Slab waveguide theory for general multi-slot waveguide

Abstract

Optical devices based on slot waveguide are of considerable interest in numerous applications due to the distinct feature of strong electric field confinement in a low-refractive index region. A theoretical model based on multi-slab waveguide theory is used to reveal the physical mechanism of the slot waveguide. The calculation results derived from the basic Helmholtz equation for the conventional single-slot waveguide with a ~2% validation of the effective refractive index are compared to the former experiment results by the Cornell University group. Moreover, we extend the theoretical model to a general multi-slot waveguide. Its electric field distribution and key properties such as optical power confinement factor and enhancement factor in slot are deduced theoretically and fully discussed.     

Design of polarization-independent optical couplers composed of three parallel slot waveguides

The characteristics of directional couplers and power splitters based on three-guide optical couplers in slot waveguide structures are analyzed in detail by a three-dimensional full-vectorial beam propagation method. The numerical results show the achievement of a compact three-guide directional coupler operating as polarization independent with a length of 58.0 microm and having almost evenly spaced propagation constants of the three lowest order modes for quasi-TE and quasi-TM modes. Thus, a high coupling efficiency from one outside waveguide to the other outside waveguide is demonstrated and is over 99.5% for both polarization states. For a three-guide power splitter, multiple sets of waveguide parameters for achieving polarization-independent operation are presented. Tolerances to operating wavelength and structural parameters are also analyzed, and the evolution of the injected field along the propagation distance through the proposed devices is demonstrated.     

Effect of intensity dependent higher-order dispersion on femtosecond pulse propagation in quantum well waveguides

We have studied theoretically the dispersion of ultrafast coherent pulses in GaAs/AlGaAs quantum well waveguide structures as a function of optical intensity. Semiconductor Bloch equations are used to obtain the polarization induced in the medium due to an incident Gaussian electromagnetic beam. The partial differential equation describing the pulse propagation in the presence of group velocity dispersion is used to analyze the role of higher-order dispersion on femtosecond pulse propagation in the waveguide. Due consideration has been given to the intensity dependent optical susceptibility of the medium. The results of the numerical analysis manifest significant influence of higher-order dispersion on femtosecond pulse propagation over short waveguide distance.     

Power Absorption Efficiency in Superconducting Box-Shaped Optical Waveguides

A very sensitive superconducting traveling wave photodetector made of a modified box-shaped waveguide, which includes two high index layers and an active superconducting layer, is studied. The optical propagation constants and the power absorption efficiency for guided modes are determined using the finite element method; the results show that by acting only on the waveguide geometry, different confinement regimes of the light in the absorption superconducting layer can be achieved and optimized.     

Sensitivity of Traveling Wave Photodetectors in Superconducting Box-Shaped Plasmon-Polariton Optical Waveguides

A very sensitive superconducting traveling wave photodetector made of a modified box-shaped waveguide, which includes two high index layers and an active superconducting layer, is studied. The optical propagation constants and the power absorption efficiency for guided modes are determined using the finite element method; the results show that by acting only on the waveguide geometry, different confinement regimes of the light in the absorption superconducting layer can be achieved and optimized.     

Bright,dark, and singular solitons in optical fibers with spatio-temporal dispersion and spatially dependent coefficients

Abstract

Investigated in this work is the nonlinear Schrödinger equation with space-dependent parameters, which models the propagation of optical solitons in spatially inhomogeneous optical fiber with detuning, spatiotemporal dispersion, intermodal dispersion, and fiber gain or loss. Through the ansatz scheme, analytical bell, kink, and singular soliton solutions under certain coefficient constraints are obtained.     

A lossless negative dielectric constant from quantum dot exciton polaritons

Prospects for a lossless negative dielectric constant material for optical devices are studied. Simulations show that with sufficient gain, a mixture of two semiconductor quantum dots (QDs) can produce an effective dielectric constant that is lossless and negative. This permits, in concept, arbitrarily small scaling of the optical mode volume, a major goal in the field of nanophotonics. The proposed implementation of a lossless negative dielectric constant material based on colloidal QDs opens a tractable path.     

Non-paraxial Propagation of Gaussian Beam Rays in Active Graded-index Waveguides with Gain or Losses

Non-paraxial propagation of Gaussian beam rays in active multi-mode graded-index waveguides with polynomial refractive index profile is investigated analytically using algebraic perturbation theory and the generalized coherent states formalism. Explicit expressions are obtained for the beam ray trajectories and widths in a waveguide with gain or losses. The influence of the gain or losses of the waveguide on the recently found effect of large-scale periodical revival (reconstruction) of initial beam field distribution is investigated in detail. The results obtained may be useful for large distance image transmission through optical fibres.     

