Conventional Dielectric Waveguides Research Articles

Subwavelength confinement in an all-dielectric slot waveguide with bound states in the continuum for ultrahigh-Q microresonators

Bound state in the continuum (BIC) has been considered a promising strategy to reduce fabrication complexity and radiative loss. Despite the successful demonstration of many BIC-based configurations, their mode confinement is comparable to conventional dielectric waveguides. Even though plasmonics with strong light localization have been introduced into BICs, the intrinsic ohmic loss is still a facing challenge to be conquered. Herein, we propose a new approach, to our knowledge for the first time, that integrates subwavelength confinement and optical BIC in all dielectric slot waveguides. A nanogap with low refractive index is utilized to separate the top waveguide and high-index substrate, leading to strong mode localization and energy confinement (∼ λ2/30). Surprisingly, leveraging on the concept of optical BICs, the leakage channels of the waveguide bound mode can be readily engineered to cancel each other through destructive interference, which effectively suppresses the optical dissipation. Moreover, the subwavelength confinement is observed in ultrahigh-Q microring resonators, resulting in a giant enhancement of the Purcell factor (> 108). Our findings introduce a new concept into the BIC family and provide novel thoughts to design BIC photonics, which may show superiority in achieving low threshold lasers and highly sensitive sensors.

The perspective of topological photonics for on-chip terahertz modulation and sensing

Terahertz (THz) technology has seen significant advancements in the past decades, encompassing both fundamental scientific research, such as THz quantum optics, and highly applied areas like sixth-generation communications, medical imaging, and biosensing. However, the progress of on-chip THz integrated waveguides still lags behind that of THz sources and detectors. This is attributed to issues such as ohmic losses in microstrip lines, coplanar and hollow waveguides, bulky footprints, and reflection and scattering losses occurring at sharp bends or defects in conventional dielectric waveguides. Inspired by the quantum Hall effects and topological insulators in condensed matter systems, recent discoveries of topological phases of light have led to the development of topological waveguides. These waveguides exhibit remarkable phenomena, such as robust unidirectional propagation and reflectionless behavior against impurities or defects. As a result, they hold tremendous promise for THz on-chip applications. While THz photonic topological insulators (PTIs), including wave division, multiport couplers, and resonant cavities, have been demonstrated to cover a wavelength range of 800–2500 nm, research on tunable THz PTIs remains limited. In this perspective, we briefly reviewed a few examples of tunable PTIs, primarily concentrated in the infrared range. Furthermore, we proposed how these designs could benefit the development of THz on-chip PTIs. We explore the potential methods for achieving tunable THz PTIs through optical, electrical, and thermal means. Additionally, we present a design of THz PTIs for potential on-chip sensing applications. To support our speculation, several simulations were performed, providing valuable insights for future THz on-chip PTI designs.

Evaluation of the coupling characteristics of a sandwiched plasmonic waveguide between two dielectric waveguides using air-gap couplers to achieve high-performance optical interconnects

To achieve ultrahigh-density nanoplasmonic integrated circuits, low-loss conventional dielectric waveguides (CDWs) are used to transfer light into and out of the metal–dielectric–metal (MDM) plasmonic waveguides. We show that sandwiching the MDM plasmonic waveguide between two CDWs forms a Fabry–Perot cavity-like structure, which causes oscillations in the transmission coupling efficiency (TCE) into the output CDW as the length of the MDM waveguide is increased. Three types of compact air-gap couplers (AGCs) were used at the interface between those two types of waveguides to enhance the coupling efficiency between them. Our simulation results indicate that tapering CDW before it is connected to AGC not only enhances TCE into the output CDW by 9% over a wide spectrum range but also reduces the need for high-precision fabrication techniques to align AGC at the interface. Moreover, we achieved a TCE into the output CDW of 77% optimized for the optical communications wavelength of 1550 nm when the length of the MDM plasmonic waveguide is 600 nm. In addition, we showed that our proposed design has large fabrication tolerance by investigating the change in its spectrum response as various parameters of the design dimensions differ from that of the targeted optimum dimensions. We found that increasing the dimensions of AGC reduces the width of the spectrum, whereas increasing the width of CDW with its tapered part shifts the spectrum to the right by 100 nm per 40 nm increase in the width. We also found that increasing the refractive index of the MDM plasmonic waveguide with AGC controls the cutoff wavelength, in which TCE is almost zero at wavelengths shorter than the cutoff wavelength, and consequently provides a unique advantage in using our proposed design in sensing applications.

