Anti-Reflection Coatings & Quartz Wave plates
Reflection at the surface of receiver optical components inevitably implies a loss of sensitivity and can lead to unwanted effects like fringes, multiple images and ghosting. These effects can be greatly reduced by efficient anti-reflection coatings. Coatings can be tuned both to maximise and minimise the reflection coefficient of an optical component at given frequencies.
Our surface coatings can be applied to a range of optical materials to generate prescribed transmission and reflection properties throughout the THz range for components such as:
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cryostat vacuum windows
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lenses
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quarter wave and half wave plates (see below)
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filters
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ferrites
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The figure below shows the effect of a coating layer applied to z-cut quartz. Broad-band transmission across the observation band (in this case 200 to 300 GHz) increases from 75% for uncoated quartz (blue line) to 98% (red line).
Transmission improvement due to ARC on z-cut quartz window optimised for 200-300 GHz
Our coating technology, like so much of our product range, was originally developed for astronomical receivers in the far-infrared and sub-millimetre range of the spectrum, particularly for vacuum windows and wave-plates.
Materials
Materials that we can apply coatings to include:
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Polypropylene (PP)
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Polyethylene
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Sapphire
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Quartz (crystalline and vitreous)
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Diamond
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Silicon
Benefits of our coating service
Cheaper Materials
Our coating technology makes it possible to substitute cheaper synthetic optical materials for natural quartz crystals in wave plates, windows and lenses, where the size of components is limited by the availability of natural crystals.
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Reduced Weight
An optical component in sapphire can be seven times thinner than its quartz counterpart resulting in substantial savings in materials costs, mass and thermal load.
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More Efficient Optical Designs
The use of coatings on materials which have a relatively high refractive index at terahertz frequencies such as Quartz (n=2.1), Silicon (n=3.4) and Germanium (n=4) permits a larger radius of curvature in the design of lenses. This can reduce optical aberration and improve the broad band performance of optics.
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Better Performance
The graph below shows how the surface treatment of y-cut sapphire can dramatically reduce the variation in transmission of a wave-plate over a broad frequency range to 0.99-1.0 (depending on frequency) compared with 0.3-1.0 for untreated material.
Uncoated (light blue) and coated (dark blue) transmission of the QUEST instrument sapphire wave-plate
Quartz Waveplates
In addition to our windows, we can also supply AR coated half and quartz quarter wave plates. These waveplates can be used to change circular polarized light into linear,or vice versa (quarter wave plates) or rotate linearly polarised light (half waveplate). We take great care to ensure that our quartz is cut and mounted so that its crystalline axis can easily be identified. Like our other products, we don't make "standard waveplates" but work with our customers to design and tune the centre frequency to the customers requirements. The graph below shows the transmission of a quarter waveplate (QWP) designed to operate at 35.46 cm-1 (marked in black).
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