Multi-channel terahertz wavelength division demultiplexer with defects-coupled photonic crystal waveguide

Abstract

Terahertz (THz) wavelength division demultiplexer based on a compact defects-coupled photonic crystal waveguide is proposed and demonstrated numerically. This device consists of an input waveguide that perpendicularly coupled with a series of defects cavities, each of which captures the resonance frequency from the input waveguide. Coupled-mode theory and finite element method are used to analyze the transmission properties of the structure. It is found that the transmission wavelength centered around 1 THz can be adjusted by changing the geometrical parameters of defects cavities, which equals to THz waves generated by optical methods such as difference frequency generation and optical rectification. Applications in this frequency range are urgently needed. Furthermore, the highest transmission efficiency of 0.94 can be achieved when a perfect wavelength-selective mirror is set in the output waveguide.     

A Simple Model of Gain Saturation in Homogeneously CW Lasers with Distributed Losses

Abstract

A simple approximate method for the analysis of two-mirror lasers including longitudinal electric field dependence, distributed loss and point losses at the mirrors is presented. Two kinds of the approximate expression are obtained with the help of the energy theorem and the threshold field approximation which require only a programmable calculator for appropriate calculation. They relate the normalized small signal gain in the active medium to laser parameters and they are found to be in a good agreement with exact solutions. A stable optical resonator and a homogeneously broadened medium is assumed.     

Geometries and materials for subwavelength surface plasmon modes

Plasmonic waveguides can guide light along metal-dielectric interfaces with propagating wave vectors of greater magnitude than are available in free space and hence with propagating wavelengths shorter than those in vacuum. This is a necessary, rather than sufficient, condition for subwavelength confinement of the optical mode. By use of the reflection pole method, the two-dimensional modal solutions for single planar waveguides as well as adjacent waveguide systems are solved. We demonstrate that, to achieve subwavelength pitches, a metal-insulator-metal geometry is required with higher confinement factors and smaller spatial extent than conventional insulator-metal-insulator structures. The resulting trade-off between propagation and confinement for surface plasmons is discussed, and optimization by materials selection is described.     

Enhanced Exciton and Photon Confinement in Ruddlesden–Popper Perovskite Microplatelets for Highly Stable Low‐Threshold Polarized Lasing

At the heart of electrically driven semiconductors lasers lies their gain medium that typically comprises epitaxially grown double heterostuctures or multiple quantum wells. The simultaneous spatial confinement of charge carriers and photons afforded by the smaller bandgaps and higher refractive index of the active layers as compared to the cladding layers in these structures is essential for the optical‐gain enhancement favorable for device operation. Emulating these inorganic gain media, superb properties of highly stable low‐threshold (as low as ≈8 µJ cm^?2) linearly polarized lasing from solution‐processed Ruddlesden–Popper (RP) perovskite microplatelets are realized. Detailed investigations using microarea transient spectroscopies together with finite‐difference time‐domain simulations validate that the mixed lower‐dimensional RP perovskites (functioning as cladding layers) within the microplatelets provide both enhanced exciton and photon confinement for the higher‐dimensional RP perovskites (functioning as the active gain media). Furthermore, structure–lasing‐threshold relationship (i.e., correlating the content of lower‐dimensional RP perovskites in a single microplatelet) vital for design and performance optimization is established. Dual‐wavelength lasing from these quasi‐2D RP perovskite microplatelets can also be achieved. These unique properties distinguish RP perovskite microplatelets as a new family of self‐assembled multilayer planar waveguide gain media favorable for developing efficient lasers.     

On the Design of Dielectric Strip Plasmonic Structures for Subwavelength Waveguiding Applications

In this paper, we develop an efficient method for designing the geometry of dielectric strip plasmonic structures for future subwavelength waveguiding applications. Leveraging an efficient finite-difference field solver, we investigate the impact of dielectric strip geometry on propagation loss and spatial light confinement. We demonstrate that a dielectric strip embedded in a metallic medium can support single-mode propagation while achieving subwavelength light confinement. We then formulate and solve the dielectric strip design optimization problem to ensure monomode propagation while balancing propagation losses and light confinement. The results indicate that the design technique can locate optimal dielectric strip geometries more efficiently than enumerating over the complete design space, which is crucial for the realization of subwavelength on-chip waveguiding applications.     

