Abstract

Conventional diffractive optical components require fabrication of an approximation to a surface shape and to achieve high efficiency, it is necessary to use multiple levels to build up the desired shape. For say 16 steps, four masking steps are required and each has to be accurately aligned. The overall process is time-consuming and does not have good yield. However the phase of the light can be controlled locally by using a series of slots of different widths provided these slots are narrow enough to avoid diffraction - thus forming an artificial dielectric. The key point is that the slots all have same depth and so only a single lithographic step is required.

We have designed artificial dielectric components using a hybrid technique that combines Rigorous Coupled Wave Theory (RCWA) and Effective Medium Theory (EMT). The model allows for the constraints imposed by fabrication (minimum linewidth and maximum aspect ratio of features) to be included automatically in the optimisation of the design. The first design is for a wedge, to be built in Gallium Arsenide for operation at a wavelength of 1.15 μm. The design has a theoretically diffraction efficiency greater than 90%.

The designed pattern is written in electron beam resist (PMMA) using a Leica Beamwriter. After development the pattern is transferred into an 100 nm thick layer of silicon oxide deposited by plasma enhanced chemical vapour deposition. This is then used as the masking layer in the transfer of the pattern into an epitaxial GaAs film on a GaAs substrate. The oxide layer is removed by wet etching prior to testing.

The wedge has an overall period of 2.3 μm and is divided into seven slots with a minimum linewidth of 70 nm. The etch depth is 740 nm. Initial devices gave a diffraction efficiency of greater than 80% - further details of the device performance will be given.

© 1998 Optical Society of America