Waveguide Geometry Research Articles

Robust modeling of acoustic phonon transmission in nanomechanical structures

The transmission of acoustic phonons is an important element in the design and performance of nano-mechanical devices operating in the mesoscopic limit. Analytic expressions for the power transmission coefficient, T, exist only in the low-frequency limit, in cases described by thin-plate elasticity theory, and for well-defined elastic waveguiding geometries. We compare two numerical techniques based on finite-element computations to determine the frequency dependence of T for arbitrary phonon scattering structures. Both methods take into account acoustic mode conversion to acoustic and optical modes. In one case, the phase and amplitude of complex-valued reflected waves are determined and related to transmission through a Fresnel equation, while in the other, the magnitude of the transmitted mechanical power is directly calculated. The numerical robustness of these methods is demonstrated by considering the transmission across an abrupt junction in a rectangular elastic beam, a well-known problem of considerable importance in mesoscopic device physics. The simulations presented extend the standard results for acoustic phonon transmission at an abrupt junction, and are in good agreement with analytic predictions in the long-wavelength limit. More generally, the numerical methods developed provide an effective tool for calculating acoustic mode energy loss in nano-mechanical resonators through mode conversion and heat transfer in arbitrary mesoscopic structures.

Photonic devices based on optical forces

A self-aligned photonic coupler, consisting of a tapered fiber and a suspended rectangular waveguide, is presented as a new approach for optical interfacing from SMF fiber domain to integrated photonic domain. The alignment occurs due to optical forces and a theoretical maximum transmitance of -1.4dB as well as significant attractive optical force range of 1.25μm for 1mW of input power are demonstrated. For a non-optimized geometry of 660nm taper diameter and 240x220nm waveguide, this result already matches transmitance for typical approaches for this interfacing with advantages of coarser alignment complexity and polarization and bandwidth flexibility.

Crosstalk Suppressed High Efficient Mode-Selective Four-Wave Mixing Through Tailoring Waveguide Geometry

We propose and demonstrate improved and accurate design criteria for mode-selective nonlinear waveguides, using a complete multimode theoretical model. Being different from previously reported schemes where only the cross section was considered, the nonlinear conversion efficiency can be further enhanced in the proposed scheme by means of elaborately designing both the waveguide length and cross section. Furthermore, the modal crosstalks introduced by the intermode nonlinear processes are thoroughly discussed and significantly suppressed. A two-mode nonlinear circuit using asymmetric directional coupler based multiplexer/de-multiplexer and an optimized multimode waveguide is fabricated for demonstration, utilizing a silicon integrated platform. Experimental results show that the highest conversion efficiencies of fundamental transverse electric and first order transverse electric modes are −20.44 and −23.15 dB, respectively, with nonlinear crosstalks less than −19 dB.

Dispersion-engineered silicon nitride waveguides for mid-infrared supercontinuum generation covering the wavelength range 0.8–6.5 μm

We numerically demonstrate the generation of a mid-infrared supercontinuum (SC) through the design of an on-chip complementary metal oxide semiconductor compatible 10 mm-long air-clad rectangular waveguide made using stoichiometric silicon nitride (Si3N4) as the core and MgF2 glass as the lower cladding. The proposed waveguide is designed for pumping in both the anomalous and all-normal dispersion regimes. A number of waveguide geometries are tailored for pumping at 1.55 m with ultrashort pulses of 50 fs duration and a peak power of 5 kW. By initially keeping the thickness constant at 0.8 m, four different structures are engineered with varying widths between 3 m and 6 m. The largest SC spectral evolution covering a region of 0.8 m to beyond 6.5 m could be realized by a waveguide geometry with a width of 3 m. Numerical analysis shows that increasing width beyond 3 m while keeping thickness constant at 0.8 m results in a reduction of the SC extension in the long wavelength side. However, the SC spectrum can be enhanced beyond 6.5 m by increasing the waveguide thickness beyond 0.9 m with the same peak power and pump source. To the best of our knowledge, this is the first report of a broad SC spectral evolution through numerical demonstration in the mid-infrared region by the Si3N4 waveguide. In the case of all-normal dispersion pumping, a flatter SC spectra can be predicted with the same peak power and pump pulse but with a reduced bandwidth spanning 950–2100 nm.

Demonstration of stimulated photon echo isolation using interferometric cancellation.

