Optical devices enter third dimension
The prospect of replacing electronic computers with optically based devices has come a step closer with a novel laser technique.
Lead Researcher Aniwat Tandaechanurat at the University of Tokyo’s Institute for Nano Quantum Information Electronics, Japan, explains, ‘In order for an optical chip to work, it is necessary to integrate a variety of optical devices including lasers, waveguides, modulators and detectors onto one chip. These devices are fabricated and functioned separately because of difficulty in their integration. If we can integrate them [easily] and densely enough with proper optical logics, we may be able to replace electronic computers with optical ones.’
The team has demonstrated that lasing from a 3D photonic crystal nanostructure could lead to integrated 3D electronic and photonic circuits, lasers and waveguides onto a single chip. To produce the new laser they combined a gallium arsenide-based 3D photonic nanostructure with indium arsenide (InAs) light-emitting quantum dots embedded inside a nanocavity.
The crystal’s resonance band is narrow enough to provide the feedback for lasing, and the quantum dots can be tuned to allow light of about 1.2-1.3m in wavelength – the optical communications band – to be emitted from the device.
The team reported the coupling of quantum dots to 3D photonic crystals back in 2008, however, Tandaechanurat says lasing was not obtained at the time. It is only after improving the structural design, crystal fabrication and dot quality that lasing has been achieved.
Previous integration attempts have been met with limited success owing to the difficulty of fabricating structures with feature sizes (about 100nm) of the same order as wavelength, but Tandaechanurat said the team has overcome this by developing a ‘manipulation’ fabrication technique to insert quantum dots or a cavity into the 3D structure.
The technique also allows the structure itself to be made from different semiconductor materials to suit the particular device. For example, the team has recently fabricated a crystal made from silicon with a quality factor of 10,000, and plans to embed InAs dots in it soon to achieve lasing.
Commercial manufacture is not expected for several years yet, partly because fabrication still has to be carried out by humans. To obtain manufacturing-scale yields at a practical cost, robotic systems are likely to be needed.
Professor Martyn Pemble, Head of the Advanced Materials and Surfaces Group at Tyndall National Institute, in Cork, Ireland, sees this work as very promising. ‘Photonic crystals have a band gap where light may not travel with a particular energy, in a particular direction, making it follow particular pathways. To achieve this, the material must show a periodic – and large – change in refractive index on a length scale comparable to the wavelength of the light. The researchers have made such a structure by etching into a slab of silicon (large refractive index), so that the overall structure consists of air (low refractive index).’
For Pemble though, the limitations are the costs of the process – with making the crystal and placing the quantum dots within the cavity. He adds, ‘Making the crystal is not a challenge for today’s lithographic processing methods, but placing the dots reliably and reproducibly could be an issue.’