Plasmonic Waveguide Array Research Articles

Ultra-short bilayer graphene optical fiber intensity modulator based on silver-semicircular hybrid plasmonic waveguide

The intensity modulator is a key component in optical fiber communication. In this paper, an ultra-short graphene light intensity modulator based on hybrid plasmonic waveguide (HPW) array is proposed. Al2O3 embedded with double-layer graphene at a certain interval is placed on the side polishing surface of the microfiber Mach–Zehnder modulator (MZM), and the silver-semicircular array is placed on the surface of the transition material as hybrid plasmonic waveguide for the first time in all-fiber modulator. Analyzing by building a 3D model in finite difference time domain software, the hybrid surface plasmon polariton (HSPP) between silver-semicircles significantly enhances the subwavelength confinement and the light–graphene interaction. The modulation range of the real part of effective refractive index (Neff) can reach to 0.01984 by adjusting the Fermi level of graphene. Therefore, π phase change can be achieved only in a modulation length of 45 μm, and the insert loss can be controlled below 0.0159 dB/μm through the analysis of the imaginary part of the Neff, which also leads to a low total loss of 0.7155 dB. Owing to the excellent phase modulation efficiency, high extinction ratios of 39.39 dB and 47.77 dB can be reached for port 1 and port 2, respectively. Double-layer graphene forms the flat capacitor, which brings high modulation efficiency while achieving a high modulation bandwidth of 59.945 GHz. The application of HPW in graphene optical fiber modulator greatly improves the working efficiency, reduces the difficulty of integration, overcomes high insert loss, improves the extinction ratio, this modulator can be widely used in the future optical fiber communication system.

A local area quantum teleportation network based on an array of electrically activated graphene waveguides.

We present a scheme to generate a continuous variable (CV) multipartite entangled state using an array of plasmonic graphene waveguides that are activated by nonclassical driving microwave modes. Within this scheme, we can exploit the interaction of two light fields coupled to the same microwave mode in each waveguide to produce any type of multipartite Gaussian entangled state. A teleportation network is illustrated using the resultant CV multipartite entangled state. In particular, the proposed setup enables coherent state teleportation across remotely connected nodes with fidelity above a threshold limit of 2/3, providing secure quantum teleportation networking even in the presence of losses.

Topological Bloch–Zener oscillations in non-Hermitian graphene plasmonic waveguide arrays

We investigate the topological Bloch–Zener oscillations for surface plasmon polaritons (SPPs) propagating in a Su–Schrieffer–Heeger plasmonic system, which composed of graphene dimer arrays with a potential gradient. The topological trivial and non-trivial band gaps are achieved to investigate the topological effect to plasmonic Bloch oscillations. The topological transition from trivial phase to non-trivial one through the exceptional point is realized by alternating the gain and loss in the graphene waveguide arrays. Zener tunneling of the SPP beam was observed in the parity-time (PT) symmetric region, which was prohibited in the PT-symmetric broken region. The numerical calculations demonstrated reasonable agreements with theoretical tight-binding model. Furthermore, a single dynamically stable zero-energy edge state was also found in these non-Hermitian plasmonic waveguide arrays.

Dissipation engineered directional filter for quantum ratchets

We demonstrate transport rectification in a hermitian Hamiltonian quantum ratchet by a dissipative, dynamic impurity. While the bulk of the ratchet supports transport in both directions, the properly designed loss function of the local impurity acts as a direction-dependent filter for the moving states. We analyse this scheme theoretically by making use of Floquet-S-Matrix theory. In addition, we provide the direct experimental observation of one-way transmittance in periodically modulated plasmonic waveguide arrays containing a local impurity with engineered losses.

Formation of nanostructures and optical analogues of massless Dirac particles via femtosecond lasers.

Subwavelength-scale surface structures have many important engineering and nanotechnology applications, e.g., superhydrophobicity and light-trapping. However, an effective and economic nanofabrication solution for general engineering materials, e.g., metals or silicon, is still not available to date. In this paper, we present an experimental and theoretical study of the nanostructure formation mechanism based on double time-delayed femtosecond laser beams and the coupled mode theory (CMT), demonstrating the use of an optical analogue of massless Dirac particles for high-throughput nanofabrication for the first time. In the experiments, a variety of complex periodic structures, including hexagonally arranged nanoholes, nano-square array, and periodic ripples, have been fabricated. The formation mechanisms of these nanostructures are explained by the CMT, where a transient plasmonic waveguide array (TPWA) is formed by the interference between the preceding laser and the induced surface plasmon polaritons (SPPs). The SPPs induced by the subsequent laser propagates through the TPWA, resulting in conical diffraction. This result shows the first practical application of the massless Dirac dynamics in nanofabrication.

