High-index Waveguides Research Articles

Phase Composition of HiVac-VPE Lithium Niobate Optical Waveguides Identified by Spectroscopic Investigations.

High-index contrast lithium niobate waveguides, fabricated by the High Vacuum Vapor-phase Proton Exchange (HiVac-VPE) technique, are very promising for increasing both the optical nonlinear and electro-optical efficiencies of photonic integrated devices. So as to play this role effectively, it is mandatory to know the crystallographic phase composition of waveguides and the position of protonated layers for appropriate tailoring and optimization based on the intended applications. In addition, the estimation of structural disorder and electro-optical properties of the waveguides are also of high interest. Benefiting from Raman spectroscopy, IR reflection, IR absorption, and UV-VIS absorption, the HxLi1-xNbO3 phase compositions, as well as the structural disorder in waveguides, were determined. Based on experimental data on the shift of the fundamental absorption edge, we have quantitatively estimated the electro-optic coefficient r13 in as-exchanged waveguides. The electro-optical properties of the waveguides have been found to be depending on the phase composition. The obtained results allow for reconsidering the proton exchange fabricating process of photonic nonlinear devices and electro-optic modulators based on high-index contrast channel waveguides on the LiNbO3 platform.

Nano antenna-assisted quantum dots emission into high-index planar waveguide

Integrated quantum photonic circuits require the efficient coupling of photon sources to photonic waveguides. Hybrid plasmonic/photonic platforms are a promising approach, taking advantage of both plasmon modal confinement for efficient coupling to a nearby emitter and photonic circuitry for optical data transfer and processing. In this work, we established directional quantum dot (QD) emission coupling to a planar TiO2 waveguide assisted by a Yagi-Uda antenna. Antenna on waveguide is first designed by scaling radio frequency dimensions to nano-optics, taking into account the hybrid plasmonic/photonic platform. Design is then optimized by full numerical simulations. We fabricate the antenna on a TiO2 planar waveguide and deposit a few QDs close to the Yagi-Uda antenna. The optical characterization shows clear directional coupling originating from antenna effect. We estimate the coupling efficiency and directivity of the light emitted into the waveguide.

External chirality enhancing downconversion in a waveguide-coupled nonlinear plasmonic metasurface.

Metasurfaces, typically constructed from spatial arrangements of localized building blocks, can enhance light-matter interactions through local field enhancement or by coherent coupling to extended photonic modes. Recent works have explored how guided mode resonances influence the performance of nonlinear metasurfaces. Here we investigate the modal impact on difference-frequency generation in a waveguide-coupled metasurface platform. The system is constructed from gold split-ring resonators on a high-index TiO2 waveguide. We find that a symmetric configuration of the metasurface's localized modes and the extended waveguide modes lead to a modest enhancement of the downconversion process. However, when the mirror symmetry of the localized modes with respect to the guided mode propagation breaks, it introduces external chirality. This enables coupling to a higher quality mode, resulting in a 70-fold enhancement of the difference-frequency generation. The capacity to manipulate the nonlocal modes through the design offers broader control over the interaction and new avenues to tailor the nonlinear processes.

Coupling strategy between high-index and mid-index micro-metric waveguides for O-band applications

The integration of fast and power efficient electro-absorption modulators on silicon is of utmost importance for a wide range of applications. To date, Franz-Keldysh modulators formed of bulk Ge or GeSi have been widely adopted due to the simplicity of integration required by the modulation scheme. Nevertheless, to obtain operation for a wider range of wavelengths (O to C band) a thick stack of Ge/GeSi layers forming quantum wells is required, leading to a dramatic increase in the complexity linked to sub-micron waveguide coupling. In this work, we present a proof-of-concept integration between micro-metric waveguides, through the butt-coupling of a {1.25},{upmu hbox {m}} thick N-rich silicon nitride (SiN) waveguide with a {1.25},{upmu hbox {m}} thick silicon waveguide for O-band operation. A numerical analysis is conducted for the design of the waveguide-to-waveguide interface, with the aim to minimize the power coupling loss and back-reflection levels. The theoretical results are compared to the measured data, demonstrating a coupling loss level of {0.5},hbox {dB} for TE and TM polarisation. Based on the SiN-SOI interconnection simulation strategy, the simulation results of a quantum-confined Stark effect (QCSE) stack waveguide coupled to a SiN waveguide are then presented.

