In the fields of electronics and optics, researchers are constantly looking for applications that increase the efficiency and cost-effectiveness of existing technologies. Nanostructured materials are opening new fields of research and application in this area. One area of this research is the use of photonic nanostructures in semiconductor lasers.
Semiconductor Lasers: An Overview
Semiconductor lasers are a class of laser-based on semiconductor gain media. They directly convert electrical energy into light. Since their invention in 1962, they have evolved and are now used in a variety of applications including fiber optic cabling, 3D sensors, CD players, and medicine (especially dentistry.) They can also be used to cut soft tissue.
Semiconductor lasers are also known as laser diodes. The first commercial semiconductor lasers emitted light in the infrared wavelength, but over the past few decades, semiconductor lasers have been developed which emit light in a range of wavelengths. Until the turn of the century, many semiconductor lasers used, emitted red light, but with the development of the first true blue and violet laser diodes based upon gallium arsenide semiconductor chemistry by Shuji Nakamura at Nichia, the functionality and range of uses increased exponentially. Violet laser diodes are commonly used in flow cytometers.
Common materials used for semiconductor lasers include gallium arsenide (GaAs) indium phosphide (InP) gallium indium phosphide (GaInP) and gallium nitride (GaN.) Semiconductor lasers are small and inexpensive alternatives to larger, bulkier commercial lasers.
What are Photonic Nanostructures?
Photonic nanostructures, otherwise known as photonic crystals, are structures that manipulate and confine light on the nanometer scale. Light is confined to small volumes by resonant recirculation in optical microcavities within the nanostructures. They are commonly used in solar panels, but their use in other applications has been explored in recent years.
Photonic crystals exist in nature, including being contained in the wings of some species of butterfly and gemstones including opals. Research into artificial photonic crystals began in 1887 when the English physicist Lord Rayleigh performed experiments with multi-layer dielectric stacks. The results of his research demonstrated that these structures have a photonic band-gap in one dimension.
It was not until a century later, when Eli Yablonovitch and Sajeev John experimented on periodic optical structures with more than one dimension, that the first viable photonic crystals were created. Since then their use has grown exponentially with further research into them being performed by scientists.
The Application of Photonic Nanostructures to Semiconductor Lasers
Applying photonic nanostructures to semiconductor lasers provides benefits for miniaturization and control. Nanostructures can dramatically increase the optical sensor sensitivity, as the nanostructure greatly amplifies or reduces the electromagnetic field of light.
Over the past few decades, research has been carried out, which has pushed the boundaries of these applications. One particularly notable research project in 2003 carried out by a team led by Raffaelle Colombelli of Bell Laboratories produced a surface-emitting quantum cascade microcavity laser.
Quantum cascade lasers emit light when electrons transition from a higher to lower energy level in a quantum well. A single electron can produce many photons as it cascades through the many quantum levels. As the wavelength depends on the width of the quantum well, photons of more than one wavelength can be emitted at the same time.
They used a nanostructured high-index contrast 2D photonic crystal to form a micro-resonator. The micro-resonator provides both feedback for the action of the laser and vertical diffraction of light from the surface of the semiconductor. One of the limitations of quantum cascade lasers that researchers had previously encountered was that they only emitted light in certain directions and did not include vertically from the surface of the semiconductor. Thus, the use of the photonic nanostructure to provide this functionality represents a major step forward in the development of these lasers.
There has also been research into the use of nanostructured photonic crystals as miniaturized laser mirrors and their use to create more efficient tunable semiconductor lasers. Ongoing research is providing some fascinating solutions and pushing the field of semiconductor lasers and their uses to increasingly interesting areas.
Sources and Further Reading
Colombelli, R et al. (2003) Quantum Cascade Surface- Emitting Photonic Crystal Laser Science Vol. 302 Issue 5649, pp. 1374-1377
Kamp, M et al. (2004) Semiconductor photonic crystals for optoelectronics Physica E: Low-dimensional Systems and Nanostructures Vol. 21 Issues 2-4
Diode Laser – Topic Overview ScienceDirect