An alternative to phase-matched angled beam spatial-spectral holographic grating geometries for separating stimulated photon echoes (SPEs) from the probing pulse is proposed and demonstrated. By use of a Mach-Zehnder geometry with inhomogeneously broadened medium in both paths, the SPE can be interferometrically isolated from the generating probe pulse. This interferometric-based technique is well suited to waveguide geometries, which have benefits for future quantum and classical optical signal processing applications such as quantum memories, correlation, and efficient cryogenic microwave-to-optical conversion. Experimental demonstrations showing interferometric-based isolation in Tm3+-doped LiNbO3 at 3.2K are performed and analyzed.

Effect of Planar Waveguide Geometry on the Formation of Optical Bullets

The formation of optical bullets in propagating light beam pulses inside a planar waveguide with quadratic nonlinearity is studied. Cases of a waveguide with saturation and an unlimited waveguide with a quadratic profile of the change in the refractive index are considered.

Wide subwavelength grating waveguide photonic integrated circuit sensor system

We are studying the viability of a chip-scale integrated photonics based sensor assay array for wearable soldier health monitoring and environmental hazard warning applications. We propose using label-free analyte capture material clad wide subwavelength grating (SWG) waveguides as the sensor elements in a multiplexed array to probe the myriad variety and concentration ranges of chemical hazards and biomarker health molecules expected in these applications. We present here the simulated behavior of both rectangular and tapered wide SWG geometries. We discuss their transmission spectra, effective index, bulk sensitivity, and cladding dependence on waveguide width alongside strip and slot waveguide geometries. Experimental transmission and effective index measurements validating our simulation techniques are also given.

Modal classification in optical waveguides using deep learning

ABSTRACTSingle-mode operation is crucial in many on-chip integrated photonic devices, and thus the identification of single-mode geometries is an inevitable design requirement. In this article, we develop deep learning (DL) models for ultra-quick classifications of optical waveguide geometries into single- and multi-modal geometries. The DL model accurately predicts the boundary in the parameter space for the geometry of the waveguide that splits the space into single- and multi-modal regions. Using silicon nitride channel waveguide, and targeting both visible and telecommunication wavelengths, we illustrate how DL models can be developed with a minimal number of exact numerical simulations to Maxwell’s equations.

A Broadband Circularly Polarized Wide-Slot Antenna With a Miniaturized Footprint

This letter presents a novel and simple feeding technique for exciting orthogonal components in a wide-slot antenna. In this technique, a rectangular bracket-shape parasitic strip is placed at the open end of the straight microstrip line to excite the fundamental horizontal and vertical components of the circular polarization (CP). The proposed technique—when employed in conjunction with the asymmetrical geometry of coplanar waveguide and a protruded stub from the ground plane—permits for maintaining axial ratio (AR) below 3 dB within a wide range in the C-band as well as a compact footprint of the antenna. All geometry parameters of the antenna are adjusted through rigorous EM-driven optimization to obtain the best performance in terms of footprint, impedance matching, and AR bandwidth (ARBW). The size of the proposed antenna is only 27 mm × 28.8 mm. The structure features a 62% impedance bandwidth (3.6–6.85 GHz) and ARBW of approximately 49% (3.6–5.93 GHz) with the average realized gain of 3.3 dB within the CP operating band. Numerical results are validated experimentally. A close agreement between simulation and measurement results has been observed.

Metallic Slit–Loaded Ring Resonator–Based Plasmonic Demultiplexer with Large Crosstalk

Photonics provides a key solution to limit the myriads of challenges offered by existing silicon technology. Here, we propose a simple 2-channel Plasmonic demultiplexer with Metal-Insulator-Metal (MIM) waveguide geometry. The demultiplexer is capable to separate two fundamental optical windows (i.e., 1310 nm and 1550 nm). An FDTD (finite difference time domain) method is used to investigate the structure numerically. The optical properties of the demultiplexer are also investigated. The performance of the proposed demultiplexer is measured using crosstalk and quality factor. The minimum value of crosstalk is − 30 dB and maximum quality factor is 60. The maximum transmission is limited to 45% due to large metallic losses. The proposed demultiplexer is ultra-compact in size and is capable to provide sub-wavelength confinement. It is suited for WDM applications and can pave a way to design the photonic integrated circuits.

