A new toolbox to tune polaritons
© Gwenolé Gasnier
EPFL scientists show for the first time a method for tuning polariton collisions, with great implications for quantum physics research and superfluidity.
Polaritons are formed when an excited electron combines with a photon. There are several types of polaritons depending on the light wavelength and type of matter particle. These can include ‘conventional’ particles like electrons or other exotic particles like phonons and excitons, which occur from the behavior of electrons in materials. Polaritons themselves appear both as waves with measurable frequencies, amplitudes and wavelengths, and can also be described as ‘particles’ with properties such as mass, spin, and velocity.
The group of Benoît Deveaud has shown how polaritons can be controlled inside a semiconductor microresonator, which is a device that can ‘hold’ polaritons for a prolonged period of time. The researchers trapped two polaritons with anti-parallel spins inside the microresonator and tuned their energies to be equal to the energy of the excitonic molecule, the so-called biexciton. When the energies of the polaritons and the molecular states “match”, the strength of collisions between polaritons strongly increases revealing large nonlinearities. Therefore polaritons might display either repulsive or attractive interactions depending on their relative energies respect to the biexciton state.
This method is called “Feshbach resonance” is a remarkable physical phenomenon allowing engineering of the fundamental interactions and the scattering properties of free particles. It is also a key tool for understanding Bose–Einstein condensates (BECs), which are unconventional states of matter that occur in extremely low temperatures. In a BEC, particles agglomerate into their lowest possible state of energy, giving rise to phenomena like superconductivity (zero electrical resistance) and superfluidity (zero liquid viscosity). The introduction of Feshbach resonance can instigate a wide range of major observations in cold-atom systems.
The observation carried out by Deveaud’s group imparts a comprehensive understanding of Feshbach resonance in spinor polariton gases, which widens the palette of Feshbach resonance techniques already explored in cold atom physics. Moreover, it provides an unprecedented tool for controlling the behavior of polaritons, which can pave the way for novel device principles such as single-photon generation based on “Feshbach blockade” or polariton switches.
Reference
Takemura N, Trebaol S, Wouters M, Portella-Oberli MT, Deveaud B. Polaritonic Feshbach resonance. Nature Physics (2014) DOI: 10.1038/nphys2999