Polariton laser reaches room temperature

28 Mar 2007

The first polariton laser to operate at room temperature has been demonstrated by physicists in the UK and Switzerland.

The researchers claim that the semiconductor device requires ten times less energy to operate than a comparable conventional solid-state laser. As a result the breakthrough could lead the way to very low power lasers for use in optical data-storage systems (Phys. Rev. Lett. 98 126405).

Polaritons are “quasiparticles” that arise from the coupling of light with an electric dipole in a semiconductor material. More precisely, they consist of a photon and an exciton, which is itself a dipolar quasiparticle comprising a bound electron-hole pair. Polaritons are part light and part matter &ndash and possess new properties not seen in either photons or excitons.

Light is emitted from a polariton laser in a process that involves the scattering of pairs of polaritons. The scattering is stimulated by a light from a separate pump laser. However, unlike conventional solid-state lasers – which consume a great deal of energy “pumping” the majority of valence electrons into the conduction band – very little energy is required to operate a polariton laser. Some physicists therefore believe that polaritons offer a way to create lasers with very low power requirements.

While polariton lasers have already been built, they only worked when cooled below 200 K. Now, Jeremy Baumberg and colleagues at Southampton University along with co-workers at Ecole Polytechnique Federale de Lausanne, have built a polariton laser that operates at room temperature (300 K).

The laser is a microcavity structure in which a layer of the semiconductor gallium nitride (GaN) several hundred nanometres thick is sandwiched between two layers of reflecting material. The size of the cavity is chosen to resonate with ultraviolet light of a certain wavelength and polaritons are created in the GaN as the light is reflected back and forth across the cavity.

Baumberg told Physics Web that GaN was chosen because the binding energy of excitons in this material is known to be very high, and therefore it is an ideal candidate for a polariton laser. Unfortunately, GaN is a very difficult material to work with and as a result Baumberg said that it took the group five years to get the lasers to work. However, he is hopeful that the performance of the lasers can be further improved as GaN processing technology matures. Indeed, Baumberg said the group has already reduced the original energy requirements of the laser by a further factor of ten and that more improvements are possible.

Baumberg also believes that the polaritons in GaN microcavities could form a Bose–Einstein condensate (BEC) at room temperature – something that has already been observed at much lower temperatures in microcavities made of other semiconductor materials. A BEC occurs when a significant number of the polaritons (which are bosons) condense into the lowest energy state, forming a macroscopic coherent quantum state. Baumberg believes that polaritons could someday form the basis of a “BEC-on-a-chip” that could be used as an interferometer.