Designing a Cir-Pol Sensitive to Left-Handed Light.

A concept which the reader may already be familiar with, is a Circular Polarizer, which first linearly polarizes light, and then renders the result circularly-polarized. But somebody might be interested, in creating a filter which is only sensitive to light circularly-polarized in one direction. Well, it turns out that this is as straightforward to achieve, as the first example, if we can assume that we have a birefringent layer, light with one known wavelength, and that we can adjust the thickness of the birefringent layer as needed.

cir-pol-inv_1-svg

 

cir-pol-inv_4

 

If we assume that light can first be linearly polarized, and then passed through the birefringent layer, whose extraordinary and ordinary axes are both at a 45⁰ angle to that of the original plane-polarization, then due to the higher refractive index of the extraordinary axis, its wave-function – i.e., dipole-moment – will become delayed with respect to that of the ordinary axis, until the former is phase-delayed by 90⁰, which is also 1/4 the wavelength of the light, with respect to the latter. In the example shown above, left-handed, circularly-polarized light has been achieved.

But the question could next be asked, what would happen if, we passed this helical beam of light, whose dipole-moments propagate as a left-handed helix, through another birefringent layer, that exactly matches the previous one. And the result which we’d obtain, is that the phase-position of the wave-function along the extraordinary axis, which has already been phase-delayed 90⁰, will be phase-delayed again, by another 90⁰, so that now its phase-position will be at 180⁰ to that, along the ordinary axis. And so where the diagram above showed full amplitude, it will consistently show zero amplitude, and full amplitude will take place perpendicularly, to where it had been before.

Thus, by controlling in which direction the extraordinary layer is followed by the transmitting direction of the linear polarizer that comes next, we can control whether the combination will be sensitive to left-handed or right-handed light.

I suppose that the mental exercise can be taken one step further, and we can ask what would happen, if directly after circularly-polarized light was achieved, said beam was bounced off a metallic mirror, with the directions of propagation before and after, ‘normal’ to the surface of that mirror.

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Mirrors

I’ve read, that essentially there exist three types of reflections in Physics:

  1. Metallic
  2. Non-Metallic
  3. Total Internal Reflection (See Below)

Metallic reflections (almost) tend to preserve the polarization of the light (except for what’s written below), while non-metallic reflections tend to polarize the light. The latter are also the basis for “polarizing mirrors”.

Beam-splitters are essentially polarizing mirrors:

  • When randomly-polarized light hits them, the deflected beam will be plane-polarized in one direction, while the transmitted beam will contain, what the deflected beam does not contain.
  • When circularly-polarized light hits them, nothing really prevents them from splitting the beam.
  • When plane-polarized light hits them, depending on the angle of polarization, the amplitude of one emerging beam can become much lower, than that of the other. This is probably also why, linear polarizers can interfere with the physical auto-focus of a DSLR-camera.

(Edit 02/25/2018 :

Even though the articles I gave above ‘seem complete’, only today I’ve learned that they need to be modified. Specifically, the deflected beam is only polarized perfectly, when the incident beam strikes a non-metallic mirror at Brewster’s Angle. And I have no reason to think, that this account is wrong. )

From what I read, reflection, according to the particle depiction, takes place, because photons couple with plasmons, to form surface-polaritons.

From what I read, refraction takes place, according to the particle depiction, because photons couple with excitons, to form photon-excition polaritons.

(Updated 02/27/2018 : )

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