We report novel phonon-polariton states induced by spatial defects in topological metasurface integrated with hexagonal boron nitride (hBN). The introduction of topological defects, created by stitching domains with different choices of unit cells leads to the emergence of spatially localized modes, while the coupling of these trapped modes with phonons in hBN gives rise to the formation of polaritonic states. We designed and fabricated a mid-IR-operating hybrid system that consists of a photonic metasurface with a thin layer of hBN on top of it. Topological defect modes of the fabricated structure were probed using direct imaging in both real- and Fourier-space.
We introduce the concept of adiabatic topological photonic structures, which allows us to overcome many of the limitations of topological photonic metasurfaces. We demonstrate that topological metasurfaces with slowly varying gauge fields significantly improve trapping of optical modes, and also offer excellent guiding features in both spin-Hall and valley-Hall topological photonic structures. Adiabatic variation of the mass terms at the domain walls makes topological boundary modes less sensitive to details of the lattice, perceiving the structure as an effectively homogeneous Dirac metasurface. As the result, the modes exhibit longer radiative lifetime and propagation distance, while retaining their topological resilience. At the same time, localized modes trapped due to the 2D variation of the mass term exhibit high quality factors and controllable radiative properties, which, along with non-zero angular momentum of their far field, makes them of great interest for applications.
Strong light-matter coupling enabled polariton states were extensively exploited for enhanced optical nonlinearities, development of low threshold lasers and quantum simulators. Here, we will report our recent work on demonstration of novel topological polaritonic phases by leveraging the strong coupling between the photonic topological boundary states and two different material degrees of freedom in 2D Van der Waals materials; first transverse optical phonons in a hexagonal boron nitride (hBN) thin film and second excitons in 2D WSe2 monolayer. Our results will demonstrate emergence of topological boundary states of phonon-polariton and exciton-polariton character, and their resilient unidirectional propagation around sharp corner with avoided backscattering.
Here we demonstrate that a unitary transformation due to nonuniform artificial gauge field enables a new class of topological boundary states carrying both spin and valley polarization. We show that such transformations also allow to tune radiative lifetimes of the hybrid spin-valley boundary modes. Then we demonstrate that gauge transformations, when applied adiabatically to the boundary modes, offer a mechanism for flipping the pseudo-spin without back reflection thus implementing an X-gate acting in synthetic Bloch subspaces spanned by pseudo-spins. Finally, we show that such adiabatic evolutions give rise to the geometrical phases, which offers a generic Phase-gate operation. Our results unveil a new versatile approach to control modes in topological photonics and also envisions topological materials as one of promising candidates for integrated quantum photonics applications.
We have designed and experimentally realized a polaritonic topological insulator based on bulk transition metal dichalcogenide crystals (TMDC, ~40nm-thick WS2 film). We have demonstrated that due to their high refractive index and the presence of exciton modes in the optical range, they represent an excellent platform for topological polaritonics, offering both excellent confinement and strong light-matter interactions in a single material. The successful patterning of TMDC into the topological crystal was demonstrated and emergence of the topological polaritonic boundary modes was directly confirmed by the back focal plane imaging and real space imaging techniques.
Geometrical phases such as Pancharatnam and Berry phase, have been playing important role in classical wave and quantum physics. In topological photonics, the geometrical phases can be controlled with artificial gauge fields by designing lattice geometry of photonic crystals. Here, we theoretically and experimentally demonstrate that geometric phases can give rise to a new class of resonant states in topological ring resonators. Our simulations and analytical model reveal a hierarchy of the resonant modes and transformation of polarization states. Also, we provide experimental observations of the resonant states by infrared spectroscopy.
