▎ 摘　　要

NOVELTY - The element (20) has a support portion (21), and a metasurface (22) is formed on a surface of the support portion. The metasurface includes a metal pattern (24) is mounted to emit an electron in response to incidence of an electromagnetic wave. A metal layer (25) contains an alkali metal and is formed on the metal pattern. The metal layer extends beyond the metal pattern to reach a region on the surface of the support portion, the region not formed with the metal pattern. The support portion is made of silicon, quartz, sapphire, or zinc selenide. A material of the metal pattern includes gold, platinum, aluminum, graphene, silver, or copper. USE - Metasurface element for electron tube (claimed). ADVANTAGE - The resistance value of the surface of the support portion can be reduced, and the occurrence of charge-up on the metal pattern can be suppressed. The probability of tunneling can be increased and the sensitivity can be improved. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following: (1) an electron tube; and (2) a method for producing an electron tube. DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of the holding unit. Metasurface element (20) Support portion (21) Metasurface (22) Metal pattern (24) Metal layer (25)