What Are Polaritons?

Polaritons are hybrid quasiparticles that arise from the strong coupling between photons confined in an optical structure and elementary excitations in a material, typically excitons, phonons, or plasmons. When the coupling rate between the electromagnetic field and the material excitation exceeds the decay rates of both, the two modes cannot be treated independently; instead, they hybridize into new eigenstates called polariton branches. These mixed light-matter states inherit properties from both constituents: the low effective mass and long coherence length of photons combine with the nonlinearity and interaction strength of matter excitations, producing phenomena that neither photons nor bare material excitations exhibit alone.

The concept originated in the 1950s with theoretical work by Huang and others on the coupling of infrared photons to optical phonons in ionic crystals, producing what are now called phonon polaritons. The corresponding framework for exciton-photon coupling in semiconductors developed through the 1990s, and the first experimental demonstrations of semiconductor microcavity polariton effects established the platform that continues to drive much of the field. Polariton physics draws on condensed matter physics, quantum optics, and photonic engineering.

Exciton-Polaritons

Exciton-polaritons form when photons in a semiconductor microcavity, typically a planar structure with distributed Bragg reflector mirrors bounding a thin active layer, couple resonantly to excitons in the active material. The resulting dispersion relation splits into upper and lower polariton branches separated by the Rabi splitting energy, which characterizes the coupling strength. At low densities, lower-branch polaritons behave as bosons with an effective mass three to four orders of magnitude smaller than the free-electron mass, enabling them to accumulate in a single ground state at relatively high temperatures. Inorganic semiconductor microcavities based on gallium arsenide and cadmium telluride were early platforms, and organic and two-dimensional-material systems have since extended strong coupling to room temperature. The nature of exciton-polariton condensates and their relationship to Bose-Einstein condensation have been studied extensively in the context of non-equilibrium physics.

Phonon Polaritons and Plasmon Polaritons

Phonon polaritons arise when electromagnetic radiation couples to optical phonon modes in polar dielectrics such as hexagonal boron nitride, silicon carbide, or calcite. These quasiparticles can be confined to sub-diffraction wavelengths in the mid-infrared spectral range, enabling field concentrations many orders of magnitude beyond what conventional optics can achieve. Surface plasmon polaritons form analogously at the interface between a metal and a dielectric, where photons couple to collective oscillations of conduction electrons. Both varieties are central to the field of nanophotonics, and their sub-wavelength confinement properties have been characterized in detail through near-field scanning optical microscopy. Research from groups including those at NIST's Center for Nanoscale Science and Technology has contributed to understanding polariton propagation and damping at the nanoscale.

Polariton Condensation and Lasing

Above a threshold excitation density, lower-branch exciton-polaritons can undergo macroscopic occupation of the lowest-energy state, forming a non-equilibrium condensate that exhibits spatial coherence, long-range order, and superfluid-like behavior. This polariton condensation was reported in semiconductor microcavities in 2006, and has since been observed in organic, perovskite, and two-dimensional material systems at room temperature. The coherent output of a polariton condensate resembles laser emission but arises without population inversion, operating at lower threshold carrier densities. The threshold behavior, nonlinear gain, and coherence properties of polariton lasers in GaN microcavities have been studied for potential low-power photonic applications.

Applications

Polaritons have applications in a wide range of disciplines, including:

• Low-threshold polariton lasers and coherent light sources in photonic integrated circuits
• Quantum simulation using polariton lattices to model correlated quantum systems
• Ultrafast optical switching based on polariton nonlinearities
• Sub-diffraction nanoscale imaging and spectroscopy using phonon polariton near-field probes
• Quantum information processing using polariton condensates as qubits