An all-Si photonic structure emulating the quantum-valley-Hall effect is proposed. We show that it acts as a photonic topological insulator (PTI), and that an interface between two such PTIs can support edge states that are free from scattering. The conservation of the valley degree of freedom enables efficient in-and out-coupling of light between the free space and the photonic structure. The topological protection of the edge waves can be utilized for designing arrays of resonant time-delay photonic cavities that do not suffer from reflections and cross-talk.

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Abstract:An all-Si photonic structure emulating the quantum-valley-Hall effect is proposed. We show that it acts as a photonic topological insulator (PTI), and that an interface between two such PTIs can support edge states that are free from scattering. The conservation of the valley degree of freedom enables efficient in-and out-coupling of light between the free space and the photonic structure. The topological protection of the edge waves can be utilized for designing arrays of resonant timedelay photonic cavities that do not suffer from reflections and cross-talk. Introduction:The discovery of topological phases of light has been one of the most exciting developments in photonics [ 1,2,3,4,5,6,7,8,9,10,11] in the past decade. It followed the time-honored path of translating the concepts from condensed matter physics into the language of optical sciences, followed by developing novel applications based on those concepts. Photonic topological insulators (PTI) can be viewed as the extension of topological insulators [ 12,13,14,15,16,17,18,19,20] into the realm of optics. One potential application of PTIs is to utilize the reflectionsfree propagation of topologically protected edge waves (TPEWs) that exist either at the PTI's edge [ 3,4,5] or at the interface between two different PTIs [ 6, 7, 10] for developing robust optical delay lines for large-scale photonic integrations.Specific implementations of PTIs vary considerably, and can utilize large coupled optical resonators [ 3,4], wavelength-scale photonic structures [ 1, 2, 10, 21], or metacrystals [ 6]. To date, most of the wavelength and sub-wavelength scale PTI concepts utilized metals. For example, metallic metamaterials comprised of split-ring resonators [ 6] and meta-waveguides comprised of an array of metal rods attached to one of the two confining metal plates [ 10,21] have been used to emulate the binary spin degrees of freedom (DOF) by ensuring that the two polarization states of light, the transverse electric (TE) and transverse magnetic (TM) modes, propagate with the same speed. This, property, known as spin-degeneracy [ 6,7], is challenging to achieve without using metals. Avoiding metals is crucial if the spectral range of sub-wavelength PTI is going to be extended beyond the THz/mid-infrared portions of the electromagnetic spectrum.In this Letter we demonstrate that an all-dielectric PTI can be developed by relying on a binary degree of freedom other than the spin. In designing the structure, we borrow the concept of the valley DOF from a rising field of valleytronics [ 22,23,24,25,26,27]. It has been theoretically shown [ 22,23,24] that the valley DOF in any graphene-like material behaves as a spin-like binary DOF. Specifically, the angular rotation of the electron wavefunction in the or ′ valleys of the band structure generates an intrinsic magnetic moment [ 23] analogous to that produced by the electron spin. This similarity between the valley and spin DOF enables the quantum-valley Hall (QVH) effect [ 27] analogous to the quantum-spin Hall...

