excitonphonon

Exciton-phonon coupling refers to the interaction between excitons—bound states of an electron and a hole in a semiconductor or insulator—and the quantized lattice vibrations, or phonons, of the host material. This coupling affects how excitons exchange energy with the lattice, altering their energies, lifetimes, and dynamics, and plays a central role in shaping optical response across a wide range of materials.

The main mechanisms include deformation-potential coupling to acoustic phonons and Fröhlich coupling to polar optical phonons. In two-dimensional materials and in organic crystals, additional vibrational modes can couple to excitons. Strong coupling can lead to the formation of exciton polarons, where the exciton becomes dressed by a cloud of phonons, as well as phonon-assisted processes for exciton creation, recombination, and relaxation.

Experimental signatures of exciton-phonon coupling include temperature-dependent linewidth broadening of spectral features, Stokes shifts between absorption and emission, and phonon sidebands in optical spectra. The coupling can reduce exciton mobility and diffusion, influence radiative and non-radiative decay rates, and affect the luminescence quantum yield. In materials such as halide perovskites, transition metal dichalcogenides, and other soft lattices, coupling to low-energy lattice modes can be particularly pronounced and contribute to anomalous temperature behavior of optical properties.

Theoretical treatments range from perturbation theory for weak coupling to more comprehensive approaches that include phonon self-energies, such as the Bethe-Salpeter equation with electron-phonon interactions, or model Hamiltonians that capture deformation potential and Fröhlich interactions. These frameworks aim to predict exciton linewidths, energy shifts, and relaxation dynamics relevant to optoelectronic devices.

Applications of exciton-phonon knowledge span light emission, absorption, and energy transfer in semiconductors, two-dimensional materials, perovskites, and organic semiconductors, informing material design to optimize exciton lifetimes and transport.