We demonstrate the emergence of a new class of guided modes in photonic metasurfaces with a gradient change of mass term across boundary between topological and trivial domains. These modes possess spin degeneracy and exhibit splitting in their quality factors due to spin-dependent radiative losses. In experiment we probe our spin-full guided modes by selectively exciting them with circularly polarized light of opposite handedness and we confirm significant difference in the radiative losses for selected k-vectors. Metasurfaces supporting these modes can be used for spin-full waveguiding and can find applications in integrated photonics due to the possibility of spin multiplexing.
In this work we propose a method to achieve improved topological edge sates by engineering an optimal profile of the transition at the boundary between topological and trivial domains. From experiment and simulation results we confirmed that the quality factor of edge state for smooth transition profile can be increased by more than an order compared to the edge state of a conventional step profile. At the same time the modes retained their topological resilience, which, when combined with the reduced radiative leakage, enables robust photonic transport over long distances even above the light line.
We report applications of monolayer and multilayer Van der Waals (vdW) materials, transition metal dichalcogenides (TMDCs) and hexagonal boron nitride (hBN), for enhanced and enriched light-matter interactions in topological metasurfaces. First, we show that one can selectively couple spin-polarized edge modes to the valley-polarized excitons in monolayer TMDCs. The resultant exciton-polaritons allow a directional transfer of the valley degree of freedom and spin of excitons. Second, we demonstrate that a similar approach can be applied to phonons in mid-infrared domain, and the transverse vibrations in an hBN film are directionally guided in a resilient manner, avoiding backscattering due to defects.
We report novel topological polaritonic states of half-light and half-phonon character in a topological mid-infrared metasurface integrating hexagonal boron nitride (hBN). A custom-built Fourier-space imaging microscope was used to investigate a three-dimensional band diagram of the photonic topological phonon-polaritonic states in our system for the first time. Our Fourier- and real-space imaging experimental findings, supported by theoretical models, demonstrate that topological properties of photonic modes are transcribed to the in-plane lattice vibrations (phonons) supported by hBN. As a result, the observed topological phonon-polaritons exhibit unidirectional propagation with the helical nature of vibrations and the resilience of the modes to sharp bends.
The WS2 monolayer encapsulated in two thin hBN layers was pumped at room temperature by a circularly polarized laser in order to excite one of the valleys (K or K’ valley). The refractivity spectra measured using both left- and right- CP probe with low intensity, revealed the nonreciprocal response at exciton resonance wavelength. Based on this effect, we propose a novel design of an isolator containing SiN ring resonator integrating an asymmetrically places WS2 monolayer. By applying the coupled mode theory and parameter extracted from the experiment, the isolation of the device was estimated to be ~20dB.
Here we directly emulate a two-dimensional Dirac equation with a position-dependent mass term in a photonic crystal and present a new type of photonic resonators with light confinement originating in relativistic Dirac physics. Some of the modes of such resonators represent eigenmodes of a supersymmetric Hamiltonian. To test our concept, we designed, fabricated, and studied a resonator operating in the mid-IR region. Direct imaging of the structure in both real and Fourier spaces confirmed existence of the modes. The demonstrated approach offers a new route for designing photonic devices and probing supersymmetric quantum physics by using a classical photonic platform.
we demonstrate 2D photonic HOTI (PHOTI) with topological states two dimensions lower than the one of the host system. We consider a photonic metacrystal of distorted Kagome lattice geometry that exhibits topological bulk polarization, leading to the emergence of 1D topological edge states and of higher order 0D states confined to the corners of the structure. Interestingly, in addition to corner states due to the nearest neighbour interactions and protected by generalized chiral symmetry 1, we discover and take advantage of a new class of topological corner states sustained by long-range interactions, available in wave-based systems, such as in photonics. Our findings demonstrate that photonic HOTIs possess richer physics compared to their condensed matter counterparts, offering opportunities for engineering novel designer electromagnetic states with unique topological robustness.
