Session K02: Topological Metamaterials

Exceptional Nanophotonics

Integrated nanophotonics promises next-generation computing platforms. In this talk, I will present our work on exceptional nanophotonics, creating “quantum” exceptional points on-a-chip and utilizing their associated topology. Based on the exceptional point-induced unidirectionality, we harness optical losses to enable unique photonics functionalities, in particular, an orbital angular momentum (OAM) microlaser that structures and twists lasing radiations at the micro/nano-scale and a low-power all-optical switch, which may address the growing demand for information capacity. Additionally, I will discuss non-Hermitian topological photonics where optical non-Hermiticity and topological physics are coupled to produce robust photonic integrated circuits and laser sources and novel microscopic thermal sensors.
  

Definite photon deflections of topological defects in metasurfaces and symmetry-breaking phase transitions with material loss

In this work [1], by using an artificial waveguide bounded with rotational metasurface, the nontrivial effects of a topological defect of spacetime are experimentally emulated. The photon deflection in the topological waveguide has a robust definite angle that does not depend on the location and momentum of incident photons. This is remarkably different from the random optical scattering in trivial space. By including material loss such a topological effect can be well understood from the symmetry breaking of photonic modes. Our technique provides a platform to investigate topological gravity in optical systems. This method can also be extended to obtain many other novel topological photonic devices.

[1] Chong Sheng, Hui Liu*, Huanyang Chen and Shining Zhu “Definite photon deflections of topological defects in metasurfaces and symmetry-breaking phase transitions with material loss”Nature Communications 9:4271, DOI: 10.1038/s41467-018-06718-9 (2018)   

Loss induced high efficient Lorentz nonreciprocal metasurfaces

Lorentz nonreciprocal metasurfaces rely on breaking all symmetries related to energy flow reciprocity. We numerically design a metasurface made of dimer unit cell to achieve nonreciprocity. The dimer consists of a gyromagnetic and lossy dielectric cylinder. Under the biased magnetic field, the TE (nonzero electric field out of plane) polarized plane wave at normal incident in different direction exhibit nonreciprocal transmission. We successfully employ the multiple scattering theory (MST) up to the dominant resonant modes of cylinders to describe the numerical nonreciprocal feature. The nonreciprocal mechanism can be revealed by the eigen-response theory that different direction incidence comprises of different portion of lossy mode which will favour to excite the lossy dielectric cylinder, leading to nonreciprocal transmission.   

Topological pumping in a magneto-mechanical system

A topological charge pump conveys quanta of charge across a gapped system while being robust against array disorder, external noise, and disorder in the pumping protocol. The implementation of topological insulators in both optical and acoustic coupled resonator systems has opened exciting paths towards new control over light and sound, however an equivalent temporal topological pump has yet to be demonstrated. In this work, we demonstrate that a 1D classical system that is adiabatically spatio-temporally modulated can produce an equivalent topological pump for bosons. Our experimental demonstration consists of an array of magneto-mechanical resonators, where the inter-resonator coupling rates are controlled by the distance over which the magnetic interaction takes place. We modulate both the coupling rates and the on-site potentials using a mechanical apparatus, and demonstrate that mechanical energy can be pumped from one edge mode of the array to the other edge mode while the system remains gapped. We show that this 1D array is the equivalent of a Chern insulator in one real dimension and one synthetic dimension.
  

Unidirectional excitation of surface plasmon polaritons in a grating system with Parity-Time symmetry

Over the past years, there has been rapidly growing interest in non-Hermitian photonic systems. A Hamiltonian with Parity-Time (PT) symmetry can show real spectrum below a certain threshold, known as the exceptional point (EP), accompanied with novel phenomena above the EP. Here we propose a plasmonic grating system with PT symmetry to excite unidirectional surface plasmon polaritons (SPPs). We have designed realistic grating array only consisting of passive rectangular gratings (i.e., without gain materials), which can significantly reduce the challenges in fabrication and optical experiments. In addition, instead of using ideal sinusoidal permittivity modulation in simulation in former paper(2), here we demonstrate in both simulation and experiment that using discrete grating array can also achieve excellent contrast between SPPs in opposite directions
In summary, we have demonstrated a sub-wavelength scale plasmonic grating structure with PT symmetry that can realize high performance for unidirectional excitation of SPPs. These results can be employed as a new approach to designing nanoscale optical devices, such as low-loss plasmonic routers and isolators for optical computation, communication, and information processing.

