CCD measures intensity prop. to z-component of C-Poynting vector OR |E-field|^2

[omg!]

intuitively, i would say that in a CCD, the photoelectrons are being "created" only by the electric field of the incident light, so that the intensity is proportional to
|E_x|^2+|E_y|^2,
with the xy-plane coinciding with the CCD plane.
But I have several papers here saying that the intensity is actually proportional to
(\textbf{E}\times\textbf{B}^*)_z.
could someone shed some light on why this should be the case? thanks!

 

[chrisbaird]

They both basically mean the same thing. The energy flow due to electrodynamic fields in general is described by the Poynting vector S:

S = E x H

For air, the magnetic field H and the magnetic field B are related according to B = μ₀ H, leading to:

S =(1/μ₀) E x B

The intensity in a certain direction is typically defined as the time-average of the Poynting vector dotted by the direction. For sinusoidal waves, the time-average is found by complex conjugating the magnetic field and multiplying by a half:

I = (<S>_t)_z =(1/2 μ₀) (E x B*)_z

If the wave is a transverse plane wave (not always the case), then the electric field and magnetic field are related according to: B= sqrt(μ₀ε₀) z x E which leads to:

I =(1/2Z₀) |E|²

For a transverse wave, there is no z-component to the electric field, only x and y components, so we have finally:

I =(1/2Z₀)(|E_x|² +|E_y|²)

So your last expression is more general, and your first expression only applies to transverse plane waves.

 

[omg!]

thank you for your clear explanation.

forgive my ignorance, but could you point out an example of a non-transversal EM wave?

i'm trying to calculate the intensity on the CCD resulting from a focused light beam. the two formulas give slightly different values, the more general one has a complex component, do you think one should simply take the real part or the modulus?

 

[chrisbaird]

If you are close enough to light sources, the fields are not necessarily transverse. This is called the near field. Also, light in a waveguide (like a fiber optic) is not necessarily transverse. If you are using the expression I =(1/2 μ₀) (E x B*)_z, then the result should be automatically real-valued only. That is the point of the complex conjugation operation ("*").