Coupled-resonator Waveguides Research Articles

Single Photon Scattering Properties in Coupled-Resonator Waveguide Coupling with a Nanocavity Interacting with an External Mirror**Supported by the National Natural Science Foundation of China under Grant Nos. 11105001 and 61472282, the Anhui Provincial Natural Science Foundation under Grant Nos. 1408085QA22, 1608085MA09, and 1508085MF129.

We investigate theoretically single photon transport properties in coupled-resonator waveguide coupling with a nanocavity interacting with an external mirror. By using the discrete coordinates approach, transmission and reflection amplitudes of the propagating single photon in the waveguide are obtained. The influence of the coupling strength between the nanocavity and the external mirror on the single photon scattering spectra is discussed. We also extend the results to the waveguide with linear and quadratic form dispersion relations.

Information-holding quantum router of single photons using natural atom

In a quantum network, a quantum router can play a key role that involves exchanging quantum information (quantum states) among different parties. In this paper, we propose a single-photon quantum router by using a four-level atom that is embedded in quantum multi-channels, which are composed of coupled-resonator waveguides. Single photons can be guided from one quantum channel to another by controlling an on-demand classical field. The fidelity of the routing atom recovers to a maximum after a time evolution cycle of the system; thus, the fidelity of the information-carrying single photon reaches unit at the same time. For the first time, we demonstrate that quantum information carried by single photons can be perfectly held after a quantum router.

Realizing of plasmon Fano resonance with a metal nanowall moving along MIM waveguide

A larger number of complicated plasmonic nanostructures have been realized to exhibit Fano interference. In this paper, we demonstrate a simple nanostructure, side coupled waveguide resonator system with a metal nanowall located in the metal–insulator–metal waveguide (MIM), which can also achieve multiple plasmonic Fano resonance. In the proposed nanostructure, the asymmetric line shape originates from the interference between the slot resonator and the new resonator. Therefore, the Fano line shape can be actively controlled by the phase difference of the two resonators and the thickness of the metal nanowall. A scattering matrix method is used to calculate the transmission spectra. Results obtained by the scattering matrix theory are consistent with those from the finite-difference time-domain simulations (FDTD). Moreover, Fano resonances in the proposed structure show high sensitivity, which may have important application in plasmonic nanosensor and modulator.

Conductive, dielectric and amalgamated obstacles coupled waveguide based highly selective multiband bandpass resonators

This paper demonstrates transverse nonplanar resonators consisting of symmetrically placed obstacles (posts which made up of same or different type of materials) coupled into rectangular waveguide. Introducing posts inside a waveguide in proposed structure creates discontinuity and become a cavity resonator. Resonance is created at X band, Ku band and Ka band frequencies are reported. Simulation results demonstrates that resonators's results are matched to free-space as well as having a superior insertion loss and better return loss of conductive posts compare to dielectric posts. Moreover, obstructing the E-field (using conductive posts) compare to the H-field (using dielectric posts) are given better insertion loss and impedance match- ing characteristics. However, insertion of amalgamated obstacles into rectangular waveguide generates good transmission and reflection zeros compared to other two topologies at resonant frequencies which are described here. Posts coupled waveguide resonators having high power handling capacity so it can be utilized for Base transceiver station, Satellite Communication, Space applications, sensors as well as the fusion plasma diagnostics system. All the proposed structures are simulated with Ansoft's finite-element High Frequency Structure Simulator (HFSS) tool to demonstrate the characteristics of the resonators using a rectangular waveguide with center frequency of 11.2 GHz for X-band. In addition, the equivalent model of resonator is simulated in ADS (Advance designing system). It is observed that offered topologies are very sensitive to the parameters devia- tion (Post coupled waveguide resonator performance depends on diameter of conductive post, elevation of post and detachment between posts).

