Quasiparticles

An exciton is a bound electron-hole pair in a semiconductor. When a valence-band electron is promoted to the conduction band, it leaves behind a positively charged hole. The electron and hole attract each other through the Coulomb interaction and can bind into a charge-neutral quasiparticle that moves through the material as a single entity. Because excitons are electrically neutral, they do not carry net charge current, but they strongly govern optical response near the band edge.

Because the electron and hole are bound by Coulomb attraction, the exciton resonance typically lies below the single-particle band gap by an amount set by the exciton binding energy. This binding energy is sensitive to dielectric screening, so exciton energies can be tuned by the surrounding dielectric environment. As a result, excitonic materials can serve as local probes, with exciton resonances shifting in response to nearby materials and interfaces.

Excitons are composite bosons formed from two fermions, and under suitable conditions they can exhibit collective phenomena, including Bose-Einstein condensation and superfluidity. Although they are bosons in the low densities, at higher densities, interactions between their constituent fermionic particles (electrons and holes) become stronger, giving rise to many-body effects such as exciton-exciton scattering and the formation of multi-exciton complexes, including biexcitons and triexcitons.

Excitons also carry internal spin and valley quantum numbers inherited from the electron and hole, which determine whether an exciton is optically “bright” or “dark” and set selection rules for absorption and emission. These spin-dependent properties enable control of exciton polarization and coherence, making excitonic systems useful for spin- and valley-based optoelectronics and for encoding and manipulating information in solid-state platforms.

Excitonic materials can mediate effective photon-photon interactions: photons can create excitons, excitons can interact while they exist in the material, and their recombination can emit photons. This light-matter conversion, together with exciton-exciton interactions, makes excitonic systems promising for nonlinear optics and optical information processing. A central goal in this area is to enhance and control excitonic interactions and reduce the decoherence pathways to improve processing efficiency and functionality.

A phonon is a collective excitation of lattice vibrations. In a crystal, atoms oscillate about equilibrium positions, and these coupled oscillations form collective normal modes that can be quantized into phonons. Acoustic phonons correspond to long-wavelength vibrations associated with sound and elastic deformations, while optical phonons involve relative motion between atoms within the basis and typically occur at higher frequencies. Phonons play central roles in thermal conductivity, heat capacity, and the temperature dependence of electrical transport, because they carry heat and scatter charge carriers. They also mediate key interactions such as conventional superconducting pairing and can couple strongly to light in polar crystals through infrared-active modes.

Since the phonon energy scales are on the order of meV, terahertz radiation ...

A magnon is a collective excitation of spin waves in magnetic materials. In a ferromagnet, for example, the ground state has spins aligned, and a magnon corresponds to a coherent precession pattern where the spin direction varies smoothly across the lattice. Quantizing these spin waves produces magnons that carry energy and angular momentum and exhibit dispersions set by exchange interactions, anisotropy, and external magnetic fields. Magnons govern low-temperature magnetic heat capacity and thermal transport, and they underlie technologies and research directions in spintronics and magnonics, where information can be carried by spin excitations rather than charge motion.

GHz-THz

Polarons describe Polarons are ...

Trions

Attractive/repulsive polaron

Dressed

Polarons describe charge carriers dressed by lattice distortion. When an electron or hole moves through an ionic or polarizable lattice, it can distort the surrounding ions and polarization cloud, and the combined object behaves as a new quasiparticle with renormalized properties. This dressing often increases the effective mass and modifies mobility, sometimes dramatically, especially in polar semiconductors and many oxides. In the small-polaron limit, the distortion is localized and transport can become thermally activated hopping. In the large-polaron limit, the distortion is more extended and the carrier can remain relatively mobile, with scattering and effective mass set by electron-phonon coupling and screening.

Plasmons are quasiparticles corresponding to collective oscillations of the electron density. In metals and doped semiconductors, electrons can oscillate coherently against the positive background, producing plasmon modes whose characteristic frequency depends on carrier density and effective mass. In reduced dimensions, plasmons can be tightly confined to surfaces, interfaces, or nanostructures, enabling strong field enhancement and subwavelength light confinement. Plasmons are central to plasmonics, where they are used to control light at the nanoscale, and they also influence optical spectra and screening in many electronic materials. 

The word “polariton” combines “polar-” from polarization and the suffice "-iton" that marks quasiparticles. The electric-dipole response of a dielectric medium gives rise to the new normal modes of Maxwell's equations. 

The concept of “dressing” is particularly useful. Whenever the

Dressed

Phonon-polariton

Exciton-polariton

Magnon-polariton

Magnon-phonon-polariton

...

Polaritons are hybrid light-matter quasiparticles that arise when an electromagnetic field couples strongly to a material excitation so that the resulting eigenmodes are mixtures of both. Common examples include exciton-polaritons (photon plus exciton) in microcavities and phonon-polaritons (photon plus optical phonon) in polar crystals. Because polaritons inherit properties from light, they can propagate rapidly and be manipulated optically, while also retaining material nonlinearities and interactions. This combination enables phenomena such as long-range energy transport, tunable dispersion engineering, and strong nonlinear responses at comparatively low power.