High performing designer glass platform to host versatile photonic devices

The aluminum to alkaline earth ratio (Al:[AE]) in borosilicate glass is introduced as a novel compositional parameter to produce high index waveguides through femtosecond laser inscription. Alkaline earths are considered to be major refractive index providers for laser written optical waveguides due to their collective migration toward the guiding region. Contrary to the existing belief that increasing the concentration of such index providers within a glass composition will help to produce high index change waveguides, we demonstrate that high index waveguides can be fabricated by tuning the Al:[AE] ratio while keeping the absolute concentration of the index provider to a minimum.

Waveguide-based total internal reflection fluorescence microscope enabling cellular imaging under cryogenic conditions.

Total internal reflection fluorescence (TIRF) microscopy is an important imaging tool for the investigation of biological structures, especially the study on cellular events near the plasma membrane. Imaging at cryogenic temperatures not only enables observing structures in a near-native and fixed state but also suppresses irreversible photo-bleaching rates, resulting in increased photo-stability of fluorophores. Traditional TIRF microscopes produce an evanescent field based on high numerical aperture immersion objective lenses with high magnification, which results in a limited field of view and is incompatible with cryogenic conditions. Here, we present a waveguide-based TIRF microscope, which is able to generate a uniform evanescent field using high refractive index waveguides on photonic chips and to obtain cellular observation at cryogenic temperatures. Our method provides an inexpensive way to achieve total-internal-reflection fluorescence imaging under cryogenic conditions.

Waveguide Enhanced Raman Spectroscopy for Biosensing: A Review.

Waveguide enhanced Raman spectroscopy (WERS) utilizes simple, robust, high-index contrast dielectric waveguides to generate a strong evanescent field, through which laser light interacts with analytes residing on the surface of the waveguide. It offers a powerful tool for the direct identification and reproducible quantification of biochemical species and an alternative to surface enhanced Raman spectroscopy (SERS) without reliance on fragile noble metal nanostructures. The advent of low-cost laser diodes, compact spectrometers, and recent progress in material engineering, nanofabrication techniques, and software modeling tools have made realizing portable and cheap WERS Raman systems with high sensitivity a realistic possibility. This review highlights the latest progress in WERS technology and summarizes recent demonstrations and applications. Following an introduction to the fundamentals of WERS, the theoretical framework that underpins the WERS principles is presented. The main WERS design considerations are then discussed, and a review of the available approaches for the modification of waveguide surfaces for the attachment of different biorecognition elements is provided. The review concludes by discussing and contrasting the performance of recent WERS implementations, thereby providing a future roadmap of WERS technology where the key opportunities and challenges are highlighted.

Highly Sensitive Sensor Structure Based on Sol-Gel Waveguide Films and Grating Couplers

The technologies of optical planar evanescent wave chemical and biochemical sensors require chemically resistant, high refractive index waveguide films having very good optical transmission properties. In this paper we present such two-compound SiOx:TiOy waveguide films fabricated by using the sol-gel method and the dip-coating technique. These films not only have high optical quality and low propagation losses but also an extremely high refractive index of >1.90 (λ = 632.8 nm). Further we demonstrate efficient and simple sensing structures, designed and fabricated based on these films. For this purpose, grating couplers with a period of Λ = 417 nm were fabricated on the interface between a waveguide film and cover using the single-step nanoimprint method. These sensing structures were tested as planar refractometers. The results of the theoretical analysis on the basis of which the structures were designed as well as results of their experimental characterization are presented in this work. Consequently, the relationship between parameters and the sensitivity of investigated sensing structures is discussed. As a result, the profitable properties of the designed grating coupler sensors are verified and excellent consistency between the results of the theoretical analysis and experimental results is achieved.