Theoretical Investigation of an Air-Slot Mode-Size Matcher between Dielectric and MDM Plasmonic Waveguides

Hybrid integration of dielectric and plasmonic waveguides is necessary to reduce the propagation losses due to the metallic interactions and support of nanofabrication of plasmonic devices that deal with large data transfer. In this paper, we propose a direct yet efficient, very short air-slot coupler (ASC) of a length of 36 nm to increase the coupling efficiency between a silicon waveguide and a silver-air-silver plasmonic waveguide. Our numerical simulation results show that having the ASC at the interface makes the fabrication process much easier and ensures that light couples from a dielectric waveguide into and out of a plasmonic waveguide. The proposed coupler works over a broad frequency range achieving a coupling efficiency of 86% from a dielectric waveguide into a metal-dielectric-metal (MDM) plasmonic waveguide and 68% from a dielectric waveguide to an MDM plasmonic waveguide and back into another dielectric waveguide. In addition, we show that even if there are no high-precision fabrication techniques, light couples from a conventional dielectric waveguide (CDW) into an MDM plasmonic waveguide as long as there is an overlap between the CDW and ASC, which reduces the fabrication process tremendously. Our proposed coupler has an impact on the miniaturization of ultracompact nanoplasmonic devices.

Efficient coupling of light from dielectric to HIMI plasmonic waveguide

This is the first report on the investigation of a hybrid plasmonic tapered coupler injecting light efficiently from conventional dielectric waveguide to a hybrid insulator-metal-insulator (HIMI) plasmonic waveguide. The proposed structure gives high transmission efficiency and suppresses the reflection losses significantly. With this tapered coupling mechanism, maximum transmittance and return loss of 98% and 26 dB have been observed, respectively. The coupler also offers very good propagation length (274 μm) along with minimal mode propagation losses (0.016 dB/μm). Apart from that, it offers decent confinement of light in low dielectric regions up to 32%. The proposed structure is designed in an ultra-short tapered length of 1.0 μm, and the entire structure occupies a footprint area of 1.4 × 2.4 μm 2 . Proposed hybrid waveguide coupler could be useful for several chip scale applications.

Analogue of the Kerker effect for localized modes of discrete high-index dielectric nanowaveguides

Recently developed field of all-dielectric nanophotonics allowed for the observation of the Kerker effect, i.e., unidirectional scattering of electromagnetic radiation by a dielectric particle in optical frequency range. In this paper, we consider the analogue of this effect for localized waves which manifests itself as an interference of the evanescent tails of the optical waveguide modes. Specifically, we design a discrete nanophotonic waveguide that supports two degenerate modes characterized by different symmetries with respect to the plane that contains the waveguide axis, leading to an asymmetric field distribution of the propagating wave under appropriate excitation conditions. We perform numerical simulations of the excitation of such a waveguide with a point dipole and predict that its polarization state can be encoded into the field pattern of the signal propagating along the waveguide and transferred for relatively large distances. We also propose a planar directional coupler that consists of a developed discrete waveguide and conventional single-mode dielectric waveguides that exploit the interference effect for routing single photons generated by circularly polarized quantum emitters.

Electromagnetic Wave Propagation Through Air-Core Waveguide With Metamaterial Cladding

Metamaterial-based waveguides offer unusual properties compared to conventional metallic and dielectric waveguides. We experimentally investigate a novel waveguide with an air-core and a cladding formed of slotted cylinder resonators, which can have an electric and/or a magnetic response at terahertz (THz) frequencies, depending on their orientation and the polarization of the light, and through these confine radiation to the hollow core. Various interesting phenomena are observed in the experiments, including confinement of radiation through the electric response as well as the magnetic response, spatial dispersion, total internal reflection with refractive indices below 1, and slit resonances. Such waveguides promise future novel THz applications, such as refractive index sensors or dispersion management.

Germanium laser with a hybrid surface plasmon mode

The possibility of creating an n++-Ge laser with a hybrid surface plasmon TM mode is theoretically studied. The distribution of electromagnetic fields and the absorbance in the mode under study, the optical confinement factor, the gain, and the threshold current density in the laser under consideration are calculated. It is shown that the threshold current density at optimal layer thicknesses of an n++-Ge laser with a hybrid surface plasmon TM mode can be 2–3 times lower than the experimentally observed threshold current density in an n++-Ge laser with a conventional dielectric waveguide.

