This invention relates gratings and their application as wavelength filters, selective polarisors and as absorbers. It has particular but not exclusive application to microwaves.
Over the past few decades, interest has grown in enhanced transmission of electromagnetic waves through periodic metallic samples such as hole arrays and deep metallic arrays. Recently this has been attributed to Surface Plasmon Polaritons (SPP""s) within the cavities of such samples causing the transmission of radiation though sample with cavity widths much smaller than the wavelength of radiation.
The study of the excitation of SPP""s on metallic gratings has been carried out for over a century. However nearly all these investigations have been carried out with relatively shallow gratings which produce real diffractive orders.
The inventors however have determined that if the pitch of a grating is made shorter than half the incident wavelength and it is made very deep, then the side of the grooves come so close together that it is possible for the evanescent fields of excited SPP""s on each side to interact across the narrow cavity. For certain depths the SPP""s set up standing waves with in the cavity, causing large field enhancement within the grooves. The deep zero order grating provides a large number of such grooves in the form of a slat structure which will then give strong transmission of long wavelength radiation provided it is incident polarised with a component of the electric field orthogonal to the groove surfaces.
Accordingly the invention comprises a grating comprising a plurality of substantially parallel members having a conducting surface of depth L, separated by a dielectric layer gap, and having of pitch xcexg and where L greater than 16xcexg.
Preferably the members are metal slats.
The slats may alternatively comprise foil covered plastic. The gaps may be filled wholly or partially with dielectric material.
In a particularly advantageous embodiment the gap is filled wholly or partially with liquid crystal whose refractive index can be controlled by suitable application of voltage across the gap. This allows for a variable i.e. selective wavelength filter/polariser.
Preferably the gap is less than 1 mm.
The inventors have moreover ascertained a number of interesting effects and applications of this phenomena which will be clear from the description.