Strong frequency-dependent beam steering dynamics, Zitterbewegung effect, and Klein tunneling in a ternary plasmonic-dielectric superlattice

Band structures are intrinsically influenced by the mode interaction in waveguide array, which can support many intriguing phenomena such as negative refraction, on-chip lensing, and quantum-analog dynamics like Bloch oscillation and topological interface transportation. Multiple branches of band may emerge due to the comprehensive mode interactions associated with high-order photonic modes supported in high refractive index dielectric waveguide. Furthermore, the number of photonic modes confined in a high-index waveguide is determined by both the width and working frequency. Hence, the mode interaction and resultant band structure can be effectively tuned by the frequency. As such, a diversity of equifrequency contours could be generated and modulated among a wide frequency range, which, however, remains largely unexplored. Here, we present several frequency-determined beam dynamics in a composite ternary plasmonic-dielectric waveguide array, including collimation effect, angle-dependent beam branching/multibranching, conicallike beam diffraction. Based on the evolution of the band structure, photonic Zitterbewegung effect and Klein tunneling can also be observed. In contrast to the coherent coupling between symmetric and antisymmetric modes that gives rise to the photonic Zitterbewegung effect, we show that three-mode coherent coupling in the ternary system can yield a superimposed extreme oscillation of the beam. This represents an extension of the general Zitterbewegung effect in photonic system. Additionally, the Klein tunneling sandwiched in two types of artificial waveguide arrays with different Dirac points is demonstrated, and approximately unimpeded penetration is exhibited. The configurations discussed here can be readily implemented in on-chip systems for a variety of potential applications including wave routing, selectively directional coupling, and multiplexing.

Hybrid electro-optic modulator combining silicon photonic slot waveguides with high-k radio-frequency slotlines

Electro-optic (EO) modulators rely on the interaction of optical and electrical signals with second-order nonlinear media. For the optical signal, this interaction can be strongly enhanced using dielectric slot–waveguide structures that exploit a field discontinuity at the interface between a high-index waveguide core and the low-index EO cladding. In contrast to this, the electrical signal is usually applied through conductive regions in the direct vicinity of the optical waveguide. To avoid excessive optical loss, the conductivity of these regions is maintained at a moderate level, thus leading to inherent RC limitations of the modulation bandwidth. In this paper, we show that these limitations can be overcome by extending the slot–waveguide concept to the modulating radio-frequency (RF) signal. Our device combines an RF slotline that relies on B a T i O 3 as a high-k dielectric material with a conventional silicon photonic slot waveguide and a highly efficient organic EO cladding material. In a proof-of-concept experiment, we demonstrate a 1 mm long Mach–Zehnder modulator that offers a 3 dB bandwidth of 76 GHz and a 6 dB bandwidth of 110 GHz along with a small π voltage of 1.3 V ( U π L = 1.3 V m m ). We further demonstrate the viability of the device in a data-transmission experiment using four-state pulse-amplitude modulation (PAM4) at line rates up to 200 Gbit/s. Our first-generation devices leave vast room for further improvement and may open an attractive route towards highly efficient silicon photonic modulators that combine sub-1 mm device lengths with sub-1 V drive voltages and modulation bandwidths of more than 100 GHz.

All-optical RF spectrum analyzer with a 5 THz bandwidth based on CMOS-compatible high-index doped silica waveguides

We report an all-optical radio-frequency (RF) spectrum analyzer with a bandwidth greater than 5 THz, based on a 50 cm long spiral waveguide in a CMOS-compatible high-index doped silica platform. By carefully mapping out the dispersion profile of the waveguides for different thicknesses, we identify the optimal design to achieve near-zero dispersion in the C-band. To demonstrate the capability of the RF spectrum analyzer, we measure the optical output of a femtosecond fiber laser with an ultrafast optical RF spectrum in the terahertz regime.

Parameters of fast and high-yield InAs/GaAs quantum dot semiconductor scintillator

InAs quantum dots (QDs) embedded into a waveguiding GaAs semiconductor matrix may produce scintillation detectors with exceptional speed and yield, making them valuable for nuclear security, medical imaging, and high-energy physics applications. In this work, we developed thick (~ 25 μm) epitaxial heterostructures with high luminescence efficiency composed of self-assembled nano-engineered InAs QDs grown by molecular beam epitaxy. The bulk GaAs acts as a stopping material for incident particles and as a waveguide when layer-transferred onto a low-index substrate. Waveguiding and self-absorption (< 1 cm−1) were studied using photoluminescence with scanning laser excitation and modeled with ray optics approximation and geometrical coupling of high-index waveguide to a collection fiber. Scintillating signals from α-particles were analyzed with an external photodiode (PD) and an integrated PD which provided an improved optical coupling. The mean charge collected by the integrated PD corresponded to 3 × 104 photoelectrons per 1 MeV of deposited energy or ~ 13% of the theoretically achievable light yield. Combined with the previously measured QD scintillation time of 0.3–0.6 ns, this makes the InAs/GaAs QD heterostructures the fastest high-yield scintillation material reported.