Modeling of Inline Transitions Between Different Waveguides as Impedance Inverters for the Use in Novel Filter Designs

In this paper, a novel approach for field coupled inline transitions between different waveguides is proposed. The immittance inverting characteristics of these transitions are simultaneously utilized for filter design. Based on scattering parameters, an exact relation is derived between the desired inverter constants and the even and odd mode characteristic impedances. The well-known approximation introduced by Cohn is improved. Further it will be shown that the introduced inline transitions can be effectively modeled with a contra-directional symmetrical coupler. The description of the field coupled waveguide transitions as inverters allows a “straight on design method” for general field coupled structures and accelerates the optimization process. The integration of the inline transitions into the filter design is applied for two different types of transitions, namely, the transition between a parallel-plate waveguide and a rectangular waveguide (RWG) and the transition between a microstrip line and a RWG, using a ridged waveguide section for the coupling. For both configurations a third-order bandpass filter was realized by using three coupled waveguide resonators. Two of the coupling structures were made up of inline transitions. To verify the approach and quantify losses, a filter was fabricated and its scattering parameters measured. With this new approach low-loss waveguide filters can be integrated into a planar RF circuit while minimizing space. No delicate manufacturing processes are required.

Realizing controlled plasmonically induced reflection in metal–insulator–metal plasmonic waveguide-resonator coupling systems

The original characteristics of plasmonically induced reflection (PIR) were investigated numerically in metal–insulator–metal (MIM) plasmonic waveguide-resonator systems. A three-level plasmonic system was utilized to verify the simulation results. The obtained ultra-narrowband PIR window is attributed to the strong destructive interference coupling between bright and dark modes. The outstanding PIR window can not only be controlled vertically but also tuned horizontally. A PIR window with a group time delay of up to 0.24 ps was achieved, exhibiting advantageous slow light features. As applications, distinctive double-channel PIR windows were realized. Undoubtedly, the proposed structures can find utility in highly integrated plasmonic circuits for optical switching.

T-shaped single-photon router.

We study the transport properties of a single photon scattered by a two-level system (TLS) in a T-shaped waveguide, which is made of two coupled-resonator waveguides (CRWs)- an infinite CRW and a semi-infinite CRW. The spontaneous emission of the TLS directs single photons from one CRW to the other. Although the transfer rate is different for the wave incident from different CRWs, due to the boundary breaking the translational symmetry, the boundary can enhance the transfer rate found in Phys. Rev. Lett. 111, 103604 (2013) and Phys. Rev. A 89, 013805 (2014), as the transfer rate could be unity for the wave incident from the semi-infinite CRW.

Silicon Graphene Reconfigurable CROWS and SCISSORS

We propose the incorporation of graphene to integrated coupled resonator waveguides and side-coupled integrated spaced sequence of resonator devices to enable reconfigurable operation. The key element to achieve this is a tunable silicon graphene Mach–Zehnder interferometer (MZI) that acts as an equivalent variable 2 $\times$ 2 coupler, where the value of its coupling constant is changed by varying the chemical potential of a graphene section placed on top of one of its arms.

Slow light in half-periodic photonic crystal coupled-resonator waveguides with tunable central rod

In photonic crystals (PCs) waveguides, the speed of light can be significantly reduced due to the weak coupling between adjacent high-quality-factor nanocavity resonators. However, little research focuses on the length between two cavities associated with slow light characteristics. A waveguide based on a PC within a square array with adjustable rods spreading regularly at the edge and center of the cavity is proposed. Slow light properties in this new structure are studied by using the plane wave expansion method. Through a series of experiments, we find the most suitable coupling length to compromise various parameters of slow light, which is just the half-period of that in a conventional structure. Furthermore, the numerical results reveal that the group velocity is below 3.532×10 −3 c in the new model. The corresponding normalized delay-bandwidth product can reach as high as 0.3068 since the group velocity dispersion is at the range of (−5 to 5)×10 6 . All of above signify that the very small size and ultraslow light, as well as outstanding dispersion performance, are realized and there are some advanced features, such as low loss and high integration, in this half-periodic structure.

Integrated waveguide coupled Si_3N_4 resonators in the ultrahigh-Q regime

The vast majority of work on waveguide-coupled resonators focuses on decreasing losses in the waveguide and coupling region. Here we present fully integrated resonators based on an ultralow-loss Si3N4 waveguide platform. By tailoring the directional coupler excitation to the resonators, we are able to achieve lower loss single-mode coupling to multimode waveguide widths compared to straight bus waveguide directional couplers. This allows us to demonstrate record-high integrated waveguide coupled intrinsic quality factor (Qint) values of 81 million at a 9.65 mm bend radius, with a future direction to both stronger and lower loss waveguide–resonator coupling. This result opens up integration possibilities for narrow linewidth integrated diode lasers, low noise feedback systems, microwave photonic research, and the ultrastable timing reference community.

