Bragg and Brillouin diffraction

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Discussion Overview

The discussion revolves around the concepts of Bragg and Brillouin diffraction, particularly in the context of an acousto-optic modulator. Participants explore the relationship between sound waves and light diffraction, the derivation of relevant equations, and the implications of varying refractive indices in crystals.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about the equation for Bragg diffraction and its application to Brillouin scattering, questioning how multiple maxima can be produced when the angles of incoming and outgoing light must be equal.
  • Another participant suggests that a perturbation expansion can be used, indicating that small changes in the refractive index lead to Bragg scattering, and that the average value of the refractive index can be used to determine maxima.
  • A participant notes that changing the frequency of the sound wave alters the angle, which raises questions about the nature of the Bragg angle in this context.
  • There is a discussion about the definition of the Bragg angle and whether it can be varied by changing the frequency of the sound wave.
  • One participant asserts that in traditional Bragg diffraction, the incident and outgoing angles must be equal, while another suggests that the setup described is fundamentally different due to the ability to change the outgoing angle independently.
  • Another participant references a textbook and a website that explain Bragg scattering, expressing confusion about their relevance to the current setup.
  • A later reply clarifies that Bragg's law applies to the angle of incident and reflected light producing constructive interference, but notes that in their setup, the incident angle is constant while the frequency of the waves is altered, leading to a different relationship between angles.

Areas of Agreement / Disagreement

Participants express differing views on the application of Bragg's law to their specific experimental setup, with some asserting that the principles of Bragg diffraction do not fully apply to the acousto-optic modulator scenario. The discussion remains unresolved regarding the exact relationship between the angles and the implications of varying the sound wave frequency.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the refractive index and its variability, as well as the specific conditions under which the equations are derived. The relationship between Bragg and Brillouin scattering is not fully clarified.

cooev769
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I'm supposed to give a seminar tomorrow for my 300 level experimental physics paper. The experiments we do our reports on are pre-determined and I pulled the short straw with the most bloody complicated on here, the acousto-optic modulator. I'm trying to get my head around it and I've spend the good part of the day and not gotten very far. I understand that you require constructive interference to produce the maxima and due to the varying refractive indices of the material we will get a weird equation. But the equation we are given is

sin (θ) = λ/2nd

Where θ the bragg angle, d is the wavelength of the sound wave in the acousto optic modulator. So it seems odd to me firstly how are the multiple maxima produced when the light in angle must equal the light out angle shouldn't that just produce on maxima. Secondly how can we just chuck n in for the quoted refractive index of the crystal when this varies at every point. How the hell is this equation derived specifically for brillouin scattering. There are no good sources of this on the internet.

Please help, thanks.
 
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You are doing kind of a perturbation expansion here. The small changes of n lead to the Bragg scattering, but the condition for the maxima can be determined using the average value of n.
 
Oh okay because the crystal came with a designated n and the Bragg equation seemed useless then because this would vary. Unlike normal Bragg diffraction though we change the frequency of the sound wave causes a change in the angle does this sound right?
 
Sorry worded poorly. A Bragg angle is a defined angle for constructive interference of the incoming and outgoing wave. But in this case I'm guessing we can change the Bragg angle by varying the frequency of the sound wave is this correct?
 
Yes, because changing the frequency of the sound wave you change also the wavelength of the sound which modulates the diffractive index and acts as a lattice for Bragg scattering.
 
Sweet but for brag diffraction the theta in must equal the theta out. Seeing as you can keep the theta in constant and change the theta out I'm assuming it's fundamentally different?
 
cooev769 said:
Sweet but for brag diffraction the theta in must equal the theta out.

Ok, it isn't a Bragg scattering, rather refraction at a grating.
 
Well that's what I was taught. It's what my textbook introduction to optical electronics and this website says, so now I'm just doubley confused.

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/bragg.html
 
  • #10
Never mind had a chat to a solid state lecturer. Well Bragg's law tells you the angle at which theta is the incident and reflected angle producing constructive interference. Changing the lattice structure will require you to change the angle to get a new Bragg angle. This is different from my setup because with my set up we had a constant incident angle and altered the frequency of the waves to alter the outgoing angle. Which obviously wasn't equal to the incident angle like Bragg's law stare, yet when we used the equation we had to define theta as the outgoing angle only.
 

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