Phonon-photon coupling in semiconductors is a fundamental process that underpins various optical and thermal phenomena. This interaction is critical for understanding energy dissipation, thermal conductivity, and optical properties in materials like silicon (Si) and gallium arsenide (GaAs). The primary mechanisms of light-phonon coupling include Raman scattering and Brillouin scattering, which provide insights into vibrational modes and their influence on material behavior.

Semiconductors exhibit distinct phonon modes due to their crystalline structure. In Si, the diamond lattice gives rise to optical and acoustic phonons, while in GaAs, the zinc-blende structure introduces additional polar phonon modes. These phonons interact with photons through inelastic scattering processes, where energy is exchanged between the electromagnetic wave and the lattice vibrations.

Raman scattering is a widely studied phenomenon where incident photons interact with phonons, resulting in a shift in the photon's energy. This shift corresponds to the energy of the involved phonon mode. In Si, the first-order Raman spectrum shows a sharp peak at approximately 520 cm⁻¹, corresponding to the triply degenerate optical phonon at the Brillouin zone center. GaAs, being a polar semiconductor, exhibits both longitudinal optical (LO) and transverse optical (TO) phonon modes, with Raman peaks near 292 cm⁻¹ (TO) and 269 cm⁻¹ (LO). The difference in intensity and frequency between these modes arises from the Fröhlich interaction, which couples photons to longitudinal phonons via the electric field generated by atomic displacements.

Brillouin scattering, on the other hand, involves interactions with acoustic phonons, leading to smaller energy shifts compared to Raman scattering. This process is sensitive to the elastic properties of the material. In Si, Brillouin scattering reveals acoustic phonon modes with velocities around 8430 m/s for longitudinal waves and 5840 m/s for transverse waves. GaAs, with its lower stiffness, exhibits slower acoustic phonon velocities—approximately 4720 m/s for longitudinal and 3340 m/s for transverse modes. These measurements are crucial for understanding thermal transport, as acoustic phonons dominate heat conduction in non-metallic solids.

The efficiency of phonon-photon coupling depends on several factors, including the material's dielectric function, phonon dispersion relations, and symmetry properties. In non-polar materials like Si, Raman scattering is governed by deformation potential coupling, where atomic displacements modulate the electronic polarizability. In polar materials like GaAs, additional coupling via the macroscopic electric field enhances the interaction strength for LO phonons. This distinction is evident in the Raman selection rules: Si exhibits a single dominant peak, while GaAs shows multiple peaks due to the breakdown of symmetry in polar crystals.

Temperature also plays a significant role in phonon-photon interactions. As temperature increases, phonon populations grow, leading to enhanced Stokes and anti-Stokes scattering intensities. The ratio of anti-Stokes to Stokes peaks provides a direct measure of phonon occupancy, following Bose-Einstein statistics. At room temperature, Si's Raman peak broadens due to anharmonic phonon-phonon interactions, while GaAs exhibits additional broadening from free carrier absorption in doped samples.

Strain and defects further modify phonon-photon coupling. In strained Si, the Raman peak shifts linearly with applied stress, enabling non-destructive strain measurements in semiconductor devices. Defects introduce localized vibrational modes, which appear as additional peaks or shoulders in the Raman spectrum. For example, oxygen impurities in Si produce a characteristic peak at 1136 cm⁻¹, while arsenic antisite defects in GaAs alter the LO phonon linewidth.

Advanced techniques like resonant Raman spectroscopy enhance the sensitivity to specific phonon modes by tuning the incident photon energy to electronic transitions. In GaAs, resonance with the bandgap energy (1.42 eV at room temperature) dramatically increases the LO phonon scattering intensity due to intermediate-state electron-phonon coupling. This effect is weaker in indirect bandgap materials like Si, where momentum conservation requires participation of multiple phonons.

Phonon-photon interactions also influence thermal properties. The coupling between optical phonons and photons contributes to the dielectric loss in polar materials, affecting their performance in high-frequency applications. In GaAs, LO phonon lifetimes extracted from Raman linewidth measurements are on the order of picoseconds, indicating rapid energy dissipation. Acoustic phonons, probed via Brillouin scattering, determine the thermal conductivity, which is approximately 150 W/m·K for Si and 55 W/m·K for GaAs at room temperature.

Emerging research explores engineered phonon-photon coupling in nanostructures. Confinement effects in Si nanowires or GaAs quantum dots modify the phonon dispersion, leading to shifts in Raman frequencies and altered scattering selection rules. Such effects are critical for designing optothermal devices where precise control over heat and light is required.

In summary, phonon-photon coupling in semiconductors is a rich field that bridges optical and thermal phenomena. Raman and Brillouin scattering serve as powerful tools to probe vibrational properties, while material-specific interactions dictate the coupling strength and spectral features. Understanding these mechanisms is essential for optimizing semiconductor performance in applications ranging from photonics to thermoelectrics.