Metallic screens for sub-wavelength focusing of electromagnetic waves
View Patent ↗A method and apparatus for near-field focusing of an incident wave, over a range of frequencies from microwaves to optical frequencies, into a sub-wavelength spot having a peak-to-null beamwidth of λ/10. The screen may be made out of closely spaced, unequal slots cut on a metallic sheet. Nano-scale focusing capability may be achieved with a simple structure of three slots on a metallic sheet, which can be readily implemented using current nanofabrication technologies. Unlike negative-refractive-index focusing implementations, this “meta-screen” does not suffer from image degradation when losses are introduced and is easily scalable from microwave to Terahertz frequencies and beyond. The slotted geometry is designed using a theory of shifted beams to determine the necessary weighting factors for each slot element, which are then converted to appropriate slot dimensions.
1. A device for focusing an incident electromagnetic plane wave having a frequency of between 10 3 and 10 15 Hertz, the device comprising a ground plane having a plurality of parallel slots cut therein selectively positioned to focus the plane wave in the near field focal plane into spots which have a maximum diameter less than the wavelength of the plane wave, wherein each slot is of differing length than each other slot.
2. The device of claim 1 characterized in that the ground plane is a metallic screen.
3. The device of claim 1 characterized in that the plurality of slots comprises at least three slots.
4. The device of claim 1 characterized in that the plurality of slots comprises a central slot and a pair of lateral slots, one on either side of the central slot, each slot having a length of between 0.95λ and 1.05λ.
5. The device of claim 1 characterized in that the arrangement of slots is selected such that inductive and capacitive slots are adjacent one another.
6. A method of focusing electromagnetic waves into spots in the near-field plane, comprising directing an incident electromagnetic plane wave towards a ground plane, the ground plane having a plurality of parallel slots cut therein, wherein each slot is of differing length than each other slot.
7. The method of claim 6 wherein the ground plane is a metallic screen.
8. The method of claim 6 characterized in that each of the plurality of slots comprises a central slot and a pair of lateral slots, one on either side of the central slot, each slot having a length of between 0.95λ and 1.05λ.
9. The method of claim 6 characterized in that the frequency of the electromagnetic waves is between 10 3 and 10 15 Hertz.
10. The method of claim 6 characterized in that variation of the amount of loss at the screen does not alter the diameter of each spot.
11. A method for fabrication of the screen of claim 1 comprising the following steps:
selecting a plate of stainless steel having of a desired thickness;
laser cutting a plurality of slots into the plate; and
stretching the plate on a frame.
12. The device of claim 1 for increasing the field strength at the image plane in an electromagnetic wave focusing system.
13. The use of the method of claim 6 for microwave imaging.
14. The use of the device of claim 1 for microwave imaging.
15. The use of the method of claim 6 for lithography.
16. The use of the device of claim 1 for lithography.
17. The use of the method of claim 6 for sensing.
18. The use of the device of claim 1 for sensing.
19. The use of the method of claim 6 for microscopy.
20. The use of the device of claim 1 for microscopy.
21. The use of the method of claim 6 for detection.
22. The use of the device of claim 1 for detection.
23. The use of the device of claim 1 for focusing an incident electromagnetic plane wave in two dimensions, both azimuth and elevation.
24. The method of claim 6 , wherein the focusing is in two dimensions, azimuth and elevation.
25. A device for focusing in two dimensions an incident electromagnetic plane wave having a frequency of between 10 3 and 10 15 Hertz, the device comprising a ground plane having a plurality of parallel slots cut therein selectively positioned to focus the plane wave in the near field focal plane into spots which have a maximum diameter less than the wavelength of the plane wave, wherein each slot is of differing length than each other slot.