Ultrathin Waveguides Research Articles

Guiding light with surface exciton–polaritons in atomically thin superlattices

Abstract Two-dimensional materials give access to the ultimate physical limits of photonics with appealing properties for ultracompact optical components such as waveguides and modulators. Specifically, in monolayer semiconductors, a strong excitonic resonance leads to a sharp oscillation in permittivity from positive to even negative values. This extreme optical response enables surface exciton–polaritons to guide visible light bound to an atomically thin layer. However, such ultrathin waveguides support a transverse electric (TE) mode with low confinement and a transverse magnetic (TM) mode with short propagation. Here, we propose that realistic semiconductor–insulator–semiconductor superlattices comprising monolayer WS2 and hexagonal boron nitride (hBN) can improve the properties of both TE and TM modes. Compared to a single monolayer, a heterostructure with a 1-nm hBN spacer separating two monolayers enhances the confinement of the TE mode from 1.2 to around 0.5 μm, while the out-of-plane extension of the TM mode increases from 25 to 50 nm. We propose two simple additivity rules for mode confinement valid in the ultrathin film approximation for heterostructures with increasing spacer thickness. Stacking additional WS2 monolayers into superlattices further enhances the waveguiding properties. Our results underscore the potential of monolayer-based superlattices as a platform for visible-range nanophotonics with promising optical, electrical, and magnetic tunability.

Ultrathin waveguides for 2D photonic integrated circuits

Ultrathin waveguides for 2D photonic integrated circuits

Strain-Engineered Wrinkles on Graphene Using Polymeric Actuators

Corrugations strongly affect the electronic and optical properties of two-dimensional materials, and could be used to develop e.g. ultrathin pressure sensors, electronic waveguides, or hydrogen-storage devices. That requires fine control of the structure and geometry of the wrinkles, and thus a reliable and flexible method to generate them. This work shows how polymeric microactuators can be used to induce ordered arrays of wrinkles on graphene. Moreover, its proof-of-concept experiment demonstrates the potential of mechanical actuators to tune the corrugation of other two-dimensional materials as well.

Realization of ultrathin waveguides by elastic metagratings

Guiding classical waves has inspired a wealth of nontrivial physics and significant applications. To date, a robust and compact way to guide energy flux traveling along an arbitrary, prescheduled trajectory in a uniform medium is still a fundamental challenge. Here we propose and experimentally realize a generic framework of ultrathin waveguides for omnidirectional wave trapping and efficient routing. The metagrating-based waveguide can totally suppress all high-order parasitic diffractions to route guided elastic waves without leakage. The proposed waveguide protype works in a broad frequency range under a full-angle radiated source. An analytical slab-waveguide model is presented to predict and tailor the diffracted patterns. Compared with existing methods based on topological edge states or defected metamaterials, our meta-waveguide strategy exhibits absolute advantages in compact size, robust performance, and easy fabrication, which may provide a design paradigm for vibration and noise control, energy harvesting, microfluidics, wave steering in acoustics and other waves.

High-performance and fabrication friendly polarization demultiplexer

A compact and fabrication friendly polarization demultiplexer (P-DEMUX) is proposed and characterized to enable wavelength-division-multiplexing and polarization-division-multiplexing simultaneously. The proposed structure is composed of a polarization-selective microring resonator in ultrathin waveguide and two bus channels in the silicon nitride-silica-silicon horizontal slot waveguides. In the slot waveguide, the transverse electric (TE) mode propagates through the silicon layer, while the transverse magnetic (TM) mode is confined in the slot region. In the designed ultra-thin waveguide, the TM mode is cut-off. The effective indexes of the TE modes for ultrathin and slot waveguides have comparable values. Thanks for these distinguishing features, the input TE mode can be efficiently filtered through the ultra-thin microring at the resonant wavelength, while the TM mode can directly output from the through port. Simulation results show that the extinction ratio of the proposed P-DEMUX for TE and TM modes are 33.21 dB and 24.97 dB, and the insertion losses are 0.346 dB and 0.324 dB, respectively, at the wavelength of 1551.64 nm. Furthermore, the device shows a broad bandwidth (>100 nm) for an extinction ratio (ER) of >20 dB. In addition, the proposed P-DEMUX also has a good fabrication tolerance for the waveguide width variation of –20 nm ≤ Δ w g ≤ 20 nm and the microring width variation of –20 nm ≤ Δ w r ≤ 20 nm for a low insertion loss of <0.75 dB and low ER of <–18 dB.

