A Metalens with a Near-Unity Numerical Aperture


Numerical aperture is defined as the product of the refractive index of the medium via which light traverses and the sine of the angle under which it is focused. Numerical aperture of any lens is significantly important in that it determines its ability to focus light and its resolving proficiency. Large numerical apertures are desirable for applications that require small light-matter interaction volumes or considerably large angular collections. In the past, in order to achieve large numerical apertures, precision bulk optics that were expensive and specialty items had to be utilized. Recently, meta-surfaces that allow the lens designer to avert these issues during the production of high-numerical apertures lenses in an ultra-flat fashion have been developed. Unfortunately, so far, the application of meta-surfaces is limited to numerical apertures on the same order of magnitude as traditional optical components.

Recently, a team of researchers led by Arseniy Kuznetsov from the Data Storage Institute and now working for the Institute of Materials Research and Engineering (Agency for Science, Technology and Research, A*STAR) in Singapore has developed a novel approach that yields in a diffraction-limited flat lens with a near-unity free-space numerical aperture and subwavelength thickness, operating with un-polarized light at 715 nm. In order to achieve this goal, the researchers developed a novel design concept, which allowed them to create arrays of nanoantennas, which can bend the incoming light at very large angles, up to 82 degrees. Their work is currently published in the research journal, Nano Letters.

The research technique employed commenced with full wave numerical simulations of the diffraction efficiencies of regular arrays of nano-antennas using a finite element method- based commercial software. Each part of the lens was designed and simulated to bend light at different angles towards the focus. Then it continued with sample fabrication where thin films of amorphous silicon were deposited on fused silica substrates via chemical vapor deposition. Next, the samples were patterned using single-step electron beam lithography and subsequently etched via reactive ion etching in an inductively coupled plasma system using chlorine gas. The research team then proceeded to conduct optical measurements in order to characterize the diffraction efficiencies in transmission of uniform arrays of silicon disks.

The research team observed that the novel approach employed resulted in a diffraction-limited flat lens with a near-unity numerical aperture (NA > 0.99) and subwavelength thickness (∼λ/3), operating with un-polarized light at 715 nm. Additionally, the research team noted that the diffractive elements could efficiently bend light at angles as large as 82°, thereby representing a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop previously associated with the standard, phase mapping approach. To demonstrate the lens performance the team applied it in a confocal configuration to map color centers in sub-diffractive diamond nanocrystals.

The Ramón Paniagua-Domínguez and colleagues study presented novel concepts in which nano-antennas with tailored scattering patterns were used to engineer the redistribution of the diffracted energy in periodic arrays. Their new research has potential to find important applications in the field of optical gratings, where a precise control of energy distribution is vital. Furthermore, the associated flat lens design, with a near-unity numerical aperture, may be useful in a wide range of applications requiring high-resolution focusing, such as photolithography. Altogether, this work represents a clear-cut example of how resonant nano-antennas may help to overcome the limitations of phase mapping approaches to diffractive optics and how meta-surface concepts may lead to truly subwavelength-thick flat optical devices that break the limitations of their traditional counterparts.

A Metalens with a Near-Unity Numerical Aperture. Advances in Engineering

About the author

Dr. Ramon Paniagua-Dominguez obtained his BSc in Physics (2008), MSc in Theoretical Physics (2009) and PhD in Physics (2013) from Universidad Complutense de Madrid. During his PhD studies he worked at the Instituto de Estructura de la Materia (Spanish National Research Council, CSIC) under the IC3P scholarship grant. Since September 2014 he has been working in the Agency for Science, Technology and Research (A*STAR) in Singapore, first in the Data Storage Institute and then in the Institute of Materials Research and Engineering. His research interests are in nano-optics and nanophotonics, in particular in the fields of optical antennas for tailored light emission and scattering, and their application to metasurfaces and flat optics. He has authored over 30 journal papers and filed 5 patents.