In this work we design and experimentally realize a photonic kagome metasurface exhibiting a Wannier-type higher-order topological phase. We demonstrate and visualize the emergence of a topological transition and opening of a Dirac cone by directly exciting the bulk modes of the HOTI metasurface via solid-state immersion spectroscopy. The open nature of the metasurface is then utilized to directly image topological boundary states. We show that, while the domain walls host 1D edge states, their bending induces 0D higher-order topological modes confined to the corners. The demonstrated metasurface hosting topological boundary modes of different dimensionality paves the way to a new generation of universal and resilient optical devices which can controllably scatter, trap and guide optical fields in a robust way.
Higher-order topological (HOT) states are topological states localized in more than one dimension of a D-dimensional system. In the recent years, HOT states have been shown to exist in classical wave-systems such as photonics and acoustics and have been used to explore a host of topological phenomena that have typically been associated with condensed matter systems. In our work, we construct the 3D acoustic metamaterial with HOT states through a rapid prototyping process and manufacture the individual metaatoms and metamolecules, which can then be snapped together to form 3D metamaterials with complex geometries. The assembled 3D topological metamaterial represents the acoustic analogue of the pyrochlore lattice with acoustic modes strongly bound to the individual resonant cavities and a design that only allows for nearest neighbor coupling. This provides us with the framework to explore the topological nature of the structure in a semi-analytical way (tight-binding model) while comparing it to the first-principles finite element method (FEM) model, and then comparing both theoretical results to the experiment. Consistent with the models, we observe the third-order (0-D) topological corner states along with second-order (1D) edge states and first-order (2D) surface states within the same topological bandgap, thus establishing a full hierarchy of HOT states in three dimensions. Additionally, we experimentally measure the field profile of each topological mode, which are in excellent agreement with the numerically calculated profiles of the HOT states.
Topological systems are inherently robust to disorder and continuous perturbations, resulting in dissipation-free edge transport of electrons in quantum solids, or reflectionless guiding of photons and phonons in classical wave systems characterized by topological invariants. These established examples of topological physics, however, do not exhaust all possible topological phases, and recently a new class of topological metamaterials characterized by bulk polarization has been introduced. In addition to edge conduction, these systems have been shown to host higher-order topological states, such as corner states. Here, we introduce topological bulk polarization in two-dimensional Kagome photonic meta-structures, and observe topological transitions as the design parameters are tuned. We demonstrate that our topological meta-structure hosts both 1D edge and Wannier-type second-order corner states with unique properties. The edge states have the angular momentum that reverses for opposite propagation direction, thus supporting directional excitation. We also observe the second order topological states protected by the generalized chiral symmetry of the meta-structure, which are localized at the corners and are pinned to ‘zero energy’. Interestingly, unlike the corner states protected by the conventional chiral symmetry, the generalized chiral symmetry of our three-atom sublattice enables their spectral overlap with the continuum of bulk states without leakage. Our findings open new directions in photonics for controlling propagation and manipulating electromagnetic waves, including within the radiative continuum.
The past few years have witnessed the discovery of photonic topological insulators, which transformed our views on propagation and scattering of electromagnetic waves. Here we show that judiciously designed all-dielectric photonic metamaterials and metasurfaces can exhibit a topological photonic state. The bianisotropy of the metamaterial is shown to result in emergence of a topological photonic band gap in the bulk and photonic states guided by edges and interfaces which appear to be one-way spin-polarized and immune to sharp bending. Experimental realizations of photonic topological metamaterials and metasurfaces for microwave and optical domains are used to test and confirm the topological properties.
We demonstrate that the distorted Kagome lattice formed by two-dimensional(2d) array of dielectric rods embedded in
air exhibits a new class of topological states characterized by a topological invariant number in Pauli vector space. The
Kagome lattice can be considered as a 2d analogue of the Su-Schrieffer–Heeger (SSH) model, which displays a phase
transition by detuning the relative amplitudes of the inter-cell and intra-cell hopping terms. The phase transition is
accompanied by the opening of a complete band gap in the Brillouin zone, which may host topological edge states on
either the truncated end of the lattice or at the domain walls between topological nontrivial and trivial domains. To
further reveal the connection between the bulk invariance and edge states, polarizations of shrunken and expanded
effects are calculated. Our first-principles simulations based on finite element method (FEM) are used to design the
lattice and confirm the analytic prediction.
