A polariton laser is a novel type of

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

source that exploits the coherent nature of Bose condensates of

exciton-polaritons In physics, the exciton–polariton is a type of polariton; a hybrid light and matter quasiparticle arising from the strong coupling of the electromagnetic dipolar oscillations of excitons (either in bulk or quantum wells) and photons. Because ligh ...

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

s to achieve ultra-low threshold lasing. In 1996, Imamoglu ''et al.'' proposed such a novel type of coherent light source and explained the concept based on an effect closely related to

Bose–Einstein condensation Bose–Einstein may refer to: * Bose–Einstein condensate, a phase of matter in quantum mechanics ** Bose–Einstein condensation (network theory), the application of this model in network theory ** Bose–Einstein condensation of polaritons ** B ...

of atoms: A large number of bosonic particles (here:

polaritons In physics, polaritons are bosonic quasiparticles resulting from strong coupling of electromagnetic waves (photon) with an electric or magnetic dipole-carrying excitation (state) of solid or liquid matter (such as a phonon, plasmon, or an exc ...

) form a condensate in a macroscopically occupied quantum state via stimulated scattering. The condensate of polaritons finally provides coherent emission of light. Thus, it is a coherent light source that owns a different working mechanism compared to conventional laser devices. Owing to its principle, a polariton-laser promises a more energy-efficient laser operation. The typical semiconductor structure for such a laser consists of an

optical microcavity An optical microcavity or microresonator is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium, or by wrapping a waveguide in a circular fashion to form a ring. The former type is a standing wave cavity, and ...

placed between

distributed Bragg reflector A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with different refractive index, or by periodic variation of some characteri ...

s. An early demonstration of polaritonic lasing and a comparison to conventional lasing was achieved in 2003 by H. Deng ''et al.'' at Stanford University under optical excitation (Polaritonic condensation was later fully linked to dynamical

in 2006 by Kasprzak ''et al.''). However, electrical pumping of a

polariton In physics, polaritons are bosonic quasiparticles resulting from strong coupling of electromagnetic waves (photon) with an electric or magnetic dipole-carrying excitation (state) of solid or liquid matter (such as a phonon, plasmon, or an exc ...

laser—crucial for a practical use of polaritonic light sources—was not demonstrated until 2013 when the first and unambiguous demonstration of an electrically pumped polariton-laser was presented by a team of researchers from the

University of Michigan The University of Michigan (U-M, U of M, or Michigan) is a public university, public research university in Ann Arbor, Michigan, United States. Founded in 1817, it is the oldest institution of higher education in the state. The University of Mi ...

and by a team from University of Würzburg together with their international partners using the similar techniques. At this stage, the electrically driven device operates at very low temperatures around 10 K and needs a magnetic field applied in the Faraday geometry. In 2007, even room temperature operation of an optically pumped polariton laser was demonstrated, promising the development of future electrically pumped polariton lasers for room temperature application. It is important, and challenging, to distinguish polaritonic lasing from conventional (photonic) lasing, owing to the similar emission characteristics. A crucial element of the success by both teams lies in the hybrid nature of polaritons whose matter component (excitons) exhibits a sensitive response to an external magnetic field. The Michigan team led by Pallab Bhattacharya used a combination of modulation doping of the quantum wells in the active region, to enhance polariton-electron scattering, and an external magnetic field to enhance the polariton-phonon scattering and the exciton -polariton saturation density. With these measures they achieved a comparably low polariton lasing threshold of 12 A/cm² (published in ''Physical Review Letters'' in May 2013). The investigations performed by the team in Würzburg, having started with the idea of engineering an electrical device in 2007, led to the desired effect after a few years in cooperation with their international partners from the U.S., Japan, Russia, Singapore, Iceland and Germany. Finally, their studies were complemented by a crucial experiment in a magnetic field: an unambiguous verification of the emission-mode's matter component in the polaritonic laser regime was given, yielding a first-time experimental demonstration of an electrically pumped polariton laser by C. Schneider, A. Rahimi-Iman and co-authors in the team of S. Höfling (published in ''Nature'' in May 2013). On June 5, 2014, Bhattacharya's team succeeded in creating what's believed to be the first polariton laser that is fueled by electric current as opposed to light, and also works at room temperature, rather than far below zero.

References

Semiconductor lasers Laser science {{Semiconductor laser Laser types

See also

References