We propose a set of three simple photonic platforms capable of emulating quantum topologically insulating phases corresponding to Hall, spin-Hall, and valley-Hall effects. It is shown that an interface between any two of these heterogeneous photonic topological insulators supports scattering-free surface states. Spin and valley degrees of freedom characterizing such topologically protected surface waves determine their unique pathways through complex photonic circuits comprised of multiple heterogeneous interfaces.Light propagation through waveguides, photonic crystals, and other photonic system can often be reduced to a simple scalar wave equation that imposes fundamental limitations on how optical energy can be transported in space. For example, it is generally believed to be impossible to guide light along sharply bent trajectories without reflections. A new paradigm that calls into question this conventional wisdom has been recently introduced with the realization of a new class of photonic structures: photonic topological insulators (PTIs) [ 1, 2, 3, 4, 5, 6, 7, 8,9]. Just as their condensed matter counterparts, topological insulators (TIs) [ 10,11,12] from which they have been derived by analogy, PTIs enable reflections-free propagation of topologically protected surfaces waves (TPSWs) [ 5, 8] In this Letter, we demonstrate that these three condensed matter systems can be emulated by novel PTI structures. More significantly, while it is nearly impossible to realize lateral heterojunctions between different classes (e.g., between QSH and QH) of TIs in naturally occurring electronic materials, interfacing heterogeneous PTIs is relatively straightforward and, in fact, highly beneficial to developing novel photonic circuits. We demonstrate that reflectionsfree TPSWs are supported by the interfaces between such domains. It is shown that topological protection emerges not only from the conservation of the spin degree of freedom (DOF) [ 4, 5, 8], but also from the conservation of another binary DOF: the valley. While it has been recently recognized that the valley DOF can produce novel topological phases and chiral edge states of electrons [ 18,19,20], here we demonstrate its significance to reflections-free propagation of surface waves in photonic structures.The photonic analogues of the three topological phases are obtained by imposing three types of distinct symmetry-breaking perturbations on a simple symmetric "photonic graphene" (PhG) [ 21] structure shown in Fig.1(a). The PhG is comprised of a hexagonal arrays of metal posts symmetrically placed between two metal plates and separated from them by the gap 0 . A similar photonic structure was considered earlier in the narrower context of QSH-type [ 8] PTIs

Plasmonic cavities represent a promising platform for controlling light-matter interaction due to their exceptionally small mode volume and high density of photonic states. Using plasmonic cavities for enhancing light's coupling to individual two-level systems, such as single semiconductor quantum dots (QD), is particularly desirable for exploring cavity quantum electrodynamic (QED) effects and using them in quantum information applications. The lack of experimental progress in this area is in part due to the difficulty of precisely placing a QD within nanometers of the plasmonic cavity. Here, we study the simplest plasmonic cavity in the form of a spherical metallic nanoparticle (MNP). By controllably positioning a semiconductor QD in the close proximity of the MNP cavity via atomic force microscope (AFM) manipulation, the scattering spectrum of the MNP is dramatically modified due to Fano interference between the classical plasmonic resonance of the MNP and the quantized exciton resonance in the QD. Moreover, our experiment demonstrates that a single two-level system can render a spherical MNP strongly anisotropic. These findings represent an important step toward realizing quantum plasmonic devices.optical spectroscopy | hybrid nanostructures | quantum systems | plasmonic cavities | Fano resonance M any quantum network and quantum information processing schemes build upon the enhanced light-matter interaction between a single quantum emitter and a cavity, enabling the effective conversion between photonic and matterbased quantum states (1-4). For example, if the absorption of a photon by a single atom placed inside a cavity can render it transparent to a second photon, then a variety of promising quantum information processing devices can be envisioned including quantum phase gates and repeaters (5). Such QED effects require a high atomic cooperativity c ¼ g 2 γk , where the coupling strength g 2 ∝ 1=V is inversely proportional to the volume of the cavity mode V (6). γ and k are the linewidth of the atomic transition and the cavity mode, respectively. Typically, a high cavity quality factor Q (or low k) of conventional photonic cavities is required to compensate for relatively large (diffraction-limited) mode volumes and comes at a cost: The narrow linewidth of cavity modes places stringent requirements on their spectral alignment with the frequencies of quantum transitions. Plasmonic cavities, on the other hand, achieve high values of C while maintaining moderate Q values because of their ultrasmall modal volume (7-10). The relaxed spectral alignment requirements facilitate the experimental realization of various quantum phenomena, such as collective photon emission from a small ensemble of emitters (11) and single photon sources with tunable statistical properties (12).Prior experiments exploring cavity QED effects associated with single emitters coupled to plasmonic cavities or waveguides focused almost exclusively on the observations of reducing the emitter's lifetime due to the enhanced radiative (propor...