Bulk spectrum and edge modes of 2D photonic crystals with parity and time-reversal symmetries broken in a different way are investigated. It is shown that for specific values of parameter of the symmetry reduction the bulk modes exhibit a peculiar one-way Dirac-like dispersion. The domain wall formed by two crystals with the symmetry reduction parameter reversed is shown to exhibit an edge mode which coexists with the one-way bulk Dirac regime. In addition, we demonstrate that parity-time symmetric interfaces between photonic crystals with gain and loss support a new class of dissipation-less surface modes.
The concept of symmetry pervades modern physics. Through the conservation laws derived from various symmetries, high-level restrictions and selection rules can be derived for a variety of physical systems without any need for detailed investigations of their specific properties. The spatial symmetries of electric charge distribution on the metamaterial’s surface determine whether the EM resonance is “bright” (radiatively coupled to) or “dark” (radiatively de-coupled from) the EM continuum. As we demonstrate in this talk, other (non-spatial) symmetries and their breaking can also be crucial to determine the properties of EM resonances and enable their mutual coupling, which in turn can give rise to EM Fano interferences.
I will consider a meta-surface formed by a two-dimensional array of double-antenna meta-molecules resting on a gyromagnetic ferrite substrate. In conclusion, I will use simple symmetry considerations to predict and numerically demonstrate two phenomena that occur in meta-surfaces when symmetry of the system is reduced by a gyromagnetic substrate: gyromagnetically induced transparency and nonreciprocal Fano interference. These phenomena hold significant promise for practical applications such as the dynamic control of resonant EM interactions using magnetic fields produced by the external currents, mitigation of co-site interference and improving isolation. Spectral positions, radiative lifetimes and quality factors of Fano resonances can be controlled by the magnitude of the external magnetic field. This class of effects may lead to a new generation of tunable and nonreciprocal Fano resonant systems for various applications where strong field enhancement, tunability and nonreciprocity are simultaneously required.
Science thrives on analogies, and a considerable number of inventions and discoveries have been made by pursuing an unexpected connection to a very different field of inquiry. For example, photonic crystals have been referred to as “semiconductors of light” because of the far-reaching analogies between electron propagation in a crystal lattice and light propagation in a periodically modulated photonic environment. However, two aspects of electron behavior, its spin and helicity, escaped emulation by photonic systems until recent invention of photonic topological insulators (PTIs). The impetus for these developments in photonics came from the discovery of topologically nontrivial phases in condensed matter physics enabling edge states immune to scattering. The realization of topologically protected transport in photonics would circumvent a fundamental limitation imposed by the wave equation: inability of reflections-free light propagation along sharply bent pathway. Topologically protected electromagnetic states could be used for transporting photons without any scattering, potentially underpinning new revolutionary concepts in applied science and engineering.
I will demonstrate that a PTI can be constructed by applying three types of perturbations: (a) finite bianisotropy, (b) gyromagnetic inclusion breaking the time-reversal (T) symmetry, and (c) asymmetric rods breaking the parity (P) symmetry. We will experimentally demonstrate (i) the existence of the full topological bandgap in a bianisotropic, and (ii) the reflectionless nature of wave propagation along the interface between two PTIs with opposite signs of the bianisotropy.