Observation of topological transport quantization by dissipation in fast Thouless pumps

Quantized dynamics is essential for natural processes and technological applications alike. The work of Thouless on quantized particle transport in slowly varying potentials (Thouless pumping) has played a key role in understanding that such quantization may be caused not only by discrete eigenvalues of a quantum system, but also by invariants associated with the nontrivial topology of the Hamiltonian parameter space. Since its discovery, quantized Thouless pumping has been believed to be restricted to the limit of slow driving, a fundamental obstacle for experimental applications. Here, we introduce non-Hermitian Floquet engineering as a new concept to overcome this problem. We predict that a topological band structure and associated quantized transport can be restored at driving frequencies as large as the system’s band gap. The underlying mechanism is suppression of non-adiabatic transitions by tailored, time-periodic dissipation. We confirm the theoretical predictions by experiments on topological transport quantization in plasmonic waveguide arrays.

LSPR Excitation on Au Nanorings From Integrated Hybrid Plasmonic Aperture Waveguide and Its Application in Methanol Detection in the IR-Band

The presented work deals with an infra-red (IR)-band methanol sensor, which has been realized in an integrated device having hybrid plasmonic aperture waveguide and Au-nanoring array over broad wall of it covered by porous silicon clad. Through modeling and numerical simulations, we have shown that suitable synthesized aperture in the waveguide can excite localized surface plasmon resonance (LSPR) on the nanorings of suitable diameter via radiative field coupling, whereas LSPR can be tuned with the received methanol by the porous silicon clad. Thus, by contrasting the power available at the sense port of the waveguide with null and finite methanol doses, we have determined sensitivity (STM) and figure of merit (FOM) of the sensor, and our further analyses suggest that the sensor response can be tailored by diameter of aperture and ring, and by the porosity of the silicon cladding medium. In this respect, we have found that, for 20%-45% clad porosity, S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sub> = 77.95-66.66 nm/RIU and FOM = 17.51-7.66/RIU can be achieved during methanol detection.

Exceptional points and spectral singularities in active epsilon-near-zero plasmonic waveguides

We present a nanoscale active plasmonic waveguide system consisting of an array of periodic slits that can exhibit exceptional points and spectral singularities leading to several novel functionalities. The proposed symmetric active system operates near its cut-off wavelength and behaves as an effective epsilon-near-zero (ENZ) medium. We demonstrate the formation of an exceptional point (EP) that is accessed with very low gain coefficient values, a unique feature of the proposed nanoscale symmetric plasmonic configuration. Reflectionless ENZ transmission and perfect loss-compensation are realized at the EP which coincides with the effective ENZ resonance wavelength of the proposed array of active plasmonic waveguides. When we further increase the gain coefficient of the dielectric material loaded in the slits, a spectral singularity occurs at the ENZ resonance leading to super scattering (lasing) response at both forward and backward directions. These interesting effects are achieved by materials characterized by very small gain coefficients with practical values and at subwavelength scales due to the strong and homogeneous field enhancement inside the active slits at the ENZ resonance leading to enhanced light-matter interaction. We theoretically analyze the obtained EP, as well as the divergent spectral singularity, using a transmission-line model and investigate the addition of a second incident wave and nonlinearities in the response of the proposed active ENZ plasmonic system. Our findings provide a novel route towards interesting nanophotonic applications, such as reflectionless active ENZ media, unidirectional coherent perfect absorbers, nanolasers, and strong optical bistability and all-optical switching nanodevices.

Strong absorption near exceptional points in plasmonic waveguide arrays

We investigate the exceptional points (EPs) in plasmonic waveguide arrays, including metallic waveguide arrays (MWAs) and graphene sheet arrays (GSAs). The EPs emerge at the boundary of strong and weak coupling ranges in both systems. The cross conversion of Bloch modes and variation of geometric phase can be observed by encircling an EP in the parametric space. We also show the Bloch modes exhibit strong absorption in the vicinity of EPs in GSAs, which originates from the enhanced longitude electric field along the propagation direction. The abnormal absorption and field enhancement also arise in ultrathin MWAs and disappear when the thickness of metal film increases. Our results may find applications in optical switches and sensors at the nanoscale.