Resonant waveguide grating fabrication on planar and cylindrical substrates using a photosensitive TiO2 sol-gel approach

A single-step sol-gel technological approach combining sol-gel layer development and UV lithography is demonstrated for the fabrication of resonant waveguide gratings (RWG) on planar and cylindrical substrates. The aim of this article is the demonstration of a resonant reflection in TE and TM polarization in the near-infrared region (NIR) in a planar and in cylindrical-based resonant waveguide gratings (RWG). In this work, we start with a planar corrugated waveguide structure excited by a planar wave and demonstrate this concept to a circularly symmetrical waveguide applied to the inside wall of an 8 mm diameter tube, excited by a cylindrical wave. For both configurations, the same TiO2 sol-gel layer is used for the high index waveguide layer and for the grating printing thanks to the UV photosensitivity property of the sol-gel layers, avoiding any etching processes. The reflection spectrum was measured in the near-infrared range and compared to the modeling, showing the expected resonant behavior.

Low-loss, high-bandwidth fiber-to-chip coupling using capped adiabatic tapered fibers

We demonstrate adiabatically tapered fibers terminating in sub-micron tips that are clad with a higher-index material for coupling to an on-chip waveguide. This cladding enables coupling to a high-index waveguide without losing light to the buried oxide. A technique to clad the tip of the tapered fiber with a higher-index polymer is introduced. Conventional tapered waveguides and forked tapered waveguide structures are investigated for coupling from the clad fiber to the on-chip waveguide. We find the forked waveguide facilitates alignment and packaging, while the conventional taper leads to higher bandwidth. The insertion loss from a fiber through a forked coupler to a sub-micron silicon nitride waveguide is 1.1 dB and the 3 dB bandwidth is 90 nm. The coupling loss in the packaged device is 1.3 dB. With a fiber coupled to a conventional tapered waveguide, the loss is 1.4 dB with a 3 dB bandwidth extending beyond the range of the measurement apparatus, estimated to exceed 250 nm.

Broadband transparency with all-dielectric metasurfaces engraved on silicon waveguide facets: effect of inverted and extruded features based on Babinet's principle.

Building blocks of photonic integrated circuitry (PIC), optical waveguides, have long been considered transparent. However, the inevitable Fresnel reflection from waveguide facets limits their transparency. This limitation becomes more severe in high-index waveguides in which the transparency may drop to 65%. We overcome this inherent optical property of high-index waveguides by engineering an appropriate facet landscape made of sub-wavelength artificial features unit cells. For this, we develop a semi-analytical formalism for predicting the metasurface parameters made of high-index dielectric materials, to be engraved on the facets of optical waveguides, based on Babinet's principle: either extruded from the waveguide facet or etched into it. Our semi-analytical model predicts the shape of anti-reflective metasurface unit cells to achieve transmission as high as 98.5% in near-infrared from 1 μm to 2 μm. This new class of metasurfaces may be used for the improvement of PIC devices for communication and sensing, where device transparency is crucial for high signal-to-noise ratios.

Ultra-compact, low-loss terahertz waveguide based on graphene plasmonic technology

In order to meet the demand for highly integrated terahertz chips with ultra-low transmission loss, a novel hybrid graphene plasmonic waveguide is proposed, wherein low-index insulators are embedded between a slot-rib (covered by graphene) structure and a high-index dielectric waveguide. This designed waveguide combines the plasmonic response of graphene, energy distribution mechanism of materials with large refractive index difference, and modal properties of hybrid plasmonic waveguide to yield superior waveguiding performance. We achieved 50 times stronger mode confinement along with five times smaller propagation loss, as compared to the traditional graphene hybrid plasmonic waveguide operating at 3 THz. Investigation on the influence of modal properties on chemical potential of graphene revealed an active approach to control mode confinement or propagation distance. In addition, owing to the ultra-strong energy confinement of modes, the crosstalk caused by coupling between two parallel waveguides is negligible, which paves the way for realizing ultra-compact, chip-level terahertz devices.

