Fabrication Of Waveguides Research Articles

Light-matter interaction during and post polymerization in self-written polymer waveguide integrated with optical fibers

Self-written polymer waveguide is a polymer interconnection structure often integrated with optical fibers. The structure is fabricated by photopolymerization, through which a photon beam initiates a polymerization reaction to change liquid monomer into a solid polymer. During light-matter interaction photoinitiator molecules are excited to the singlet and then to the third state to form free radicals by attaching to the adjacent monomer molecules. The free radicals in turn form polymer chains and cross-linked structures. Photo-induced polymerization leads to a raised refractive index of the monomer and forms an optical waveguide. The change is permanent to allow the process of self-focusing and self-trapping of the propagation light beam and hence guide it to the aligned optical fiber. In this article, light matter interaction during polymer waveguide fabrication and post polymerization has been investigated. A photopolymer system of acrylate-based monomer, co-initiator amine and photo initiator is used to study the absorption of the monomer mixture at the spectral range of 450–550 nm. Then photobleaching was investigated after waveguide fabrication. The optical transmission efficiency of the polymer bridge increases as absorption ceases within the polymer structure such that optical loss reduces from 1.1 dB to about 0.65 dB.

(Invited) Spatially–Resolved Luminescence Properties of Etched Quantum Well Microstructures

Fabrication processes for photonic devices, especially micro- or nano-photonic devices, call for some kind of control of the materials’ optical properties at the stage of the final device. Our study focusses on measuring the optical quality of quantum well (QW) containing materials after device processing, using steps such as etching which allows fabrication of optical waveguides for example. We first designed and realized specific material structures based on an InP substrate, with a series of InAsxP1-x quantum wells such as illustrated on the figure. The samples were grown by gas-phase molecular beam epitaxy. The samples were characterized by spatially-resolved photoluminescence (PL) at low temperature (10 K). The top right part shows a typical spectrum measured from the sample we have grown: sharp lines associated with each QW can be clearly identified thanks to a careful grading of the As/P composition in each QW, while keeping its thickness to a constant value (typically 7 to 8 nm). Then we etched the samples (in the form of rectangular ridges as shown) using different etching processes, based either on wet etching or reactive ion etching (RIE), and recorded the PL spectra with high spatial resolution across the obtained structures. In parallel, the samples after etching were also characterized by cross-sectional scanning electron microscopy. Data processing from the series of PL spectra was then performed in order the characterize the effects of etching on the QW materials.We show PL intensity profiles across a series of etched stripes of different widths resulting from a RIE process. These intensity profiles were obtained from the lines associated with the different QWs in the structure. One can observe, first of all, that the PL intensity gradually increases from the edges to the center of the stripes, over a range of up to 10 µm. One can also observe that for “wide” stripes, i.e. 30 or 50 µm, the PL intensity for each line saturates at the same level in the central part. However, for the narrower stripes (10 or 20 µm), the PL signal does not reach the same level. This is the signature of some PL-limitation phenomenon, which extends its effects far from the etched sidewalls. A “standard” phenomenon resulting solely from non-radiative recombination at the etched sidewalls is excluded. Non-radiative recombination can only affect regions which are less than one charge carrier diffusion length from the sidewalls. Diffusion length in our materials is less than 1 µm. Therefore, some kind of “long range” interaction seems to affect our samples. We propose that this long range interaction is due to some stray static electric field resulting from the presence of residual ions left by the etching process at the sidewalls. The electric field affects the PL intensity through dissociation of excitons. Moreover, we observed that different conditions for the etching process (wet etching versus RIE, crystallographic orientation of the stripes) impact the magnitude of this PL intensity quenching.[1] Pearton S 1997 Materials Science and Engineering: B 44 1–7[2] Landesman J P, Isik-Goktas N, LaPierre R R, Levallois C, Ghanad-Tavakoli S, Pargon E, Petit-Etienne C and Jiménez J 2021 Journal of Physics D: Applied Physics 54 445106[3] Fiore A, Rossetti M, Alloing B, Paranthoen C, Chen J X, Geelhaar L and Riechert H 2004 Physical Review B 70 205311[4] Bludau W and Wagner E 1976 Physical Review B 13 5410–5414 Figure 1

Waveguide fabrication with integrated coupling optic

We present the fabrication of a waveguide with integrated injection optics using a laser photoinscription technique in the bulk of a chalcogenide glass. The injection optic is a gradient index lens with a parabolic refractive index profile. Its focusing characteristics depend on the amplitude of the index gradient and the thickness of the lens. These two parameters can be used to optimize light injection when the lens is written in front of a waveguide. We show experimentally that the coupling efficiency can be superior to that obtained by conventional injection with an external lens. Numerical simulations show that this coupling efficiency can be as high as 85%. The simultaneous inscription of a waveguide and its injection lens result in a monolithic component that is compact, lightweight and insensitive to mechanical or thermal perturbations. Performance has been evaluated in the mid-infrared range, but these results can be extended to the visible.

