A miniature jail for photons: that is the nanocavity that researchers from the University of Twente have discovered. It is an extremely small cavity that is surrounded on all sides by a photonic crystal – a structure of pores that are perpendicular to each other. The confinement of photons in a 3D cavity could lead to efficient miniature lasers and LEDs, to optical storage of data or to sensitive sensors for biomedical applications.

Photonics

Techniques for ‘catching’ light are at the basics of photonics. A well-known cavity consists of two mirrors between which a standing wave is formed of a particular colour of light, depending on the distance between the mirrors. This is the principle of the laser. But light that leaks away through the sides, is no longer reflected by the mirrors. This can be prevented by surrounding the photon with mirrors in three dimensions, according to the UT researchers. The mirrors in this case are three-dimensional photonic crystals. They comprise pores that are deeply etched into silicon, in two directions, perpendicular to each other.

Deliberate disturbance

Photonic crystals are known for their remarkable light properties. The structure and the periodicity of the pores are such that only light of specific wavelengths can propagate. But how then, do you create deep in this structure, an irregularity or ‘defect’ in this crystal, so that the photon is trapped? In their publication, the UT researchers show that this can be achieved by giving the two pores, perpendicular to each other, deliberately a different diameter. Where they cross, a cavity is created. On all sides the cavity is surrounded by the regular crystal structure: for the photon there is no escape.

Lightweight

By altering the periodic structure locally, the crystal also shows considerable absorption of light in the visible range, up to 10 times the absorption of ‘untreated’ silicon. This is a great property for sensors. The great density of pores also makes the crystal very light in weight – the researchers also call this ‘holeyness’. Furthermore, the crystals can be integrated with current silicon technology.

Light chips

Earlier, the researchers demonstrated that photonic crystals that have a diamond structure can reflect light over a very broad colour spectrum and for all angles. This research was the basis for the new invention that was presented here. The cavities are expected to play an important role for the storage and processing of light signals.

Source: University of Twente