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Beam propagation in anizotropic crystals
Optic axis of a crystal is the direction in which a ray of transmitted
light suffers no birefringence (double refraction). Light propagates
along that axis with a speed independent of its polarization
However, if the light beam is not parallel to the optical axis, then,
when passing through the crystal the beam is split into two rays: the
ordinary and extraordinary, to be mutually perpendicular polarized.
A crystal which has only one optic axis is called uniaxial crystal. An uniaxial
crystal is isotropic within the plane orthogonal to the optical axis of the crystal.
A crystal which has only two optic axis is called biaxial crystal.
The refractive index of the ordinary ray is constant for any direction in the
crystal, and of the extraordinary ray is variable and depends on the direction.
In a uniaxial crystal for the direction parallel to the optical axis the refractive
indices are equal.
Positive
birefringence
Negative
birefringence
BBO is negative uniaxial crystal
Fast Axis
Direction having a low refractive index is the fast axis; at right angles to it is
the slow axis, with a high index of refraction.
Type I cut
kp =ks +ki
npwp=2nswscosqs
for ws = wp/2
np=nscosqs
For collinear down-conversion the index
of refraction at wavelengths differing by a
factor of two must be equal. For isotropic
material it is impossible, but by using
birefringent material we can achieve this
condition via the different indices of
refraction
of
orthogonal
linear
polarizations.
Index of refraction of BBO (negative
uniaxial crystal): blue solid curve:
ordinary polarization; red/dash-dot curve:
full extraordinary polarization; green/dash
curve: extraordinary polarization at the
phase matching angle (29o)
Calculating Characteristics of Noncollinear Phase Matching
in Uniaxial and Biaxial Crystals
N. Boeuf, D. Branning, I. Chaperot, E. Dauler, S. Guérin, G. Jaeger, A. Muller, A. Migdall
Originally published in: Opt. Eng. 39(4), 1016-1024 (April 2000).
http://units.nist.gov/Divisions/Div844/facilities/cprad/index.html
Polarization of light
Malus' law, which is named after Etienne-Louis Malus(1775 –
1812) , says that when a perfect polarizer is placed in a polarized
beam of light, the intensity, I, of the light that passes through is
given by
where
I0 is the initial intensity, and θi is the angle between the light's
initial polarization direction and the axis of the polarizer.
His discovery of the polarisation of light by reflection was
published in 1809 and his theory of double refraction of light in
crystals, in 1810.
Retarders
A retarder, or waveplate, is an optical device that resolves a light wave
into two orthogonal linear polarization components and produces a
phase shift between them. The resulting light wave generally is of a
different polarization form.
Ideally, retarders do not polarize, nor do they induce an intensity
change in the light beam. They simply change its polarization form.
Retarders are used in applications where control or analysis of
polarization states is required.
Half wave plate
A retarder that produces a λ/2 phase shift is known as a half wave
retarder. Half wave retarders can rotate the polarization of linearly
polarized light to twice the angle between the retarder fast axis and the
plane of polarization.
Placing the fast axis of a half wave retarder at 45° to the polarization
plane results in a polarization rotation of 90°. Passing circularly polarized
light through a half wave plate changes the "handedness" of the
polarization.
Quarter waveplate
If the orthogonal electric field components are equivalent, a phase
shift λ/4 in one component will result in circularly polarized light.
Retarders that cause this shift are known as quarter wave retarders.
They have the unique property of turning elliptically polarized light into
linearly polarized light or of transforming linearly polarized light into
circularly polarized light when the fast axis of the quarter wave plate at
45° to the incoming polarization plane. (Light polarized along the
direction with the smaller index travels faster and thus this axis is
termed the fast axis. The other axis is the slow axis).