@article{60891, abstract = {{Straight dielectric waveguide channels made from slabs of thin-film lithium niobate (TFLN), or lithium niobate on insulator (LNOI), are investigated in the linear regime, for channels of rib and strip type with common trapezoidal cross sections, in Z-cut and X-cut samples at varying on-chip orientation. We clarify the theoretical basis for the waveguides with potentially non-diagonal core permittivity. Symmetry classes can be distinguished that differ in their consequences for potential modal degeneracy and polarization conversion. Our rigorous numerical analysis by means of a finite-element solver takes the anisotropy of the lithium niobate cores rigorously into account. We discuss extensive data for effective indices, polarization properties, and hybridization of guided modes, in single- and multimode channels. Scans over the waveguide width and orientation as primary parameters are complemented by a series of illustrations of vectorial mode profiles. These turn out to be essentially complex in cases of X-cut channels at non-crystal-axis-aligned orientations.}}, author = {{Hammer, Manfred and Khan, Shahriar and Taheri, Behnood and Farheen, Henna and Förstner, Jens}}, issn = {{2770-0208}}, journal = {{Optics Continuum}}, keywords = {{tet_topic_waveguide}}, number = {{10}}, pages = {{2356}}, publisher = {{Optica Publishing Group}}, title = {{{TFLN channel waveguides of rib and strip type: Properties of guided modes}}}, doi = {{10.1364/optcon.569959}}, volume = {{4}}, year = {{2025}}, } @inproceedings{61760, abstract = {{We present a topology-optimized silicon nitride (Si3N4) coupler designed to enhance the coupling efficiency between integrated single-photon emitters and photonic waveguide modes. By leveraging inverse design techniques, we optimize the coupler’s geometry to maximize power transfer while maintaining fabrication feasibility by improving mode overlap and directional emission, addressing the challenge of low coupling efficiency caused by size mismatch and material incompatibility. Simulations demonstrate a substantial enhancement in photon extraction and waveguide coupling. This approach can be extended to other photonic devices, offering a versatile framework for improving quantum light-matter interactions in integrated photonics.}}, author = {{Farheen, Henna and Chen, Yuheng and Chen, Peigang and Kryvobok, Artem and Peana, Samuel and Senichev, Alexander and Shalaev, Vladimir M. and Boltasseva, Alexandra and Förstner, Jens and Kildishev, Alexander V.}}, booktitle = {{Photonic Computing: From Materials and Devices to Systems and Applications II}}, editor = {{Ni, Xingjie and Cai, Wenshan}}, keywords = {{tet_topic_waveguide}}, publisher = {{SPIE}}, title = {{{Topology-optimized silicon nitride coupler for integrated single-photon emitters}}}, doi = {{10.1117/12.3065734}}, year = {{2025}}, } @article{54668, abstract = {{Samples of dielectric optical waveguides of rib or strip type in thin-film lithium niobate (TFLN) technology are characterized with respect to their optical loss using the Fabry-Pérot method. Attributing the losses mainly to sidewall roughness, we employ a simple perturbational procedure, based on rigorously computed mode profiles of idealized channels, to estimate the attenuation for waveguides with different cross sections. A single fit parameter suffices for an adequate modelling of the effect of the waveguide geometry on the loss levels.}}, author = {{Hammer, Manfred and Babel, Silia and Farheen, Henna and Padberg, Laura and Scheytt, J. Christoph and Silberhorn, Christine and Förstner, Jens}}, issn = {{1094-4087}}, journal = {{Optics Express}}, keywords = {{tet_topic_waveguide}}, number = {{13}}, pages = {{22878}}, publisher = {{Optica Publishing Group}}, title = {{{Estimation of losses caused by sidewall roughness in thin-film lithium niobate rib and strip waveguides}}}, doi = {{10.1364/oe.521766}}, volume = {{32}}, year = {{2024}}, } @article{55751, abstract = {{Lateral leakage of TM modes in dielectric optical waveguides of rib/ridge or strip-loaded types can be fully suppressed, if the waveguide core is formed not through a strip that protrudes at one side (up) from the remaining lateral guiding slab, but through parallel strips on both sides (up and down), such that the resulting cross section becomes vertically symmetric. The fairly general arguments underlying the leakage suppression apply to TM modes of all orders simultaneously, and are independent of wavelength. These plus-shaped waveguides support strictly guided, non-leaky TM modes for, in principle, arbitrarily shallow etching.