About the author

Ye Feng Yu holds a PhD in Electronics, Optronics and Systems (University of Paris-Est, 2012), a Master in Condensed Physics (Wuhan University, 2006) and a Bachelor in Physics of Materials (Wuhan University, 2002). He has worked as a Project Officer (2006-2007) and a Research Associate (2007-2011) at Nanyang Technological University, Singapore. Then he has spent seven years (2011-2018) at Data Storage Institute as a Research Scientist. Now he is a professor at Nanjing University of Science and Technology, China.
His main research domains are dielectric nanoantennas and metasurfaces, as well as their applications in flat lens, vortex, fluorescence enhancement, lasers, and optical forces. Dr. Yu has a wide experience in optical designs, nano-device fabrications, and optical measurements.

About the author

Egor Khaidarov is currently pursuing his PhD in electrical and electronic engineering at Nanyang Technological University, Singapore, as a joint project with Institute of Materials Research and Engineering, A*STAR. He recieved his Bachelors degree from Moscow Institute of Physics and Technology, Russia, Master of Science from Abbe School of Photonics, Jena, Germany. His interests include metasurfaces, LED, plasmonics, biosensing, tunable optics, 2D materials, nanophotonics.

About the author

Dr Sumin Choi studied Physics and Nanotechnology at University of Technology Sydney (Australia) and received his PhD in nanophotonics from the same institution in 2017, where his doctoral thesis entitled ‘Zinc oxide (ZnO) nanophotonics: toward quantum photonic technologies’. He established ZnO for quantum photonics leading to high quality publications; discoveries of resonators, single photon emitters and electroluminescence from localized defects in n-ZnO/p-Si heterojunctions. He has also explored quantum emitters in wide bandgap materials and aim to fabricate quantum nanophotonic devices for next generation of quantum information science.

His research expertise includes the experimental physics on the single photon sources from semiconductor or 2D materials such as diamond, ZnO and hexagonal boron nitride (hBN). Currently he is a research scientist at Agency for Science, Technology and Research (A*STAR) at Singapore, and his main project involves design, fabrication and integration of nanophotonic devices based on nanodiamonds entitled ‘Engineering of a scalable photonics platform for quantum enabled technologies’. Main expertise areas include quantum information science in spin states of atomic defects in diamond nanoparticles. He has explored the ability to control the SiV colour centres in diamond and spatially manipulated and precisely positioned for enhanced coupling to other nanophotonics structures focusing on the practical implementation of quantum networks by using integrated photonic technologies.

He has published one of the most cited papers regarding ZnO single photon sources and more recently studied a road map for engineering an integrated photonics platform, which will allow generation, transfer and detection of quantum information on the same optical chip.

About the author

Dr. Victor Leong is a scientist at the Institute of Materials Research and Engineering, A*STAR, Singapore. His current research is based on nanophotonics platforms, focusing on the integration of quantum emitters (color centers in diamond), optical characterization of devices, and development of on-chip single-photon detectors.

His research experience includes the study how single photons interact with single atoms, the coherent interfacing of multiple atomic quantum systems, and the design and characterization of nano-electro-mechanical switches. He obtained his Ph.D. degree in physics from the National University of Singapore, and his B.A. (Hons) in physics from Cambridge University, UK.

About the author

Reuben Bakker holds a PhD in Electrical and Computer Engineering (Purdue University, USA, 2008), a Masters in Electrical and Computer Engineering (Purdue University, USA, 2004) and a Bachelors in Engineering Physics (Queens’ University, Canada, 2002). He joined the Data Storage Institute, Singapore in 2008 and is currently with its sister institute, the Institute of Materials Science and Engineering.

Reuben’s scientific career has been spent in the realm of nanophotonics, first in near-field optics and plasmonics, then nanofabrication and now dielectric nanoantennas. He is also working on transferring technology out of the lab and into companies.

About the author

Dr. Xinan Liang is currently a scientist in institute of materials research and engineering (IMRE), ASTAR Singapore. He obtained his bachelor degree of material science from Lanzhou University (China) in 1992, master degree of electron, ion and vacuum physics from Chinese Academy of Space Technology (CAST) in 1997 and Ph.D from Shanghai Institute of Ceramics, Chinese Academy of Science (SICCAS) in 2000. From 2000 to 2002, he worked in SICCAS as an assistant professor focused on in-situ observation of crystal growth process under microgravity environment. Since 2002, he held different positions as a senior research fellow, scientist in Data Storage Institute (DSI), ASTAR Singapore. In DSI, he worked on various projects on the development of holographic data storage media and related recording techniques, electronic holographic 3D display and submicron spatial light modulators.