1. W. Wang et al., Phys Rev Lett 119, 077401 (2017).   

Atomic-scale nanophotonics and quantum optics through 2D phonon polaritonics

Recent developments have demonstrated extreme confinement of electromagnetic energy in the mid-infrared via phonon polaritons in polar insulators. Phonon polaritons hold promise for low-loss nanophotonics, as well as realizing extreme interactions with quantum emitters at the single-photon level. To bring these materials to the ultimate limit of optical nano-confinement, it is critical to consider phonon polaritons in low-dimensional polar materials, where the physics of optical phonons is fundamentally different due to the lack of LO-TO splitting. In this talk, we find universal forms for the properties of phonon-polaritons in one- and two-dimensions which result from the unique character of Coulomb interactions in low-dimensional systems. Leveraging first-principles calculations of optical phonons in 2D, we calculate the polaritonic properties of polar insulator monolayers, presenting specific results for hexagonal boron nitride. We find regimes of low-loss and high-confinement of electromagnetic energy, and show how these regimes can be probed with EELS. We then show how nanostructuring can be used to develop ultrafast quantum emitters of phonon polaritons in the mid-IR.   

Zak phase of 1-D electrostatically gated graphene grating and optical properties of plasmonic edge state

We design periodic metal gates on top of a hBN/Graphene/hBN sandwich heterostructure to modulate local graphene conductivity. We investigated properties of graphene surface plasmon(GSP) of a doubly gated graphene. Stop bands due to GSP modes mixing can be controlled by electrostatic doping. Topology of each plasmon band can be characterized by a Zak phase. When two insulators of a same bandgap but distinct Zak phases are joined together, an edge state in the bandgap has been observed in many physical systems. The edge state observed on our graphene grating platform presents an anisotropic dipole moment. While it radiating into the left or right in the forward direction offers an interrogation approach for band topologies of component graphene gratings, the angle of radiation remains a tunable property that depends on an interplay of the mode's resonant frequency, the bandgap width, its mid-gap frequency and the dimensions of metal gates. Due to its capability of active tuning, this may serve as a stepping stone on the way toward active control of surface plasmon band structures for optical communication, wavefront steering or sensing applications.   

Broadside Coupled Resonators for Real-time Tunable Metamaterials

Metamaterials based on broadside coupled split ring resonators (BC-SRRs) exhibit large tunability induced by modulating the coupling that arises from mutual capacitance and inductance. We design and fabricate a real-time tunable terahertz metamaterial by integrating BC-SRRs with comb-drive actuators, and characterize the dynamic response using terahertz time domain spectroscopy. The experimental results reveal that we can control the amplitude and phase of the transmission coefficient dynamically by driving the comb-drive actuators. We also observe remarkable modulation in the group delay. A two-port resonator model, derived from coupled mode theory, can analytically explain the experimental results and unveils the critical role of coupling between BC-SRRs. The results demonstrate that the propagation of electromagnetic waves can be dynamically controlled through modulating coupled resonators in metamaterials.   

Ultrahigh Numerical Aperture Silicon Metalens at Visible Wavelengths

High Numerical Aperture (NA) Metalens is among the promising optical elements for ultrathin microscope objective [1, 2]. Meanwhile, crystalline silicon (c-Si) has been shown to be used in the high transmission metasurfaces at visible wavelength because of its low loss and low cost [3, 4].
Here, we demonstrate an oil front-immersion metalens with NA = 1.48, a bandwidth of 211 nm and a focusing efficiency of 48% at 532 nm wavelength, base on a 500 nm thick c-Si on sapphire [2] (compared with 50% for back-immersion TiO2 metalenses with NA = 1.1 [1]). Thereby demonstrating the highest NA of any metalens at visible wavelengths reported to the best of our knowledge. We further implement our metalens onto the laser scanning confocal microscopy as the immersion objective to demonstrate the imaging quality.
We note that this fabrication process is fully compatible with mature CMOS technologies. We envision this design also to be beneficial for immersion metalens doublets, thereby pushing metasurfaces into practical applications such as confocal microscopy and achromatic lenses.
References
[1] W.T. Chen, et. al., Nano Letters, 17, 3188, 2017
[2] H. Liang, et. al., Nano Letters, 18, 4460, 2018
[3] Z. Zhou, et. al., ACS Photonics, 4, 544, 2017
[4] D. Sell, et. al., ACS Photonics, 3, 1919, 2016   

Spectral properties of aperiodic Vogel spirals

By using the dyadic Green's matrix spectral method, we demonstrate that aperiodic deterministic Vogel spirals made of electric dipoles support light localization in three dimensions, an effect that does not occur in traditional uniform random media. We discover a transition from an extended to a localized regime by evaluating the Thouless conductance and by performing a finite-size scaling. Vogel spirals are suitable photonic platforms to localize light thanks to their distinctive structural correlation properties that enable collective electromagnetic excitations with strong light-matter coupling. By decomposing the dyadic field propagator in its different components we show that light localization in Vogel arrays originates from collective electromagnetic coupling involving the contributions of multiple length scales. Our results unveil the importance of structural correlations in deterministic aperiodic photonic media for the design of localized states with strongly enhanced light-matter interactions. In addition, our findings may open new vistas for the engineering of mesoscopic transport and localization phenomena and should encourage deeper investigations of photonic devices based on deterministic aperiodic architectures.   

Topological Metamaterials

Selected by Focus Topic Organizer Xiaobo Yin, Natalia Litchinister, Nanfang Yu