Monday, September 23, 2013

CST interviews Dr Miguel Navarro-Cía, Imperial College London

Dr. Miguel Navarro-Cía, currently a Junior Research Fellow in the Department of Electrical and Electronic Engineering at Imperial College London, won a CST University Publication Award in 2012 for work on a metamaterial lens as part of a team in the Millimeter & Terahertz Waves Laboratory at Universidad Pública de Navarra. As part of our university interview series, we caught up with him to ask him about his work.

CST: Hello Miguel, thank you for taking the time to answer a few questions.
Let’s start with the work that won the 2012 University Publication Award, the metallic lens. Could you give a bit of background into the lens – for example, what applications the lens might have?

Navarro-Cía: The field of metallic lenses traces back to 1940s when they were investigated for radar. It has regained attention now in plasmonics and in metamaterials for other type of applications such as near-field beam shaping at optical frequencies. The advantage of metallic lenses is that they can be easily made graded-index, and thus, planar. For our particular metamaterial design, they can also be free-space matched. We believe that these two features make the fishnet metamaterial lens suitable for terahertz applications where standard dielectric lenses made of silicon, for instance, are bulky, relatively lossy and extremely mismatched with free space. In fact, because of the large mismatch, a silicon lens might behave as a resonator in many terahertz applications, which is not desirable. We are also looking at applications involving high power or for hazardous conditions where dielectric lenses cannot be used.

CST: How did simulation help the group to develop the lens?
Navarro-Cía: When we started the project, the metamaterial structure that we were considering for the lens (the fishnet metamaterial, which we also call “stacked subwavelength hole array” or “extraordinary transmission metamaterial”) did not have an analytical solution. The analytical solution was developed afterwards. Therefore, the numerical approach was the only way to compute its dispersion diagram. From the dispersion diagram, we were able to calculate the effective index of refraction of the metamaterial, which is of course key to defining the profile of a lens. We validated experimentally the effective refractive index via the Snell's law by constructing a prism and measuring the angle of refraction of the output beam. However, this is not the end of the story: the fishnet metamaterial is anisotropic, and it imposes a non-negligible step-wise approximation to the profile. Therefore, we needed to simulate the 2D and 3D model to confirm that all these issues were not going to affect the response of the lens. Finally, we also used CST to model classical metallic lenses and compare their performance with our fishnet metamaterial lens.
CST: The paper covers basically the entire development process, from the first analytic calculations, through simulation right up to building and testing a prototype. Where do you see the line between simulation and prototyping? In other words, at what point do you say “We’re ready to build a model”?
Navarro-Cía: Given our experience with the prism (simulation and experiment agreed very well), we were confident that the lens was going to work. Thus, when the 2D simulations were confirming our assumptions, we felt ready to build the prototype. It has been a long time (the project started in 2007), but I would say that the final 3D simulation (with best mesh and accuracy) was running in parallel to the prototyping.

CST: Has the CST University Publication Award helped you and your group in carrying out research?
Navarro-Cía: The license for one year went to Universidad Pública de Navarra, Spain, where I did my PhD and the work of the awarded paper. The group there has had a significant budget cut down since 2012, due to the economic situation in Spain, and this meant that the number of licenses, among other expenses, had been cut.
In addition, the head of the group and co-author of the awarded paper, Prof. Mario Sorolla, passed away in November 2012. I can tell that receiving an upgraded license has helped raise their spirits and to maintain the research activity that they had with Prof. Mario Sorolla, who was indeed the one who introduced CST software into Pamplona.

CST: What is the latest on the lens?
Navarro-Cía: In collaboration with Dr Miguel Beruete and his group (formerly Prof. Mario Sorolla's group), I have recently submitted a paper where we apply a zoning technique to the lens in order to make it ultra-compact and quasi-planar. The results are excellent. And actually, they will be shown in the upcoming CST webinar “Modeling and simulation of metamaterial-based devices for industrial applications”. This is the last step before introducing the lens in a real application.

CST: Is there any other work you’re currently involved in that you would like to highlight?
Navarro-Cía: At the present moment I am very active on THz waveguides in collaboration with Dr Oleg Mitrofanov from University College London, and I use CST MICROWAVE STUDIO® for three purposes:
(i) To compute the eigenmodes of dielectric-lined cylindrical metallic waveguides. CST tools help us to estimate losses and the origin of them, unveil the mode mixture in multi-mode situations, etc.
(ii) To study the propagation of THz pulses along a tapered parallel-plate waveguide. CST MWS (specifically, the time monitor) was key to understanding our initial experiments and to shed light into unusual dispersions reported in the literature.
(iii)  To understand and optimize an imaging setup developed by Dr Oleg Mitrofanov where Zenneck surface waves are launched by hitting the edge of a metal structure with the HE11 mode of a dielectric-lined cylindrical metallic waveguide.
Finally, my PhD student is working on nanoantennas and one aspect that we are right now interested in is the local temperature increase because of the plasmonic resonance. For this work we use the time-domain solver of CST MWS along with the thermal solver from CST MPHYSICS® STUDIO.

CST: We look forward to seeing this research soon. Thank you again for your time, and best of luck in your work.

For more information, see Miguel Navarro-Cía’s homepage and Google Scholar page.

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