Postmelting Encapsulation of Glass Microwires for Multipath Light Waveguiding within Phosphate Glasses.

Glass waveguides are the fundamental component of advanced photonic circuits and play a pivotal role in diverse applications, including quantum information processing, light generation, imaging, data storage, and sensing platforms. Up to date, the fabrication of glass waveguides relies mainly on demanding chemical processes or on the employment of expensive ultrafast laser equipment. In this work, we demonstrate an advanced, simple, low-temperature, postmelting encapsulation procedure for the development of glass waveguides. Specifically, silver iodide phosphate glass microwires (MWs) are drawn from splat-quenched glasses. These MWs are then incorporated in a controlled manner within transparent silver phosphate glass matrices. The judicious selection of glass compositions ensures that the refractive index of the host phosphate glass is lower than that of the embedded MWs. This facilitates the propagation of light inside the encapsulated higher refractive index MWs, leading to the facile development of waveguides. Importantly, we substantially enhance the light transmission within the MWs by leveraging the plasmon resonance effects due to the presence of silver nanoparticles spontaneously generated owing to the silver iodide phosphate glass composition. Employing this innovative approach, we have successfully engineered waveguide devices incorporating either one or two MWs. Remarkably, the dual MW devices are capable of transmitting light of different colors and in multipath direction, rendering the developed waveguides outstanding candidates for extending the functionalities of diverse photonic and optoelectronic circuits, as well as in intelligent signaling applications in smart glass technologies.

Feasibility study on fabrication of Ka-band waveguides via 3D printing and post-metal coating

Feasibility study on fabrication of Ka-band waveguides via 3D printing and post-metal coating

Characterizations of two-photon absorption process induced by defects in aluminum nitride using Z-scan method

In this work, we reported two-photon absorption (TPA) measurements for aluminum vacancies in Aluminum nitride single crystals. We measured the linear transmission and identified the defect levels. Using the Z-scan method, we measured the TPA coefficients of the transitions between defect levels from 380 nm to 735 nm. The transition occurs between the aluminum vacancies defect levels. Furthermore, the power dependence shows good linear fitting, confirming the TPA mechanism. These results will be helpful for the design and fabrication of ultra-low loss waveguides and integrated photonics in the ultraviolet spectral range.

Optimization of waveguide fabrication processes in lithium-niobate-on-insulator platform.

Lithium niobate (LN) is used in diverse applications such as spectroscopy, remote sensing, and quantum communications. The emergence of lithium-niobate-on-insulator (LNOI) technology and its commercial accessibility represent significant milestones. This technology aids in harnessing the full potential of LN's properties, such as achieving tight mode confinement and strong overlap with applied electric fields, which has enabled LNOI-based electro-optic modulators to have ultra-broad bandwidths with low-voltage operation and low power consumption. Consequently, LNOI devices are emerging as competitive contenders in the integrated photonics landscape. However, the nanofabrication, particularly LN etching, presents a notable challenge. LN is hard, dense, and chemically inert. It has anisotropic etch behavior and a propensity to produce material redeposition during the reactive-ion plasma etch process. These factors make fabricating low-loss LNOI waveguides (WGs) challenging. Recognizing the pivotal role of addressing these fabrication challenges for obtaining low-loss WGs, our research focuses on a systematic study of various process steps in fabricating LNOI WGs and other photonic structures. In particular, our study involves (i) careful selection of hard mask materials, (ii) optimization of inductively coupled plasma etch parameters, and finally, (iii) determining the optimal post-etch cleaning approach to remove redeposited material on the sidewalls of the etched photonic structures. Using the recipe established, we realized optical WGs with total (propagation and coupling) loss value of -10.5 dB, comparable to established values found in the literature. Our findings broaden our understanding of optimizing fabrication processes for low-loss lithium-niobate waveguides and can serve as an accessible resource in advancing LNOI technology.

Precise Characterization of a Waveguide Fiber Interface in Silicon Carbide.

