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.
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.
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 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.
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.
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 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.
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.
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.
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.
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