Coupling Between Silicon Waveguide and Metal-Dielectric-Metal Plasmonic Waveguide with Lens-Funnel Structure

Various photonic integrated components have been implemented by ultra-thin silicon-on-insulator (SOI) waveguides; therefore, it is desirable to couple ultra-thin SOI waveguides to plasmonic waveguides. In this paper, we present an ultra-thin SOI waveguide to a metal-dielectric-metal plasmonic waveguide based on a lens-funnel structure consisting of truncated Luneburg lens and metallic parabolic funnel. The lens is implemented by varying the guiding layer thickness. The effect of different parameters of the coupler’s geometry is studied using the finite-difference time-domain method. The 1.13-μm-long coupler improves the average coupling efficiency in the C-band from 66.4 to 82.1%. The numerical simulations indicate that the coupling efficiency is higher than 69% in the entire O, E, S, C, L, and U bands of optical communication.

Ultrathin and flexible directional coupler with arbitrary coupling level using s-shaped spoof surface plasmon polariton coupled-line

Here, we study the coupling effect between two identical ultrathin plasmonic waveguides and present an ultra-flexible directional coupler by printing a pair of corrugated metallic strips with mirror symmetry on the top of an ultrathin dielectric substrate. The proposed coupler supports the transmission and coupling of spoof surface plasmon polariton waves efficiently. We experimentally demonstrate that the coupler possesses excellent flexibility, arbitrary coupling level and wide bandwidth simultaneously. Hence, the proposed coupler paves the way to develop flexible plasmonic devices, conformal circuits, human body sensors and wearable devices in microwave and terahertz frequencies.

Low-loss and low-crosstalk multi-channel mode (de)multiplexer with ultrathin silicon waveguides.

A multi-channel mode (de)multiplexer based on the supermode evolution in cascaded dual-core adiabatic tapers is designed and realized with ultrathin (∼50 nm) silicon-on-insulator nanowire waveguides. This dual-core adiabatic taper is designed optimally, so that all the supermodes are confined in the wide core at the input end and the highest-order supermode is localized in the narrow core at the output end. In this way, the highest-order supermode is dropped as the fundamental mode at the cross port. As an example, a (de)multiplexer with three mode channels is designed and fabricated. From the measured spectral responses, the fabricated device has a crosstalk lower than -20 dB and an excess loss less than 0.2dB in a bandwidth of ∼65 nm.

Surface Plasmon Polaritons Propagation Along Armchair and Zigzag Single-Wall Carbon Nanotubes With Different Radii

In this paper, we have studied chirality dependent optical properties of single-wall carbon nanotubes (CNTs), such as dynamic conductance and permittivity functions in THz, IR, and optical telecommunication bands. It is observed that CNTs with different radii yield different permittivity functions. We have presented the effects of phenomenological relaxation frequency, longitudinal surface plasmon resonance, and hybridization of π-orbitals on optical traits of armchair and zigzag CNTs. After their characterization, single-wall CNTs are utilized for guiding plasmonic surface modes, each is frequency dependent on its geometrical dimensions. It is demonstrated that surface plasmon polaritons can be excited along metallic nanotubes with very small radii. It is also revealed that as one choose the nanotube with smaller radius the propagation length and surface plasma frequency of that nanotube increase. The confinement of these modes and also their propagation length are studied and it is found that CNTs could become an excellent candidate for ultrathin plasmonic waveguides.

60-nm-thick basic photonic components and Bragg gratings on the silicon-on-insulator platform

We demonstrate integrated basic photonic components and Bragg gratings using 60-nm-thick silicon-on-insulator strip waveguides. The ultra-thin waveguides exhibit a propagation loss of 0.61 dB/cm and a bending loss of approximately 0.015 dB/180° with a 30 μm bending radius (including two straight-bend waveguide junctions). Basic structures based on the ultra-thin waveguides, including micro-ring resonators, 1 × 2 MMI couplers, and Mach-Zehnder interferometers are realized. Upon thinning-down, the waveguide effective refractive index is reduced, making the fabrication of Bragg gratings possible using the standard 248-nm deep ultra-violet (DUV) photolithography process. The Bragg grating exhibits a stopband width of 1 nm and an extinction ratio of 35 dB, which is practically applicable as an optical filter or a delay line. The transmission spectrum can be thermally tuned via an integrated resistive micro-heater formed by a heavily doped silicon slab beside the waveguide.

