The production and properties of holographic diffraction gratings

The techniques for producing high quality holographic diffraction gratings are investigated. These gratings are formed by recording the fringes of an optical interference pattern in such a way as to give grooves in a highly reflecting surface, and have been found to be superior to ruled gratings in terms of resolving power, although the sinusoidal groove profile means that the efficiency cannot be controlled easily.
In order to produce gratings of high quality, care must be taken in the design of the apparatus used to form the fringes. It is shown that the two interfering wavefronts must be identical in curvature and accurately plane (r ≫ 20m) in order to form an accurate plane grating.
The effects of the light source and environmental changes on the quality of the pattern are investigated, and the apparatus is designed to minimise these effects. Stability tests of the fringe system show that it is satisfactory.
The fringes are recorded in positive photoresist (Shipley AZ1350), which becomes soluble on exposure to light. The techniques for preparing the blanks by spin coating and developing the exposed gratings are discussed.
The exposure characteristics of the photoresist are measured and the groove profile for various combinations of exposure and development time is calculated. It is found that the minimum distortion from a sinusoidal groove profile results when the gratings are given a small, constant exposure to the fringes and the groove depth is changed by varying the development time. The resist surface is metallised to give a high quality reflection grating.
The efficiency of the gratings as a function of angle of incidence, wavelength and groove depth is investigated in various configurations. It is found that for coarse gratings (λ/d ≲0.25) with highly reflecting surfaces, simple scalar theory is sufficient to explain the observed behaviour.
For gratings with larger values of λ/d, there are fewer propagating orders and a corresponding increase in the intensity of the high orders. This introduces Wood's anomalies, which are rapid variations of efficiency occurring when one of the orders passes off the grating. These are polarisation dependant and cannot be explained on simple theories.
The basic properties of rigorous theories which assume a perfect metal are considered. These are found to be inaccurate in the region of the S polarised (E perpendicular to the grooves) anomaly.
The recent theories which take account of the optical properties of the metal are shown to predict accurately the observed variation of efficiency even in the region of the anomalies. The dependence of the grating properties on the metal coating has been shown to be due to an interaction between the radiation field and the electrons of the metal coating. This surface plasmon resonance is investigated in detail. The phenomena caused by the resonance are found to be:
i) A net absorption of energy by the grating;
ii) Redistribution of energy between the orders of the grating;
iii) An increase in the scattered light from the grating;
iv) A well defined angular distribution of scattered light.
The phenomena only occur with S-polarised incident light.
The conditions under which the resonance occurs are investigated and the effects are observed. The dependence of the resonance on groove depth is also investigated. It is shown how observations of the surface plasmon resonance may provide information concerning the properties of electron waves on periodic surfaces.
Finally, some applications of the holographic gratings made during this work are discussed. These are:
i) Radiation resistant gratings for use in synchrotron radiation;
ii) Gratings for tunable lasers;
iii) A multichannel spectrometer using a composite grating;
iv) Holographic zone plates as imaging elements.