Apparatus for Sub-Wavelength Near-Field Focusing of Electromagnetic Waves

Abstract

Planar sub-wavelength structures provide superlensing, i.e., electromagnetic focusing beyond the diffraction limit. The planar structures use diffraction to force the input field to converge to a spot on the focal plane. The sub-wavelength patterned structures manipulate the output wave in such a manner as to form a sub-wavelength focus in the near field. In some examples, the sub-wavelength structures may be linear grating-like structures that can focus electromagnetic radiation to lines of arbitrarily small sub-wavelength dimension, or two dimensional grating-like structures and Bessel (azimuthally symmetric) structures that can focus to spots of arbitrarily small sub-wavelength dimensions. The particular pattern for the sub-wavelength structures may be derived from the desired focus. Some examples describe sub-wavelength structures that have been implemented to focus microwave radiation to sub-wavelength dimensions in the near field.

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. 
     
     
         2 . The device of  claim 1 , wherein the ground plane is a metallic screen. 
     
     
         3 . The device of  claim 1 , wherein the plurality of slots comprises a central slot and a pair of lateral slots, at least one on either side of the central slot, each slot having a length of between 0.95λ and 1.05λ. 
     
     
         4 . The device of  claim 1 , wherein the arrangement of slots is selected such that inductive and capacitive slots are adjacent one another. 
     
     
         5 . The device of  claim 1 , wherein the frequency is between 100 Hz and 300 GHz. 
     
     
         6 . The device of  claim 1 , wherein the frequency is between 300 GHz and 3 THz. 
     
     
         7 . A method of focusing electromagnetic waves into spots in the near-field plane, comprising directing an incident electromagnet plane wave towards a ground plane, the ground plane having a plurality of parallel slots cut therein. 
     
     
         8 . The method of  claim 7  wherein the ground plane is a metallic screen. 
     
     
         9 . The method of  claim 7  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. 
     
     
         10 . An imaging device for receiving an incident optical radiation and producing an image from the incident optical radiation, the imaging device comprising:
 a planar structure having a sub-wavelength pattern that under illumination by the incident optical radiation forms the image at a near-field focal plane, located at a distance, L, from the planar structure, where the image has a resolution below, λ, and where L is below λ.   
     
     
         11 . The imaging device of  claim 10 , wherein λ is between 0.8-1000 μm. 
     
     
         12 . The imaging device of  claim 10 , wherein λ is between 400-800 nm. 
     
     
         13 . The imaging device of  claim 10 , wherein λ is between 400-200 nm. 
     
     
         14 . The imaging device of  claim 10 , wherein the sub-wavelength pattern is of a plurality of parallel slots. 
     
     
         15 . The imaging device of  claim 14 , wherein, across the sub-wavelength pattern, adjacent slots differ in slot height.