Phonons are quantized sound waves that represent the collective lattice vibrations within solids, analogous to how photons represent electromagnetic waves. They are bundles of energy associated with the oscillations of atoms in a solid's lattice structure, leading to collective behaviors that define the material's properties, such as being a metal, insulator, or semiconductor. The interaction between photons and phonons can result in the absorption of photons by solids, converting their energy into heat, which is a manifestation of phonon vibrations. Understanding phonons is essential for grasping the thermal and electrical properties of materials.
PREREQUISITESPhysicists, materials scientists, and engineers interested in the thermal and electrical properties of solids, as well as students studying solid-state physics and quantum mechanics.
...When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.
A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.