Deep-subwavelength plasmonic mode converter with large size reduction for Si-wire waveguide

If we are to utilize deep-subwavelength plasmonic waveguides in photonic integrated circuit applications, highly efficient three-dimensional (3D) mode conversion must be achieved between deep-subwavelength plasmonic waveguides and conventional dielectric waveguides such as Si-wire waveguides. Here, we describe 3D mode conversion from a Si-wire waveguide (the core size is 400 nm×200 nm) to a plasmonic slot waveguide (the air core size is 50 nm×20 nm) with a coupling loss of 1.7 dB. Our mode converter has only a two-dimensional laterally tapered structure even with the presence of a large discontinuity in the thickness, and can still produce efficient full 3D mode conversion with a very short taper length (600 nm). Calculation results obtained with the finite element method agreed well with the experimental results. We believe our mode converter will provide a new deep-subwavelength photonic platform.

Local field enhancement on demand based on hybrid plasmonic-dielectric directional coupler.

The concept of local field enhancement using conductor-gap-dielectric-substrate (CGDS) waveguide structure is proposed. The dispersion equation is derived analytically and solved numerically. The solution of the dispersion equation reveals the anti-crossing behavior of coupled modes. the optimal gap layer thickness and the coupling length of the guided modes are obtained. The mechanism of the CGDS works as follows: Light waves are guided by conventional low-loss dielectric waveguides and, upon demand, they are transformed into highly confined plasmonic modes with strong local field enhancement, and get transformed back into low-loss dielectric modes. As an example, in a representative CGDS structure, the optimal plasmonic gap size is 17 nm, the local light intensity is found to be more than one order of magnitude stronger than the intensity of the dielectric mode at the film surface. The coupling length is only 2.1 μm at a wavelength of 632.8 nm. Such a local field confinement on demand is expected to facilitate efficient light-matter interaction in integrated photonic devices while minimizing losses typical for plasmonic structures.

Silicon-Organic Hybrid (SOH) and Plasmonic-Organic Hybrid (POH) Integration

Silicon photonics offers tremendous potential for inexpensive high-yield photonic-electronic integration. Besides conventional dielectric waveguides, plasmonic structures can also be efficiently realized on the silicon photonic platform, reducing device footprint by more than an order of magnitude. However, neither silicon nor metals exhibit appreciable second-order optical nonlinearities, thereby making efficient electro-optic modulators challenging to realize. These deficiencies can be overcome by the concepts of silicon-organic hybrid (SOH) and plasmonic-organic hybrid integration, which combine SOI waveguides and plasmonic nanostructures with organic electro-optic cladding materials.

Linear guided waves in a hyperbolic planar waveguide. Dispersion relations

We have theoretically investigated waveguide modes propagating in a planar waveguide formed by a layer of an isotropic dielectric surrounded by hyperbolic media. The case, when the optical axis of hyperbolic media is perpendicular to the interface, is considered. Dispersion relations are derived for the cases of TE and TM waves. The differences in the characteristics of a hyperbolic and a conventional dielectric waveguide are found. In particular, it is shown that in hyperbolic waveguides for each TM mode there are two cut-off frequencies and the number of propagating modes is always limited.

On-chip plasmonic waveguide optical waveplate.

Polarization manipulation is essential in almost every photonic system ranging from telecommunications to bio-sensing to quantum information. This is traditionally achieved using bulk waveplates. With the developing trend of photonic systems towards integration and miniaturization, the need for an on-chip waveguide type waveplate becomes extremely urgent. However, this is very challenging using conventional dielectric waveguides, which usually require complex 3D geometries to alter the waveguide symmetry and are also difficult to create an arbitrary optical axis. Recently, a waveguide waveplate was realized using femtosecond laser writing, but the device length is in millimeter range. Here, for the first time we propose and experimentally demonstrate an ultracompact, on-chip waveplate using an asymmetric hybrid plasmonic waveguide to create an arbitrary optical axis. The device is only in several microns length and produced in a flexible integratable IC compatible format, thus opening up the potential for integration into a broad range of systems.

Discrete plasmonic Talbot effect in finite metal waveguide arrays.

We introduce supermode theory into the propagation of surface plasmon polaritons (SPPs) in nanoscale metal waveguide arrays (MWGAs). The SPP supermodes in finite MWGAs are analyzed and the coefficient of excited supermodes can be determined quantitatively. The field intensity distributions in finite MWGAs can be explained by the superposition of the excited SPP supermodes. The discrete plasmonic Talbot effect in a finite MWGA is achieved successfully by adjusting different intensity for each input field. We also find that the period condition of the input fields in MWGAs is not the same with conventional dielectric waveguides. The theory is verified by the finite difference time-domain (FDTD) method.

Dual-channel rainbow trapping of terahertz surface plasmon-polariton in graded metallic grating structures

We numerically show that slowing down and even stopping Terahertz waves can be achieved in a one-dimensional graded-grating-depth metal structure. Since the dispersion curves are dependent on the graded grating depths, waves of different wavelengths can be localized at different spatial positions of the structure, leading to rainbow trapping. Especially, dual-channel trapped rainbows with different modes can be realized in a single structure as two bands appear in the dispersion curve within the analyzed frequency range. In addition, due to the fact that the bands with different order modes do not overlap one another in the frequency domain, for each mode, its single-mode propagation can be available by properly choosing operation frequency. Such a phenomenon never happens for conventional dielectric or metal waveguides.

