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Surface resonant structures can be used as spectral filtering elements. Both band-stop and band-pass designs are possible using capacitive and inductive mesh concepts respectively. With the current evolution in electron lithographic technology to allow the realisation of sub-micron features, it has become possible in recent years to produce resonant mesh arrays to high degrees of fidelity and performance over significant areas of optical substrates. These can provide, for example, responses in the infra-red spectral region using elements with a dimensional scale of a few microns. The unit cell design in the mesh can be relatively complex and can be tailored to provide specific spectral responses in the waveband range of interest. Such structures can find a wide variety of applications, especially as dichroic beam splitter elements for the separation of infra-red and RF radiation. The possibility of using such structures for spectral filtering has been known for many years, particularly for the microwave, millimeter and far infra-red wavelength regime. Some of the earlier applications in the far JR have been discussed by UJriCh [1,21 More recently, Byrne et al [31 fabricated capacitive and inductive mesh filters for the infra-red using electron beam lithography and demonstrated the bandstop and bandpass transmissive properties achievable. Byrne and coworkers used a lift-off technique to create capacitive mesh patterns having linewidths of less than O.25j.tm in O.lp.m thick aluminium and gold on calcium fluoride. Crossed capacitive dipoles of aluminium 2.6.tm in length with an aspect ratio of 10:1 produced a broad reflection band at 6.25tm. The measured bandwidth (FWHM) was about 2.5pm. For inductive designs, a two level lift-off approach was adopted, involving pattern transfer from an initial capacitive design using oxygen reactive ion etch techniques. A 1.8pm length slot with aspect ratio of 10:1 produced a resonance at 6.5.tm with peak transmittance of about 80% and bandwidth of 2.75p.m. Low levels of intrinsic absorption in the slot material of the inductive mesh are enhanced by the resonant effect of the mesh and reduce the level of transmittance achievable. It is of interest from the point of view of potential high power laser applications to explore the relationship between the level of optical absorbance and the ensuing laser damage threshold of such meshes. Mohebi [71 explored the response of wire grid polarisers to pulsed C02 laser irradiation at 10.6j.m and found that damage thresholds were largest for the case when the polarisation of the incident radiation was perpendicular to the wires. Antirerfiection coatings reduced the damage threshold. This work assesses the case of crossed dipole meshes and explores the role of absorption by incorporating weakly absorbing films in the meshes and by the addition of dielectric material between the absorber and the antenna plane. It also explores the role of resistive loss in determining the properties of capacitive meshes. The absorber used has optical constants of n = 2.9 - 0.25i which are typical of some transition metal oxides and chalcogenide materials.
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