# Sigma polarisation

1. Mar 17, 2006

### edallica

can anyone point me in the right direction to somewhere i can find out about sigma polarisation.

i want to know how it is defined.

thanks

2. Mar 17, 2006

### Tide

Basically p-polarization (pi) refers to the component of the wave parallel to a surface while s-polarization (sigma) refers to the component perpendicular to the s-polarized component.

3. Mar 19, 2006

### Claude Bile

Depends on the context.

When dealing with polarisation effects, it is necessary to adopt a frame of reference. For example, when dealing with angled reflections off surfaces, p is used to define the component parallel to the plane of incidence and s is used to describe the component perpendicular to the plane of incidence (the denotion s comes from the german(?) word for perpendicular). There are many other examples where p and s refer to different components.

Pi and Sigma are simply greek designations for p and s.

Claude.

4. Mar 20, 2006

### edallica

sorry guys i should have been more specific. im trying to get my head round the definitions for laser cooling. i always thought that sigma plus and sigma minus were just names for circularly polarized light, and described the way the E-vector rotated out a circle if you looked down the path which the light travels. but i know this is not the case, it is defined as the the rotation about the local magnetic field rather than about the direction of propagation.

what i really wanted was a place that told me why it is defined as the rotation about he local magnetic field and not the direction propagation.

sorry for not being clear!

if anyone can help here i would be really grateful

5. Nov 29, 2008

### xsemoi

What follows can be read in Demtröder "atomic phyiscs", keyword "Zeeman effect".

The B-field defines the quantization axis. That is, because the atom does not care, in which direction the photon travels, but just interacts with its electric field. Therefore all what matters to the atom is the direction the electric field of the photon oscillates.

Now, put the B-field along the z-axis and look at the emission proceess instead of the absorption. The atom has several ways to emit the photon.
1,2) into the z-direction (=axial direction) + circular polarized light. Depending on the rotation direction (clockwise or anti-clockwise) you call these photons sigma- or sigma+
3,4) into the radial direction + linear polarized light. Depending on the polarization the photon is either pi or sigma polarized. ONCE AGAIN: sigma (withou plus or minus) means LINEAR polarized.

It's not trivial, so you better check out the book.

Cheers.
semoi