Single-Photon Transport Properties in Coupled-Resonator Waveguide with Nonlocal Coupling to a Whispering-Gallery Resonator Interacting with Two Separated Atoms

We investigate the single-photon transport properties in a one-dimensional coupled-resonator waveguide non-locally coupled to a whispering-gallery resonator embedded with two separated two-level atoms based on the real-space approach. The control of single-photon transport is realized by modulating the detuning between two separated atoms, phase differences, and the symmetric/asymmetric nonlocal couplings. The multi-frequency photon-filter is obtained, and the influences of dissipation effects on single-photon transmission spectra and the phase shift are also discussed.

Phonon waveguides for electromechanical circuits.

Nanoelectromechanical systems (NEMS), utilizing localized mechanical vibrations, have found application in sensors, signal processors and in the study of macroscopic quantum mechanics. The integration of multiple mechanical elements via electrical or optical means remains a challenge in the realization of NEMS circuits. Here, we develop a phonon waveguide using a one-dimensional array of suspended membranes that offers purely mechanical means to integrate isolated NEMS resonators. We demonstrate that the phonon waveguide can support and guide mechanical vibrations and that the periodic membrane arrangement also creates a phonon bandgap that enables control of the phonon propagation velocity. Furthermore, embedding a phonon cavity into the phonon waveguide allows mobile mechanical vibrations to be dynamically switched or transferred from the waveguide to the cavity, thereby illustrating the viability of waveguide-resonator coupling. These highly functional traits of the phonon waveguide architecture exhibit all the components necessary to permit the realization of all-phononic NEMS circuits.

Controllable single-photon frequency converter via a one-dimensional waveguide

We propose a single-photon frequency converter via a one-dimensional waveguide coupled to a $V$-type atom. The on-demand classical field allows the atom to absorb a photon with a given frequency, then emit a photon with a carried frequency different from the absorbed one. The absorption and re-emission process is formulated as a two-channel scattering process. We study the single-photon frequency conversion mechanism in two kinds of realistic physical system: coupled resonator waveguide with cosine dispersion relation and an optical waveguide with linear dispersion relation respectively. We find that the driving field prefers weak in coupled resonator waveguide but arbitrarily strong in optical waveguide to achieve an optical transfer efficiency.

An analysis of waveguide resonator coupling in a microsphere and confocal cavity whispering gallery system

There are two kinds of coupling method in an optical-fiber taper coupled to a silica-microsphere whispering gallery mode (WGM) system. One is indirect coupling; another is direct coupling. In the case of indirect coupling, pump light is injected from the side of the micro-cavity. Thus, a ring mode results from the total internal reflection excited by spherical ring standing wave. In the case of direct coupling, pump light is injected from the front of the micro-cavity. Thus, a concentric mode or confocal mode result from the transmission input excited by spherical axial-oscillating standing wave. The ring mode and concentric mode have both been adopted in microsphere cavities. However, the confocal mode has not been reported, either in theory or experiment. In this paper, we adopt a micro-spheroid resonator as the confocal cavity, which can be obtained by exerting a pressure on the microsphere cavity. Thus, the confocal mode is produced in a micro-spheroid resonator. Then we investigate waveguide resonator coupling in a microsphere and confocal cavity WGM system. Finally, we make a detailed analysis of the full width at half maximum (FWHM), ζ, and quality factor, Q, between the indirect coupling and the direct coupling. We find that for the concentric mode, , which is nearly the same as that of the ring mode, Q = 7.2 × 104. For large diameter of injection laser beam, the confocal mode can obtain higher quality factor (Q > 1.202 × 106) than that of the ring mode (Q = 7.2 × 104).

