Two-photon polymerization (2PP) is now an established technique for nanofabrication. Conventional fabrication
processes using laser 2PP commonly use a single point beam delivery system in order to write artifacts in the volume of
a resin. Complex shapes such as micro-models, woodpile photonic structures and spiral structures have been realized in
several material systems including, sol-gels, organically modified ceramics and resins. One area of current interest in
2PP micro fabrication is the introduction of more complex beam delivery systems, aimed at introducing a degree of
parallel processing to the writing method. In this paper we describe two alternative parallel processing approaches to beam delivery that demonstrate the use of
diffractive and refractive optics to shape the laser before focusing. Firstly a Fraunhofer diffractive optical element to
generate a linear array of 4 spots of equal intensity thus writing four structures simultaneously. Secondly an axicon lens
is used to form an annulus in the focal plane of the focusing element. This enables complete three dimensional annular
structure fabrication without the use of scanning stages.
For all experiments, a Ti:sapphire laser was used to initiate 2PP microfabrication. The materials system used was a
Zr/PMMA hybrid prepared by the sol-gel method on a glass substrate.
We report on the design of phase only diffractive optical elements (DOEs) with a locally improved signal to noise ratio. The formation of low noise regions is particularly useful for designs where it is difficult to achieve low background noise throughout the entire reconstruction. Such a case may arise in highly quantized elements or elements with a small spatial bandwidth. The DOE design is achieved by a direct search algorithm which allows for the definition of arbitrary low-noise areas in the reconstruction plane. However, the method is not restricted to direct search and should work with all direct schemes. While the local suppression of noise works rather well other performance criteria like diffraction efficiency and reconstruction error do, in practice, not suffer much from the additional constraints. This is due to the advantage that amplitude freedom and phase freedom can be maintained in large areas of the reconstruction. It is shown how this approach may be used in the design of beam splitters to prevent the formation of unwanted repeated spot patterns.
We present the fabrication of phase only diffractive optical elements (DOEs) by contact prints using a novel technique. Phase only DOEs consist of a transparent slide with a surface profile that modulates the phase of an incident laser beam in the desired manner. This surface profile can be exploited to split or to shape the laser beam. In our process a Perspex sheet (PMMA) is exposed through a chrome-on-quartz mask that carries the DOE pattern. The UV light used in the exposure is generated by an ArF excimer laser (l=193nm). The high energy radiation causes photomodification and the phase pattern of the DOE is transferred to the PMMA. While the primary investigations concentrate on binary DOEs, which require one exposure only, we have also attempted to produce multilevel designs which need mask alignment and tighter control of the exposure. At the moment, we study the capability of the method to produce relatively large structures but it might also be possible to use it to manufacture finer structures.