Transverse Waveguide Modes Research Articles

Simulation of two-qubit quantum gate based on classical transverse spatial waveguide modes encoding

Simulation of two-qubit quantum gate based on classical transverse spatial waveguide modes encoding

Coherent Spontaneous Emission from Short Electron Bunches: Competition between Different Transverse Waveguide Modes

In this paper, we study spontaneous coherent undulator emission from a short dense electron bunch. A special “negative-mass” undulator ensuring stabilization of the axial size of the bunch is considered. We focus on the situation where the competition between two different transverse waveguide modes takes place, so that the undulator resonance is provided simultaneously for three waves placed on the dispersion diagram symmetrically with respect to electrons (namely, the group velocity of the higher waveguide mode coincides with the electron velocity, whereas two resonance frequencies of the lower transverse mode correspond to wave group velocities, one of which slightly exceeds the translational velocity of electrons, with the other being less than the electron velocity). We show that the higher waveguide mode can win this competition due to its proximity to group synchronization with the electron bunch.

Mode-Entanglement in Silicon Waveguides Through Intermodal Four-Wave Mixing

Silicon photonics is rapidly emerging as a promising, scalable platform for quantum information applications. As the technology matures, sources of high-quality quantum-optical states are increasingly important. We discuss the use of intermodal four-wave mixing in silicon waveguides for creating entangled photon pairs. We describe a framework to numerically analyze the state of produced photon pairs and discuss a scheme for generating photon pairs that are purely entangled in their transverse waveguide mode. Using this scheme, we find a theoretical fidelity of 99.6 % to a true Bell state in the transverse mode with no spectral correlations.

Probing mid-infrared surface interface states based on thermal emission.

Probing mid-infrared surface wave radiation remains a big challenge for a long time. The lack of convenient and quick mid-infrared surface wave radiation probing methods limits the development of the integrated mid-infrared materials and devices. In this work, we propose a scheme to construct and probe the mid-infrared surface wave radiation of interface state in the waveguide through thermal emission. A superlattice composed of alternately placed periodic meta-crystals is designed to construct an array of interfaces to realize the interface states through the transverse electrical waveguide modes with a tolerance in structural parameters. By heating the structure, we employ angular resolved thermal emission spectroscopy to directly and quickly verify the dispersion of mid-infrared interface states, which have specific frequencies, angles, and polarizations. Moreover, we establish a thermal imaging microscopy to probe the local waveguide interface state directly for the first time. This proposed infrared probing method based on thermal emission can be generalized to probe the mid-infrared surface wave in other systems, such as surface plasmon waves in graphene or surface phonon waves in two-dimensional materials in the mid-infrared range.

Dual-Wavelength Lasing in Quantum-Dot Plasmonic Lattice Lasers

Arrays of metallic particles patterned on a substrate have emerged as a promising design for on-chip plasmonic lasers. In past examples of such devices, the periodic particles provided feedback at a single resonance wavelength, and organic dye molecules were used as the gain material. Here, we introduce a flexible template-based fabrication method that allows a broader design space for Ag particle-array lasers. Instead of dye molecules, we integrate colloidal quantum dots (QDs), which offer better photostability and wavelength tunability. Our fabrication approach also allows us to easily adjust the refractive index of the substrate and the QD-film thickness. Exploiting these capabilities, we demonstrate not only single-wavelength lasing but dual-wavelength lasing via two distinct strategies. First, by using particle arrays with rectangular lattice symmetries, we obtain feedback from two orthogonal directions. The two output wavelengths from this laser can be selected individually using a linear polarizer. Second, by adjusting the QD-film thickness, we use higher-order transverse waveguide modes in the QD film to obtain dual-wavelength lasing at normal and off-normal angles from a symmetric square array. We thus show that our approach offers various design possibilities to tune the laser output.

Dependence of the Spontaneous Luminescence Intensity in ZnO Nanorods on their Length

The influence of the length of ZnO nanorods (500 nm in diameter) on the mode structure and spontaneous luminescence in the ultraviolet spectral region is studied by optical luminescence microscopy. It is shown that individual nanorods with a metal mirror on one face exhibit only two or three laser modes in the case of short nanocavity lengths (8–30 μm). Different values of the optical losses of the longitudinal and transverse waveguide modes are established for a ZnO nanorod lying on a glassy substrate. The quadratic dependence of the spontaneous luminescence intensity on the rod length can be attributed to improvement of the optical quality factor Q of bound longitudinal modes of light within longer rods (the Purcell effect).

