Dielectric Optical Waveguides Research Articles

Compact and efficient lossy mode resonant refractive index sensor for aqueous environment

A very short length high sensitivity, large figure of merit and very high resolution integrated-photonic refractometer for aqueous environment operating in visible region of wavelength is proposed. The sensor design depends on the periodic coupling of the guided dielectric optical waveguide mode field and the lossy mode field of the conducting indium-tin-oxide thin film. Various layer thicknesses can be optimized to provide power transfer to the lossy layer and lossy mode resonance, resulting in a strong guided mode power absorption in the lossy layer occurs. The sensor has been designed to operate in both the TE and TM polarizations with different optimized layer frthicknesses. The optimized thicknesses are different for TE and TM polarizations. The obtained numerical results show that a spectral sensitivity of 2200 nm/RIU/2645 nm/RIU for TE/TM mode could be achieved with a very high resolution. Also, the sensors can operate in power interrogation mode with a maximum sensitivity nearly 5 × 107 dB RIU−1.

Weak optical modes for high-density and low-loss photonic circuits

Dielectric optical waveguides constitute the main building blocks of photonic integrated circuits (PICs). Channels with high refractive index contrast can provide very compact PIC components, whereas structures with lower index exhibit less propagation loss. A hybrid concept that can combine the best of high- and low-index materials is highly required. Here, we devise a new approach to realize compact and low-loss hybrid optical waveguides based on the interaction of weak optical modes. This is a rather universal approach that can be applied to a wide range of optical materials. To prove the principle, the hybrid waveguide structure is formed by combining a low-index polymer and a thin layer of silicon nitride. For this material combination, a minimum bending radius of 90 µm (for a bending loss of 0.005 dB/90°) and a propagation loss of 0.7 dB/cm are achieved. The viability of this platform is demonstrated through a series of high-performance novel PIC components. This hybrid waveguide platform enabled by a powerful and simple design concept holds great promise for high-density and low-loss PICs.

Creation and preservation of superoscillation in a dielectric optical waveguide.

Superoscillation refers to a phenomenon where a band-limited wave locally oscillates faster than its highest Fourier component. Current research on optical superoscillations predominantly lies on the basis of free-space waves. As the optical waveguides play a key role in energy and information transportation, guided waves with precisely controlled deep-subwavelength features offers unprecedented flexibility for applications. In this Letter, we numerically show that, by superimposing eigenmodes of a multimode SiO2 waveguide and forcing the resultant field to pass through a set of predetermined points, superoscillatory fields in various shapes can be formed in preset cross-sectional planes. Furthermore, by padding prescribed intensities in multiple cross sections, we successfully create a persistent superoscillatory saddle.

All-optical switch with ultrahigh switching efficiency and ultralow threshold energy based on a one-dimensional PT-symmetric ring optical waveguide network

In this work, we propose an all-optical switch based on one-dimensional (1D) parity-time-symmetric (PT-symmetric) ring optical waveguide network (PTSROWN). By use of the resonant coupling effect of the PT-symmetric structure, the all-optical switch based on our designed 1D PTSROWN generates an ultrahigh transmissivity of 1.42×1018 at ON state, which breaks the bottleneck of the transmissivity of 1 at ON state produced by the all-optical switches made of dielectric optical waveguide networks, photonic crystals, microring resonators, and meanwhile, creates an ultralow transmissivity of 1.01×10−18 at OFF state. It causes our designing all-optical switch reproducing extraordinary large switch efficiency of 1.41×1036, which is 10 orders of magnitude greater than the optimal results reported yet. On the other hand, the super resonant coupling effects can also make the all-optical switch generate ultrastrong photonic localization at ON state, and then drastically reduce the threshold energy. In addition, we have deeply optimized the PT-symmetric optical waveguide network and obtain better extremum spontaneous PT-symmetric breaking points. On this condition, our designing all-optical switch can produce an ultralow threshold energy of 4.3×10−34 J, which is 12 orders of magnitude smaller than the best reported results. This not only provides a better choice for developing all-optical switching devices with better performance, but also possesses a broad application prospect in designing high-efficiency optical amplifiers, optical energy saver devices, etc.

Local laser oxidation of titanium film for the post-fabrication trimming of photonic integrated circuits

Local laser oxidation of a thin titanium film is considered as a means of a precise adjustment of losses and effective refractive index of dielectric optical waveguides. A fine phase control of an operating point and extinction ratio enhancement up to 57 dB were demonstrated using an integrated optical Ti:LiNbO3 Mach-Zehnder modulator. This technique only slightly affects the dielectric waveguide material and is very promising for a high precision permanent trimming of photonic devices based on dielectric waveguides of different material platforms and fabrication technologies.