Spin-active optical emitters in silicon carbide are excellent candidates toward the development of scalable quantum technologies. However, efficient photon collection is challenged by undirected emission patterns from optical dipoles, as well as low total internal reflection angles due to the high refractive index of silicon carbide. Based on recent advances with emitters in silicon carbide waveguides, we now demonstrate a comprehensive study of nanophotonic waveguide-to-fiber interfaces in silicon carbide. We find that across a large range of fabrication parameters, our experimental collection efficiencies remain above 90%. Further, by integrating silicon vacancy color centers into these waveguides, we demonstrate an overall photon count rate of 181 kilo-counts per second, which is an order of magnitude higher compared to standard setups. We also quantify the shift of the ground state spin states due to strain fields, which can be introduced by waveguide fabrication techniques. Finally, we show coherent electron spin manipulation with waveguide-integrated emitters with state-of-the-art coherence times of T 2 ∼ 42 μs. The robustness of our methods is very promising for quantum networks based on multiple orchestrated emitters.

Low temperature PECVD processes for the fabrication of integrated symmetrical resonant UV210 organic/semiconductor structures

The resonant structures used as sensors for investigating specific dark and opaque substances may pose difficulties due to their potential aggressiveness towards photonic chips and the risk of destruction upon contact. To avoid it, this study presents a detailed description of a novel symmetrical waveguide structure with UV210 enclosed between Si/SiO2 bi-layers implemented for integrated photonics. The fabrication process is thoroughly explained, accompanied by various analyses conducted for characterizations purposes. The structure is composed of an organic UV210 material developed using deep UV lithography at 248 nm and fabricated onto an oxidized silicon layer, resulting in a Si/SiO2 bi-layer configuration. This process enables the fabrication of µm-scale access waveguides and racetrack Micro-Resonators (MRs) with a 400-nanometers gap. Several multilayer families have been produced using various low-temperature PECVD processes to obtain symmetrical Si/SiO2/UV210/Si/SiO2 structures and then properly characterized using non-destructive analyses and imaging techniques. The advantage of structural symmetry is primarily related to the electromagnetism and guidance equations, which eliminate the requirement for a cut-off thickness (or frequency), enabling significant miniaturization. Additionally, the upper cladding composed of a Si/SiO2 bi-layer offers protection against potentially aggressive substances directly in contact with the organic waveguide core and MRs. Different bi-layer upper cladding families, produced under various PECVD conditions, were characterized using XPS to understand the bonding mechanism between the silicon and UV210 organic. In addition, ellipsometric analyses were conducted to determine the real and imaginary components of the refractive index of the global structure and the upper cladding bi-layer. Structure imaging were obtained by Raman analyses and optical statistical measurements were conducted on four different types of heterostructures, with silica temperatures ranging from 120°C to 150°C during the second PECVD process. The resonance analysis, performed through Free Spectral Range (FSR) measurements, allowed us to draw conclusions regarding the stability and reproducibility of the fabrication process, as well as the impact of PECVD processes conditions on the optical measurements. Such optoelectronic resonant elements and hybrid heterostructures enable the investigation of aggressive substances in direct contact.

Overcoming oxygen inhibition in UV photolithography for the fabrication of low-loss polymer waveguides.

Perfluorinated acrylate polymer materials exhibit low absorption loss at 1310 and 1550 nm, but molecular oxygen inhibits their photocuring. We propose a novel, to our knowledge, UV photolithography method incorporating a pre-exposure process for fabricating low-loss perfluorinated acrylate polymer waveguides. During the pre-exposure process, a partially cured thin layer forms on the core layer, effectively overcoming oxygen inhibition in subsequent lithography. Furthermore, the functional group contents of the polymerized materials were characterized by a Raman spectrometer to analyze the development reaction under the pre-exposure layer. Utilizing this improved method, we fabricated a straight waveguide with a length of 21 cm. The experiments showed that the propagation losses are 0.14 dB/cm at 1310 nm and 0.51 dB/cm at 1550 nm. The inter-channel cross talk for a core pitch of 250 µm was measured as low as -49 dB at 1310 nm. Error-free NRZ data transmission over this waveguide at 25 Gb/s was achieved, showcasing the potential in optical interconnect and communication applications.

On-Chip Broadband, Compact TM Mode Mach-Zehnder Optical Isolator Based on InP-on-Insulator Platforms.