Simulation of massless Dirac dynamics in plasmonic waveguide arrays.

The recent simulation of quantum behavior in condensed matter physics by well-designed photonic structures has opened unprecedented opportunities to steer the flow of light in novel manners. Here, we propose a design to simulate massless Dirac dynamics in evanescently coupled plasmonic ridge waveguide arrays with alternating positive and negative coupling coefficients. The coupling coefficients are carefully tailored by adjusting the geometric parameters of the waveguide in arrays, which are supposed to be feasible with advanced nanofabrication techniques. Further theoretical studies are also carried out based on the coupled mode theory to discuss the influence of excitation condition on beam propagation in this massless Dirac dynamics simulation case.

Discrete Talbot effect in dielectric graphene plasmonic waveguide arrays

We theoretically predict and numerically simulate the discrete plasmonic Talbot effect in dielectric graphene plasmonic waveguide (DGPW) arrays at THz wavelengths. The super-modes in DGPW arrays are analyzed by using super-mode theory and the field intensity distributions in the designed DGPW arrays are explained by the superposition of different super-modes and simulated by using commercial software (COMSOL) based on finite element method (FEM). The discrete plasmonic Talbot effect is realized by superposition of different super-modes. The Talbot distance can be actively tuned by varying the Fermi energy of graphene and it decreases monotonically with increasing Fermi energy. In addition, it is found that the Talbot distance increases with the period of the DGPW arrays. Simulation results show a good agreement with the theoretical analysis.

Nonlinear infrared plasmonic waveguide arrays

The large negative permittivity of noble metals in the infrared region prevents the possibility of highly confined plasmons in simple waveguide structures such as thin films or rods. This is a critical obstacle to applications of nonlinear plasmonics in the telecommunication wavelength region. We theoretically propose and numerically demonstrate that such limitation can be overcome by exploiting inter-element coupling effects in a plasmonic waveguide array. The supermodes of a plasmonic array span a large range of effective indices, making these structures ideal for broadband mode-multiplexed interconnects for integrated photonic devices. We show such plasmonic waveguide arrays can significantly enhance nonlinear optical interactions when operating in a high-index, tightly bound supermode. For example, a third-order nonlinear coefficient in such a waveguide can be more than three orders of magnitude larger compared to silicon waveguides of similar dimensions. These findings open new design possibilities towards the application of plasmonics in integrated optical devices in the telecommunications spectral region.

Topologically protected interface mode in plasmonic waveguide arrays

AbstractRecent realization of nontrivial topological phases in photonic systems has provided unprecedented opportunities in steering light flow in novel manners. Based on the Su–Schriffer–Heeger (SSH) model, a topologically protected optical mode was successfully demonstrated in a plasmonic waveguide array with a kinked interface that exhibits a robust nonspreading feature. However, under the same excitation conditions, another antikinked structure seemingly cannot support such a topological interface mode, which appears to be inconsistent with the SSH model. Theoretical calculations are carried out based on the coupled‐mode theory, in which the mode properties, excitation conditions, and the robustness are studied in detail. It is revealed that under the exact eigenstate excitations, both kinked and antikinked structures do support such robust topological interface modes; however, for a realistic single‐waveguide input only the kinked structure does so. It is concluded that the symmetry of interface eigenmodes plays a crucial role, and the odd eigenmode in a kinked structure offers the capacity to excite the nonspreading interface mode in the realistic excitation of a one‐waveguide input. Our finding deepens the understanding of mode excitation and propagation in coupled waveguide systems, and could open a new avenue in optical simulations and photonic designs. image

Discrete gap solitons in nonlinear binary plasmonic waveguide arrays

We investigate plasmonic mode propagation in nonlinear binary plasmonic waveguide arrays composed of two kinds of metal slabs alternately embedded in nonlinear dielectric materials, and numerically obtain soliton solutions with different symmetries. In particular, we find discrete gap solitons with strong transverse confinements reaching the scale of 10 nm. The equations to describe the mode propagations indicate that the system also provides a plasmonic platform to simulate aspects of quantum dynamics.

