Control of Polarised Light Flashcards
What are the main principles used by crystal polarisers?
Birefringence, Brewster’s angle and Total internal reflection
How might 2 birefringent prisms be combined
rotate prisms so that ordinary polarisation in first prism is extraordinary in the second.
The perpendicular polarisation goes from high to low refractive index and undergoes TIR
Parallel polarisation transmitted near brewster’s angle
Describe the Nicol Prism
2 prisms combined with parallel optic axes glued with material n between no and ne.
snells law separated beams at entrance and perpendicular polarisation undergoes TIR so it can be absorbed
parallel polarisation is transmitted near Brewster’s angle
Describe the Wollaston Prism polarising beam splitter
2 rotated birefringent prisms use refraction alone to generate orthogonally polarised components
(in words) what is the extinction ratio
the ratio of the transmitted irradiance through polarisers oriented parallel then crossed is the extinction ratio - ideally it would be infinity
What does a retarder do?
Changes the polarisation of an incident wave because one polarisation sees more phase delay than the other and this changes the relative phase.
How is a birefringent crystal cut to make a retarder?
so that its optic axis is parallel to both back and front surfaces of the plate
What is the formula for the phase difference or retardation
Δ𝜙 = 2𝜋/𝜆 [|𝑛𝑜 − 𝑛𝑒|] 𝑑 (radians)
what is the jones vector for waveplate output polarisation
1 |
|exp 𝑖 2𝜋/𝜆 [𝑛𝑜 − 𝑛𝑒] 𝑑|
Assuming a 45 degree input plane polarisation, what will a quarter waveplate do?
create circular polarisation
Assuming a 45 degree input plane polarisation, what will a half wave plate do?
rotate linear polarisation by 90 degrees
What is the effect of a 𝜆/4 plate if linearly polarised light is incident parallel to either principal axis
there is no effect
what are the principal axes for a uniaxial crystal
the polarisation directions of the ordinary and extraordinary ray
What happens to a beam propagating through a 𝜆/2 plate
one polarisation experiences half a wavelength more phase delay than the other
the handedness of the beam will be inverted
What is the phase difference of o and e rays emerging from a quarter wave plate
pi/2
Describe a system that could be used to block reflected light
A 𝜆/4 plate placed between a linear polariser and a reflecting surface
the 𝜆/4 place converts the LPL to RCPL which is then reflected, changing the handedness.
The linear polariser now blocks the light that passes through the 𝜆/4 plate
Incoming light 45 degree polarised
wave plate with axes at 0 or 90 degrees
Describe the output for a 𝜆/4 and 𝜆/2 plate
𝜆/4 - circular
𝜆/2 linear in opposite sign to incoming
Incoming light 0 or 90 degree polarised
wave plate with axes at +/- 45 degrees
Describe the output for a 𝜆/4 and 𝜆/2 plate
𝜆/4 - circular
𝜆/2 - 90 or 0 degree polarisation
When does no polarisation rotation occur for a wave plate
if the input polarisation is parallel to the wave-plate principal axes
When can a wave-plate be called a zero-order waveplate?
If it has less than 2pi relative phase delay
what is the formula for the thickness of a zero order 𝜆/4 plate
d = 𝜆/(4|n0-ne|)
derived from |n0-ne|d2pi/𝜆=pi/2
what are the advantages and disadvantages of a multi-order waveplate
it had more than 2 pi relative phase delay
for a 𝜆/4 plate |n0-ne|d2pi/𝜆=pi/2 + N(2pi)
N is the number of waves of relative phase delay
it is thicker but much more wavelength dependent
How do you make a thick zero order wave plate?
combine 2 plates with fast and slow axes opposite to each other
as long as d1-d2 is equal to d for the zero order plate, it will behave like one
What are the components of a circular polariser?
a linear polariser followed by a quarter waveplate
How could a right circular polariser be made into a right circular analyser
by exchanging the order of the elements
what is the matrix representing a mirror
|-1 0|
| 0 1|
what is optical activity
the ability of materials to rotate the direction of linear polarisation as the light travels through
what happens to linearly polarised light in an optically active material
the plane of vibration is continuously rotated as it propagates along the optic axis
what is the rotation angle in an optically active material
𝛽 = (𝑛𝐿 − 𝑛𝑅)(𝜋𝑑/𝜆0)
What can happen from a Fresnel reflection
polarisation rotation
When will reflection cause depolarisation
if the interface has different parallel x and y reflection coefficients and the light is not purely parallel or perpendicularly polarised