Microwave band gap and cavity mode in spoof–insulator–spoof waveguide with multiscale structured surface

We propose a multiscale spoof–insulator–spoof (SIS) waveguide by introducing periodic geometry modulation in the wavelength scale to a SIS waveguide made of a perfect electric conductor. The MSIS consists of multiple SIS subcells. The dispersion relationship of the fundamental guided mode of the spoof surface plasmon polaritons (SSPPs) is studied analytically within the small gap approximation. It is shown that the multiscale SIS possesses microwave band gap (MBG) due to the Bragg scattering. The ‘gap maps’ in the design parameter space are provided. We demonstrate that the geometry of the subcells can efficiently adjust the effective refraction index of the elementary SIS and therefore further control the width and the position of the MBG. The results are in good agreement with numerical calculations by the finite element method (FEM). For finite-sized MSIS of given geometry in the millimeter scale, FEM calculations show that the first-order symmetric SSPP mode has zero transmission in the MBG within frequency range from 4.29 to 5.1 GHz. A cavity mode is observed inside the gap at 4.58 GHz, which comes from a designer ‘point defect’ in the multiscale SIS waveguide. Furthermore, ultrathin MSIS waveguides are shown to have both symmetric and antisymmetric modes with their own MBGs, respectively. The deep-subwavelength confinement and the great degree of control of the propagation of SSPPs in such structures promise potential applications in miniaturized microwave device.

Mid-infrared integrated waveguide modulators based on silicon-on-lithium-niobate photonics

Heterogeneous integration techniques, such as direct bonding, have enabled solutions to many problems facing integrated photonics. In particular, the relatively new field of mid-infrared (mid-IR) integrated photonics has been hindered by the availability of functional, transparent substrates in this wavelength range. The key to achieving compact, high-performance optical modulation and frequency conversion is the monolithic integration of silicon photonics with a material with high second-order nonlinear susceptibility. By transferring large areas of thin, monocrystalline silicon to bulk lithium niobate (LiNbO3) substrates, the first silicon-based platform to exploit the Pockels or linear electro-optic effect in the mid-IR range is achieved. Integrated Mach–Zehnder interferometer modulators with an extinction ratio of ∼8 dB, a half-wave voltage-length product of 26 V·cm, and an on-chip insertion loss of 3.3 dB are demonstrated at a wavelength of 3.39 μm. Ultrathin optical waveguides fabricated and characterized on this platform exhibit a low transverse electric mode linear propagation loss of 2.5 dB/cm. Future capabilities such as wideband difference frequency generation for integrated mid-IR sources are envisioned for the demonstrated silicon-on-lithium-niobate platform.

Performance of ultra-thin SOI-based resonators for sensing applications.

This work presents simulation and experimental results of ultra-thin optical ring resonators, having larger Evanescent Field (EF) penetration depths, and therefore larger sensitivities, as compared to conventional Silicon-on-Insulator (SOI)-based resonator sensors. Having higher sensitivities to the changes in the refractive indices of the cladding media is desirable for sensing applications, as the interactions of interest take place in this region. Using ultra-thin waveguides (<100 nm thick) shows promise to enhance sensitivity for both bulk and surface sensing, due to increased penetration of the EF into the cladding. In this work, the designs and characterization of ultra-thin resonator sensors, within the constraints of a multi-project wafer service that offers three waveguide thicknesses (90 nm, 150 nm, and 220 nm), are presented. These services typically allow efficient integration of biosensors with on-chip detectors, moving towards the implementation of lab-on-chip (LoC) systems. Also, higher temperature stability of ultra-thin resonator sensors were characterized and, in the presence of intentional environmental (temperature) fluctuations, were compared to standard transverse electric SOI-based resonator sensors.

Efficient Sub-wavelength Plasmonic Superconducting Photodetectors in Ultra-Thin Planar Waveguides and in Optical Fibers

By using a finite-element method we investigate the efficiency of sub-wavelength plasmonic superconducting photodetectors in ultra-thin planar waveguides (the incident wave is P-polarized) and in optical fibers (a traveling wave). The unit cell of a periodic sub-wavelength planar waveguide is made by a gold strip that is encapsulated in an YBCO layer and further enclosed above by an air layer and beyond by a thick gold plate substrate. For a planar structure with a single unit cell, the power absorption in YBCO layer is very large (between 0.929 and 0.975) for a fixed wavelength λ=1.55 μm when the distance between the strips and gold substrate is varied between 17 nm and 32 nm. When the wavelength is varied between 1.1 μm and 1.9 μm, the power absorption in YBCO layer is between 0.944 and 0.965 with a maximum of 0.976 at λ=1.40 μm. The superconducting traveling wave photodetector is made by an optical fiber, which includes a gold core surrounded by a number of smaller gold cylinders at a small distance from the central gold region and that are encapsulated in an active YBCO layer and further enclosed by an air layer. In our structure of the fiber with 18 smaller gold cylinders, the imaginary parts of the effective index of plasmon modes (two nondegenerate modes and eight twofold degenerate modes) and the power absorption (between 0.904 and 0.976) in active YBCO layer are very large. The first nine modes are highly confined in the YBCO layer in the space between the neighboring gold cylinders and the other nine modes are highly confined in the YBCO layer but in the space between the small gold cylinders and the gold core. Although in our structure, the thickness of the YBCO layer is 10 times smaller in comparison with that of a recently published model with 32 small gold cylinders, the losses in active YBCO layer are also very large.

