Wireframe polarizer
A wire grid polarizer (or Hertzian grating ) is a polarizer for electromagnetic waves , the parallel arrangement of a well conducting metal wires is. It is permeable to waves whose electric field oscillates perpendicular to the wires. In the case of parallelism , the grating reflects like a conductive surface. Waves polarized obliquely or circularly or unpolarized are separated into a perpendicular and a parallel component. The wire grid polarizer does not absorb energy.
The distance and diameter of the wires should be significantly smaller than the wavelength. Mutual isolation is important.
functionality
When the electric field vector oscillates parallel to the wires - or other good electrical conductors - an alternating current is generated in them without the original electromagnetic wave being weakened. This alternating current radiates waves in all directions ( Huygens principle ), which have opposite phases to the incident wave.
- Both fields cancel each other out behind the grid. The reloading of the wires only acts like a slightly increased dielectric constant.
- In front of the grid, both waves are superimposed to form a standing wave . The wire mesh acts like a reflector.
- If a sufficient number of parallel wires are hit by the incident wave, no waves are emitted in "oblique" directions or parallel to the grating plane (phase compensation as with the optical grating ). This does not apply if the grid has too few wires. Their radiation diagram then resembles that of a dipole antenna .
In the limit of vanishing distances between the wires, a metal plate is obtained that reflects the wave.
If the electric field vector of the incoming wave forms a right angle with the wires , the electrons (if the wires are sufficiently thin, d << λ) cannot make any movements worth mentioning. Since a negligible alternating current does not emit a wave, the wave can pass unhindered.
Applications
In addition to demonstration experiments in teaching, grid polarizers are used to measure the direction of polarization of electromagnetic waves, e.g. B. on astronomical radio telescopes .
In 1960, a wire mesh with 2160 wires per millimeter was produced that also worked with infrared light. Today polarization films are available for light in the visible to the near infrared range.
Reflectors of linearly polarized antennas designed as rods or wire grids reduce their wind load - no closed plate is required as a reflector.
Individual evidence
- ↑ George R. Bird, Maxfield Parrish, Jr .: The wire grid as a near-infrared polarizer . In: Journal of the Optical Society of America . tape 50 , no. 9 , 1960, pp. 886-891 , doi : 10.1364 / JOSA.50.000886 .
- ↑ Wire grid polarizer with 100 nm wire spacing