Analytic Mode Normalization for the Kerr Nonlinearity Parameter: Prediction of Nonlinear Gain for Leaky Modes.

Based on the resonant-state expansion with analytic mode normalization, we derive a general master equation for the nonlinear pulse propagation in waveguide geometries that is valid for bound and leaky modes. In the single-mode approximation, this equation transforms into the well-known nonlinear Schrödinger equation with a closed expression for the Kerr nonlinearity parameter. The expression for the Kerr nonlinearity parameter can be calculated on the minimal spatial domain that spans only across the regions of spatial inhomogeneities. It agrees with previous vectorial formulations for bound modes, while for leaky modes the Kerr nonlinearity parameter turns out to be a complex number with the imaginary part providing either nonlinear loss or even gain for the overall attenuating pulses. This nonlinear gain results in more intense pulse compression and stronger spectral broadening, which is demonstrated here on the example of liquid-filled capillary-type fibers.

Compact silicon TE-pass polarizer using adiabatically-bent fully-etched waveguides.

A high-performance integrated silicon TE-pass polarizer is proposed and demonstrated. The polarizer uses a series of adiabatic waveguide bends that yield high extinction ratio for the TM polarization and low insertion loss for the TE polarization, and does not require special materials or complex fabrication steps. The polarizer, implemented on a silicon-on-insulator platform with a 220 nm silicon thickness, is measured to have insertion loss ≤ 0.37 dB (average 0.12 dB) and extinction ratio ≥ 27.6 dB (average 36.0 dB) over a 1.5 μm to 1.6 μm wavelength range, with a footprint of 63 μm × 9.5 μm. The trade-off between the footprint of the polarizer and its performance is established. While the analysis was done for a silicon-on-insulator platform, the concept is applicable to other waveguide geometries and integrated photonic platforms.

Significant reduction in propagation loss in electrospun polymer fibers by adopting thermal drawing

Electrospun polymer submicroscale fibers normally have flexible cylindrical waveguide geometries and can be produced with ultra-long and functional materials. Hence, they are promising building blocks for small optical devices. However, the propagation loss in the fibers is still higher than that in submicroscale polymer fibers produced by melt and solution drawing. In this study, thermal drawing of electrospun fibers was found to reduce their propagation loss. Fivefold thermal drawing of as-electrospun polymer fibers significantly reduced their diameters (e.g., from 1.121 μm to 0.497 μm) and propagation loss (e.g., from 16 dB cm−1 to 6.6 dB cm−1 at 521-nm wavelength). Because fluctuations in the fiber diameter significantly reduced after thermal drawing, the propagation loss reduction could be attributed to the reduced fluctuations in addition to the reduced density inhomogeneity in the fibers. These findings pave the way for low-loss electrospun fiber waveguides and their application in small optical devices.

A ghost imaging modality in a random waveguide

We study the imaging of a penetrable scatterer, aka target, in a waveguide with randomly perturbed boundary. The target is located between a partially coherent source which transmits the wave, and a detector which measures the spatially integrated energy flux of the wave. The imaging is impeded by random boundary scattering effects that accumulate as the wave propagates. We consider a very large distance (range) between the target and the detector, where that cumulative scattering is so strong that it distributes the energy evenly among the components (modes) of the wave. Conventional imaging is impossible in this equipartition regime. Nevertheless, we show that the target can be located with a ghost imaging modality. This forms an image using the cross-correlation of the measured energy flux, integrated over the aperture of the detector, with the time and space resolved energy flux in a reference waveguide, at the search range. We consider two reference waveguides: the waveguide with unperturbed boundary, in which we can calculate the energy flux, and the actual random waveguide, before the presence of the target, in which the energy flux should be measured. We analyze the ghost imaging modality from first principles and show that it can be efficient in a random waveguide geometry in which there is both strong modal dispersion and mode coupling induced by scattering, provided that the standard ghost imaging function is modified and integrated over a suitable time offset window in order to compensate for dispersion and diffusion. The analysis quantifies the resolution of the image in terms of the source and detector aperture, the range offset between the source and the target, and the duration of the measurements.