We have studied magneto-optical responses of gold-bismuth-substituted yttrium iron garnet (Au-Bi:YIG) composite films in which Au particles are embedded into Bi:YIG in two different ways. First type were samples in which planar arrays of Au particles were introduced into the middle of Bi:YIG films using a step-by-step sputtering technique. Second type were granular films fabricated using simultaneous co-sputtering of Au and Bi:YIG; in these films Au particles occupy positions inside composite films in a random way. Absorption bands associated with localized surface plasmon resonances (LSPRs) were observed in transmission spectra of films of both types. In the spectral range of LSPRs, samples of Au-array type exhibited larger Faraday rotation angles as compared with that for reference Bi:YIG films of the same thickness. However, given that the volume fraction of Au particles was nearly the same for both types, the
enhancement of Faraday rotation for samples of Au-granular type was insignificantly altered. Experiments showed that of the primary importance is the role of the interfaces between Au particles and Bi:YIG. Theoretical estimations showed that, in samples of
Au-granular type, air shells appeared between Au particles and Bi:YIG during fabrication. In fact, annealing needed for crystallization of Bi:YIG films is always accompanied with an expansion of their thicknesses.
We report on an analytical study of the photonic band structure of 2D and 3D multi-component photonic crystals. It is found that both types of crystal demonstrate a quasiperiodic resonant behavior of (hkl) photonic stop-bands as a function of the reciprocal lattice vector, providing a selective ON/OFF switching of nonresonant (hkl) stop-bands. Our predictions are compared with the results of conventional numerical studies using the photonic Korringa-Kohn-Rostocker method. Experimental transmission spectra of a-SiO2 synthetic opals show the OFF-switching of the {111} stop-bands at the filler permittivity of ~1.82, the {200} bands at ~1.63, the {220} bands at ~1.93, and the {311} bands at ~1.75. The (222) photonic stop-band, which is due to the second order diffraction from the (111) planes, cannot be switched OFF in a wide range of filler permittivity values, thus indicating a resonant behavior. The experimental data demonstrate an excellent agreement with the theoretical predictions.
We develop a method to study the optical and magneto-optical response of perforated metallic films with magneto-optic
media embedded within their holes. Due to the strong electromagnetic field confinement associated with the excitation
of the transmission resonances appearing in this type of structures and nonreciprocal nature of the magneto-optical
phenomena, strongly enhanced conversion of the polarization state in both reflected (magneto-optical Kerr effect) and
transmitted (magneto-optical Faraday effect) waves was found. Possibility to reach huge rotation angles is demonstrated
by increasing magneto-optic holes filler/background optical contrast, which appears to be responsible for the resonance
quality.
Optical properties of mesoporous three-dimensional photonic crystals (3D PCs) based on thin opal films were found to be extremely sensitive to a humidity of the surrounding air. It was manifested that the internal structure of a single SiO2 sphere together with the net of voids between them in a thin opal film acts as a sponge for wet steams. Our experimental data have shown that hydrophilic internal structure of a mesoporous film sponges up (and lose) water (dry or wet steams) that influences dielectric permittivity, the latter causes significant changes in transmission spectra. High sensitivity, quick response and possibility of contactless measurements makes sensors based on optical effects in mesoporous PCs to be very promising. It concerns not only humidity sensors, but also sensors of various gases, temperature, deformation and other environmental impacts.
We present a new method for calculation of optical and magneto-optical properties of three-dimensional magnetophotonic crystal heterostructures, composed from a sequence of homogeneous plates of a magneto-optical material and photonic crystal slabs. The algorithm is based on the layer KKR technique. As examples we consider the Bi:YIG (bismuth-substituted yttrium-iron-garnet) magneto-optical plate sandwiched by photonic crystal slabs consists of (i) simple cubic/face centered cubic lattices of SiO2 spheres in the air; (ii) air spheres in silicon background (inverse opals). The enhanced Faraday rotation appears at the resonant transmission frequency in analogy with one-dimensional structures with magneto-optical microcavity. However, the calculated spectral behavior of the Faraday rotation as well as its dependence on defect thickness is quite different and unusual. For instance, the Faraday rotation angle changes its sign within the photonic band gap that is due to complicated reflection of waves from 3D photonic crystal slabs.
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