Photocathode

Abstract

An electron-emitting photocathode includes a base and a large number of projecting elements such as microscopic wires projecting from a surface of the base. The photocathode has high quantum efficiency, and hence can be used as the emitting element in a sensitive phototube.

1 . A photocathode comprising: 
 (a) a base having horizontal directions;    (b) a set of elements projecting from the base in a vertical direction transverse to said horizontal directions, said elements being adapted to emit electrons upon absorption of light by said elements.    
     
     
         2 . A photocathode as claimed in  claim 1  wherein said elements are substantially parallel to one another.  
     
     
         3 . A photocathode as claimed in  claim 1  wherein said projections have substantially equal height in said vertical direction.  
     
     
         4 . A photocathode as claimed in  claim 3  wherein said height is between 0.1 and 201 μm.  
     
     
         5 . A photocathode as claimed in  claim 1  wherein said elements are substantially evenly distributed in at least one of said horizontal directions.  
     
     
         6 . A photocathode as claimed in  claim 5  wherein said elements are substantially evenly distributed in at least two of said horizontal directions.  
     
     
         7 . A photocathode as claimed in  claim 1  wherein said elements are elongated structures.  
     
     
         8 . A photocathode as claimed in  claim 7  wherein said elements are wires.  
     
     
         9 . A photocathode as claimed in  claim 1  wherein said set of elements has an aggregate surface area, and said base has a surface area, said aggregate surface area of said set of elements being substantially greater than said surface area of said base.  
     
     
         10 . A photocathode as claimed in  claim 1  wherein said set of elements is provided at a density of greater than 10 9  elements per square centimeter over at least a part of said base.  
     
     
         11 . A photocathode as claimed in  claim 1  wherein said set of elements includes an ordered array of said elements in a hexagonal arrangement.  
     
     
         12 . A photocathode as claimed in  claim 1  wherein said elements have surfaces formed from a low-work function material.  
     
     
         13 . A photocathode as claimed in  claim 1  wherein said low-work-function material is selected from the group consisting of alkali metals, alkali metal antimony compounds, cesium, cesium oxide, diamond, silicon, carbon nanotubes, III-V compound semiconductors and combinations thereof.  
     
     
         14 . A photocathode as claimed in  claim 1  wherein said base and said elements are formed integrally with one another.  
     
     
         15 . A method of converting light to an electrical signal comprising: 
 (a) maintaining a photocathode as claimed in  claim 3  at a negative potential relative to an anode structure in a vacuum, and    (b) applying the light to the photocathode, whereby electrons are emitted from said element to said anode structure.    
     
     
         16 . A method as claimed in  claim 15  wherein the height of said projections is substantially equal to:  
         nλ/4  where: 
 n is an integer; and  
 λ is the wavelength of the light.  
   
     
     
         17 . A phototube comprising: 
 (b) an anode structure;    (a) a photocathode for receiving light, said photocathode having a base and a plurality of elements that project from a surface of said base toward said anode structure, said photocathode being capable of emitting electrons upon receipt of said light;    (c) an enclosure maintaining said photocathode and said anode in a vacuum.    
     
     
         18 . A system comprising a phototube as claimed in  claim 17  and a potential source electrically coupled to said anode and said photocathode, said source maintaining said photocathode at a negative potential relative to said anode.  
     
     
         19 . A system as claimed in  claim 18  further comprising a light source for providing said light.  
     
     
         20 . A phototube as claimed in  claim 17  wherein said anode structure includes an electron multiplying device for receiving said emitted electrons.