An optical fiber can transmit far more information than a conventional copper cable. The same is true when it comes to computer microchips: the integration of photonic devices on silicon electronic circuits is a long-standing goal towards faster electronics that can process large amounts of data.

Small, low-power lasers are essential components for the development of on-chip communications systems. However, existing semiconductor-based lasers are relatively large and thus have large operating powers. Researchers from NTT Corporation in Japan have now constructed a photonic-crystal laser that is compact, high-speed and has extremely low power consumption.1

Two-dimensional photonic crystals are known for their good waveguiding properties: a narrow channel in a photonic crystal’s regular array of holes allows the light to be confined to a very small area. When the photonic crystal is made from a light-emitting semiconductor material such as indium gallium arsenide (InGaAs), a laser can be realized simply by pumping light into the waveguide region. However, this means that the entire photonic crystal is made from a single light-emitting semiconductor. Because these materials have poor thermal conductivity, losses occur when the optical pumping increases the temperature of the active region, thus reducing the output power.

Fig. 1: Buried heterostructure photonic-crystal lasers. The laser design consists of an InP photonic crystal with an InGaAsP single-quantum well (SQW) active region. From Ref. 1. © S. Matsuo

The photonic-crystal laser developed by the NTT researchers now solves this problem by using a ‘buried heterostructure’ (Fig. 1). In this design, the bulk of the photonic crystal is made from a material with a large bandgap, indium phosphide (InP), whereas the waveguiding region consists of a material with a small bandgap, indium gallium arsenide phosphide (InGaAsP).

The optical contrast between these two materials leads to an efficient confinement of light solely within the buried heterostructure region. The energy of the exciting laser beam is chosen such that it only excites carriers from the buried heterostructure, thereby minimizing losses. “The confinement of carriers as well as photons in this ultrasmall region leads to promising characteristics of the lasers, such as the high-speed modulation of light and extremely small laser thresholds,” says Shinji Matsuo, who led the research team.

According to Matsuo, the combined advantages of small size and efficient lasing make these photonic crystals suitable for incorporating into silicon chips, thus providing a template for future devices. “In the next step, we will try to fabricate lasers that are electrically pumped. Once integrated with silicon chips, these photonic crystals will open a new market for photonic devices.”