Plasmonic-Induced Transparencies in an Integrated Metaphotonic System.

In this contribution, we numerically demonstrate the generation of plasmonic transparency windows in the transmission spectrum of an integrated metaphotonic device. The hybrid photonic–plasmonic structure consists of two rectangular-shaped gold nanoparticles fully embedded in the core of a multimode dielectric optical waveguide, with their major axis aligned to the electric field lines of transverse electric guided modes. We show that these transparencies arise from different phenomena depending on the symmetry of the guided modes. For the TE mode, the quadrupolar and dipolar plasmonic resonances of the nanoparticles are weakly coupled, and the transparency window is due to the plasmonic analogue of electromagnetically induced transparency. For the TE mode, the quadrupolar and dipolar resonances of the nanoparticles are strongly coupled, and the transparency is originated from the classical analogue of the Autler–Townes effect. This analysis contributes to the understanding of plasmonic transparency windows, opening new perspectives in the design of on-chip devices for optical communications, sensing, and signal filtering applications.

Full-Vector Analysis of Bending Waveguides by Using Meshless Finite Cloud Method in a Local Cylindrical Coordinate System

By using a meshless finite cloud method, a full-vector mode solver in terms of transverse-magnetic-field components for optical dielectric waveguide bends is developed in a local cylindrical coordinate system. To construct the interpolation function, both the point collocation and the fixed reproducing kernel technique are utilized, and then with the help of the continuity conditions of the longitudinal field components and the perfectly matched layer absorbing boundary conditions, a standard matrix eigenvalue is obtained. Compared with the conventional meshed numerical techniques, the distributions of solution points of the present meshless method can be applied to the area of complexity in a completely free manner. Consequently, excellent convergence with high accuracy can be realized. Moreover, the present method can be applied to both the uniform and non-uniform nodal distributions, thus the computational procedure is relatively flexible. To validate the proposed method, both fundamental and high-order leaky modes for a typical rib bending waveguide are analyzed and their modal field distributions and corresponding effective refractive indexes are presented. Results are in good agreement with those published earlier, showing the effectiveness of the present method.

In Situ Monitoring of Rolling Circle Amplification on a Solid Support by Surface Plasmon Resonance and Optical Waveguide Spectroscopy.

The growth of surface-attached single-stranded deoxyribonucleic acid (ssDNA) chains is monitored in situ using an evanescent wave optical biosensor that combines surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The "grafting-from" growth of ssDNA chains is facilitated by rolling circle amplification (RCA), and the gradual prolongation of ssDNA chains anchored to a gold sensor surface is optically tracked in time. At a sufficient density of the polymer chains, the ssDNA takes on a brush architecture with a thickness exceeding 10 μm, supporting a spectrum of guided optical waves traveling along the metallic sensor surface. The simultaneous probing of this interface with the confined optical field of surface plasmons and additional more delocalized dielectric optical waveguide modes enables accurate in situ measurement of the ssDNA brush thickness, polymer volume content, and density gradients. We report for the first time on the utilization of the SPR/OWS technique for the measurement of the RCA speed on a solid surface that can be compared to that in bulk solutions. In addition, the control of ssDNA brush properties by changing the grafting density and ionic strength and post-modification via affinity reaction with complementary short ssDNA staples is discussed. These observations may provide important leads for tailoring RCA toward sensitive and rapid assays in affinity-based biosensors.

Circular Optical Arrays Using Waveguide Fed 45° Angled Mirrors

An array of optical waveguide fed 45° metal-coated mirrors are proposed and validated by simulation, for generating optical scalar and vector vortices. Subwavelength element-to-element spacings are achieved, which is not possible in traditional grating-based phased arrays. This vortex source can be superior over other methods like phase masks which are not planar process integrable with lasers and phase modulators. A simple plane wave reflection model is used to analyze the mirror and the results are matched with reasonable tolerance against simulation results. The spin and orbital angular momentum charges are directly calculated from the field values for quantifying the performance. Four- and eight-element arrays are studied. The matching of prediction with simulation has been good for linear, azimuthal and radial polarization. Good angular momentum conversion efficiencies are obtained for up to orbital angular momentum charges ${l}$ = ±2. Important aspects of generating vortices with circular polarization are brought out. Directional beams with >10 dB gains are achieved. This is the first report in which a planar dielectric optical waveguide array with subwavelength spacing has achieved good angular momentum control for linearly, azimuthally, radially and circularly polarized beams, in the same geometry.

