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Reactive Plasmonics researchers at King’s College London have engineered a new nanoscale device which creates a controlled stream of ‘hot electrons’ – high energy electrons which allow unusual chemical reactions to take place. This could open up new research avenues for pharma, chemicals and materials industries.

The research, published in Nature Nanotechnology, describes how a metamaterial – a material with properties not found in nature – can be constructed which uses quantum effects to turn electrons flowing through a circuit into hot electrons and light, in a highly controlled manner. This has a wide variety of novel applications in opto-electronics, sensing, and chemistry.

The device takes advantage of a quantum effect called electron tunnelling to produce streams of particles which can have important applications, when properly controlled.

A voltage is applied across the device, causing a flow of electrons from one material (eutectic gallium indium) to another (gold nanorods).  These are separated by an air gap, which would usually stop the electron flow. But when the air gap is less than a nanometer, quantum mechanical rules apply, which allow the electrons to ‘tunnel’ through.  This tunnelling produces two useful effects.

Firstly most of the tunnelling electrons arrive in the gold nanorod tips in the form of ‘hot electrons’. Hot electrons are of great interest to chemical industries, since their high energies allow chemical reactions to occur between molecules which would not normally react with each other.

Secondly, a small proportion of the tunnelling electrons excite other particles (called plasmons) in the metamaterial. These excitations emit light, the wavelength of which is directly related to the applied voltage. Usually this conversion is very inefficient, but the King’s metamaterial uses array gold nanorods providing one hundred billion tunnel junctions to improve the electron-to-plasmon conversion, making the emitted light visible to the naked eye.

The creation of hot elections is very useful to a wide range of industries that are interested in creating new chemicals that do not occur under normal conditions. For example they could be used to synthesise new molecules in pharma and chemical industries which require high energies. It can work as a lab-on-a-chip device for developing and understanding new chemical reactions where precise stimulation and monitoring are paramount.

To demonstrate the device, the team performed oxidisation and reduction reactions involving oxygen and hydrogen molecules.  Hot electrons have previously been produced using light. But this method leads to a broad distribution of energy across the excited electrons. The new tunnelling technique creates hot electrons with consistent and controllable energy levels, allowing for very precise control of reactions. This new material therefore allows highly tunable production of hot electrons and so can be used to control chemical reactions with much greater precision.

The tunnel junctions (the air gap) in the material are very sensitive to changes. For example the new products from a chemical reaction in the junctions will dramatically affect the tunnelling properties, changing the flow of electrons, so its presence can be detected by monitoring the change in current or light. This means the materials can also be used to closely monitor the reactions it is enabling.

Similar principles can be applied for monitoring the presence of gasses. The King’s material was designed to be sensitive to hydrogen and oxygen, two chemicals where there is a great need for sensitive monitoring, and was shown to reliably detect these chemicals. Detecting hydrogen leaks is important in the production of fuel cells, and monitoring the presence of oxygen is vital in a wide variety of controlled chemical reactions, such as in the manufacture of drugs.

The material can be modified to be sensitive to different molecules, making it viable as a highly sensitive, cheap, and easy to use sensor which provides visual feedback when a molecule is detected.  A further benefit of the material is in small scale electronics. Since light is generated by applying a voltage along the nanorod which is only ten nanometers thick, it can be used to optically transmit information (1s and 0s produced by rapidly turning the light on and off) between or within microchips.

This is usually done with semiconductor lasers, but as electronics get smaller the semiconductor devices become too bulky. The King’s metamaterial allows the optical signals to be produced with a much smaller device, bringing advantages to chip manufacturers and facilitating the development of ever faster electronics.

Dr Pan Wang, lead author of the paper, says: “This one tiny device offers several amazing applications: plasmon excitation, light generation, and chemical reaction activation. And all this is achieved by a small, easy to produce material which only requires a small voltage to function.”

RPLAS Principal Investigator Professor Anatoly Zayats concludes: “When we began these studies we expected to generate some weak light which we thought should be enough for various nanophotonic applications on a chip. But as sometimes happens in the research, the applications are much richer. We believe the potential of the approach for designing chemical reactions stimulated with hot electrons and monitoring chemical processes for drug and materials discovery is huge.”

 

Researchers from the Reactive Plasmonics team at King’s College London have developed a new method for rapidly changing the polarisation of light, one of its fundamental properties. The research, published in Nature Photonics, could lead to much faster data transfer and advance research into nano-materials.

A light wave undulates in different ways – known as its polarisation. The polarisation of light is changed by the material it passes through, so we can use it to learn about unseen nano-scale worlds such as drug chemistry and quantum electronics. Switching polarisation is also used to transfer digital information along fibre optic cables.

The electronic methods currently used to control the light polarisation in such applications is reaching its physical speed limit.  Researchers at King’s have overcome this problem, allowing polarisation to be switched at timescales of less than a millionth of a millionth of a second – hundreds of times faster than current electronic methods.

This will allow us to ‘see’ very fast nano-scale processes such as chemical reactions for the first time, by illuminating them with rapidly changing light. This helps us to understand the difference in formation of nasty chemicals and life-saving drugs, and allows us to study new materials that will bring about the next technological revolutions.  This will also represent a major advance in data transfer speeds. By rapidly changing the polarisation of light – to represent a one or a zero – data can be passed along fibre optic cables and into your living room more rapidly. This will help meet growing data sharing demands driven by streaming and cloud services.

The team designed nano-structured materials that can control light polarisation using light itself – a technique known as ‘all-optical polarisation control’.  These nano-structures are known as metamaterials: materials with optical properties not available in nature. These thin, lightweight materials are constructed from elements smaller than a thousandth of a millimetre in order to create exotic optical effects.

