Aug 29, 2018
(Nanowerk News) A large machine with all manner of assorted protruding pipes stands ready for action in one of the labs at the Norwegian University of Science and Technology's (NTNU) Department of Electronic Systems. Some of the pipes are protected by insulating material, while others are wrapped in silver paper.
This new “MBE” machine has allowed researchers to make an excitingbreakthrough. MBE is an abbreviation for molecular beam epitaxy.
Molecular beam epitaxy (MBE), which is what happens inside this machine, has helped researchers create a nanowire with a special property that allows it to work as a nanolaser. (Image: Idun Haugan / NTNU)
Welcome to the world of nanotechnology, where quantum structures rule and constituents are so small that they’re measured in billionths of a metre: one nanometre (nm) is equal to 10-9 meters (one billionth of a metre). The average human hair is approximately 100 000 nm thick. Nanometres are often used to measure the wavelength of light, and this breakthrough is about just that, specifically infrared light.
The NTNU researchers who have been working with these miniscule units have managed to produce a nanowire with a very special superlattice. The result is a miniature laser in the form of a nanowire. It’s the uniformity of the superlattice that makes this miniature laser exceptional.
“The challenge is to get the superlattice structure consistent and even, so that the nanowire produces light at the same wavelength the whole way. Now we’ve managed to create this special superlattice inside the nanowires with the necessary regularity,” says Professor Helge Weman. He heads a research group that is working with the nanomaterials for this project.
Weman’s colleagues Professor Bjørn-Ove Fimland, Ton van Helvoort, and the research team have made numerous nanowire-related research breakthroughs in recent years. In this latest breakthrough, PhD candidates Dingding Ren and Lyubomir Ahtapodov conducted the experiments that led to their promising results.
“They have a very good handle on this [process], and that control is the key,” said Weman.
The online journal Nano Letters published an article on their research findings earlier this year ("Single-Mode Near-Infrared Lasing in a GaAsSb-Based Nanowire Superlattice at Room Temperature").
Schematic drawing of nanowires with six superlattices consisting of a total of 60 quantum wells. The laser emits infrared laser light (red arrows) from the ends of the nanowire when illuminated with a “pump laser” (green arrow). (Image: NTNU)
“Surplus electrons fall into quantum wells and create light. When the electrons fall from one level to another inside the wells, the energy is converted to infrared light,” explains Fimland.
The infrared light consists of photons, which are the building blocks of all light. In this case, the photons clone each other so that they generate more and more identical photons.
The ends of the nanowire act like a mirror so that the light is reflected and sent back and forth through the nanowire. The uniform superlattice keeps the light’s wavelength steady, clear and sharp.
“One characteristic of a laser is that it shines at a very clearly defined wavelength. Our laser is in the infrared area at around 950 nanometres and has a very narrow wavelength,” said Weman.
![Molecular beam epitaxy machine](https://www.nanowerk.com/nanotechnology-news2/id50975_1.jpg)
Structure of atoms inside the nanowires
A nanowire is several hundred times smaller than a human hair. Within each nanowire, the research group set up six superlattices consisting of ten quantum wells each. In order to obtain the uniform structure that forms the superlattice, the researchers created a very special structure using atoms. The nanowires are built – or “grown” – by spraying the structure with different types of atoms. The atomic elements gallium and arsenic have created the basic structure, and the quantum wells contain antimony atoms as well. This atomic combination, plus semiconductors used to conduct power and create light, create the superlattice. “The basic constituents are from two different groups in the Periodic Table: Group III and Group V. When we mix atoms from the two different groups we get what’s called three–five semiconductors. They’re well suited for generating light,” says Fimland.Creating light in a quantum well
By using a pump laser to transmit energy to the nanowires, electrons are released from the electron cloud surrounding the nuclei in the nanowires. The released electrons wander around – and many of them fall into the quantum wells. The electrons only have a short life span, and under certain circumstances the energy from them is transformed into infrared light. Now we’re finally approaching the heart of this new miniature laser.![Schematic drawing of nanowires with six superlattices consisting of a total of 60 quantum wells](https://www.nanowerk.com/nanotechnology-news2/id50975.jpg)