Single-photon router: Coherent control of multichannel scattering for single photons with quantum interferences

We propose a single-photon router using a single atom with an inversion center coupled to quantum multichannels made of coupled-resonator waveguides. We show that the spontaneous emission of the atom can direct single photons from one quantum channel into another. The on-demand classical field perfectly switches-off the single-photon routing due to the quantum interference in the atomic amplitudes of optical transitions. Total reflections in the incident channel are due to the photonic bound state in the continuum. Two virtual channels, named as scatter-free and controllable channels, are found, which are coherent superpositions of quantum channels. Any incident photon in the scatter-free channel is totally transmitted. The propagating states of the controllable channel are orthogonal to those of the scatter-free channel. Single photons in the controllable channel can be perfectly reflected or transmitted by the atom.

Dispersion relation of coupled-resonator acoustic waveguides formed by defect cavities in a phononic crystal

We investigate the dispersion of phononic crystal waveguides formed by evanescent coupling of a chain of defect cavities and supporting slow-wave propagation. These coupled-resonator acoustic waveguides (CRAWs) are analogous to the coupled-resonator optical waveguides formed in photonic crystals. CRAW dispersion can be controlled by increasing the distance between cavities, with the result of decreasing their coupling, and hence flattening the dispersion relation. Based on the tight-binding model, the dispersion relation is found in the form of a Fourier series expansion with explicitly given coefficients. This model is tested against the exact dispersion relation of a two-dimensional solid–solid phononic crystal of tungsten inclusions in a silicon matrix and only partial agreement is found. An alternative model of a linear chain of coupled resonators, resting only on the hypotheses of linearity and periodicity, is then proposed. While the Fourier coefficients in this model are a priori unspecified, they can be fitted against the exact dispersion relation, resulting in an excellent agreement with only a few terms in the Fourier series expansion. The Fourier coefficients are shown to be a direct measure of the coupling of neighbouring resonators.

Quantum routing of single photons with a cyclic three-level system.

We propose an experimentally accessible single-photon routing scheme using a △-type three-level atom embedded in quantum multichannels composed of coupled-resonator waveguides. Via the on-demand classical field being applied to the atom, the router can extract a single photon from the incident channel, and then redirect it into another. The efficient function of the perfect reflection of the single-photon signal in the incident channel is rooted in the coherent resonance and the existence of photonic bound states.

Polarization-dependent photon switch in a one-dimensional coupled-resonator waveguide

Polarization-dependent photon switch is one of the most important ingredients in building future large-scale all-optical quantum network. We present a scheme for a single-photon switch in a one-dimensional coupled-resonator waveguide, where N(a) Λ-type three-level atoms are individually embedded in each of the resonator. By tuning the interaction between atom and field, we show that an initial incident photon with a certain polarization can be transformed into its orthogonal polarization state. Finally, we use the fidelity as a figure of merit and numerically evaluate the performance of our photon switch scheme in varieties of system parameters, such as number of atoms, energy detuning and dipole couplings.

Single-Photon Transmission Characteristics in a Pair of Coupled-Resonator Waveguides Linked by a Nanocavity Containing a Quantum Emitter

Single-photon scattering in a pair of coupled-resonator arrays of waveguides linked by a nanocavity embedded with a two-level system is investigated theoretically. By using the discrete coordinates approach, we deduce the analytical expressions for the transmission and reflection amplitudes. The calculations reveal that the transport properties of the single photon can be controlled by adjusting the coupled strength between the nanocavity and the coupled-resonator array. Quantum switching and a 1/4 beam splitter for the band-edge photon based on this structure are also discussed.

Oscillatory Vertical Coupling between a Whispering-Gallery Resonator and a Bus Waveguide

We report on a theoretical and experimental study of the optical coupling between a whispering-gallery type resonator and a waveguide lying on different planes. In contrast to the usual in-plane geometry, the present vertical one is characterized by an oscillatory behavior of the effective coupling as a function of the vertical gap. This behavior manifests itself as oscillations in both the resonance peak waveguide transmission and the mode quality factor. An analytical description based on coupled-mode theory and a two-port beam-splitter model of the waveguide-resonator vertical coupling is developed for arbitrary phase-matching conditions and is successfully used to interpret the experimental observations.