Optical Anapole Metamaterial


The first document ever published about static toroidal dipole in the context of nuclear physics was in the late 1950s’. Since then subsequent studies have established that dynamic toroidal dipoles are an independent entity in the family of electro dynamic multipole expansion. This therefore means that the toroidal dipole is naturally different from the electric and magnetic multipoles that are associated with nonvanishing induced charge density and nonzero induced transverse current density, respectively. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a non-radiating charge current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Unfortunately, the resulting excitations have proven difficult to couple to external radiation without resorting to complex excitation geometries.

Recently, a team of researchers led by Professor Din Ping Tsai from the Department of Physics, at National Taiwan University demonstrated a quasi-planar plasmonic metamaterial exhibiting induced transverse toroidal response and associated resonant anapole response in the optical part of the spectrum, driven by normal incident light. In particular, they reported on a toroidal metamaterial comprised of a multilayered structure containing a planar array of vertical split-ring resonators (VSRRs) suspended in a dielectric medium and covered with a perforated gold film. Their work is currently published in the research journal, ACS Nano.

In brief, the research method employed commenced with sample fabrication where a ZEP520A layer was spin coated onto a fused silica substrate and then was baked on a hot plate. Subsequently, an E-spacer layer was spin coated onto the ZEP520A layer. The researchers then fabricated two prongs of vertical split-ring resonators using e-beam exposure and lift-off process. Once the samples were fabricated, measurements were undertaken using techniques such as the Fourier-transform infrared spectrometer and near-infrared polarizer. Lastly, simulations, that enabled the researchers obtain the optical reflection as well as the transmission spectra for the samples, were carried out.

The authors observed that all the significant properties of the anapole metamaterial could be tuned through a single well-defined geometrical parameter, which could be controlled to a great precision during fabrication. Consequently, the latter was seen to benefit the design of the optical metamaterials with the desired multipole response, while the others were suppressed on purpose. Moreover, they realized that their technique offered fine control over the radiation loss, and near-field enhancement in nano-photonics.

In summary, the study presented successful design, analysis and fabrication of the planar plasmonic metamaterial, which supports transverse toroidal dipole and anapole excitations that dominate the metamaterial’s resonant response in the optical part of the spectrum. In general, the results obtained here prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional transmission and reflection experiments. Altogether, their work has potential immediate applications in sensing, nonlinear optics and opto-mechanics.

About the author

Din Ping Tsai received Ph.D. in Physics from University of Cincinnati, USA in 1990. He worked at Micro Lithography Inc., California, USA, Ontario Laser and Lightwave Research Center, Toronto, Canada, and National Chung Cheng University, Taiwan from 1990 to 1999. He joined National Taiwan University (NTU) as an Associate Professor in 1999, and became Professor and Distinguished Professor of Department of Physics at NTU in 2001 and 2006, respectively. He served as the Director General of the Instrument Technology Research Center (NARL) located in Hsinchu Science Park, Taiwan from 2008 to 2012. He is the Director and Distinguished Research Fellow of Research Center for Applied Sciences, Academia Sinica since 2012.

Professor Tsai is a Fellow of American Association for the Advancement of Science (AAAS), American Physical Society (APS), Institute of Electrical and Electronics Engineers (IEEE), Optical Society of America (OSA), International Society of Optical Engineering (SPIE), Electro Magnetics Academy (EMA), Asia-Pacific Academy of Materials, The Japan Society of Applied Physics (JSAP), and The Physical Society of R.O.C. He is author and coauthor of 293 SCI journal papers (more than 8262 cited times, H-index 46), 65 book chapters and conference papers, and 39 technical reports and articles. He had 66 patents in USA, Japan, Canada, Germany and Taiwan.

Professor Tsai is the President of Taiwan Information Storage Association (2015-), Taiwan Photonics Society (2014-2016). He also serves as committee member for IEEE Joseph F. Keithley Award in Instrumentation & Measurement (2013-2016), and chairman of the award committee for OSA and IS&T Edwin H. Land Medal (2014-2016). He was the Director of the Board of SPIE from 2012 to 2014, and Chair (2009-2013) of IEEE Instrument and Measurement Society Taipei Chapter, Member of SPIE Fellow Committee for three years (2010-2013), and Member of OSA Fellows and Honorary Members Committee for 2008 & 2009, respectively. He was the chair and vice chair of the International Society for Optical Engineering (SPIE) Taiwan chapter for 2004 & 2005 and 1996 & 1997. He was also a member of technical committee of IEEE/LEOS nanophotonics.

He currently serves as Editor of Progress in Quantum Electronics, Associate Editor of Journal of Lightwave Technology, Member of Editorial boards of Small Methods, Advanced Quantum Technologies, Opto-Electronic Advances, APL Photonics, Physical Review Applied, Optics Communications, Plasmonics, ACS photonics, and Optoelectronics Letters, respectively.

He is member of the boards of Taiwan Photonics Society, Taiwan Nanotechnology Industrialization Development Association, and Taiwan Information Storage Association.


Pin Chieh Wu, Chun Yen Liao, Vassili Savinov, Tsung Lin Chung, Wei Ting Chen, Yao-Wei Huang, Pei Ru Wu, Yi-Hao Chen, Ai-Qun Liu, Nikolay I. Zheludev, Din Ping Tsai. Optical Anapole Metamaterial. ACS Nano 2018, volume 12, page 1920−1927.

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