}}, author = {{Üstün, Necati and Farheen, Henna and Hammer, Manfred and Förstner, Jens}}, issn = {{0740-3224}}, journal = {{Journal of the Optical Society of America B}}, keywords = {{tet_topic_waveguide}}, number = {{9}}, pages = {{2077}}, publisher = {{Optica Publishing Group}}, title = {{{Symmetry-protected TM modes in rib-like, plus-shaped optical waveguides with shallow etching}}}, doi = {{10.1364/josab.528729}}, volume = {{41}}, year = {{2024}}, } @article{56193, abstract = {{Dielectric slab waveguides made of thin-film-lithium-niobate (TFLN) media are consid-ered, for operation in the linear regime. We outline and implement a largely analytic procedure forrigorous modal analysis of three-layer slabs with birefringent, anisotropic core. For Z-cut wave-guides, the slab eigenmode problem separates into uncoupled sets of scalar equations for TE andTM modes. Slabs in X-cut configuration support mostly mildly hybrid eigenmodes, with clear pre-dominant TE or TM polarization, and with effective indices that depend on the propagation directionof the modes, relative to the crystal axes. Strong hybridization can be observed for near degeneratemodes in singular configurations without vertical symmetry, or in symmetric slabs where two nearlydegenerate modes are of the same symmetry class. Dispersion curves for slab thickness and propa-gation angle are discussed, for slabs with oxide and air cover. }}, author = {{Hammer, Manfred and Farheen, Henna and Förstner, Jens}}, issn = {{2770-0208}}, journal = {{Optics Continuum}}, keywords = {{tet_topic_waveguide}}, pages = {{532822}}, publisher = {{Optica Publishing Group}}, title = {{{Guided modes of thin-film lithium niobate slabs}}}, doi = {{10.1364/optcon.532822}}, year = {{2024}}, } @article{43245, abstract = {{High-contrast slab waveguide Bragg gratings with 1D periodicity are investigated. For specific oblique excitation by semi-guided waves at sufficiently high angles of incidence, the idealized structures do not exhibit any radiative losses, such that reflectance and transmittance for the single port mode add strictly up to one. We consider a series of symmetric, fully and partly etched finite gratings, for parameters found in integrated silicon photonics. These can act as spectral filters with a reasonably flattop response. Apodization can lead to more box shaped reflectance and transmittance spectra. Together with a narrowband Fabry–Perot filter, these configurations are characterized by reflection bands, or transmittance peaks, with widths that span three orders of magnitude.}}, author = {{Hammer, Manfred and Farheen, Henna and Förstner, Jens}}, issn = {{0740-3224}}, journal = {{Journal of the Optical Society of America B}}, keywords = {{tet_topic_waveguide}}, number = {{4}}, pages = {{862}}, publisher = {{Optica Publishing Group}}, title = {{{How to suppress radiative losses in high-contrast integrated Bragg gratings}}}, doi = {{10.1364/josab.485725}}, volume = {{40}}, year = {{2023}}, } @inproceedings{30387, abstract = {{Resonant evanescent coupling can be utilized to selectively excite orbital angular momentum (OAM) modes of high angular order supported by a thin circular dielectric rod. Our 2.5-D hybrid-analytical coupled mode model combines the vectorial fields associated with the fundamental TE- and TM-modes of a standard silicon photonics slab waveguide, propagating at oblique angles with respect to the rod axis, and the hybrid modes supported by the rod. One observes an efficient resonant interaction in cases where the common axial wavenumber of the waves in the slab matches the propagation constant of one or more modes of the rod. For certain modes of high angular order, the incident wave is able to transfer its directionality to the field in the fiber, exciting effectively only one of a pair of degenerate OAM modes}}, author = {{Hammer, Manfred and Ebers, Lena and Förstner, Jens}}, booktitle = {{Complex Light and Optical Forces XVI}}, editor = {{Andrews, David L. and Galvez, Enrique J. and Rubinsztein-Dunlop, Halina}}, keywords = {{tet_topic_waveguide}}, pages = {{120170F}}, publisher = {{SPIE}}, title = {{{Resonant evanescent excitation of OAM modes in a high-contrast circular step-index fiber}}}, doi = {{10.1117/12.2612179}}, year = {{2022}}, } @phdthesis{30722, abstract = {{In dieser Arbeit wird die elektromagnetische Wellenausbreitung in integrierten optischen Wellenleitern mit Hilfe von halb analytischen und numerischen Simulationsmethoden untersucht. Im ersten Teil werden 2-D Si/SiO2-Wellenleiterkonfigurationen mit hohem Brechungsindexkontrast betrachtet. Die Strukturen werden mit halb geführten Wellen unter schrägen Ausbreitungswinkeln angeregt. Dadurch kann die Leistungsübertragung zu bestimmten ausgehenden Moden unterdrückt werden, wodurch vollständig verlustfreie Systeme entstehen. Zusätzlich dient die Anregung mit einem seitlich begrenzten, einfallenden Wellenbündel aus halb geführten Wellen dazu, praktisch relevantere 3-D Konfigurationen zu realisieren. Darüber hinaus wird eine schrittweise Winkelspektrum-Methode vorgestellt, die es ermöglicht, in Kombination mit voll vektoriellen 2-D Finite-Elemente-Lösungen für Teilprobleme mit geringerer Komplexität, die Wellenausbreitung in planaren, linsenförmigen Wellenleitern numerisch in drei Raumrichtungen zu berechnen. Im zweiten Teil dieser Arbeit wird die Ausbreitung in Wellenleiterstrukturen aus Lithiumniobat untersucht, welche für quantenoptische Effekte genutzt werden. Zur Detektion einzelner Photonen werden supraleitende Nanodrähte auf eindiffundierten Lithiumniobat Wellenleitern mit zusätzlicher Taperschicht aus Silizium betrachtet. Um die Wellenausbreitung in diesen 3-D Wellenleitern zu beschreiben, wird eine einseitig gerichtete Finite-Elemente „Modal Matching“ Methode eingeführt. Abschließend werden Rippenwellenleiter aus Lithiumniobat analysiert, die auf Siliziumdioxid Plattformen aufgebracht sind. Der Schwerpunkt liegt hier auf dem nichtlinearen „Parametric Down-Conversion“ Prozess, der für die Erzeugung verschränkter Photonen verwendet wird.}}, author = {{Ebers, Lena}}, keywords = {{tet_topic_waveguide}}, title = {{{Semi-guided waves in integrated optical waveguide structures}}}, doi = {{10.17619/UNIPB/1-1288}}, year = {{2022}}, } @article{35128, abstract = {{Here we demonstrate a new, to the best of our knowledge, type of 3-dB coupler that has an ultra-broadband operational range from 1300 to 1600 nm with low fabrication sensitivity. The overall device size is 800 µm including in/out S-bend waveguides. The coupler is an asymmetric non-uniform directional coupler that consists of two tapered waveguides. One of the coupler arms is shifted by 100 µm in the propagation direction, which results in a more wavelength-insensitive 3-dB response compared to a standard (not shifted) coupler. Moreover, compared to a long adiabatic coupler, we achieved a similar wavelength response at a 16-times-smaller device length. The couplers were fabricated using the silicon nitride platform of Lionix International. We also experimentally demonstrated an optical switch that is made by using two of these couplers in a Mach–Zehnder interferometer configuration. According to experimental results, this optical switch exhibits –10 dB of extinction ratio over the 1500–1600 nm wavelength range. Our results indicate that this new type of coupler holds great promise for various applications, including optical imaging, telecommunications, and reconfigurable photonic processors where compact, fabrication-tolerant, and wavelength-insensitive couplers are essential.}}, author = {{Nikbakht, Hamed and Khoshmehr, Mohammad Talebi and van Someren, Bob and Teichrib, Dieter and Hammer, Manfred and Förstner, Jens and Akca, B. Imran}}, issn = {{0146-9592}}, journal = {{Optics Letters}}, keywords = {{tet_topic_waveguide}}, number = {{2}}, pages = {{207}}, publisher = {{Optica Publishing Group}}, title = {{{Asymmetric, non-uniform 3-dB directional coupler with 300-nm bandwidth and a small footprint}}}, doi = {{10.1364/ol.476537}}, volume = {{48}}, year = {{2022}}, } @inproceedings{30389, abstract = {{Online solvers for a series of standard 1-D or 2-D problems in integrated optics will be discussed. Implemented on the basis of HTML/JavaScript/SVG with core routines compiled from well tested C++-sources, the quasi-analytical algorithms require a computational load that can be handled easily even by current mobile devices. So far the series covers the 1-D guided modes of dielectric multilayer slab waveguides and the oblique plane wave reflection from these, the modes of rectangular channel waveguides (in an approximation of effective indices), bend modes of curved multilayer slabs, whispering-gallery resonances (“Quasi-Normal-Modes”) supported by circular dielectric cavities, the hybrid modes of circular multi-step-index optical fibers, bound and leaky modes of 1-D complex multilayers, including plasmonic surface modes, and, with restrictions, quite general rectangular scattering problems in 2-D.