Lack of control of the variables determining the response of a system is usually seen as a curse in science and technology. But what about a slight pinch of imperfection and disorder? Imperfections and irregularities are unavoidable in nanoscience due to our limited level of control of nanofabrication processes. Disorder is potentially detrimental to nanosystems, but if well-contained, disorder might not be an intruder after all, leading to novel physical concepts and applications.

Published in the prestigious journal Physical Review Letters, scientists from TU/e and the Dutch Institute for Fundamental Energy Research (DIFFER) have investigated the role of imperfections and disorder in nanolasers. By introducing a slight degree of disorder, they have observed a dramatic change: the laser no longer emits in one specific direction, but in all directions.

Development of nanoscale lasers (smaller than the thickness of a human hair) is a very active field of research. In a normal laser, each photon (light particle) is ‘cloned’ many times in a medium that is located inside a cavity (e.g. a pair of mirrors between which the photon moves back and forth producing other photons with the same characteristics). This process is known as Light Amplification by Stimulated Emission of Radiation (LASER). To achieve laser emission an electrical current is usually injected through the medium, or it is illuminated with high energy light. The minimum energy needed for a laser to emit is called the lasing threshold.

Condensation

A different kind of laser is the so-called polariton laser. This works on the principle not of cloning photons but making non-identical photons identical in much the same way as water vapor molecules, moving in all directions with different velocities, are condensed into a single drop. Condensation of photons gives rise to the intense and directional emission characteristic of a laser. An important advantage of polariton lasers is that they have a much lower lasing threshold, which makes them excellent candidates for many applications.

However, a major problem of polariton lasers has been that they need to operate at very low temperatures (like vapor condensation that takes place only when the temperature is lowered) but by using organic materials, it is possible to obtain polariton laser emission even at ambient temperature. The Eindhoven researchers demonstrated last year that they can realize nanoscale polariton lasers that function at ambient temperature, using metallic nanoparticles instead of mirrors as in normal lasers.