Mongrel Particles Act like Bosons

Daniel S. Burgess

Strange quasiparticles made of photons and electrons may lead to applications in quantum optoelectronics, including high-efficiency microlasers, optical amplifiers and switches. The particles, called polaritons, display properties of bosons such as the propensity to clump into a single quantum state and stimulated scattering, which could allow them to be the basis of a bosonic laser.

"Polaritons are an intimate mixture of light and matter," explained Jeremy J. Baumberg, a professor of physics and astronomy at the University of Southampton.

The particles are formed by confining the photon-exciton cascade in a semiconductor. Normally, photons create electron-hole pairs that decay and yield photons. If these re-emitted photons are trapped within the semiconductor, however, they repeat the cycle, and the photons and excitons blend into polaritons, with properties unlike either parent.

Baumberg and his colleagues, who described their work in the Feb. 14 issue of Physical Review Letters, created a microcavity by sandwiching InGaAs quantum wells between GaAs/AlGaAs distributed Bragg reflectors. They illuminated the cooled cavity with pulses from a 100-fs mode-locked Ti:sapphire laser. They used Baumberg's coherent control technique to produce 2-ps pump and probe pulses by filtering the laser's emission with liquid crystal modulators, and they designed a femtosecond goniometer to control the pulses' angles of incidence without changing their time of arrival.

A bosonic sea of polaritons in a semiconductor microcavity amplifies an incoming laser pulse by nearly two orders of magnitude. Courtesy of Jeremy J. Baumberg, University of Southampton.

Photons from the pump pulses created polaritons in the optical cavity. Probe pulses followed 5 ps later, and the researchers measured the reflected light. The polaritons amplified the probe pulses by up to nearly two orders of magnitude. The dependence of this effect on the angle of incidence indicates that the polaritons exhibit stimulated scattering characteristic of bosons.

Beyond expanding our understanding of the physical world, investigations into polaritons have practical applications. The researchers are building a continuous-wave version of the system, which they say will be the smallest parametric oscillator, and they hope to combine their work with photonic crystal technology. Because the particles behave like bosons, there may be a polariton equivalent of Bose-Einstein condensates, with unusual optical and quantum properties.

"A whole new array of interactions between light and matter can be engineered," said Baumberg. "We can make better low-threshold lasers, more efficient microlasers, and understand more of the mysterious way in which light and matter are entangled."