Waveguide Approach Research Articles

Design and analysis of 3D-printed hybrid couplers for D-band applications

AbstractThis work focuses on the impact of the build orientation on additively manufactured waveguide-based hybrid couplers for D-band frequency range and relates it to other sources of uncertainty within the overall manufacturing process and measurement instrumentation for the D-band frequency range. The designed specimens are first printed from UV curable photopolymer resin and subsequently metal coated by an electroless silver plating process, which in turn is improved by making use of the slotted waveguide approach. Although the requirements toward geometrical precision to achieve phase errors below 10° are in an order of 0.1 mm, a desktop grade DLP printer is utilized in this work in order to point out the prospects and limitations of additive manufacturing. Furthermore, waveguide paths with bends are part of the model and their impact on the measured attenuation is estimated explicitly.Despite this narrow field of tolerances, one specimen could have been realized, which achieves a measured output magnitude imbalance of 0.7 dB over the frequency range from 120 to 155 GHz while at the same time exhibiting a phase deviation of only <10° from the desired 90°. With these demonstrated results, the proposed approach provides suitability for future applications in the D-band frequency range.

Planar Plasmonic Terahertz Waveguide Based Upon One Dimensional Array of Pyramidal Corrugations and Refractive Index Sensing

We examine the refractive index sensor ability of the terahertz surface plasmon modes supported by a planar plasmonic terahertz (THz) waveguide. The waveguide is comprised of one-dimensional array of periodically arranged subwavelength scale pyramidal corrugations resulting in a plasmonic metamaterials waveguide. The waveguide exhibits strong field confinement along with the corrugated pattern. We have analyzed the dispersion property and quality factor of the fundamental as well as higher-order modes of the waveguide. In order to examine sensor capability, we used analytes of different refractive indices and observed a corresponding frequency shift. We further analyze the sensitivity of all the modes for the given volume of analyte and compare their sensing performance. We have employed the semi-analytical model to understand the numerical observations. The proposed waveguide approach can potentially result in highly sensitive refractive index sensors at terahertz frequencies.

On the behaviour of infinite, periodic, mono-coupled waveguides using a transmission coefficient phase method

Abstract Wave propagation in a one dimensional, mono-coupled waveguide composed of an infinite wave-bearing structure supported by periodic discontinuities is analysed in terms of the transmission and reflection coefficients of a single discontinuity, expressions for which are often available in the literature. The general solution is presented and is applicable for arbitrary transmission and reflection coefficients. For conservative discontinuities, conservation of energy imposes relationships on the complex values of these coefficients and yields a simplified expression for the propagation constant, which is a function of the phase of the transmission coefficient, and a second phase angle which arises from the conditions of continuity and equilibrium at the discontinuity. Expressions for the bounding frequencies between pass and stop bands, the bandwidth of a stop band, and the upper bound of attenuation within a stop band are also given in terms of these phases. For some common systems, the phase of the transmission coefficient determines the behaviour of the entire system and thus provides a straightforward analysis. The proposed approach for mono-coupled waveguides can also be applied to some multi-coupled waveguides such as beams under the assumption that nearfield waves are negligible. The approach is then applied to an infinite periodic string on arbitrary supports, an Euler-Bernoulli beam on periodic arbitrary supports, and a system of thin beams that are hinged at periodic intervals with arbitrary supports.

Comparison of Free-Space and Waveguide-Based SERS Platforms.

Surface-Enhanced Raman Spectroscopy (SERS) allows for the highly specific detection of molecules by enhancing the inherently weak Raman signals near the surface of plasmonic nanostructures. A variety of plasmonic nanostructures have been developed for SERS signal excitation and collection in a conventional free-space microscope, among which the gold nanodomes offer one of the highest SERS enhancements. Nanophotonic waveguides have recently emerged as an alternative to the conventional Raman microscope as they can be used to efficiently excite and collect Raman signals. Integration of plasmonic structures on nanophotonic waveguides enables reproducible waveguide-based excitation and collection of SERS spectra, such as in nanoplasmonic slot waveguides. In this paper, we compare the SERS performance of gold nanodomes, in which the signal is excited and collected in free space, and waveguide-based nanoplasmonic slot waveguide. We evaluate the SERS signal enhancement and the SERS background of the different SERS platforms using a monolayer of nitrothiophenol. We show that the nanoplasmonic slot waveguide approaches the gold nanodomes in terms of the signal-to-background ratio. We additionally demonstrate the first-time detection of a peptide monolayer on a waveguide-based SERS platform, paving the way towards the SERS monitoring of biologically relevant molecules on an integrated lab-on-a-chip platform.