Modeling liquid-crystal devices with the three-dimensional full-vector beam propagation method

Simulation of light propagation within nematic liquid-crystal (LC) devices is considered, of which the director is aligned normal to the z axis. A three-dimensional full-vector finite-difference beam propagation method for an anisotropic medium is presented and an alternating direction implicit scheme is adopted. Simulations of light propagation in a bulk polarization converter, a waveguide with a LC covering layer, and an integrated polarization splitter and optical switch are presented. Comparison with an existing simulation method is carried out for beam behavior within the bulk polarization converter. The effect of strong surface anchoring of a LC cell on the beam behaviors within the integrated switch is also demonstrated.     

Numerical analysis of step-index channel waveguides designed with a formally-equivalent separable index profile

Abstract

A numerical test is proposed to evaluate the separability of a modal field supported by an inverted-rib waveguide, which was properly designed to make its general non-separable index profile formally equivalent to a separable one. The possibility of obtaining such a separable equivalent profile leads to much more simplicity in the design of channel waveguides and other more complicated integrated devices, since it is possible to obtain a quasi-exact solution to the modal propagation problem by using the effective index method. The influence of the different waveguide parameters on the field separability is also investigated in order to evaluate the flexibility of the design method.     

Cylindrical hybrid plasmonic waveguide for subwavelength confinement of light

A novel cylindrical hybrid plasmonic waveguide is proposed to achieve subwavelength confinement of light. With a metal core surrounded by a silica layer and a silicon layer, the proposed cylindrical hybrid plasmonic waveguide can achieve a ring-structure mode profile at the operating wavelength (1550 nm). Most mode power locates in the silica layer with a nanoscale thickness (e.g., 50, 20, or even 5 nm), which is due to the effects of both a strong discontinuity of the normal component of the electric field at the silicon-silica interface and the exited surface plasmon wave at the silica-metal interface. Cylindrical hybrid plasmonic waveguides with different structure parameters are investigated and a relatively long propagation distance of tens of micrometers (or even hundreds of micrometers) is observed.     

Magnetophotonic crystals: nonreciprocity,birefringence and confinement

Photonic crystals in magnetic materials open up a number of interesting possibilities for the study of nonreciprocal effects in confined geometries, including enhanced Faraday rotation and optical unidirectionality. The development of integrated devices based on nonreciprocal magneto-optic phenomena requires understanding the effects of geometrical confinement on light propagation and magnetization in these systems. This article introduces a model for controlling the band gap by exploiting the birefringence in film-based planar magneto-optic structures. It also presents a study of magnetic remanence control through magnetization confinement in the resonant microcavity of one-dimensional planar magnetophotonic crystals.     

Derivation of dimensional and material requirements for propagation and processing of temporal optical solitons in planar geometry channel waveguides

We investigate the dispersion of the group velocity of light in slab and rectangular waveguides and obtain analytic expressions for the first derivative of the propagation vector with respect to the angular frequency and numerical values for the second derivative for both geometries. The last quantity is an important parameter in the temporal soliton propagation equation, which motivates this research. Provided the dispersion within the channel can be adjusted properly, planar geometry waveguides can represent good candidates for the optical processing of temporal solitons carried by optical fibers. We discuss the manner in which the dispersion coefficient depends on waveguide dimensions and material constants, and we determine the parameters that optimize soliton processing.     

High-index-contrast waveguides and devices

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.     

Efficient All-Optical Plasmonic Modulators with Atomically Thin Van Der Waals Heterostructures

All-optical modulators are attracting significant attention due to their intrinsic perspective on high-speed, low-loss, and broadband performance, which are promising to replace their electrical counterparts for future information communication technology. However, high-power consumption and large footprint remain obstacles for the prevailing nonlinear optical methods due to the weak photon–photon interaction. Here, efficient all-optical mid-infrared plasmonic waveguide and free-space modulators in atomically thin graphene-MoS₂ heterostructures based on the ultrafast and efficient doping of graphene with the photogenerated carrier in the monolayer MoS₂ are reported. Plasmonic modulation of 44 cm⁻¹ is demonstrated by an LED with light intensity down to 0.15 mW cm⁻², which is four orders of magnitude smaller than the prevailing graphene nonlinear all-optical modulators (≈10³ mW cm⁻²). The ultrafast carrier transfer and recombination time of photogenerated carriers in the heterostructure may achieve ultrafast modulation of the graphene plasmon. The demonstration of the efficient all-optical mid-infrared plasmonic modulators, with chip-scale integrability and deep-sub wavelength light field confinement derived from the van der Waals heterostructures, may be an important step toward on-chip all-optical devices.