Focusing optics

The beam is focused by means of Frensel zone plates (ZP). In a simplest case a zone plate consists of a set of radially symmetric rings, known as Fresnel zones, which alternate between opaque and transparent. Light hitting the zone plate will diffract around the opaque zones. The zones can be spaced so that the diffracted light constructively interferes at the desired focus, creating an image there. Zone plates produce equivalent diffraction patterns no matter whether the central disk is opaque or transparent. In order to get complete constructive interference at the focus, the amplitude of the diffracted light waves from each zone in the zone plate must be the same. This means that for an evenly illuminated zone plate, the area of each zone is equal. Because the area of each zone is equal, the width of the zones must decrease farther from the centre. The maximum possible resolution of a zone plate depends on the smallest zone width.

Actual zone plate has an infinite number of focuses the distance of which in the case of incident plane wave is described in the following formula:
 

F_n=Dd / nl

where n is diffraction order, D is zone plate diameter, d is outmost zone width and l is wavelength of the incident light. The utmost resolution of the zone plate (smallest spot diameter) is determined by diffraction limit and equal to 1.22d. Additionally one should include projection of the source size and account for variation of the focal distance with beam nonmonochromatisity to determine the total spatial resolution of the instrument.

To select the necessary focus order zone plate is used in a combination with order selecting aperture (OSA), i.e. a pinhole that blocks unwanted radiation from other diffraction orders. The zero order (straight light) is blocked with the circular central stop mounted at the centre of the zone plate, which should br bigger than pinhole diameter. Typical dimensions of optics at ESCA microscopy are:

The OSA and zone plate should be in the same axes with micrometric precision. The distance between them is changing with photon energy. Therefore they are moved with two separate XYZ steppermotor stages as shown in the following figure. The OSA and ZP are mounted on separate L-shaped holders attached to the appropriate stage. The positioning controlled by the channelplate image of the beam passed through the optics. When OSA and ZP are concentric the photon cone from the ZP with a central stop is imaged as a symmetric ring.

Frensel zone plates

Optics XYZ steppermotor stages