3D Printed E-Band Hybrid Coupler

In this letter, an additively manufactured 3dB hybrid coupler is presented. While the geometrical structure is generated by a stereolithography (SLA) printer from UV curable photopolymer resin as one single part, conductive coating is provided by an electroless silverplating process. The utilized slotted waveguide approach improves the solvent flow during metal plating and therefore enables the fabrication of complex waveguide structures. Measured imbalance between the through and coupling ports stays below 1.5 dB over the entire E-band, while the respective phase difference ranges only between 87° and 91° despite the challenges of additive manufacturing in such frequency regions.

Real‐Time Techniques for Interpretation of Ionospheric Backscatter Sounding Data

AbstractWe present techniques and algorithms for automatic processing and interpretation of data of ionospheric backscatter sounding (BS) using a continuous chirp signal. Experimental ionograms are processed based on data filtration with further compression using the cellular automaton method. Interpretation of signals in a backscatter ionogram is carried out by calculating the backscatter leading edge. Slightly changing ratios of frequency dependences for group characteristics of oblique sounding and BS signals in various heliogeophysical conditions are used. Parameters of radio wave propagation are calculated within the waveguide approach using the International Reference Ionosphere model. The results of interpretation for a BS ionogram allow quick determination of maximum usable frequencies, from which characteristics of oblique sounding signals are corrected for given radio paths in the sounding sector of interest.

Excitation of multi-order guided mode resonance for multiple color fluorescence enhancement

Higher-order Guided Mode Resonance (GMR) in a symmetric two-dimensional embedded grating-waveguide structure is theoretically demonstrated in this paper through a phase matching mechanism of leaky modes along second-order diffraction planes. In addition to the fundamental-order, multiple resonances are introduced at normal incidence and they are polarization independent. A strong localization of the resonance modes exists in both orders. The device is proposed for multiple color surface enhanced fluorescence that is compatible to a conventional fluorescence reader. The enhancement scheme is by increasing light absorption via the excitation of a strong evanescent wave as an extension of a resonance mode. GMR device is designed to have resonances overlapping fluorophores’ absorption spectra. The concept is verified using numerical calculations, Rigorous Coupled Wave Analysis and Homogeneous Waveguide Approach. The device is fabricated and experimentally performed for fluorescence enhancement using a microarray of Cyanine 3-labeled goat antibody and Cyanine 5-labeled goat antibody. The fluorescence signal is characterized using a low-cost CMOS array. This implementation can be very useful in a multiplex detection, where the cost of the reader can be minimized.

Coupled‐Waveguides for Dispersion Compensation in Semiconductor Lasers

AbstractA dual waveguide for intracavity dispersion compensation in semiconductor lasers is presented and applied to a short mid‐infrared wavelength quantum cascade laser operating in the first atmospheric window ( 4.6 μm). As a result, stable comb operation on the full multi‐mode dynamical range is achieved in this device. Unlike previously proposed schemes, the dual waveguide approach can be applied to various types of semiconductor lasers and material systems. In particular, it could enable efficient group velocity dispersion compensation at near‐infrared wavelengths where semiconductor materials exhibit a large value of that parameter.