An integrated optical isolator is a crucial part of photonic integrated circuits (PICs). Existing optical isolators, predominantly based on the silicon-on-insulator (SOI) platform, face challenges in integrating with active devices. We propose a broadband, compact TM mode Mach-Zehnder optical isolator based on InP-on-insulator platforms. We designed two distinct magneto-optical waveguide structures, employing different methods for bonding Ce:YIG and InP, namely O2 plasma surface activation direct wafer bonding and DVS-benzocyclobutene (BCB) adhesive bonding. Detailed calculations and optimizations were conducted to enhance their non-reciprocal phase shift (NRPS). At a wavelength of 1550 nm, the direct-bonded waveguide structure achieved a 30 dB bandwidth of 72 nm with a length difference of 0.256 µm. The effects of waveguide arm length, fabrication accuracy, and dimensional errors on the device performance are discussed. Additionally, manufacturing tolerances for three types of lithographic processes were calculated, serving as references for practical manufacturing purposes.

The impact of various factors on the surface of X-cut lithium niobate and properties of proton-exchanged waveguides

This work describes the effects of various impacts (thermal, plasma, chemical) on X-cut lithium niobate surface and on the structure and optical characteristics of proton-exchanged waveguides. Pre-annealing at 500 °C improved the structural homogeneity of congruent lithium niobate and thus enabled fabricating waveguides of a higher quality. Ar-plasma pretreatment damaged crystal surface and increased lattice deformations. Chemical treatment influenced the surface composition of lithium niobate and the refractive index in annealed proton exchange waveguides. The results provide better understanding of the role of crystal surface in the fabrication of diffusion waveguides and photonic integrated circuits.

Surface relief structures for light coupling at air/liquid interface

Relief patterns were photoinduced on the surface of a thin film floating on water. The structures have a length of several millimeters with a width of 33 μm. Both the intensity and the polarization of the irradiated light change the characteristics of the generated patterns. The light induced structuration is the result of simultaneously a reversible material transport originated by photoisomerization of azobenzene units and a process of self-organization of the molecules. The structures were used as waveguide or grating to couple light at the air/liquid interface on a short or long distance. The fabrication of waveguides can be extended to any liquid substrates.

Laser Glass deposition of single-mode glass fibers for the fabrication of chip-scale photonic circuits

The use of glass, particularly fused silica (FS), instead of polymers or semiconductors as optical waveguide material is advantageous due to broadband transmission, reduced propagation losses, and enhanced thermal and mechanical stability. In this contribution, laser glass deposition for the chip-scale fabrication of FS-based optical waveguides is investigated. We use a CO2 laser with emission at a wavelength of 10.6 µm to weld conventional single-mode glass fibers onto a FS substrate, with the aim of maintaining the waveguide properties of the fibers. Synchronized translational and rotational axis movements allow for positioning of the waveguides in arbitrary geometries. Furthermore, a CO2 laser-based cleaving method is introduced, which facilitates on-chip creation of waveguide end facets for optical coupling. An analysis of the cleave geometry in dependence on process parameters as well as optical coupling losses are presented. Coupling losses of 0.88 dB for a laser-cleave and propagation losses of 0.56 dB/cm for a 10 cm on-chip welded fiber were achieved. The results pave the way for on-chip integration of fiber-based systems like lasers, sensing devices or optical communication networks.

Design and Fabrication of 2D Exit Pupil Expansion Holographic Grating Waveguide with Large Field of View

Design and Fabrication of 2D Exit Pupil Expansion Holographic Grating Waveguide with Large Field of View

X-Cut Lithium Niobate Optical Waveguide with High-Index Contrast and Low Loss Fabricated by Vapor Proton Exchange

Highly integrated and stable devices are appealing in optical communication and sensing. This appeal arises from the presence of high refractive index contrast and high-quality waveguides. In this study, we improved the vapor proton exchange (VPE) process, enabling large-scale waveguide fabrication and addressing the issue of liquid exchange during cooling. Additionally, we have prepared and characterized planar waveguides on X-cut lithium niobate (LN) crystals. The exchanged samples exhibit α and k1 phases, refractive index contrasts as high as 0.082, and exceptional refractive index uniformity. Furthermore, we utilized the same process to fabricate channel waveguides and Y-branch waveguides. We achieved low propagation losses in channel waveguides, accompanied by small mode sizes, and low-loss Y-branch waveguides with a highly uniform beam splitting ratio. All waveguides exhibited consistent performance across multiple preparations and tests, remaining free from aging effects for three months. Our results underscore the promising potential of VPE for creating Y-branch splitters and modulators in LN crystals.