Cladding stress induced performance variation of silicon MMI coupler

In this paper, we have shown the effect of stress induced by cladding layer on the performance of silicon MMI coupler. The cladding material is assumed to be SiO2. From the numerical analysis using finite element method (FEM), we have demonstrated that the length of the MMI coupler for a given power splitting ratio is a strong function of stress. Moreover, with temperature the power splitting ratio changes as high as 50% for a MMI coupler having cladding induced stress, compared to a MMI coupler without having cladding induced stress. In this study, two cross-sectional geometries, namely rectangular and trapezoidal are considered to investigate the effect of waveguide cross section on the cladding induced stress. Here, the SiO2 cladding thickness is also varied from 400 nm to 800 nm for both types of geometries. It is found that out-of-plane stress increases with cladding thickness for both waveguide cross-sections and the opposite nature of variation is found for in-plane stress. Trapezoidal waveguide introduces higher in-plane as well as out-of-plane stress compared to rectangular waveguide for any given cladding thickness. Further, the temperature sensitivity of MMI coupler without and with stress having two different power splitting ratios and waveguide cross-sections are studied. The stress induced birefringence is also calculated for the fundamental mode (MMI under consideration supports 16 modes in total) as a function of cladding thickness and operating temperature. The results show that like previously studied waveguide structure, in MMI coupler also the birefringence increases with increased cladding thickness, whereas it decreases with the operating temperature. It is also observed that the stress induced birefringence is more for trapezoidal waveguide cross-section compared to rectangular cross-section. Simulation results reveal that the effective thermo-optic coefficient for a given mode supported by the MMI coupler is independent of cladding thickness, waveguide geometry and stress.

Hollow-Core-Integrated Optical Waveguides for Mid-IR Sensors

Integrated hollow-core antiresonant reflecting optical waveguides are designed and analyzed for mid-infrared propagation. With a proper selection of waveguide materials and geometry, low loss propagation lower than 1.5 dB/cm in the whole 2.5–14 μm range could be achieved. Single-mode waveguides, with a modal purity above 95% after propagation lengths of few millimeters, have been designed at 4 and 9 μm. The waveguides could be realized using fabrication and materials fully compatible with standard Silicon technology, enabling the chip-scale integration of Mid-IR devices for gas sensing applications.

Oriented zinc oxide nanorods: A novel saturable absorber for lasers in the near-infrared.

Zinc oxide (ZnO) nanorods (NRs) oriented along the crystallographic [001] axis are grown by the hydrothermal method on glass substrates. The ZnO NRs exhibit a broadband (1–2 µm) near-IR absorption ascribed to the singly charged zinc vacancy VZn−1. The saturable absorption of the ZnO NRs is studied at ≈1 µm under picosecond excitation, revealing a low saturation intensity, ≈10 kW/cm2, and high fraction of the saturable losses. The ZnO NRs are applied as saturable absorbers in diode-pumped Yb (≈1.03 µm) and Tm (≈1.94 µm) lasers generating nanosecond pulses. The ZnO NRs grown on various optical surfaces are promising broadband saturable absorbers for nanosecond near-IR lasers in bulk and waveguide geometries.

On-Chip Hybrid Photonic-Plasmonic Waveguides with Ultrathin Titanium Nitride Films

A solid-state hybrid photonic–plasmonic (HPP) waveguide geometry has been experimentally demonstrated with plasmonic titanium nitride. The configuration is made with robust fabrication techniques, ...

Accurate extraction of fabricated geometry using optical measurement

We experimentally demonstrate extraction of silicon waveguide geometry with subnanometer accuracy using optical measurements. Effective and group indices of silicon-on-insulator (SOI) waveguides are extracted from the optical measurements. An accurate model linking the geometry of an SOI waveguide to its effective and group indices is used to extract the linewidths and thicknesses within respective errors of 0.37 and 0.26 nm on a die fabricated by IMEC multiproject wafer services. A detailed analysis of the setting of the bounds for the effective and group indices is presented to get the right extraction with improved accuracy.

Microstrip Antenna–Oscillators Integrated with a Waveguide Built in a Dielectric Substrate

The design of a microwave oscillator based on a microstrip log-periodic antenna integrated with a field effect transistor and a waveguide built in a dielectric substrate has been developed and analyzed. The waveguide geometry provides the possibility of propagation and radiation at both the fundamental frequency of the log-periodic antenna and harmonics. Computer simulation of the oscillator design in a frequency range near resonance frequencies of the log-periodic antenna is conducted. The possibility of summation of powers of several antenna–oscillators arranged ias a linear phased array is investigated.