His works on holographic 3D display were featured by various media, such as SPIE News Room, Physics Today, ScienceDaily etc. His research interests include dielectric nano antennas, tunable submicron spatial light modulator, digital holography and 3D holographic display.

About the author

Yuan Hsing Fu is a scientist in Data Storage Institute, A*STAR, Singapore, from 2010-2018, and currently in Institute of Microelectronics, A*STAR, Singapore. Prior, he had been a post-doctoral fellow (Department of Physics, National Taiwan University, Taiwan, from 2006 to 2010), visiting research fellows (Optoelectronics Research Centre, Southampton University, UK, 2008, and Department of Physics, Imperial College London, UK, 2009) and a research fellow (School of Electrical & Electronics Engineering, Nanyang Technological University, Singapore, 2010). He received his Ph.D. and M.Sc. degrees in Physics (National Taiwan University, Taiwan, 2005 and 2002, respectively).

His research interests include light-matter interaction, nano-optics, plasmonics, metamaterials and metasurfaces, and to develop novel type nanoantenna, nano-optics devices and flat optics. Dr. Fu has in-depth knowledge in optics/photonics area and wide experiences in microscope, near-field scanning optical microscope, spectrum, laser system, nanostructure fabrication and design/simulations.

About the author

Vytautas Valuckas got his bachelor’s in physics in 2012 at Vilnius University, Lithuania. After that, he joined A*STAR, Singapore, as a research engineer, first at the Data Storage Institute and later at the Institute of Materials Research and Engineering. He works mostly with dielectric nanoantennas and metasurfaces, specializing in femtosecond laser nanofabrication and characterization, electron beam lithography, optical characterization and scanning electron microscopy.

About the author

Dr. Leonid Krivitsky obtained his Ph.D. degree from the Department of Physics of Lomonosov Moscow State University in 2005. He spent a few years as a postdoc in National Metrology Institute (INRiM, Italy), Max Planck Institute for the Science of Light (MPL, Germany), and Technical University of Denmark (DTU, Denmark). Currently, he is leading the Quantum Photonics group at the Institute of Materials Research and Engineering (A*STAR, Singapore), which he started from scratch in 2008, being awarded a prestigious A*STAR Investigatorship. His research is focused on experimental studies of quantum light, and its applications in quantum information, nano-photonics, and biology.

About the author

Dr. Arseniy Kuznetsov graduated from Nizhniy Novgorod State University (Russia) in 2002. He received his PhD in Processes Engineering from University Paris 13 (France) in 2005 and in Laser Physics from Institute of Applied Physics RAS (Russia) in 2006. Since 2007 till 2011 he worked at the Laser Zentrum Hannover (Germany) as a Humboldt Research Fellow. Since October 2011 till now he has been working in A*STAR, Singapore first in the Data Storage Institute and now in the Institute of Materials Research and Engineering.

He is currently appointed as Senior Scientist, Manager of Dielectric Nanoantennas Program and Head of Advanced Concept and Nanotechnology Department. Current activities of his group are devoted to development of novel nanodevices based on dielectric nanoantennas and metasurfaces. He is an author of over 50 journal papers and a co-inventor of 10 filed patent applications. He is the recipient of 2016 IET A F Harvey Engineering Research Prize for his pioneering research on optically resonant dielectric nanostructures and dielectric nanoantennas.


Ramón Paniagua-Domínguez, Ye Feng Yu, Egor Khaidarov, Sumin Choi, Victor Leong, Reuben M. Bakker, Xinan Liang, Yuan Hsing Fu, Vytautas Valuckas, Leonid A. Krivitsky, and Arseniy I. Kuznetsov. A Metalens with a Near-Unity Numerical Aperture. Nano Letters. 2018, volume 18, pages 2124−2132

Go To Nano Letters

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