Molecules are groups of atoms. Depending on the mobility of molecules, the state of a material can be classified as solid, liquid, or gas. Gases have very high mobility, and the reason we can smell delicious food is because food molecules fly into our noses. Liquids have moderate mobility, allowing them to adapt their shape to the container they are in. Solids, on the other hand, have limited mobility, which is why they maintain a fixed shape. However, even in solids, molecules are not still; they vibrate continuously, even at a temperature close to absolute zero. We can image the vibration of molecules in a solid using the following model. Atoms are connected by chemical bonds, which can be likened to springs. The atoms vibrate around their positions, with the chemical bonds providing a restoring force. The term used to describe the vibrational motion of atoms or molecules is ‘phonon’, which is used in the quantum world to describe the quantized vibration of a group of atoms/molecules in a solid. When the vibration is related to thermal effects, it is called an optical phonon, and when it is related to mechanical waves, it is called an acoustic phonon.
When light interacts with optical phonon, it induces Raman scattering. Raman scattering is inelastic scattering and there will be energy exchange between the photon and phonon. This exchange leads to a frequency shift in the scattered light wave. On the other hand, when light interacts with acoustic phonons, it induces Brillouin scattering, which is also an inelastic scattering process resulting in a frequency shift.
In contrast, Rayleigh scattering is not caused by light interacting with phonons. Instead, it occurs when light encounters irregularities in the material. For example, quartz and glass are both made of silicon dioxide, but their atomic structure differs. Quartz has a well-ordered atomic structure, while glass has a disordered atomic structure. When glass is cooled from its liquid state to its solid state, the disordered structure is fixed, leading to irregularities that induce small refractive index differences between different places in the material. This causes Rayleigh scattering to occur. However, in high-purity quartz, Rayleigh scattering is greatly reduced.
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Reference
- Krohn D A, MacDougall T, Mendez A. Fiber optic sensors: fundamentals and applications. Bellingham, WA: SPIE Press, 2014.