ) to the surface is in the water and for shallow impact angle is reflected back into the water.

For this reason, X-ray mirrors only in grazing incidence, or at larger angles of incidence by multiple reflection as so-called multilayer mirror feasible.

The direction of propagation of electromagnetic radiation can, in principle, by utilizing change of refraction, reflection and diffraction, such as focusing.

For X-ray wavelengths - ie in particular for wavelengths shorter than 100 nm - there is no completely transparent to radiation ( " transparent " ) media. As a result, X-ray lenses have to be as thin as possible. The easier ways to combine X-rays, are mirrors and Fresnel zone plates. X-ray mirrors have a much planer surface show as a mirror for visible light. Diffuse scattering on a surface is generated by irregularities, the surface roughness can be mentioned. Is the average distance, or the size of the bumps is much smaller than the wavelength, the surface roughness plays only a minor role. However, if this distance is similar in size to the wavelength of light, an incident beam is mainly diffusely scattered and barely reflected as beam. For X-ray radiation, which has very small wavelengths, are surfaces that look perfectly flat in the visible light, often very rough.

X-ray mirrors

To compensate for the low reflectivity in the x-ray region, to use essentially three different methods:

Roaming incidence

The reflectivity of surfaces increases with flat werdendem angle of incidence. With a refractive index that is less than 1, there may be a total reflection at very shallow angles of incidence as shown. That is why mirrors are used under grazing incidence X-ray optics in common. An example of an optical device that works with grazing incidence, is the Wolter telescope.

Multilayer systems

If you need mirrors that provide high reflectivity at normal incidence angles and have to work at only one wavelength, mirrors are often used in multi-layer systems. They consist of two different materials that are in alternating layers one above the other. These multilayer systems are always built for a specific wavelength and a particular angle. Typically used in a corresponding optical wavelength thick, and a thin optical medium. The layer thicknesses are coordinated so that for the intended angle of incidence, the period always corresponds to the wavelength. It then arrives at the reflection at the optically denser layer to a constructive interference. A popular multi-layer system, for example, the combination of silicon and molybdenum for wavelengths around 13.5 nm silicon Here is the optically thin medium and molybdenum the optically denser.

Bragg reflection

For hard X-rays described by the Bragg equation constructive interference of the waves can be utilized in the crystal lattice. Such a crystal can act at a specific angle and a specific wavelength as a mirror.

X-ray optical devices

  • EUV lithography
  • X-ray diffractometer
  • X-ray microscope
  • Röntgenmonochromator
  • X-ray telescope
  • Refractive X-ray lenses
Resonant frequency Extreme ultraviolet lithography
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