Fano approximation for coupled modes in metal-dielectric multilayer structures

We study the response of coupled modes in metal-dielectric multilayer planar structures consisting of a metal layer attached to a high-index waveguide by a low-index spacer layer. The structure supports excitation and coupling of a surface plasmon polariton mode and a waveguide mode. A coupled-mode model is developed to analyse the field transfer processes and describe the appearance of Fano resonances as a result of the modes interaction. It is demonstrated that the response of the plasmonic mode can be well fitted to a Fano function that allows extraction of width, height, sensitivity, and field enhancement.

CMOS-compatible high-index doped silica waveguide with an embedded silicon-nanocrystal strip for all-optical analog-to-digital conversion

Passive all-optical signal processors that overcome the electronic bottleneck can potentially be the enabling components for the next-generation high-speed and lower power consumption systems. Here, we propose and experimentally demonstrate a CMOS-compatible waveguide and its application to the all-optical analog-to-digital converter (ADC) under the nonlinear spectral splitting and filtering scheme. As the key component of the proposed ADC, a 50&#x00A0;cm long high-index doped silica glass spiral waveguide is composed of a thin silicon-nanocrystal (Si-nc) layer embedded in the core center for enhanced nonlinearity. The device simultaneously possesses low loss (0.16&#x00A0;dB/cm at 1550&#x00A0;nm), large nonlinearity (305&#x2009;&#x2009;W&#x2212;1/km at 1550&#x00A0;nm), and negligible nonlinear absorption. A 2-bit ADC basic unit is achieved when pumped by the proposed waveguide structure at the telecom band and without any additional amplification. Simulation results that are consistent with the experimental ones are also demonstrated, which further confirm the feasibility of the proposed scheme for larger quantization resolution. This demonstrated approach enables a fully monolithic solution for all-optical ADC in the future, which can digitize broadband optical signals directly at low power consumption. This has great potential on the applications of high-speed optical communications, networks, and signal processing systems.

Spectral near-field thermal emission extraction by optical waveguides

As far-field thermal emission is limited by blackbody radiation, the significant enhancement of thermal radiation in the near-field plays a vital role in a variety of applications such as infrared sensing, radiation cooling, and thermophotovoltaics. Yet the techniques of exporting the near-field signal to the far field are still not mature, typically relying on complex instrumentation capable of being applied only to the surfaces of model systems and structures. Here we develop an efficient method of extracting near-field thermal radiation to the far field by integrating a nanoscale thermal emitter with a high-index optical waveguide. By directly depositing the emitter onto the optical waveguide, it requires no vacuum gap and enables efficient coupling of near-field thermal radiation into propagating wave-guided modes. A single-mode planar waveguide incorporated with an indium-tin oxide (ITO) film as a thermal emitter is theoretically investigated to prove the feasibility of thermal extraction. The Wiener-chaos expansion method is applied to directly calculate the thermal radiation from the emitter-waveguide system. We experimentally demonstrate the fidelity of the optical-waveguide-assisted near-field thermal extraction by measuring the emission spectrum of an ITO-coated optical fiber and comparing with theoretical predictions. Our experimental demonstration in conjunction with the direct simulation paves the way for effectively extracting near-field thermal radiation with applications in chemical sensing, infrared imaging, and near-field thermal energy management.

Light confinement in a 90° double high mesa slot bend waveguide

In this work, we propose a technique to enhance the confinement of light at 90o sharp bend of a double high mesa slot (DHMS) waveguide based on Silicon on Insulator (SOI). These waveguides deliver high electric field and optical power density in low refractive index Nano-metric slot. The slot is displaced to the inner and outer periphery of the bend and explores the deviation in the relative power. The maximum relative power is attained by shifting the slot towards the outer periphery of the bend. This is only conceivable by choosing the precise slot position where the two evanescent tails of the high index waveguide modes have maximum overlap.