Polarizer
An optical element that transmits one polarization state and rejects the orthogonal one, by absorption, reflection, or refraction into a different direction. Specified by its extinction ratio, transmission, acceptance angle and damage threshold.
A linear polarizer passes light whose electric field lies along its transmission axis and removes the orthogonal component. For linearly polarized input at angle to the axis, an ideal polarizer transmits the fraction of the power (Malus's law): 75% at 30°, 50% at 45°, 25% at 60°. A real polarizer is characterized by how far it departs from this: the transmission of the wanted polarization, and the extinction ratio, the ratio of the transmitted powers for input polarized along and across the axis, which for a ratio of is 50 dB. The extinction measured through a polarizer pair is the upper limit on any polarization extinction ratio a setup can verify.
The physical mechanisms divide polarizers into families with different strengths.
| Type | Mechanism | Strengths | Limits |
|---|---|---|---|
| Dichroic film or glass | Aligned absorbers (polymer chains, silver nanoparticles) absorb one polarization | Thin, large aperture, wide angle | Absorbs the rejected light; lower damage threshold |
| Wire-grid | Metal lines with period well below the wavelength reflect the field parallel to the wires | Broadband, wide acceptance angle, works into the infrared | Moderate extinction; some absorption |
| Birefringent crystal prism (Glan-Taylor, Glan-Thompson, Wollaston) | Birefringence separates the two polarizations, by total internal reflection in Glan prisms or by refraction into different angles in a Wollaston | Highest extinction, high damage threshold | Small acceptance angle, bulky, costly |
| Thin-film and Brewster-angle | Reflection at or near the Brewster angle, often in a multilayer polarizing beamsplitter | High power handling, both outputs usable | Narrowband, angle sensitive |
| In-fiber and on-chip | Mode filtering, bend loss or metal cladding that removes one polarization | No free-space alignment | Extinction limited by the guide's own polarization mixing |
The Brewster angle is where the reflection of p-polarized light vanishes, , which is 56.3° for glass of index 1.5. A single surface at that angle reflects only s-polarized light, weakly, so pile-of-plates polarizers stack many surfaces, and laser cavities use Brewster windows to select a polarization at no loss for the other.
Choosing among them is an engineering decision more than a physics one. A measurement of high extinction ratio needs a crystal polarizer, and the extinction it achieves depends on alignment and on the beam's divergence within the acceptance angle. A high-power laser line needs something that reflects rather than absorbs the rejected light. A camera or display needs a large, thin film. In each case the polarizer is described by the same few numbers, and its datasheet ratio applies only at the stated wavelength, angle and beam size. With a waveplate ahead of it, a polarizer also becomes an analyzer for the polarization state, the basis of Stokes-parameter measurement.
References: J. M. Bennett, "Polarizers," in M. Bass, ed., Handbook of Optics, 3rd ed., Vol. I (McGraw-Hill, 2010); E. Hecht, Optics, 5th ed. (Pearson, 2017).