Plasmon tomography

1. A method comprising:

producing plasmon energy by exciting a plasmon resonance at at least one excitation position on a first surface of a first material;

detecting the plasmon energy at at least one measurement position on the first surface after the plasmon energy has propagated from the at least one excitation position to the at least one measurement position;

determining an attenuation of plasmon energy along a plurality of paths between the at least one excitation position and the at least one measurement position;

determining relative distances between the first surface and a second surface of a second material at a plurality of points on at least one of the surfaces based on the determined attenuation of plasmon energy along the plurality of paths; and

wherein determining an attenuation of plasmon energy along a plurality of paths between the at least one excitation position and the at least one measurement position includes determining an attenuation of plasmon energy along a plurality of paths between the at least one excitation position and the at least one measurement position for a selected relative position of the first and second surfaces.

2. The method of claim 1 , further comprising determining absolute distances between the first surface and the second surface at the plurality of points based on the relative distances and on a known distance between the first surface and the second surface at one of the points.

3. The method of claim 1 , wherein the first surface defines a part of a material comprising a photonic crystal.

4. The method of claim 1 , further comprising detecting plasmon energy at a plurality of measurement positions along a periphery of the first surface.

5. The method of claim 1 , further comprising exciting plasmon resonances at a plurality of excitation positions along a periphery of the first surface.

6. The method of claim 1 , wherein exciting a plasmon resonance comprises providing optical energy to the first surface.

7. The method of claim 1 , wherein at least one of the first or second surfaces defines a portion of a mask and the other of the first or second surfaces defines a portion of a substrate.

8. The method of claim 7 , wherein the mask comprises a plurality of plasmon guides on the first surface and wherein the plasmon guides define the plurality of paths on the first surface.

9. The method of claim 1 , further comprising altering the first or second surface after determining relative distances between the first surface and the second surface.

10. An apparatus comprising:

positioning structures configured to align a first surface of a first material with a second surface of a second material;

an optical energy source, wherein the source is alignable to provide electromagnetic radiation at a frequency responsive to excite a plasmon resonance at at least one excitation position on a first surface of the first material;

a detector assembly configured to produce signals corresponding to excited plasmon energy at a plurality of measurement position on the first surface spatially separated from the at least one excitation position by respective plasmon paths, wherein the plurality of measurement positions correspond to a fixed relative position of the first and second surfaces; and

a processor responsive to the signal corresponding to excited plasmon energy at at least one measurement position and the respective plasmon path and configured to determine at least one separation distance between the first material and the second material.

11. The apparatus of claim 10 , wherein the processor is further configured to determine an attenuation of plasmon energy along at least one of the respective plasmon paths and is configured to determine at least one separation distance between the first surface and the second surface based at least in part on the determined attenuation of plasmon energy along the at least one respective plasmon path.

12. The apparatus of claim 10 , wherein the processor is further configured to determine an attenuation of plasmon energy along a plurality of the respective plasmon paths and is configured to determine at least one separation distance between the first surface and the second surface based at least in part on the determined attenuation of plasmon energy along the plurality of the respective plasmon paths.

13. The apparatus of claim 10 , wherein the first surface defines a part of a material comprising a photonic crystal.

14. The apparatus of claim 10 , wherein the optical energy source is further configured to excite plasmon resonances at a plurality of excitation positions along a periphery of the first surface.

15. The apparatus of claim 10 , wherein the detector assembly is further configured to produce signals corresponding to excited plasmon energy at a plurality of measurement positions along a periphery of the first surface spatially separated from the at least one excitation position.

16. The apparatus of claim 10 , wherein at least one of the first or second surfaces defines a portion of a mask and the other of the first or second surfaces defines a portion of a substrate.

17. The apparatus of claim 16 , wherein the mask comprises a plurality of plasmon guides on the first surface and wherein the plasmon guides define the plurality of paths on the first surface.

18. The apparatus of claim 17 , wherein

the optical energy source is further configured to excite plasmon resonances at excitation positions substantially located at first ends of each of the plurality of plasmon guides; and wherein

the detector assembly is further configured to produce signals corresponding to excited plasmon energy at measurement positions substantially located at second ends of each of the plurality of plasmon guides.

19. The apparatus of claim 10 , wherein the detector comprises a coupler adapted for coupling plasmon energy at at least one of the plurality of measurement position into an electromagnetic wave.