Ultrathin Silicon-on-Insulator Grating Couplers

We demonstrate a novel grating coupler fabricated on a silicon-on-insulator (SOI) wafer operating at 1550 nm, based on an ultrathin 50-nm silicon geometry. The devices are fabricated in a CMOS-compatible process with a single etch step. A low insertion loss of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${-}{\rm 3.7}~{\rm dB}$</tex></formula> is achieved. We also calculate the backreflection loss to be <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${-}{\rm 14}~{\rm dB}$</tex></formula> . The devices are likely to be useful in terms of light coupling between optical fibers and ultrathin silicon waveguides.

Ultra-thin silicon-on-insulator strip waveguides and mode couplers

We demonstrate an ultra-thin silicon waveguide for wavelengths around 1.55 μm, and mode converters designed for transitions to and from standard 500 nm × 220 nm strip waveguides. The devices were fabricated in a CMOS-compatible process requiring two photolithography and etch steps. The ultra-thin waveguides exhibited losses of 2.01 ± 0.231 dB/cm, exhibited bend radii as small as 30 μm with losses of 0.05 ± 0.005 dB per bend, and exhibited coupling losses of 0.66 ± 0.014 dB to standard strip waveguides.

Bottom‐Up Tailoring of Plasmonic Nanopeapods Making Use of the Periodical Topography of Carbon Nanocoil Templates

AbstractAu nanoparticle chains embedded in helical Al2O3 nanotubes (“nanopeapods”) are synthesized by annealing carbon nanocoils coated with Au by sputtering and Al2O3 by atomic layer deposition. Regular spacing between nanoparticles with a sharp size distribution is achieved by fragmentation of the Au coating in agreement with the pitch of the nanocoils arising from three‐dimensional periodical topography of the carbon nanocoil templates. A strong plasmonic resonance behavior of the fabricated nanopeapods manifests itself in confocal laser scanning microscopy by a clear polarization contrast at red wavelengths, which is absent in the blue. Numerical simulations confirm an incisive resonance enhancement for longitudinal polarization and suggest the nanopeapods as promising candidates for highly efficient, ultrathin waveguides. The waveguiding properties of the nanopeapods are investigated by electron energy‐loss spectroscopy and energy‐filtered TEM imaging.

Homogenization of waveguide-based metamaterials by energy averaging

We develop a homogenization technique based on electromagnetic energy averaging that is suitable for waveguide-based metamaterials. We first derive general homogenization formulations using Maxwell's equations and the Poynting theorem, and then apply the technique to a common waveguide-based metamaterial implementation consisting of layered media, yielding simple expressions for the effective permittivity and effective permeability of the waveguide based on energy-density-weighted averages of the local permittivity of the layers. Examples are presented in which we evaluate the effective parameters of ultrathin metal-clad waveguides sustaining a single surface-plasmon polariton mode, using realistic material parameters and geometries that have been recently explored in the literature.

Bloch surface waves in ultrathin waveguides: near-field investigation of mode polarization and propagation

In this work, we use a multi-heterodyne scanning near-field optical microscope to investigate the polarization and propagation of Bloch surface waves in an ultrathin (∼λ∕10) ridge waveguide. First, we show that the structure sustains three surface modes, and demonstrate selective excitation of each. Then, by numerically processing the experimental data, we retrieve the transverse and longitudinal components of each of the modes, in good agreement with the calculated fields. Finally, we provide an experimental estimation of the effective indices and the dispersion relations of the modes.

Nonlinear optical pulse propagation simulation: From Maxwell’s to unidirectional equations

Spatial- and time-domain versions of the unidirectional pulse propagation equation (UPPE) are derived and compared from the point of view of their practical application in simulations of nonlinear optical pulse dynamics. A modification of the UPPE suitable for ultrathin optical waveguides, such as submicron silica wires, is also presented. We show in detail how various, previously published propagation equations follow from the UPPE in a unified way that clearly elucidates their underlying approximations and areas of applicability.