Method of defining an optical filter element and optical device comprising a filter structure

A method applied to an optical device, of defining the optical response of an optical filter element of the optical device comprising an optical cavity, the optical cavity comprising a first metallic layer, a dielectric layer over said first metallic layer, and a second metallic layer over said dielectric layer, the method comprising:

• configuring a structure of the second metallic layer to couple light incident on the optical cavity simultaneously into two different mode peaks of the optical cavity, to absorb said incident light at two different wavelengths, one to either side of a target wavelength, such that said structure appears coloured at said target wavelength in reflected or transmitted light;
• and
• controlling a polarisation of said incident light to select said optical response of the optical filer element, wherein said selection of the optical response comprises controlling the colour of the optical filer element;
• wherein a dominant mode of a first of said absorption mode peaks comprises a surface plasmon mode, and wherein a dominant mode of a second of said absorption mode peaks comprises a simultaneously present guided mode.

A method applied to an optical device, of defining an optical response of an optical filter element of the optical device comprising an optical cavity, the optical cavity comprising a first metallic layer, a dielectric layer over said first metallic layer, and a second metallic layer over said dielectric layer, the method comprising:

• configuring the second metallic layer as a 1D or 2D array of apertures in a metallic sheet or as a 1D or 2D array of metallic islands to couple light incident on the optical cavity into at least one surface plasmon resonant mode of the optical cavity, and varying a geometry of said 1D or 2D array over a lateral area of the optical filter element to vary an effective near or far field shape of the optical filter element, wherein said configuring comprises configuring a structure of the second metallic layer to couple light incident on the optical cavity simultaneously into two different mode peaks of the optical cavity, to absorb said incident light at two different wavelengths, one to either side of a target wavelength, such that said structure appears coloured at said target wavelength in reflected or transmitted light;
• and
• controlling a polarisation of said incident light to select said optical response of the optical filer element, wherein said selection of the optical response comprises controlling the colour of the optical filer element;
• wherein a dominant mode of a first of said absorption mode peaks comprises a surface plasmon mode, and wherein a dominant mode of a second of said absorption mode peaks comprises a simultaneously present guided mode.