Polarization of Light from Basics to Instruments

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Complex index of refraction

The index of refraction is actually a complex quantity:

real part

optical path length, refraction: speed of light depends on media

birefringence: speed of light also depends on P

imaginary part

absorption, attenuation, extinction: depends on media

dichroism/diattenuation: also depends on P

Part III: Optical components

Slide 34

Polarizers

Polarizers absorb one component of the polarization but not the other.

The input is natural light, the output is polarized light (linear, circular, elliptical). They work by dichroism, birefringence, reflection, or scattering.

Part III: Optical components, polarizers

Slide 35

Wire-grid polarizers (I) [dichroism]

Mainly used in the IR and longer wavelengths

Grid of parallel conducting wires with a spacing comparable to the wavelength of observation

Electric field vector parallel to the wires is attenuated because of currents induced in the wires

Part III: Optical components, polarizers

Slide 36

Wide-grid polarizers (II) [dichroism]

Part III: Optical components, polarizers

Slide 37

Dichroic crystals [dichroism]

Dichroic crystals absorb one polarization state over the other one.

Example: tourmaline.

Part III: Optical components, polarizers

Slide 38

Polaroids [dichroism]

Made by heating and stretching a sheet of PVA laminated to a supporting sheet of cellulose acetate treated with iodine solution (H-type polaroid). Invented in 1928.

Part III: Optical components, polarizers – Polaroids, like in sunglasses!

Slide 39

Crystal polarizers (I) [birefringence]

Optically anisotropic crystals

Mechanical model:

the crystal is anisotropic, which means that the electrons are bound with different ‘springs’ depending on the orientation

different ‘spring constants’ gives different propagation speeds, therefore different indices of refraction, therefore 2 output beams

Part III: Optical components, polarizers

Slide 40

Crystal polarizers (II) [birefringence]

The 2 output beams are polarized (orthogonally).

Part III: Optical components, polarizers

Slide 41

Crystal polarizers (IV) [birefringence]

Crystal polarizers used as: