A lucky break has shown silicon is a powerful material for use in the manipulation of light.
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Few people could doubt the amazing capability of silicon when it comes to applications in electronics and across a range of industrial uses. The second most abundant element of Earth’s crust has become an important component of computer chips with its unique electrical properties forming the backbone of the computing revolution.
It is no mere coincidence that the later 20th century to the early 21st century has been nicknamed the silicon age. Yet, as technology has advanced, it has increasingly seemed that silicon may not play as fundamental a role in future devices.
This is because silicon is not well adapted to all applications. Thus far this wonder material has shown itself to be a poor choice for photonics — the application of light generation, detection, and manipulation which can be used to perform tasks that fall outside the purview of ‘traditional’ electronics.
This lack of applicability in photonics is reflected by the fact that there are, as of yet, no commercially available silicon-based displays, lasers, or diodes.
This could be about to change thanks to research conducted by an international team led by scientists from the University of Surrey. The team has discovered that silicon is actually a very good fit for a device that can manipulate multiple beams of light.
The discovery could provide a significant boost to both the speed and efficiency of electronic communications. It could also grant an important leg-up for quantum computing. And all this came about through a large amount of good fortune.
“Our finding was lucky because we weren’t looking for it. We were trying to understand how a very small number of phosphorus atoms in a silicon crystal could be used for making a quantum computer and how to use light beams to control quantum information stored in the phosphorus atoms.”
Ben Murdin, Professor of Physics at the University of Surrey
Burdin is co-author of a paper documenting the team’s research published in the April edition of the journal Light: Science & Applications.
A Non-Linear Silicon Breakthrough
The team’s silicon optics breakthrough relies on the use of light in the far-infrared band of the electromagnetic spectrum and a phenomenon known as nonlinearity.
In normal ‘linear’ optics when a light wave interacts with a molecule it causes it to vibrate and emit its own lightwave. This second wave interferes with the first. In optics, nonlinear effects refer to a situation when the incident light is irradiant enough to excite many molecules to a high-energy state.
This results in the emission of light at a variety of frequencies. In terms of real-world effects, that means that a beam of infrared light can be passed through a crystal from which a green light emerges.
This is how green laser pointers work. An affordable infrared generating diode sends light into a non-linear crystal which then emits green light by effectively doubling its frequency (and in turn halving its wavelength).
So non-linear optics can be used to change the color of laser light, to shape such light, and to create some of the shortest events ever recorded by humans in terms of both time and space.
It also makes nonlinear optical phenomena of vital importance as the basis of optical communication systems and optical sensing.
Controlling Light with Light
The ability to change color through the alteration of wavelength and frequency is not the only power possessed by non-linear materials. A certain type of non-linearity can actually be used to control the information emitted in a beam. importantly, the stronger the nonlinearity the easier is to control this information, beam with weaker beams of light.
And that is where silicon comes in.
The University of Surrey-led team found that the strongest example of this nonlinearity yet discovered is possessed by silicon. Whilst the team obtained their results by using silicon-doped crystal cooled to low temperatures, they say that the strength of the nonlinearity they observed means that impressive results could be achieved with extremely weak beams of light.
“We were astonished to find that the phosphorus atoms were re-emitting light beams that were almost as bright as the very intense laser we were shining on them,” says Burdin. The University of Surrey physicist concludes by reflecting on how this research speaks to the way that science proceeds and the value of collaboration: “We shelved the data for a couple of years while we thought about proving where the beams were coming from.
“It’s a great example of the way science proceeds by accident, and also how pan-European teams can still work together very effectively.”
Dessmann. N., Le. N. H., Eless. V., et al, , ‘Highly efficient THz four-wave mixing in doped silicon,’ Light: Science & Applications, [https://doi.org/10.1038/s41377-021-00509-6]
Leonhardt. U., , ‘Essential Quantum Optics,’ Cambridge University Press.