Highly Efficient Nonlinear Optical Conversion in Waveguiding GaSe Nanoribbons with Pump Pulses Down to a Femto‐Joule Level

AbstractLayered GaSe flakes have revealed strongest second‐harmonic (SH) generation among all the two‐dimensional (2D) atomic crystals measured up to now. However, the unique feature of physical atomic thickness at nanometer scale prevents their practical applications in efficient nonlinear optical conversion due to limited light–matter interaction length. In this work, high quality single‐crystal GaSe nanoribbons (NRs) are fabricated and demonstrated as waveguides with good performance. By taking advantage of the strong confinement and long interaction length of the waveguiding approach, significantly enhanced nonlinear light–matter interaction is observed in GaSe NRs. Based on transverse SH generation, a single GaSe NR‐configured optical auto‐correlator is constructed for ultrashort pulse characterization. Because of the large second‐order nonlinear susceptibility, high sensitivity down to few femto‐joule (fJ) is realized. To the best of our knowledge, this is the highest energy sensitivity among the reported nanoscale auto‐correlators. Such NR waveguides and devices are highly compatible with standard optical fiber systems. The presented results may enable the construction of future energy efficient nonlinear photonic circuitry, chips, and devices.

Formation of hotspots in partially filled ferrite-loaded rectangular waveguides

We theoretically investigate the formation of hotspots at microwave frequencies in a partially filled, ferrite-loaded rectangular waveguide. When the unidirectional mode of this waveguide approaches an impenetrable conducting barrier, its magnetic field rises sharply due to the absence of reflection. To study this phenomenon, we use the magnetostatic approximation and derive a surface integral equation for the normal component of magnetic flux density on the surface of the ferrite layer. For a half-filled waveguide, an analytic solution is found, which shows the magnetic field to diverge near the barrier as |x|−q, where |x| is the distance from the barrier edge and q is slightly less than unity. For filling factors other than one-half, the numerical solution of the surface integral equation again confirms the appearance of a hotspot near the barrier edge.

High-Capacity and Dispersionless Delay Lines Based on Plasmonic Waveguide Periodically Coupled With Bilaterally Located Ring and Slot Resonators

We propose and study optical delay lines with high capacity and low dispersion based on a plasmonic waveguide periodically coupled with bilaterally located ring and slot resonators. The repetitive unit exhibits plasmon-induced transparency (PIT) with a large delay-bandwidth product due to the interference between the two-mode ring and single-mode slot resonators. By choosing appropriate parameters, the cascaded PIT units can possess a broad transparency window with moderate fluctuations and produce wideband dispersionless slow light. A theoretical model is developed to analyze the proposed structure and the results are validated by finite-difference time-domain simulations. The pulse propagation results indicate that the spectral fluctuations have a negligible effect on the delayed pulse shape. Using multiple cascaded PIT units, we demonstrate that the group index of the waveguide approaches 36, the 3-dB bandwidth is larger than 21 nm, and a storage efficiency of 0.5 bit/unit can be realized with negligible distortion. Due to the high performance, the proposed delay line may find potential applications in future nanoscale integrated photonic systems.

Ultra-wide band dispersionless slow light waveguides

In conventional slow light waveguide, the delay time of signal in a waveguide is inversely proportional to its bandwidth. As a measure of storage density of optical pulse, the delay-bandwidth product is limited to a small constant depending on waveguiding approach. Breaking the limit is fundamental problem in optical signal and quantum information processing fields. The solution to this problem is becoming increasingly important, especially with high-speed signal processing system. So far, there have been neither practical nor realizable techniques that can solve this problem. In this paper, we propose a mechanism to explore the possibility for breaking the delay-bandwidth limit by using ultra-flat photonic band cluster. We show that based on multiple micro-cavities having corresponding multiple ultra-flat photonic bands, a slow light with arbitrary bandwidth in frequency can be achieved with ultraslow group velocity and ultralow group velocity dispersion, leading to a broken delay-bandwidth limit. This work demonstrates that arbitrary bandwidth dispersionless slow light can be realized with a two-dimensional multiple-microcavity photonic-crystal line-defect waveguide.