Зависимость интенсивности спонтанной люминесценции наностержней ZnO от их длины

AbstractThe influence of the length of ZnO nanorods (500 nm in diameter) on the mode structure and spontaneous luminescence in the ultraviolet spectral region is studied by optical luminescence microscopy. It is shown that individual nanorods with a metal mirror on one face exhibit only two or three laser modes in the case of short nanocavity lengths (8–30 μm). Different values of the optical losses of the longitudinal and transverse waveguide modes are established for a ZnO nanorod lying on a glassy substrate. The quadratic dependence of the spontaneous luminescence intensity on the rod length can be attributed to improvement of the optical quality factor Q of bound longitudinal modes of light within longer rods (the Purcell effect).

How Ultranarrow Gap Symmetries Control Plasmonic Nanocavity Modes: From Cubes to Spheres in the Nanoparticle-on-Mirror

Plasmonic nanocavities with sub-5-nm gaps between nanoparticles support multiple resonances possessing ultra-high-field confinement and enhancements. Here we systematically compare the two fundamentally different resonant gap modes: transverse waveguide (s) and antenna modes (l), which, despite both tightly confining light within the gap, have completely different near-field and far-field radiation patterns. By varying the gap size, both experimentally and theoretically, we show how changing the nanoparticle shape from sphere to cube alters coupling of s and l modes, resulting in strongly hybridized (j) modes. Through rigorous group representation analysis we identify their composition and coupling. This systematic analysis of the Purcell factors shows that modes with optical field perpendicular to the gap are best to probe the optical properties of cavity-bound emitters, such as single molecules.

Quantum interference between transverse spatial waveguide modes

Integrated quantum optics has the potential to markedly reduce the footprint and resource requirements of quantum information processing systems, but its practical implementation demands broader utilization of the available degrees of freedom within the optical field. To date, integrated photonic quantum systems have primarily relied on path encoding. However, in the classical regime, the transverse spatial modes of a multi-mode waveguide have been easily manipulated using the waveguide geometry to densely encode information. Here, we demonstrate quantum interference between the transverse spatial modes within a single multi-mode waveguide using quantum circuit-building blocks. This work shows that spatial modes can be controlled to an unprecedented level and have the potential to enable practical and robust quantum information processing.

Erratum to: The Main Principles of Formation of the Transverse Modes in the Multilayered Waveguides of Surface Acoustic Waves

We develop a self-consistent model allowing one to analyze the properties of the interdigital transducer of the surface acoustic waves as a symmetric five-layered waveguide on a piezoelectric substrate with three possible values of the phase velocity of the acoustic-wave propagation along the longitudinal axis of the system. The transcendental dispersion relation for describing the waves in such a system is derived and the method for its instructive graphic analysis is proposed. The condition under which only the fundamental transverse mode is excited in the waveguide is formulated. The method for calculating the normalized power and the transverse distribution of the field of the continuous-spectrum waves radiated from the considered waveguide is described. It is shown that the characteristic spatial scale of the longitudinal damping of the amplitude of this field at the waveguide center can be a qualitative estimate of the transverse-mode formation length. The efficiency of a new method for suppressing the higher-order transverse waveguide modes is demonstrated.

Scattering of the Transverse Waveguide Modes of Surface Acoustic Waves by the Finite-Aperture Electrode Structures

We develop a physical model allowing one to analyze reflection of the inhomogeneous beams of surface acoustic waves from metal strips in a planar waveguide on the piezoelectric substrate. Analytical relationships for determining the coefficients of scattering and mutual conversion of the transverse waveguide modes during their interaction with the spatially limited Bragg reflectors are obtained. The waveguide-reflector characteristics are shown to depend on the ratio of the waveguide aperture to its maximum value for which only the fundamental transverse mode is excited. It is established that the developed model strictly corresponds to the energy conservation law, i.e., in the absence of dissipation, the power of the inhomogeneous beam, which is incident on the finite reflector, is equal to the total power of all the scattered fields of the discrete and continuous waveguide spectra.

