Interview with: Dr Alex Minovich from Kings College London about reflective metasurfaces.

Researchers at King’s College London, alongside Rheinische Friedrich-Wilhelms-Universität Bonn, have created new 2D nanostructured surfaces which appear as realistic 3D objects – including shading and shadows - using cutting edge nano-engineering. The paper has been published in the  journal Nano Letters.

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Stewart Bland:        I’d like to kick off by asking if you can start by introducing yourself and your group and telling us about your background.

Alex Minovich:                  My name is Alexander Minovich.  I’m originally from Belarus.  I did my undergraduate study at the Belarusian State University in Minsk.  After that, I went to Australia where I did my PhD and my first postdoc at the Australian National University in Canberra.  Last year, I received International Newton Fellowship from the Royal Society for a two-year project with King’s College, London, in the group led by Professor Anatoly Zayats.

I work in nanophotonics, which means that I fabricate and study nanostructures with properties not occurring in nature and I look for, obviously, properties that can be useful for practical applications.

Stewart Bland:        That’s fantastic, thank you.  In your study, published in Nano Letters, you apply the technique of normal mapping to a flat metasurface, to imitate a 3-D cube.  But, let’s start at the beginning:  What is normal mapping?

Alex Minovich:        Normal mapping is a computer-modelling trick, which is implemented to split up the calculation of three-dimensional things.  It can be, computationally, expensive to calculate all small three-dimensional features in complicated 3D objects.  That is why the 3D objects usually use the triangular mesh before entering.  At this step, small geometric features, such as bumps and ripples are just neglected.  Instead, they are encoded in the form of a normal map.  A normal is a vector, which is perpendicular to a surface element and it basically determines the orientation of the surface at a given point.

          The normal map is projected onto this coarse triangular mesh and this way, the information about surface orientation is contained in the normal map.  It allows to significantly increase the time calculation of 3D objects and it gives a quite realistic appearance, in the terms of shading and lighting, of three-dimensional things.

It works well for low-profile features or features that are located at larger distances.

Stewart Bland:        Okay, fantastic.  So, can you tell us a bit about the materials that you use, these metasurfaces?

Alex Minovich:        Metasurfaces are closely related to the metamaterials concept.  Metamaterials are artificial materials, which exhibit properties not occurring in nature.  Metamaterials consist of tiny elements, which are much smaller than the wavelength of light.  Electromagnetic waves sense them as artificial atoms, so-called meta-atoms.  What’s important in the metamaterial concept, that the properties of materials are determined via the geometry of these meta-atoms, but not by the chemical composition.  So, basically, designing different meta-atoms working to achieve material properties which we require, which we are looking for.

          Metasurfaces are a just a thin layer of metamaterials.  They are free of the main disadvantage of bulk metamaterials hylosis while they still interact with light quite strongly to achieve such effects as face and amplitude control of light, polarisation control, spectroselectivity and even the enhancement of non-linear effects.  So, the metasurfaces can be used for beam-steering, focusing, the fabrication of tiny lenses for mobile devices and so on.

Stewart Bland:        Fantastic.  So, how did you go about creating the 3D image and how does the image perform?

Alex Minovich:        We have chosen a cube image as a visual and simple demonstration of the proof of principles.  And, in order to implement it, we first need to design a face distribution, which we’ll encode by the metasurface elements.  First, we need to retrieve the distribution of our surface normals and encode it into the linear component of the face.  It’s different for all three cube faces.  However, if, at this point, we illuminate our sample, it shows the cube face will be bright only at the fixed illumination angle.  Thus, we need to diffuse the face companion to enlarge the scattering angles.  We do it in the form of same-colour patches, randomly positioned, which have parabolic face distribution and that corresponds and it works very similar to industrial diffusers where microlenses are used.

          So, when we combine the normal mapping with the diffuse pattern, the brightness of cube-faces change, mostly when we start varying the angle of illumination.  Next, we need to encode the face distribution, using metasurface elements and we use a filament structure to do it.  We have a gold layer, then a dielectric magnesium biphorite layer, on top of the gold layer and then an array of nanorod antenna, on top of the dielectric.  The face is encoded via the orientation of this gold nanorod antenna and the structure works, with circular polarised light. 

The elements perform well in the broadband, within a few hundred nanometres.  They work well at oblique incidence, up to 45 degrees.  The structure we fabricated also can work with interfering light, so it doesn’t require lasers to see the effect.

Stewart Bland:        Fantastic.  So, how does this approach compare to other techniques to render 3D objects, such as holography, for example?

Alex Minovich:        Holograms are, in fact, static 3D photos which are recorded at a fixed illumination which is usually done by a laser source.  Holograms have to be illuminated at a certain, fixed angle and when you start changing the position of the light source, the efficiency dramatically drops or you can get distortion of the images.  So, holograms’ 3D effects are achieved via stereo effect, which means that different eyes receive different images which correspond to the different viewing angles at the 3D scene.  Usually, holograms require a coherent light source laser to reconstruct the image.  Our normal mapping technique doesn’t produce a stereoscopic image, it doesn’t produce a stereo effect.  Instead, the volume and depth effects are created so the shading and lighting, as it’s usually done in drawings or two-dimensional projections. 

What’s important then, is that we can change the position of the light source, when we use this normal mapping technique.  And, for example, if we illuminate this 3D picture from the left, the bright areas would be located on the left, closer to the light source and dark areas will be on the right, opposite to the light source.  But, when we move the light source to the other side, lighting and shading will change accordingly.

Stewart Bland:        So, are there any applications of this technique?

Alex Minovich:        The most straightforward application is in security features, similar to security holograms.  The field requires visual effects, which are quite distinguishable and which are difficult to fabricate without know-how.  The 3D images created via normal mapping can be used as security features for IDs, notes and protecting print, packages.  Also, the diffuse metasurfaces we demonstrated can be used in any area where currently optical diffusers are utilised, such as, for example, computer displays, etalons for meteorology and so on.  Also, I believe it’s possible to use the 3D effects in artistic pictures and advertising.  When the nanotechnology advances enough to provide quick and cheap ways to fabricate light area nanostructures.

Stewart Bland:        So, what’s next for the project?

Alex Minovich:        Currently, our pattern performs well in the red part of the spectrum.  We aim to create structures which will perform well across the full visible spectrum.  Also, we want to try the structures which work with generally polarised light, because linear polarised light is a bit easier to achieve than circular polarised light.  Also, I would want to demonstrate optical diffusers in different configurations.

Stewart Bland:        Okay, excellent.  So, to finish, I’d like to ask, as always: In your opinion, what are the hot topics in material science right now?

Alex Minovich:        Metasurfaces are definitely one of the hot topics.  Because, according to Google Scholar, they run now about 9,000 articles and conference papers in this area, including about 2,000 published since the beginning of the current year.  Then I think topological insulators and grapheme would be hot topics related to optical research.