Optical Device
An optical device has a deformable solid substrate, and a two dimensional array of metal particles which is carried by the substrate. The array provides a controlled separation between nearest-neighbour particles. Deformation of the substrate produces corresponding variation in the controlled separation such that the two dimensional array undergoes a transition between metallic and insulator surface reflectance.
1 . An optical device having:
a deformable solid substrate, and
a two dimensional array of metal particles which is carried by the substrate, the array providing a controlled separation between nearest-neighbour particles;
wherein deformation of the substrate produces corresponding variation in the controlled separation such that the two dimensional array undergoes a transition between metallic and insulator surface reflectance.
2 . An optical device according to claim 1 , wherein the substrate is a reversibly deformable substrate, and wherein reversible deformation of the substrate produces corresponding reversible variation in the controlled separation such that the transition between metallic and insulator surface reflectance undergone by the two dimensional array is also reversible.
3 . An optical device according to claim 1 , wherein the metallic surface reflectance is produced by substrate deformation states in which the controlled separation is less than that required for electronic tunnelling between the particles, and the insulator surface reflectance is produced by substrate deformation states in which the controlled separation is greater than that required for electronic tunnelling between the particles.
4 . An optical device according to claim 1 , wherein the two dimensional array of metal particles is a mono-layer of metal particles.
5 . An optical device according to claim 1 , wherein, in an undeformed state of the substrate, the two dimensional array has a metallic surface reflectance.
6 . An optical device according to claim 1 , wherein the metal particles have a largest dimension which on average is greater than about 3 nm.
7 . An optical device according to claim 1 , wherein the metal particles have a largest dimension which on average is less than about 300 nm.
8 . An optical device according to claim 1 , wherein the metal particles are formed of copper or a noble metal such as gold, silver or platinum.
9 . An optical device according to claim 1 , wherein the substrate is formed of an elastomeric material.
10 . An optical device according to claim 1 , wherein the substrate is transparent.
11 . An optical device according to claim 1 , wherein the metal particles are coated to prevent direct contact between the particles.
12 . A method of producing the optical device of claim 1 , the method including the steps of:
forming a self-assembling mono-layer of metal particles at an interface between a liquid and an immiscible fluid, the liquid being located on a support, and the metal particles having a coating which promotes migration of the metal particles to the interface and monolayer assembly, and
evaporating the liquids to leave the layer of metal particles as a two dimensional array lying on the support, the coating preventing direct contact between the particles.
13 . A method of according to claim 12 , wherein the immiscible fluid is a second liquid, both liquids being located on the support and being evaporated to leave the layer of metal particles as a two dimensional array lying on the support.
14 . A method of according to claim 12 including the further step of:
transferring the two dimensional array from the support to a surface of the deformable substrate.
15 . Use of the optical device of claim 1 as a sensor sensitive to an external stimulus which deforms the substrate, the transition between metallic and insulator surface reflectance being indicative of the presence or absence of the external stimulus.