In this case, the metamaterial is constructed of gold nano-particles. A high intensity light pulse is fired into the metamaterial, injecting energy into electrons in gold particles, which in turn changes the refractive index of the material.

A second pulse is fired at the metamaterial at the same time. As this pulse passes through the material, the change in refractive index changes its polarisation. This all happens instantaneously, allowing polarisation to be changed trillions of times per second.  By simply shining two beams of light through the material, one beam is able to control the polarisation of the other at ultrafast speed.

The effect can be observed even with one beam. In this case, the polarisation of the light transmitted through the metamaterial changes with the intensity. It is like polaroid sunglasses which adjust themselves to remove glare whenever it is too much sunlight.

Luke Nicholls, the PhD student who carried out these experiments, said, “With everybody using more and more data, streaming videos, music and sharing pictures, we are fast approaching a point where the current internet infrastructure will not be able to cope. All-optical control provides an answer to this looming problem and hopefully sees an end to staring at the infuriating buffer wheel.”

 This research also has potential beyond how many box sets we can download. Control of light at such short time scales could also feed into quantum information processing, where controlling the polarisation of light is integral for building successful quantum computing devices.

 

Reactive Plasmonic’s PI Anatoly Zayats of King’s College concludes: “This effect opens up many opportunities for new applications which can directly impact everyday life. The faster you can control light polarisation, the faster you can use light to transmit data and make measurements.”

 

This groundbreaking research has been featured in the following publications

New Electronics

Drug Discovery Today

Electronics Weekly

Photonics

Electro Optics

Engineer

 

Professor Anatoly Zayat’s and Dr Alex Minovich from King’s College London have developed new 2D nano-surfaces which appear as realistic 3D objects – including shading and shadows – using cutting edge nano-engineering. The research was carried out  alongside Rheinische Friedrich-Wilhelms-Universität Bonn, and is published in the American Chemical Society journal Nano Letters.

When light hits an object, the colour, texture, and shape affect how light is absorbed and reflected, allowing you to make out the object in front of you. By altering the surface to change how light is reflected, it is possible to manipulate how it appears.

The researchers developed layered materials, incorporating precisely designed nano-features smaller than the wavelength of light – called metasurfaces. This allowed them to control how light is reflected in highly precise ways, so that a 2D surface reflects light just as a 3D object would.

Borrowing a technique from 3D graphics called Normal Mapping, researchers encoded shadow effects into the image, creating 3D images more realistic than holograms or 3D cinema. As a proof of concept, the researchers fabricated a flat metasurface imitating lighting and shading effects of a 3D cube.

The technique could have huge implications for the optical industries, including in TV screens and photography.

Professor Anatoly Zayats of King’s College says: “Metasurfaces are amazing. They open up unprecedented freedom in directing and manipulating light. One might ultimately imagine a TV screen which appears exactly the same as you move around it, or a new movement of 3D art”.

The ability to control light could bring new functionality to small camera lenses. A flat surface can be made to appear optically convex by designing appropriate metasurface properties. Future generations of smartphone cameras could use the tiny flat metasurfaces which mimic the properties of sophisticated curved camera lenses, allowing much greater control of angle and depth field.

The wafer thin materials could also replace heavy optical lenses in applications such as satellites, where weight and size have a big impact on efficiency.

More immediately, the novel nano-materials could already be used to create unique complex 3D images for security and anti-counterfeiting applications, as well as for new measurement applications requiring precise control of light.

Nanostrand is the newsletter from the Experimental Biophysics & Nanotechnology Group at King’s College London.

You can download the latest copy here or read it below.

NanostrandExport

 

‘You can’t use up creativity, the more you use the more you have’ Maya Angelou

 

Creativity @ Home is an Engineering and Physical Sciences Research Council (EPSRC) initiative to generate and nurture new and creative ways of thinking that can potentially lead to transformative research.    The Reactive Plasmonics Creativity @ Home journey started on a sunny October day at the Strand Campus of King’s College London.

Dennis Sherwood from Silver Bullet Machine led a fantastic day where creativity was explained using Koestler’s Law.  Many people believe that creativity strikes out of the blue, like a bolt of lightning and that genius just happens. This means that some people tend go through life believing that they ‘aren’t creative’ and therefore don’t get the chance to present new ideas. Dennis explained that being creative is taking things that already exist and asking the question ‘How might things be different?’

‘The creative act is not an act of creation in the sense of the Old Testament. It does not create something out of nothing; it uncovers, selects, re-shuffles, combines, synthesises already existing facts, ideas, faculties, skills. The more familiar the parts, the more striking the new whole.’

The Act of Creation, Arthur Koestler

 

 

To ask the question ‘How might things be different?’ you have to define how things are. Once you have defined reality as it currently is, you can come up with ways to see how it can be different. It’s only when you’ve asked the question do you know if it has an answer and if that answer is viable.

An iPhone was not a brand new idea, Apple took things that already existed then shuffled and combined different elements to create something new.  What Apple did so wonderfully was to give a solution that brought together a phone, a music player, a camera, a camcorder and a diary (plus many other elements). This was a solution to something we didn’t know we needed. Rather than carry separate elements, we now carry one piece of tech be it Apple or Android or and other smart phone.

A few weeks after the first event the RPLAS team decamped to the stunning location of Chicheley Hall near Milton Keynes.  This event had the team thinking about how things can be done differently in relation to the Reactive Plasmonics programme grant and we will be reporting on potential projects and collaborations that spin off from this event in the near future.

Taking part in the event has changed the way that many of the researchers think about things. To be creative you should be observant and be willing to share and listen to ideas. You will never know how things can be improved, if you never ask the question of how if might be different.