Label-free optical biosensors based on a planar optical waveguide

We review optical label-free biosensing platforms based on planar optical waveguides with their operation principles and performance characteristics. As the building blocks of refractometric optical transducers and plasmonic optical devices, optical planar waveguides are widely adopted to be core sensing platforms for label-free optical assay of biological and chemical interactions due to the high sensitivity with their invulnerability to external noise such as electromagnetic interference. The review scope includes the sensing schemes of conventional dielectric waveguides, the reverse symmetry waveguides, the anti-resonant reflecting optical waveguides (ARROW), and metal-clad leaky waveguides (MCLW). The configurations of the sensing systems with their operation features will be discussed together with the recent development and progress. In particular, we address in more detail the reverse symmetry waveguide and the MCLW based biosensors which have a capability of detecting micrometer-scale biological objects.

Quantum entanglement in plasmonic waveguides with near-zero mode indices

We investigate the quantum entanglement between two quantum dots (QDs) in a plasmonic waveguide with a near-zero mode index, considering the dependence of concurrence on interdot distance, QD waveguide frequency detuning, and coupling strength ratio. High concurrence is achieved for a wide range of interdot distances due to the near-zero mode index, which largely relaxes the strict requirement of interdot distance in conventional dielectric waveguides or metal nanowires. The proposed QD waveguide system with near-zero phase variation along the waveguide near the mode cutoff frequency shows very promising potential in quantum optics and quantum information processing.

Dielectric‐loaded plasmonic waveguide components: Going practical

AbstractSurface plasmon propagating modes supported by metal/dielectric interfaces in various configurations can be used for radiation guiding similarly to conventional dielectric waveguides. Plasmonic waveguides offer two attractive features: subdiffraction mode confinement and the presence of conducting elements at the mode‐field maximum. The first feature can be exploited to realize ultrahigh density of nanophotonics components, whereas the second feature enables the development of dynamic components controlling the plasmon propagation with ultralow signals, minimizing heat dissipation in switching elements. While the first feature is yet to be brought close to the domain of practical applications because of high propagation losses, the second one is already being investigated for bringing down power requirements in optical communication systems. In this review, the latest application‐oriented research on radiation modulation and routing using thermo‐optic dielectric‐loaded plasmonic waveguide components integrated with silicon‐based photonic waveguides is overviewed. Their employment under conditions of real telecommunications is addressed, highlighting challenges and perspectives.

Metal-cavity quantum-dot lasers with enhanced thermal performance

We designed, fabricated, and characterized thermal performances of Fabry-Pérot quantum-dot lasers with both metal-coated and conventional dielectric waveguides. With proper design, metals, such as Ag, Au, Cu, and Al can function as a low loss waveguide wall as well as an efficient heat remover. Metal-cavity waveguide lasers showed excellent threshold and characteristic temperature working above 120 °C, while dielectric waveguide lasers ceased operation near 80 °C under the same conditions. The thermal analysis of these lasers showed that metal-cavity lasers have approximately 1.5 times higher thermal conductivity compared with those of the dielectric lasers. We believe that the metal-coating of waveguides and the proper selection of metal efficiently remove the heat from the active region and enable stable lasing operation at high temperature.

Fully complementary metal-oxide-semiconductor compatible nanoplasmonic slot waveguides for silicon electronic photonic integrated circuits

Horizontal Al/SiO2/Si/SiO2/Al nanoplasmonic slot waveguides with the SiO2 width at each side of ∼15 nm and the Si core width of ∼136–43 nm were fabricated using a fully silicon complementary metal-oxide-semiconductor compatible technology. The propagation losses were measured to be ∼1.07–1.63 dB/μm at the telecommunication wavelength of 1550 nm, in agreement with those predicted from numerical simulation. A simple tapered coupler with length of ∼0.3–1 μm provides a high coupling efficiency of ∼−0.6–−1.5 dB between the plasmonic waveguide and the conventional Si dielectric waveguide. The plasmonic slot waveguide can achieve a low-loss ultracompact bend. A direct 90° bend was demonstrated to have the pure bending loss as low as ∼0.2–0.4 dB. The losses of propagation, coupling, and bending depend weakly on wavelength in the c-band. These results demonstrate the potential for seamless integration of functional plasmonic devices in existing silicon electronic photonic integrated circuits.