Polarizing materials

Question 1

Which 2 types of linear polarizers are there?

Answer 1

1. Absorptive: unwanted polarization states are absorbed
2. Beam splitting: unpolarized beam is split into two beams with opposite polarization states

Question 2

What is birefringence?

Answer 2

The polarization induced by an incident EM wave in an anisotropic material depends on the polarization of the incident wave. Because χ determines εr, which in turn determines n, the propagation of, e.g., x- and z-polarized waves in such a material will proceed according to different refractive indexes. One very striking manifestation of birefringence is the phenomenon of double refraction e.g. in calcium carbonate.

Question 3

What is an opic axis?

Answer 3

An optic axis of a crystal is a direction in which a ray of transmitted light suffers no birefringence (double refraction). An optic axis is a direction rather than a single line: all rays that are parallel to that direction exhibit the same lack of birefringence.

Question 4

Example of beam splitting linear polarizer

Answer 4

Birefringent polarizer: split of unpolarized light into:
* ordinary ray, polarization ⊥ optic axis
* extraordinary ray, polarization // optic axis

Question 5

What is the optical anisotropy?

Answer 5

Materials with anisotropic binding forces (chemical bonds) present anisotropic properties.
* Velocity of propagation
* Polarization
* Absorption
of an EM wave in a crystal depends on the direction of propagation k and polarization of the incident wave

Question 6

What is dichroism?

Answer 6

A material is dichroic if light in different polarization states travelling through, experiences a different absorption coefficient
* soft axis have low resonant frequency
* stiff axis have high resonant frequency

Selective absorption of one of the two orthogonal polarisation components of an incident beam (definition from script)

Question 7

How can polarization be obtained by reflection?

Answer 7

Stack of plates at Brewster’s angle

Question 8

What are liquid crystals?

Answer 8

Substance that can flow like a liquid but its molecule may be oriented in a crystal-like way.

The modulation of a liquid crystal’s optical activity, viewed between two sheet polarizers, is the principle operation of a LCD

Question 9

What is the difference between Isotropic, nematic, cholesteric and smectic structures?

Answer 9

• Isotropic: random orientation and no periodicity
• Nematic: orientation but no periodicity
  * Cholesteric: nematic twisted structure –> OFF (V=0) LC state (bright), polarization of light.
  * Smectic: orientation and some periodicity –> ON (V≠0) LC state (dark), optical activity is lost.

Question 10

Where does plasma frequency quantity come from?

Answer 10

Metals can be treated as plasmas,
i.e., an ensemble of charged particles composed by an equal number of positive ions and free electrons. If the electrons in the plasma are displaced from the uniform background of ions, electric fields are built up in such a direction as to restore equilibrium by pulling the electrons back to their original positions.

Because of their inertia, the electrons will overshoot and oscillate around their equilibrium positions with a characteristic frequency ωp, typically around 10-30 eV (deep UV region), known as the plasma frequency. As a result, electromagnetic waves with frequency ω < ωp cannot enter a metal and are totally reflected, whereas electromagnetic waves with frequency ω > ωp are transmitted.