Why do we need polariton lasers?
The word ‘laser’ originally stood for ‘light amplification by stimulated emission of radiation.’ In polariton lasers, the radiation is emitted spontaneously, but it possesses all the characteristic properties of laser light: it is coherent in the first and second order and monochromatic. The concept of polariton lasers was formulated by Imamoğlu and coauthors in 1996.1 They are based on a type of quasiparticle called the exciton-polariton, which is made of light and matter and occurs in suitably designed semiconductor crystal structures.
Exciton-polaritons arise from interactions between excitons (neutral quasiparticles formed from bound electron-hole pairs) and photons (for instance, modes of visible light trapped in a semiconductor structure). Being bosons, exciton-polaritons can form condensates, similar to the Bose-Einstein condensates observed in gases of cold atoms. These condensates, in which large quantities of exciton-polaritons accumulate in a single quantum state, form the basis of the polariton laser. The exciton-polariton lifetime is much shorter than a nanosecond, and they decay by passing their energy to photons, which escape from the crystal. Being generated by identical exciton-polaritons, these emitted photons form monochromatic, coherent light.
Polariton lasers have been realized in semiconductor microcavities: multilayer crystal structures in which light confined between two parallel mirrors strongly interacts with the elementary excitons in the crystal. In 1998, Le Si Dang and coauthors observed polariton lasing at liquid-helium temperature.2 The first room-temperature polariton laser with optical pumping was realized in 2007 by a collaboration of the Southampton and Lausanne groups.3 At present, optimized schemes of electrically pumped polariton lasers based on gallium nitride (GaN) microcavities are discussed in the literature4 (see Figure 1).
Polariton lasers are expected to have lower thresholds than conventional semiconductor lasers.4 Their output power is quite limited, however, because exciton-polaritons dissociate at strong pumping. Consequently, the area of application for polariton lasers still needs to be defined. Two promising directions stand out: high-speed optical polarization switches and compact sources of terahertz radiation.
Optical polarization switches, or spin switches, enable light of a chosen circular polarization to be switched on and off in an optoelectronic device. Conventional approaches to such switching are typically based on nonlinear optical effects, which require high power and external optical elements. Spin switches based on polariton lasers instead exploit the spin properties of exciton-polaritons and the strong interactions induced between exciton-polaritons by their matter (exciton) component. Like photons, exciton-polaritons have two values of spin polarization, corresponding to left and right circularly polarized light, respectively. The polarization of light emitted by a polariton laser is governed by the spin of its exciton-polariton condensate, which can be controlled by external fields. The first optical spin switches based on semiconductor microcavities were realized by Amo and our coworkers in 2010, with gigahertz switching speeds.5 A low-power continuous-wave laser primes the system in a ready state so that a little additional ‘probe’ laser light will switch on the polariton laser. Such spin switches are important building blocks for polariton integrated circuits, which would carry information as domains of spin-polarized exciton-polaritons. Polariton integrated circuits would have an advantage of lower energy losses and faster information transfer compared to conventional electronic circuits.
For the terahertz radiation application, we recently proposed vertical-cavity surface-emitting terahertz lasers based on polariton lasers.6 Emission of terahertz radiation would be stimulated by a condensate of exciton-polaritons in these devices. Unlike many other types of terahertz laser, this design does not require a waveguide or a laser cavity for terahertz photons, allowing the whole structure to be microscopic. Tuning of the emission frequency may be achieved by shifting the optical excitation beam on the surface of a graded microcavity. Tunable terahertz lasers have wide areas of application in medicine, communication technologies, and security.
Polariton lasers will bring fundamental effects of many-body quantum physics to our everyday life. Their unique physical properties make them suitable for novel types of spin switches and terahertz lasers, which have many applications. The most promising materials for polariton lasing are the wide-band-gap semiconductors GaN and zinc oxide (ZnO), which allow for room-temperature action. We are working toward commercialization of polariton lasers. The next milestones along that path are the experimental demonstration of an electrically pumped polariton laser, the realization of polarization modulators and amplifiers based on polariton lasers, and demonstration of vertical-cavity terahertz lasing.
This work has been supported by the Royal Society and the European Union IRSES (International Research Staff Exchange Scheme) project POLAPHEN (Polarization Phenomena in Quantum Microcavities).
University of Southampton
and
Charles Coulomb Laboratory
University of Montpellier 2
Alexey Kavokin received his PhD at the Ioffe Institute of the Russian Academy of Sciences in 1993. In 1998 he became a professor at Blaise Pascal University in France, and in 2005 he moved to the UK, where he is Chair of Nanophysics and Photonics in the Department of Physics and Astronomy at the University of Southampton.
