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The ability to deliver the light from the LED source to the "target" is crucial to the overall success of the project. The light emitted from the micro-LED arrays is highly divergent (half angle ~35degrees) which is not useful for most applications. We are therefore looking to develop arrays of micro-optics to shape and control the light so that it is well suited to a particular application. Two approaches are being pursued to this aim each with its own features and benefits for particular tasks.
Refractive Optics (IoP)
Probably the most versatile form of optics and in particular lenses are ones based upon the refraction of light. Such lenses have been around for hundreds, if not thousands, of years but the main effort in this project is to produce arrays of micro-lenses specifically designed to shape and control the light form the LED arrays. As each LED element is a source in its own right we need to have the ability to control the light from each element without affecting the other elements close by. In particular there has been an emphasis on the use of materials with a high refractive index such as sapphire (the substrate material of the LED arrays), Silicon carbide and even diamond. Though the work being undertaken in the novel polymer aspect of the project we have also been working on polymer lenses suitable for use in the near UV.
| For the "hard" materials we have adopted the thermal reflow technique of fabrication followed by etching using the inductively coupled Plasma etching method. In initially a layer of photo resist is spun onto the substrate at the require thickness and subsequently patterned into pillars using conventional photolithographic methods. |
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| The resulting pillars are then heated such that the photo resist melts and due to surface tension effects forms a dome. |
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| The substrate is the placed into the inductively coupled plasma etching tool which removes both the photo resist (where present) and the substrate. Due to the differential etch rate, and the fact that one needs to remove a significant thickness of photo resist at the centre of the dome it is thus possible to produce a lens shape in the substrate. |
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Using a similar method it is also possible to produce negative lenses by embossing the photo resist to give a concave rather than dome shape and subsequent etching will produce a negative lens. In a further modification of the method we have produced self aligned lens arrays using a UV LED to cure the polymer before the thermal reflow stage.
Using these methods a wide variety of lenses have been fabricated with focal lengths from 10s of microns through to millimetres. Due to the unique size and array format we have had to develop novel methods of evaluating the lens characteristics. These have been based upon confocal microscopy in which a collimated laser beam is directed through the lens array to pass through the confocal pinhole to the detector and also a reflected beam is sent onto the array to determine the position of the top of the lens. |
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| By taking a series of confocal sections one can then build up a spatial profile of the lens performance. |
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Diffractive Optics (HW)
As well as using conventional refractive optics we have also been exploring the opportunities offered by diffraction of light to produce optical components. Here the phase of the light is controlled as it passes through different thickness of material (typically polymer) so that the light is diffracted by small features towards a focal spot. This method of beam control has several potential advantages. Simpler photolithographic methods can be adopted and in the long term low cost replication methods can be used to produce large numbers of optical elements by simply embossing a shape onto a plastic substrate.
The main difficulty of the method is the computation of the shape required to produce a particular focal power. As part of the project novel computer code has been generated towards this goal. Theoretically it is possible to produce an optical element with a very fine focal spot but there are limitations on the performance of such an element. The design has to be optimised for one wavelength and thus even with the limited spectral bandwidth of the GaN LEDs a sharp focus is not always possible. However, this wavelength dependence can also be made use of to produce optical elements with significantly different optical properties at different wavelengths. |
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