Song Han

Engineering the fano resonance and electromagnetically induced transparency in near-field coupled bright and dark metamaterial

Near-field coupling between orthogonally twisted bright and dark mode resonances gives rise to sharp Fano-like resonances and electromagnetically induced transparency. We demonstrate that by varying the orientation of the near-field coupled bright and dark metamolecule with respect to the incident electric field, the shape and linewidth of the coupling induced transparency could be easily engineered. This particular aspect of the near-field coupled metamolecules could lead to engineered slow-light devices, narrow bandpass filters and ultrasensitive sensors.

Tunable electromagnetically induced transparency in coupled three-dimensional split-ring-resonator metamaterials

Metamaterials have recently enabled coupling induced transparency due to interference effects in coupled subwavelength resonators. In this work, we present a three dimensional (3-D) metamaterial design with six-fold rotational symmetry that shows electromagnetically induced transparency with a strong polarization dependence to the incident electromagnetic wave due to the ultra-sharp resonance line width as a result of interaction between the constituent meta-atoms. However, when the six-fold rotationally symmetric unit cell design was re-arranged into a fourfold rotational symmetry, we observed the excitation of a polarization insensitive dual-band transparency. Thus, the 3-D split-ring resonators allow new schemes to observe single and multi-band classical analogues of electromagnetically induced transparencies that has huge potential applications in slowing down light, sensing modalities, and filtering functionalities either in the passive mode or the active mode where such effects could be tuned by integrating materials with dynamic properties.

Near-infrared linewidth narrowing in plasmonic Fano-resonant metamaterials via tuning of multipole contributions

We report on an experimental and computational (multipole decomposition) study of Fano resonance modes in complementary near-IR plasmonic metamaterials. Resonance wavelengths and linewidths can be controlled by changing the symmetry of the unit cell so as to manipulate the balance among multipole contributions. In the present case, geometrically inverting one half of a four-slot (paired asymmetric double bar) unit cell design changes the relative magnitude of magnetic quadrupole and toroidal dipole contributions leading to the enhanced quality factor, figure of merit, and spectral tuning of the plasmonic Fano resonance.We report on an experimental and computational (multipole decomposition) study of Fano resonance modes in complementary near-IR plasmonic metamaterials. Resonance wavelengths and linewidths can be controlled by changing the symmetry of the unit cell so as to manipulate the balance among multipole contributions. In the present case, geometrically inverting one half of a four-slot (paired asymmetric double bar) unit cell design changes the relative magnitude of magnetic quadrupole and toroidal dipole contributions leading to the enhanced quality factor, figure of merit, and spectral tuning of the plasmonic Fano resonance.

Biosensing with the singular phase of an ultrathin metal-dielectric nanophotonic cavity

The concept of point of darkness has received much attention for biosensing based on phase-sensitive detection and perfect absorption of light. The maximum phase change is possible at the point of darkness where the reflection is almost zero. To date, this has been experimentally realized using different material systems through the concept of topological darkness. However, complex nanopatterning techniques are required to realize topological darkness. Here, we report an approach to realize perfect absorption and extreme phase singularity using a simple metal-dielectric multilayer thin-film stack. The multilayer stack works on the principle of an asymmetric Fabry–Perot cavity and shows an abrupt phase change at the reflectionless point due to the presence of a highly absorbing ultrathin film of germanium in the stack. In the proof-of-concept phase-sensitive biosensing experiments, we functionalize the film surface with an ultrathin layer of biotin-thiol to capture streptavidin at a low concentration of 1u2009pM.Optical sensors generally rely on abrupt phase changes to detect the presence of an analyte, but oftentimes they require complex nanostructures. Here, Sreekanth et al. use a simple asymmetric thin-film multilayer stack to demonstrate a point of darkness and phase singularity to develop a sensitive biosensor.

Tailoring the multiple electrically resonant transparency through bi-layered metamaterial-induced coupling oscillators

Metamaterials (MMs) can be tailored to support electromagnetic interference, which is the kernel for the material-based electromagnetically induced transparency (EIT) phenomena, alternatively transparency based on electric interference can be deemed as electrically resonant transparency (ERT). Here, we experimentally and theoretically demonstrate two kinds of bi-layered MMs. The C3-C6 hybrid MM exhibits triple-mode ERT with transmission peaks of 0.84 at 9.6 GHz, 0.92 at 10.4 GHz, and 0.93 at 11.5 GHz for the horizontally polarized wave, and dual-mode ERT with transmission peaks of 0.84 at 8.8 GHz and 0.91 at 10.2 GHz for the vertically polarized wave. However, the C4-C8 hybrid MM, with two stable transparent peaks of 0.92 and 0.88 at 10.46 GHz and 11.61 GHz, is proven to be polarization independent. The measured results show excellent agreement with numerical simulations. A coupled oscillator model is employed to theoretically study the near field interference between the induced dipoles on the transmission properties. The results presented here will find their application value for multi-mode slow light devices, filters and attenuators, and so on.

