Optical Microstructure Fabrication
Fabrication of Optical Microstructures at Tyndall National Institute
Physical structures with optical properties in the visible or near IR region have dimensions that are typically in the sub-micron region. The challenge is to developed low-cost embossing techniques that can be used to form the 2D structures. These structures have a broad range of applications.
Surface plasmon resonance is important in sensor applications including surface chemistry detection and biosensing The SPR sensing technology is based on the fact that the resonance condition is very sensitive to refractive index change of the surrounding medium around the metallic film.
There is a family of sensors based on corrugated surfaces and the fabrication solution is to use hot embossing. The close-packed hexagonal array is embossed in PET and a thin (70nm) layer of silver evaporated on top. When the local refractive index on the metal surface is varied, the shift in the transmission peak around 450nm is proportional to the refractive index change. The sensor is formed by measuring the refractive index of the ambient medium or coating the metal with a material that will change refractive index in the presence of the material under diagnosis.
Beam shaping and beam distribution are two important research areas in free-space propagation. Pattern generation using diffractive optical elements (DOEs) is arousing much interest in the development of low cost photonic systems. A single lightweight, compact element can provide functionality such as beam shaping, beam splitting, beam homogenisation or pattern generation and the applications are wide reaching (e.g. array generation, metrology, interconnects, lens correction). In addition, the element can combine the functions of several optical components. For example a pattern generator can be combined with a focussing element to give an array of focussed spots in the reconstruction plane.
For example of a binary (two phase level) DOE design is given where the levels are at a depth to give either a zero or p/2 phase change to a coherent illuminating beam. At some plane in the distance the 1st order diffraction pattern is reconstructed to give the target.
Again the solution is hot embossing and recently we have made a detailed study of embossing parameters on optical performance. The design of the DOEs was made using our DOEWizard code. This is an iterative unidirectional algorithm and centrosymetric binary DOEs can be designed with efficiencies of up to ~70%.
Semiconductor light-emitting diodes (LEDs) have the potential for use as low-cost and long lifetime solid-state lighting sources for various applications, including traffic signals, full-color displays, and back light units for liquid crystal displays. Although the performance of LEDs is continuously improving, in a standard planar LED, the light extraction efficiency is still limited to several percent due to the total internal reflection (TIR) at the high-refractive-index semiconductor/air interface, which is far from ideal. One method of improving the efficiency is to form a periodic pattern on
the emitting surface. The solution is to emboss the pattern in a high refractive index sol-gel. The extraction efficiency improved by 36% when a UV cured polymer was embossed onto the output surface.
Clearly, these illustrative applications show the benefits of using embossing techniques to fabricate optical microstructures. The patterns can be replicated with good resolution and at low cost.