far infrared CVD Diamond OPtics: Properties and Applications
T P Mollart and K L Lewis
Defence Research and Evaluation Agency
St. Andrews Road, Great Malvern, Worcs. WR14 3AX UK
Abstract
Diamond is a unique optical material having the combination of high strength and transparency in the visible, near and far infrared regions. As a far infrared window it has a wide range of potential uses in demanding environments, e.g. high power CO2 lasers exit windows and airborne optics. In this paper, the general optical properties of diamond will be briefly reviewed and why it is so useful will be highlighted by a comparison to other infrared materials. The far infrared optical properties, such as absorption and scatter and how they are linked to the material characteristics will be described.
Recent progress in synthesis technology enables large area three dimensional diamond components to be fabricated. The optical properties of a polished 70 mm diameter hemispherical diamond dome will be described. When considering diamond for large area applications the mechanical properties are important. The strength of CVD diamond is determined by its polycrystalline nature, how it is one of the limits to potential applications will be discussed.
High temperature transmission spectra from 5000 cm-1 (2 μm) to 200 cm-1 (50 μm) for CVD diamond will be presented. In the far infrared region the transmittance drops with temperature and a broad peak approximately centred at ~1275 cm-1 (7.8 μm) emerges. In this experiment the temperature dependency of the absorption coefficient has been deduced by assuming there are no additional losses from the surface and calculating the reflectance using a temperature dependent value for the refractive index. Direct emissivity and photo-thermal deflection measurements have verified these finding and indicate that it is an intrinsic property of diamond.
Work is still necessary on optical coatings for diamond, since in many of these applications coatings would enhance the functionality of the optic. Good adhesion to diamond can be achieved by exploiting the surface termination of the surface. How durable antireflection oxide coatings, such as yttria (Y2O3) can be deposited on diamond and their properties will be described.
Keywords: Diamond, optical properties, domes, AR coatings
T.P. Mollart
M210
DERA Malvern
St. Andrews Road
Malvern
Worcs.
WR14 3AX
Tel: + 44 (0) 1684896649
Fax:+44 (0) 1684894311