Everything about Crystal Optics totally explained
Crystal optics is the branch of
optics that describes the behaviour of
light in
anisotropic media, that is, media (such as
crystals) in which light behaves differently depending on which direction the light is propagating. Crystals are often naturally anisotropic, and in some media (such as
liquid crystals) it's possible to induce anisotropy by applying for example an external electric field.
Isotropic media
Typical transparent media such as
glasses are
isotropic, which means that light behaves the same way no matter which direction it's travelling in the medium. In terms of
Maxwell's equations in a
dielectric, this gives a relationship between the
electric displacement field D and the
electric field E:
»
Thus these waves will see two different refractive indices and travel at different speeds. This phenomenon is known as
birefringence and occurs in some common crystals such as
calcite and
quartz.
If χ
xx = χ
yy ≠ χ
zz, the crystal is known as
uniaxial. If χ
xx ≠ χ
yy and χ
xx ≠ χ
zz the crystal is called
biaxial. A uniaxial crystal exhibits two refractive indices, an "ordinary" index (
no) for light polarised in the x or y directions, and an "extraordinary" index (
ne) for polarisation in the z direction. A uniaxial crystal is "positive" if n
e > n
o and "negative" if n
e < n
o. Light polarised at some angle to the axes will experience a different phase velocity for different polarization components, and can't be described by a single index of refraction. This is often depicted as an
index ellipsoid.
Other effects
Certain
nonlinear optical phenomena such as the
electro-optic effect cause a variation of a medium's permittivity tensor when an external electric field is applied, proportional (to lowest order) to the strength of the field. This causes a rotation of the principal axes of the medium and alters the behaviour of light travelling through it; the effect can be used to produce light modulators.
In response to a
magnetic field, some materials can have a dielectric tensor that's complex-
Hermitian; this is called a gyro-magnetic or
magneto-optic effect. In this case, the
principal axes are complex-valued vectors, corresponding to elliptically polarized light, and time-reversal symmetry can be broken. This can be used to design optical isolators, for example.
(A dielectric tensor that isn't Hermitian gives rise to complex eigenvalues, which corresponds to a material with gain or absorption at a particular frequency.)
Further Information
Get more info on 'Crystal Optics'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://crystal_optics.totallyexplained.com">Crystal optics Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