Hybrid 2D‐Material Photonics with Bound States in the Continuum

AbstractIntegration of 2D materials on dielectric planar optical waveguides can make available new functionalities from the 2D materials' enhanced optoelectronic properties, such as nonlinearity, light emission, modulation, photodetection, and saturable absorption. However, the conventional integration schemes involving either the transfer of 2D materials onto prepatterned nonplanarized topology of photonic integrated circuits (PICs) or the growth and patterning of dielectric materials on 2D materials can degrade the properties of either the dielectric or the 2D material. Here, a fundamentally new and practical scheme is introduced for integrating 2D materials with PICs on a planar surface. The scheme can maintain large optical overlap with the 2D material by harnessing bound states in the continuum. This approach applies to integration of any 2D material on any single‐crystal dielectric substrate, and inherently offers strong light–matter interactions. Here, the approach is demonstrated with various hybrid photonic devices including thermo‐optic switches and filters, ultrafast photodetectors with 40‐GHz detection bandwidth, and ultrafast electro‐optic modulators with 5‐GHz modulation bandwidth. Therefore, the new platform will significantly expand the hybrid integration between 2D materials and single‐crystal dielectrics to achieve unexpected photonic functionalities.

Hollow-Core Fiber Technology: The Rising of “Gas Photonics”

Since their inception, about 20 years ago, hollow-core photonic crystal fiber and its gas-filled form are now establishing themselves both as a platform in advancing our knowledge on how light is confined and guided in microstructured dielectric optical waveguides, and a remarkable enabler in a large and diverse range of fields. The latter spans from nonlinear and coherent optics, atom optics and laser metrology, quantum information to high optical field physics and plasma physics. Here, we give a historical account of the major seminal works, we review the physics principles underlying the different optical guidance mechanisms that have emerged and how they have been used as design tools to set the current state-of-the-art in the transmission performance of such fibers. In a second part of this review, we give a nonexhaustive, yet representative, list of the different applications where gas-filled hollow-core photonic crystal fiber played a transformative role, and how the achieved results are leading to the emergence of a new field, which could be coined “Gas photonics”. We particularly stress on the synergetic interplay between glass, gas, and light in founding this new fiber science and technology.

Formation of Bound States in the Continuum in Hybrid Plasmonic-Photonic Systems.

A bound state in the continuum (BIC) is a localized state of an open structure with access to radiation channels, yet it remains highly confined with, in theory, an infinite lifetime and quality factor. There have been many realizations of such exceptional states in dielectric systems without loss. However, realizing BICs in lossy systems such as those in plasmonics remains a challenge. In this Letter, we explore the possibility of realizing BICs in a hybrid plasmonic-photonic structure consisting of a plasmonic grating coupled to a dielectric optical waveguide with diverging radiative quality factors. The plasmonic-photonic system supports two distinct groups of BICs: symmetry-protected BICs at the Γ point and off-Γ Friedrich-Wintgen BICs. The photonic waveguide modes are strongly coupled to the gap plasmons in the grating, leading to an avoided crossing behavior with a high value of Rabi splitting of 150meV. Moreover, we show that the strong coupling significantly alters the band diagram of the hybrid system, revealing opportunities for supporting stopped light at an off-Γ wide angular span.

Modeling of evanescent-wave coupling between optical dielectric waveguides

The transverse coupling between the optical dielectric waveguides is modeled and analyzed using the Improved Coupled Mode Theory. The coupling between the modes is established via the evanescent part of the modal field propagating in one waveguide which penetrates the adjacent waveguide. The coupling phenomena is analyzed numerically by evaluating the average overlap integrals, coupling coefficients, asynchronism factor, and coupling length using the Improved Coupled Mode Theory at strong and weakly guiding conditions. The performance of the Improved Coupled Mode Theory is tested and compared to the conventional Coupled Mode Theory.

Factor of spontaneous emission into the optical dielectric waveguide mode

Within the framework of a complete vector analysis, an expression for the factor of spontaneous emission into the transverse mode of a dielectric waveguide is found. The results are compared with those previously obtained in the framework of the scalar approximation. It is shown that their most significant quantitative difference is possible for waveguides used in diode lasers and amplifiers in which the transverse beam size is commensurable with the wavelength. The conditions for the applicability of the scalar approximation for calculating the spontaneous emission factor are obtained.