Subwavelength Line Imaging Using Plasmonic Waveguides

We investigate the subwavelength imaging capacity of a two-dimensional fanned-out plasmonic waveguide array, formed by air channels surrounded by gold metal layers for operation at near-infrared wavelengths, via finite element simulations. High resolution is achieved on one side of the device by tapering down the channel width while simultaneously maintaining propagation losses of a few dB. On the other, low-resolution side, output couplers are designed to optimize coupling to free space and to minimize channel cross talk via surface plasmons. Point sources separated by {\lambda}/15 can still be clearly distinguished. Moreover, up two 90% of the power of a point dipole is coupled to the device. Applications are high-resolution linear detector arrays and, by operating the device in reverse, high-resolution optical writing.

Plasmonic bistability in the nonlinear cylindrical metal-dielectric multilayers

We study the nonlinear cylindrical plasmonic waveguide arrays composed of the metal-dielectric multilayers with the Kerr-type nonlinear dielectric materials. The theory of the nonlinear propagation mode is presented and finally expressed as two coupled nonlinear equations with the help of the Lorentz reciprocity theorem. Numerical results demonstrate there exists plasmonic bistability of the propagation modes for a given intensity. This kind of bistability corresponds to the totally different energy distributions in the system, which can be used to develop the applications in the optical switches and optical detectors.

Bloch oscillations in plasmonic waveguide arrays.

The combination of modern nanofabrication techniques and advanced computational tools has opened unprecedented opportunities to mold the flow of light. In particular, discrete photonic structures can be designed such that the resulting light dynamics mimics quantum mechanical condensed matter phenomena. By mapping the time-dependent probability distribution of an electronic wave packet to the spatial light intensity distribution in the corresponding photonic structure, the quantum mechanical evolution can be visualized directly in a coherent, yet classical wave environment. On the basis of this approach, several groups have recently observed discrete diffraction, Bloch oscillations and Zener tunnelling in different dielectric structures. Here we report the experimental observation of discrete diffraction and Bloch oscillations of surface plasmon polaritons in evanescently coupled plasmonic waveguide arrays. The effective external potential is tailored by introducing an appropriate transverse index gradient during nanofabrication of the arrays. Our experimental results are in excellent agreement with numerical calculations.

Symmetric and antisymmetric nonlinear modes supported by dual local gain in lossy lattices

We introduce a discrete lossy system, into which a double “hot spot” (HS) is inserted, i.e., two mutually symmetric sites carrying linear gain and cubic nonlinearity. The system can be implemented as an array of optical or plasmonic waveguides, with a pair of amplified nonlinear cores embedded into it. We focus on the case of self-defocusing nonlinearity and cubic losses acting at the HSs. Symmetric localized modes pinned to the double HS are constructed in an implicit analytical form, which is done separately for the cases of odd and even numbers of intermediate sites between the HSs. In the former case, some stationary solutions feature a W-like shape, with a low peak at the central site, added to tall peaks at the positions of the embedded HSs. The special case of two adjacent HSs is considered too. Stability of the solution families against small perturbations is investigated in a numerical form, which reveals stable and unstable subfamilies. The instability generated by an isolated positive eigenvalue leads to a spontaneous transformation into a co-existing stable antisymmetric mode, while a pair of complex-conjugate eigenvalues gives rise to persistent breathers. This article is a contribution to the volume dedicated to Professor Helmut Brand on the occasion of his 60th birhday.

Pinned modes in lossy lattices with local gain and nonlinearity

We introduce a discrete linear lossy system with an embedded "hot spot" (HS), i.e., a site carrying linear gain and complex cubic nonlinearity. The system can be used to model an array of optical or plasmonic waveguides, where selective excitation of particular cores is possible. Localized modes pinned to the HS are constructed in an implicit analytical form, and their stability is investigated numerically. Stability regions for the modes are obtained in the parameter space of the linear gain and cubic gain or loss. An essential result is that the interaction of the unsaturated cubic gain and self-defocusing nonlinearity can produce stable modes, although they may be destabilized by finite-amplitude perturbations. On the other hand, the interplay of the cubic loss and self-defocusing gives rise to a bistability.

Coupled-mode approach to surface plasmon polaritons in nonlinear periodic structures

We present a coupled-mode theory describing light propagation in an array of nonlinear plasmonic waveguides. Our model predicts a two-band dependence of the propagation constant versus transverse quasi-momentum and existence of discrete and gap plasmon solitons.