Toroidal and magnetic Fano resonances in planar THz metamaterials

The toroidal dipole moment, a localized electromagnetic excitation of torus magnetic fields, has been observed experimentally in metamaterials. However, the metamaterial based toroidal moment was restricted at higher frequencies by the complex three-dimensional structure. Recently, it has been shown that toroidal moment could also be excited in a planar metamaterial structure. Here, we use asymmetric Fano resonators to illustrate theoretically and experimentally the underlying physics of the toroidal coupling in an array of planar metamaterials. It is observed that the anti-parallel magnetic moment configuration shows toroidal excitation with higher quality (Q) factor Fano resonance, while the parallel magnetic moment shows relatively lower Q factor resonance. Moreover, the electric and toroidal dipole interferes destructively to give rise to an anapole excitation. The magnetic dipole-dipole interaction is employed to understand the differences between the toroidal and magnetic Fano resonances. We further...

Ge2Sb2Te5-Based Tunable Perfect Absorber Cavity with Phase Singularity at Visible Frequencies

The metal-dielectric stacks-based asymmetric Fabry-Perot (F-P) cavity systems have recently attracted much interest from the scientific community for realizing perfect absorption over the spectral bands from visible to infrared since they possess a lithography-free design that is cost-effective and scalable. This study experimentally demonstrates an asymmetric F-P cavity system for achieving tunable wide angle perfect absorption and phase singularity. The proposed system shows tunable multiband perfect absorption in the visible spectral region by incorporating an ultrathin layer of phase change material such as Ge2 Sb2 Te5 (GST) in the stack. The system shows multi-narrowband perfect absorption with a maximum of 99.8% at a specific incident angle and polarization state when the GST is in amorphous phase; however, the absorption bands blueshift and broaden after switching to the crystalline phase. More importantly, the proposed scheme shows tunable phase singularity at the reflection-less point. The obtained tunable perfect absorption and abrupt phase change are solely due to the presence of a highly absorbing ultrathin layer of GST in the stack. Experimental results are validated using an analytical simulation model based on a transfer matrix method. The proposed scheme could find potential applications in active photonic devices such as phase-sensitive biosensors and absorption filters.

Giant enhancement in Goos-Hänchen shift at the singular phase of a nanophotonic cavity

In this letter, we experimentally demonstrate thirtyfold enhancement in Goos-Hanchen shift at the Brewster angle of a nanophotonic cavity that operates at the wavelength of 632.8u2009nm. In particular, the point-of-darkness and the singular phase are achieved using a four-layered metal-dielectric-dielectric-metal asymmetric Fabry-Perot cavity. A highly absorbing ultra-thin layer of germanium in the stack gives rise to the singular phase and the enhanced Goos-Hanchen shift at the point-of-darkness. The obtained giant Goos-Hanchen shift in the lithography-free nanophotonic cavity could enable many intriguing applications including cost-effective label-free biosensors.In this letter, we experimentally demonstrate thirtyfold enhancement in Goos-Hanchen shift at the Brewster angle of a nanophotonic cavity that operates at the wavelength of 632.8u2009nm. In particular, the point-of-darkness and the singular phase are achieved using a four-layered metal-dielectric-dielectric-metal asymmetric Fabry-Perot cavity. A highly absorbing ultra-thin layer of germanium in the stack gives rise to the singular phase and the enhanced Goos-Hanchen shift at the point-of-darkness. The obtained giant Goos-Hanchen shift in the lithography-free nanophotonic cavity could enable many intriguing applications including cost-effective label-free biosensors.

Dataset: linewidth narrowing by multipole tuning

Dataset for Near-infrared linewidth narrowing in plasmonic Fano-resonant metamaterials via tuning of multipole contributions

A Metamaterial Analog of the Ising Model

The interaction between microscopic particles is always a fascinating and intriguing area of science. Direct interrogation of such interactions is often difficult. Structured electromagnetic systems offer a rich toolkit for mimicking and reproducing the key dynamics that govern the microscopic interactions, and thus provides an avenue to explore and interpret the microscopic phenomena. In particular, metamaterials offer the freedom to artificially tailor light-matter coupling and to control the interaction between unit cells in the metamaterial array. Here, a terahertz metamaterial that mimics spin-related interactions of microscopic particles in a 2D lattice via complex electromagnetic multipoles scattered within the metamaterial array is demonstrated. Fano resonances featured by distinct mode properties due to strong nearest-neighbor interactions are discussed, which draw parallels with the 2D Ising model. Interestingly, a phase transition from single Fano resonance to hyperfine splitting of the Fano spectrum is observed by manipulating the 2D interactions without applying external magnetic or electric fields, which provides a potential multispectral platform for applications in super-resolution imaging, biosensing, and selective thermal emission. The dynamic approach to reproduce static interaction between microscopic particles will enable more profound significance in exploring the unknown physical world by the macroscopic analogs.