Features of backscatter ionospheric sounding as studied with a chirp ionosonde

We present the results of the studies of backscatter ionospheric sounding (BS) on the basis of a multipurpose chirp ionosonde developed in ISTP SB RAS. We analyze BS experimental data obtained during different seasons from 2005 to 2009. The accumulated dataset allows us to investigate features of BS signal propagation in various heliogeophysical conditions. To analyze and interpret BS signals on ionograms, we use the results of modeling of characteristics for chirp signals in the backscatter and oblique ionospheric sounding under a waveguide approach. We have revealed the most characteristic types of ionograms and have established conditions of appearance of a given type depending on the time of day, season, sounding direction, and medium conditions. In winter, spring, and autumn, the prevailing types of ionograms are those with BS signals corresponding to the propagation mode through reflection from the F layer. Signals reflected by E or Es layers are recorded during summer periods. At the same time, frequencies of the received signals are sufficiently large, and sometimes there are no reflections from the F layer.

A multiscale analytical model of bronchial airway acoustics.

Sound transmission and resulting airway wall vibration in a complex multiscale viscoelastic model of the subglottal bronchial tree was calculated using a modified one-dimensional (1D) branching acoustic waveguide approach. This is an extension of previous work to enable use of complex airway trees that are partially derived from subject-specific medical images, without the need for self-similarity in the geometric structure. The approach was validated numerically for simplified airway geometries, as well as experimentally by comparison to previous studies. A comprehensive conducting airway tree with about 60 000 branches was then modified to create fibrotic, bronchoconstrictive, and pulmonary infiltrate conditions. The fibrotic case-systemic increase in soft tissue stiffness-increased the Helmholtz resonance frequency due to the increased acoustic impedance. Bronchoconstriction, with geometric changes in small conducting airways, decreased acoustic energy transmission to the peripheral airways due in part to the increased impedance mismatch between airway orders. Pulmonary infiltrate significantly altered the local acoustic field in the affected lobe. Calculation of acoustic differences between healthy versus pathologic cases can be used to enhance the understanding of vibro-acoustic changes correlated to pathology, and potentially provide improved tools for the diagnosis of pulmonary diseases that uniquely alter the acoustics of the airways.

Исследование особенностей возвратно-наклонного зондирования ионосферы на базе ЛЧМ-ионозонда

We present the results of studies of backscatter ionospheric sounding (BS) on the basis of a multipurpose chirp ionosonde developed in ISTP SB RAS. We analyze BS experimental data obtained during different seasons from 2005 to 2009. The accumulated dataset allows us to investigate features of BS signal propagation in various heliogeophysical conditions. To analyze and interpret BS signals on ionograms, we use the results of modeling of characteristics for chirp signals in the backscatter and oblique ionospheric sounding under the waveguide approach. We have revealed the most characteristic types of ionograms and have established conditions of appearance of a given type depending on the time of day, season, sounding direction, and medium conditions. In winter, spring, and autumn, the prevailing types of ionograms are those with BS signals corresponding to the propagation mode through reflection from the F layer. Signals reflected by E or Es layers are recorded during summer periods. At the same time, frequencies of the received signals are sufficiently large, and sometimes there are no reflections from the F layer.

Surface-enhanced fluorescence in metal nanoparticle-doped polymer nanofibers via waveguiding excitation

We report a waveguiding excitation-based approach for surface-enhanced fluorescence. As high as 17-fold enhanced fluorescence intensity of Rhodamine 6G molecules is realized by gold nanoparticles embedded in polymer nanofibers. The enhancement results not only from the spatial confinement of light by the nanofibers but also from the wavelength match among the excitation laser, the localized surface plasmon resonance of nanoparticles, and the absorption band of dyes. On the basis of the enhancement and high-efficient waveguiding regime, the required excitation power for detectable fluorescence is decreased to the 20 nW level, which is about 50 times lower than that by free-space excitation. These fluorophore/nanoparticle-doped nanofibers may find applications in compact and energy-efficient optical devices of chemical analysis and biosensing.

Multifunctional optofluidic lab-on-chip platform for Raman and fluorescence spectroscopic microfluidic analysis.