Transmission Electron Microscopy Characterizations of Local Amorphization of Single Crystal Silicon by Nanosecond Pulsed Laser Direct Writing

The concept for fabrication of waveguides by an in‐volume laser direct writing in single‐crystal silicon is explored using a nanosecond pulse laser. The key innovation of this technology relies on the generation of amorphous silicon, which has a higher refractive index than that of crystalline silicon. Herein, transmission electron microscopy (TEM) together with selected area electron diffraction (SAED) and high‐resolution TEM (HRTEM) characterizations are used to better understand the microstructural evolutions. TEM images reveal the core‐shell structures, while SAED patterns and HRTEM directly observe the presence of amorphous silicon in the core surrounded by a crystalline silicon shell. With a lower laser scanning speed, a higher density of defects yet less amorphous silicon is formed by laser direct writing.

Structural polymorphism, electronic, and optical properties of NaSbS2: A computational approach toward eco-friendly and emerging semiconductor

In this work, the structural polymorphism and electro-optical characteristics of NaSbS2 ternary chalcogenide are realized using density functional theory (DFT). All of the NaSbS2 polymorphs are energetically stable based on the formation energy. The optimized lattice parameters of the monoclinic and triclinic structures and direct electronic bandgap (1.695 eV) of monoclinic structure are consistent with previous studies. Besides, the trigonal and triclinic structures are indirect bandgap (0.916 and 2.015 eV) semiconductors observed from electronic band structure. The higher degree of ionicity between Na and S atoms while strong covalency in the Sb-S bonds are confirmed from the charge and bond analysis. The lower value of electron-hole effective masses indicates higher carrier mobility leading to outstanding electrical conductivity. It was suggested that the polymorphs would be extremely desired to design Bragg reflectors and waveguides based on the reported high value of the real part of the dielectric constant and refractive index. These polymorphs will contribute significantly to being used as absorbers, as evidenced by their high absorption coefficient and photoconductivity and lower transmittance. All the polymorphs reflect significant optical anisotropy and birefringence, rendering them excellent for usage in the fabrication of waveguides, polarizers, and photonic devices. Thus, the projected good features of NaSbS2 polymorphs would play an essential role in the development of lead-free photovoltaic devices, including solar cells and other optoelectronic devices, and would be able to provide practical information for future theoretical and experimental studies.

A low-loss molybdenum plasmonic waveguide: perfect single-crystal preparation and subwavelength grating optimization

Abstract Plasmonic waveguides have attracted tremendous interest due to efficiently confining photons on the subwavelength spatial scale to be beating the propagation diffraction limit. Transition metal molybdenum (Mo) exhibits outstanding properties in light trapping and electromagnetic field confining, making it potentially valuable in 1.55 μm plasmonic waveguide applications. However, the reliable fabrication of high-quality Mo plasmonic waveguides is a significant challenge. A real-space micro-imaging study of the surface plasmon on Mo structures is still absent. In this study, we successfully prepared a single-crystalline Mo microrod waveguide structure and fabricated subwavelength gratings on it. The diffraction gratings were designed, optimized, and etched to excite the surface plasmon polariton behaviour of Mo for the first time. The grating-optimized Mo microrod single-crystal reveals highly efficient waveguide performance around near-infrared spectroscopy, exhibiting a long propagation length of 32 μm and a low transmission loss of 0.067 dB μm−1. The results provide an alternative to advanced materials research and optical device applications of plasmonic waveguide systems.

Silicon-Organic Hybrid Electro-Optic Modulator and Microwave Photonics Signal Processing Applications.

Electro-optic modulator (EOM) is one of the key devices of high-speed optical fiber communication systems and ultra-wideband microwave photonic systems. Silicon-organic hybrid (SOH) integration platform combines the advantages of silicon photonics and organic materials, providing a high electro-optic effect and compact structure for photonic integrated devices. In this paper, we present an SOH-integrated EOM with comprehensive investigation of EOM structure design, silicon waveguide fabrication with Slot structure, on-chip poling of organic electro-optic material, and characterization of EO modulation response. The SOH-integrated EOM is measured with 3 dB bandwidth of over 50 GHz and half-wave voltage length product of 0.26 V·cm. Furthermore, we demonstrate a microwave photonics phase shifter by using the fabricated SOH-integrated dual parallel Mach-Zehnder modulator. The phase shift range of 410° is completed from 8 GHz to 26 GHz with a power consumption of less than 38 mW.