Extremely Short Length and High Extinction Ratio Plasmonic TE Mode Pass Polarizer

Extremely short length and ultrahigh extinction ratio TE pass polarizer is proposed for wavelength $1.55\mu \text{m}$ . The polarizer section consists of an optical dielectric waveguide separated from the thin metal film plasmonic guide by a low index buffer layer. The thin film metal layer supports only Short Range Surface Plasmon Polariton (SRSPP) because effective indices below and above the metal film are different. Periodic coupling between the SRSPP mode and the optical waveguide mode takes place and varies with the length of the polarizer. It is shown that for a properly optimized metal and the buffer layer thicknesses, an extinction ratio of 223 dB can be achieved for a polarizer of length $4.6~\mu \text{m}$ . Dependence of polarizer length on the optimized buffer and metal layers is also discussed.

Surface plasmon polariton mode in the metal-insulator-metal waveguide

Metal-insulator-metal (MIM) waveguides with subwavelength apertures exhibit unique properties that are considered breakthroughs in surface plasma technology. In this paper, we established a surface plasmon polariton (SPP) mode of MIM structure as well as a theoretical model of reflectivity and excitation coefficients to clarify the SPP mode structure. During simulation calculations, attenuation of the TM ground state mode with large-thickness dielectrics is over the oscillation mode, which is obviously different from the traditional dielectric waveguide. The TE mode exhibits characteristics of anamorphic dielectric optical waveguides, and its propagation distance is longer than that of the TM0 mode. With increasing length, the cavity quality factor Q increases, which indicates that the SPP reflection is limited. The quality factor of the cavity and the end-face have a close relationship. MIM waveguides with larger Q values can produce optical waves coupled with the SPP mode. In this paper, we analyzed SPP propagation constants and attenuation coefficients to promote the development of an integrated surface plasmon.

Fiberless multicolor neural optoelectrode for in vivo circuit analysis.

Maximizing the potential of optogenetic approaches in deep brain structures of intact animals requires optical manipulation of neurons at high spatial and temporal resolutions, while simultaneously recording electrical data from those neurons. Here, we present the first fiber-less optoelectrode with a monolithically integrated optical waveguide mixer that can deliver multicolor light at a common waveguide port to achieve multicolor modulation of the same neuronal population in vivo. We demonstrate successful device implementation by achieving efficient coupling between a side-emitting injection laser diode (ILD) and a dielectric optical waveguide mixer via a gradient-index (GRIN) lens. The use of GRIN lenses attains several design features, including high optical coupling and thermal isolation between ILDs and waveguides. We validated the packaged devices in the intact brain of anesthetized mice co-expressing Channelrhodopsin-2 and Archaerhodopsin in pyramidal cells in the hippocampal CA1 region, achieving high quality recording, activation and silencing of the exact same neurons in a given local region. This fully-integrated approach demonstrates the spatial precision and scalability needed to enable independent activation and silencing of the same or different groups of neurons in dense brain regions while simultaneously recording from them, thus considerably advancing the capabilities of currently available optogenetic toolsets.

Bound states in the continuum

Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work. The fascinating wave phenomenon of ‘bound states in the continuum’ spans different material and wave systems, including electron, electromagnetic and mechanical waves. In this Review, we focus on the common physical mechanisms underlying these bound states, whilst also discussing recent experimental realizations, current applications and future opportunities for research.

Trapping light into high orbital momentum modes of fiber tapers.

A tapered cylindrical dielectric optical waveguide acts as a high quality factor white-light cavity providing high field concentration as well as long optical group delay. It is possible to optimize shape of a lossless taper to suppress reflection of the input light and to achieve infinitely high field concentration. These tapers can be used in sensing and optoelectronics applications instead of conventional microcavities.

Fabrication and characterization of Er+3 doped SiO2/SnO2 glass-ceramic thin films for planar waveguide applications

Glass-ceramics are a kind of two-phase materials constituted by nanocrystals embedded in a glass matrix and the respective volume fractions of crystalline and amorphous phase determine the properties of the glass-ceramics. Among these properties transparency is crucial in particular when confined structures, such as, dielectric optical waveguides, are considered. Moreover, the segregation of dopant rare-earth ions, like erbium, in low phonon energy crystalline medium makes these structures more promising in the development of waveguide amplifiers. Here we are proposing a new class of low phonon energy tin oxide semiconductor medium doped silicate based planar waveguides. Er3+ doped (100-x) SiO2-xSnO2 (x= 10, 20, 25 and 30mol%), glass-ceramic planar waveguide thin films were fabricated by a simple sol-gel processing and dip coating technique. XRD and HRTEM studies indicates the glass-ceramic phase of the film and the dispersion of ~4nm diameter of tin oxide nanocrystals in the amorphous phase of silica. The spectroscopic assessment indicates the distribution of the dopant erbium ions in the crystalline medium of tin oxide. The observed low losses, 0.5±0.2 dB/cm, at 1.54 μm communication wavelength makes them a quite promising material for the development of high gain integrated optical amplifiers.