Nonreciprocal propagation of hybrid electromagnetic waves in a layered ferrite–ferroelectric structure with a finite width

Effects of the nonreciprocal propagation of electromagnetic waves in a transversally limited ferrite–ferroelectric structure with a finite width and tangential magnetization have been considered. Nonreciprocal effects are manifested in the range of frequencies corresponding to the hybridization of electromagnetic waves of the ferrite and ferroelectric waveguides. The electrodynamic characteristics of waves propagating in the layered structure with a finite width in different directions have been calculated by the finite element method. The nonreciprocity factors have been calculated for transverse waveguide modes. It has been shown that nonreciprocity owing to the gyrotropy of the ferrite waveguide leads to a change in the spatial distribution of the components of electromagnetic fields in the layered structure and to the redistribution of the power transferred by an electromagnetic wave between the layers of the structure. The results obtained can be used to create nonreciprocal microwave microdevices with double control.

Ultrafast, low-power, all-optical switching via birefringent phase-matched transverse mode conversion in integrated waveguides

We demonstrate the potential of birefringence-based, all-optical, ultrafast conversion between the transverse modes in integrated optical waveguides by modelling the conversion process by numerically solving the multi-mode coupled nonlinear Schroedinger equations. The observed conversion is induced by a control beam and due to the Kerr effect, resulting in a transient index grating which coherently scatters probe light from one transverse waveguide mode into another. We introduce birefringent phase matching to enable efficient all-optically induced mode conversion at different wavelengths of the control and probe beam. It is shown that tailoring the waveguide geometry can be exploited to explicitly minimize intermodal group delay as well as to maximize the nonlinear coefficient, under the constraint of a phase matching condition. The waveguide geometries investigated here, allow for mode conversion with over two orders of magnitude reduced control pulse energy compared to previous schemes and thereby promise nonlinear mode switching exceeding efficiencies of 90% at switching energies below 1 nJ.

Exceptional ballistic transport in epitaxial graphene nanoribbons

Graphene nanoribbons will be essential components in future graphene nanoelectronics. However, in typical nanoribbons produced from lithographically patterned exfoliated graphene, the charge carriers travel only about ten nanometres between scattering events, resulting in minimum sheet resistances of about one kilohm per square. Here we show that 40-nanometre-wide graphene nanoribbons epitaxially grown on silicon carbide are single-channel room-temperature ballistic conductors on a length scale greater than ten micrometres, which is similar to the performance of metallic carbon nanotubes. This is equivalent to sheet resistances below 1 ohm per square, surpassing theoretical predictions for perfect graphene by at least an order of magnitude. In neutral graphene ribbons, we show that transport is dominated by two modes. One is ballistic and temperature independent; the other is thermally activated. Transport is protected from back-scattering, possibly reflecting ground-state properties of neutral graphene. At room temperature, the resistance of both modes is found to increase abruptly at a particular length--the ballistic mode at 16 micrometres and the other at 160 nanometres. Our epitaxial graphene nanoribbons will be important not only in fundamental science, but also--because they can be readily produced in thousands--in advanced nanoelectronics, which can make use of their room-temperature ballistic transport properties.

Single-Slit Electron Diffraction with Aharonov-Bohm Phase: Feynman’s Thought Experiment with Quantum Point Contacts

In a "thought experiment," now a classic in physics pedagogy, Feynman visualizes Young's double-slit interference experiment with electrons in magnetic field. He shows that the addition of an Aharonov-Bohm phase is equivalent to shifting the zero-field wave interference pattern by an angle expected from the Lorentz force calculation for classical particles. We have performed this experiment with one slit, instead of two, where ballistic electrons within two-dimensional electron gas diffract through a small orifice formed by a quantum point contact (QPC). As the QPC width is comparable to the electron wavelength, the observed intensity profile is further modulated by the transverse waveguide modes present at the injector QPC. Our experiments open the way to realizing diffraction-based ideas in mesoscopic physics.