A multifunctional lab-on-a-chip platform for spectroscopic analysis of liquid samples based on an optofluidic jet waveguide is reported. The optofluidic detection scheme is achieved through the total internal reflection arising in a liquid jet of only 150 μm diameter, leading to highly efficient signal excitation and collection. This results in an optofluidic chip with an alignment-free spectroscopic detection scheme, which avoids any background from the sample container. This platform has been designed for multiwavelength fluorescence and Raman spectroscopy. The chip integrates a recirculation system that reduces the required sample volume. The evaluation of the device performance has been accomplished by means of fluorescence measurements performed on eosin Y in water solutions, achieving a limit of detection of 33 pM. The sensor has been applied in Raman spectroscopy of water-ethanol solutions, leading to a limit of detection of 0.18%. As additional application, analysis of riboflavin using fluorescence detection demonstrates the possibility of detecting this vitamin at the 560 pM level (0.21 ng l-1). Although measurements have been performed by means of a compact and low-cost spectrometer, in both cases the micro-jet optofluidic chip achieved similar performances if not better than high-end benchtop based laboratory equipment. This approach paves the way towards portable lab-on-a-chip devices for high sensitivity environmental and biochemical sensing, using optical spectroscopy.

Microwave absorption properties of FSS-impacted composites as a broadband microwave absorber

The development of a cost-effective microwave absorber with wide bandwidth corresponding to reflection loss (RL) ≤ −10 dB is still a very challenging task. A sugarcane bagasse-based agricultural waste composite has been analyzed for its elemental contents. The combination of elements is suitable for its possible usage as a cost-effective microwave absorbing material. Therefore, this composite has been subjected to morphological and electromagnetic studies to analyze its microwave absorbing behavior. The frequency dependent complex dielectric permittivity and complex magnetic permeability values were obtained using a transmission/reflection waveguide approach in the X-band. Furthermore, the effect of the Minkowski loop frequency selective surface (FSS) was studied over the absorption capability of the composite. It was found that the application of FSS leads to a reduction in thickness up to 2.9 mm and an enhancement in absorption bandwidth up to 3.6 GHz. The FSS patterned composite shows a remarkable performance with peak RL of −28.4 dB at 10.7 GHz and absorption bandwidth of 3.6 GHz.

Development of a Fast-Response Aerodynamic Pressure Probe Based on a Waveguide Approach

Currently, fast-response aerodynamic probes are widely used for advanced experimental investigations in turbomachinery applications. The most common configuration is a virtual three-hole probe. This solution is a good compromise between probe dimension and accuracy. Several authors have attempted to extend the capabilities of these probes in terms of bandwidth and operating conditions. Even though differences exist between the solutions in the literature, all of the designs involve the positioning of a dynamic pressure sensor close to the measurement point. In general terms, the higher the frequency response, the more the sensor is exposed to the flow. This physical constraint puts a limit on the probe applicability since the measurement conditions have to comply with the maximum allowed operating conditions of the sensor. In other applications, when the conditions are particularly harsh and a direct measurement is not possible, a waveguide probe is commonly used to estimate the local pressure. In this device, the sensor is connected to the measurement point through a transmitting duct which guarantees that the sensor is operating in a less critical condition. Generally, the measurement is performed through a pressure tap and particular attention must be paid to the probe design in order to have an acceptable frequency response function. In this study, the authors conceived, developed, and tested a probe which combines the concept of a fast-response aerodynamic pressure probe with that of a waveguide probe. Such a device exploits the benefits of having the sensor far from the harsh conditions while maintaining the capability to perform an accurate flow measurement.

Vibro-acoustics of porous materials - waveguide modelling approach

The porous material is considered as a compound multi-layered waveguide (i.e. a fluid layer surrounded with elastic layers) with traction free boundary conditions. The attenuation of the vibro-acoustic waves in such a material is assessed. This approach is compared with a conventional Biot's model and a qualitative agreement in phase velocities as well as damping estimates is found. The waveguide model predicts four waves, out of which two are attenuated when the viscous fluid is considered (while the elastic layer being ideally lossless). One of these waves is found to be significantly controlled by the fluid viscosity, while for the other the effect of viscosity was observed for very small frequencies. The Biot's model predicts only one of these attenuated waves, where the latter one is not predicted. Thus the proposed waveguide approach provide additional information about the wave propagation in porous materials.