Original coupled FEM/BIE numerical model for analyzing infinite periodic surface acoustic wave transducers

This paper proposes a new numerical coupled Finite Element Method/Boundary Integral Equations (FEM/BIE) technique which allows the 2D physical simulation of Surface Acoustic Waves (SAWs) transducers infinitely periodic in one direction. This new technique could be generalized to various periodic acoustic 2D simulations.This new method uses an original Variational Formulation (VF) which formally includes harmonic periodic boundary conditions, and, efficient boundary integral formulations allowing to account for the semi-infinite dielectric and piezoelectric spaces. In the case of the piezoelectric semi-space, the Green’s functions are efficiently computed using Fahmy–Adler’s method [8]. Only periodic boundary conditions are needed, which greatly simplifies the code implementation.This numerical model has been developed to analyze an Inter-Digital Transducer (IDT) with complex electrode shape (unburied, buried or raised electrodes). The use of buried electrodes in SAW transducer designs on quartz has important advantages when compared with unburied metal electrodes on the surface. One important property is the suppression of transverse waveguide modes in transducers. A second advantage is the ability to use thicker metal thereby reducing the resistive losses. Buried electrodes have also been shown to increase the quality factor of Surface Transverse Wave (STW) resonators [15]. This numerical model is a very useful tool for optimizing the electrode geometry. Analysis of raised electrodes is useful for predicting the effects of Reactive Ion Etch (RIE) on the SAW or STW electrical filter characteristics. RIE is commonly used as a frequency trimming technique for SAW or STW filters on Quartz.The first part of the paper presents the theory, and, the second part is devoted to numerical validations and numerical results.

Transverse waveguide mode suppression for Pt-electrode SAW resonators on Quartz and LGS

SAW resonators on ST-X quartz and langasite (LGS) [0°, 144°, 24°] are currently being used for hydrogen fluoride (HF) vapor sensing and high-temperature sensing, respectively. For these applications, the use of Pt-based electrodes allows the resonators to withstand the targeted harsh environments. This work reveals that for Pt-electrode resonators with conventional short-circuit gratings on the aforementioned quartz and LGS orientations, acoustic energy leaks from the grating region to the bus bars, thus degrading the resonator response. To resolve this problem, this paper proposes and implements open-circuit gratings for resonators fabricated with these substrate/metal combinations. The open-circuit gratings guide the acoustic energy within the grating region, resulting in greater quality factors and reduced losses in the resonator response. In addition, scalar potential theory is utilized in this work to identify transverse waveguide modes in the responses of open-circuit grating resonators on quartz and LGS. A transverse waveguide mode dispersion relation was derived to extend the scalar potential theory to account for asymmetry in the slowness curve around the propagation direction. This is the case for several commonly used LGS orientations, in particular LGS [0°, 144°, 24°]. Finally, this work addresses spurious transverse mode mitigation by scaling both the transducer's grating aperture and electrode overlap width. Open circuit grating resonators with appropriately scaled transducer designs were fabricated and tested, resulting in a 71% increase in quality factor and a spurious mode rejection of over 26 dBc for Pt-electrode devices on ST-X quartz. This progress directly translates into better frequency resolution and increased dynamic range for HF vapor sensors and high-temperature SAW devices.

Fundamental behavior of electric field enhancements in the gaps between closely spaced nanostructures

We demonstrate that the electric field enhancement that occurs in a gap between two closely spaced nanostructures, such as metallic nanoparticles, is the result of a transverse electromagnetic waveguide mode. We derive an explicit semianalytic equation for the enhancement as a function of gap size, which we show has a universal qualitative behavior in that it applies irrespective of the material or geometry of the nanostructures and even in the presence of surface plasmons. Examples of perfect electrically conducting and Ag thin-wire antennas and a dimer of Ag spheres are presented and discussed.

Mode structure of laser emission from ZnO Nanorods with one metal mirror

The effect of the length of ZnO nanorods (500 nm in diameter) on the mode structure of their lasing in the ultraviolet spectral region is studied by optical luminescence microscopy. It is shown that separate nanorods with a metal mirror at one of the end faces exhibit only two or three laser modes at small nanoresonator lengths (8–20 μm). The different optical losses of longitudinal and transverse waveguide modes are established for nanorods lying on a glass substrate. An increase in the optical Q factor and a decrease in the lasing thresholds can be attributed to optical reflection from the metal mirror at the end face of the nanorod.

Terahertz quantum cascade lasers with quasi-periodic resonators

Quasi-periodic surface plasmon gratings based on the Fibonacci sequence are studied as distributed feedback resonators for terahertz quantum cascade lasers. The simulations indicate a quality factor comparable with standard DFB resonators. The fabricated devices show grating-dependent emission, but suppression of higher transverse waveguide modes will be necessary to unambiguously identify the fundamental resonance frequencies of the grating.