}}, author = {{Hammer, Manfred}}, booktitle = {{Integrated Optics: Devices, Materials, and Technologies XXVI}}, editor = {{García-Blanco, Sonia M. and Cheben, Pavel}}, keywords = {{tet_topic_waveguide}}, pages = {{1200414}}, publisher = {{SPIE}}, title = {{{Small-scale online simulations in guided-wave photonics}}}, doi = {{10.1117/12.2612208}}, year = {{2022}}, } @article{30210, abstract = {{Lithium niobate on insulator (LNOI) has a great potential for photonic integrated circuits, providing substantial versatility in design of various integrated components. To properly use these components in the implementation of different quantum protocols, photons with different properties are required. In this paper, we theoretically demonstrate a flexible source of correlated photons built on the LNOI waveguide of a special geometry. This source is based on the parametric down-conversion (PDC) process, in which the signal and idler photons are generated at the telecom wavelength and have different spatial profiles and polarizations, but the same group velocities. Distinguishability in polarizations and spatial profiles facilitates the routing and manipulating individual photons, while the equality of their group velocities leads to the absence of temporal walk-off between photons. We show how the spectral properties of the generated photons and the number of their frequency modes can be controlled depending on the pump characteristics and the waveguide length. Finally, we discuss special regimes, in which narrowband light with strong frequency correlations and polarization-entangled Bell states are generated at the telecom wavelength.}}, author = {{Ebers, Lena and Ferreri, Alessandro and Hammer, Manfred and Albert, Maximilian and Meier, Cedrik and Förstner, Jens and Sharapova, Polina R.}}, issn = {{2515-7647}}, journal = {{Journal of Physics: Photonics}}, keywords = {{tet_topic_waveguide}}, pages = {{025001}}, publisher = {{IOP Publishing}}, title = {{{Flexible source of correlated photons based on LNOI rib waveguides}}}, doi = {{10.1088/2515-7647/ac5a5b}}, volume = {{4}}, year = {{2022}}, } @article{28196, abstract = {{We show that narrow trenches in a high-contrast silicon-photonics slab can act as lossless power dividers for semi-guided waves. Reflectance and transmittance can be easily configured by selecting the trench width. At sufficiently high angles of incidence, the devices are lossless, apart from material attenuation and scattering due to surface roughness. We numerically simulate a series of devices within the full 0-to-1-range of splitting ratios, for semi-guided plane wave incidence as well as for excitation by focused Gaussian wave bundles. Straightforward cascading of the trenches leads to concepts for 1×M-power dividers and a polarization beam splitter.}}, author = {{Hammer, Manfred and Ebers, Lena and Förstner, Jens}}, issn = {{2578-7519}}, journal = {{OSA Continuum}}, keywords = {{tet_topic_waveguide}}, number = {{12}}, pages = {{3081}}, title = {{{Configurable lossless broadband beam splitters for semi-guided waves in integrated silicon photonics}}}, doi = {{10.1364/osac.437549}}, volume = {{4}}, year = {{2021}}, } @inproceedings{21719, abstract = {{We fabricate silicon tapers to increase the mode overlap of superconducting detectors on Ti:LiNbO3 waveguides. Mode images show a reduction in mode size from 6 µm to 2 µm FWHM, agreeing with beam propagation simulations.}}, author = {{Protte, Maximilian and Ebers, Lena and Hammer, Manfred and Höpker, Jan Philipp and Albert, Maximilian and Quiring, Viktor and Meier, Cedrik and Förstner, Jens and Silberhorn, Christine and Bartley, Tim}}, booktitle = {{OSA Quantum 2.0 Conference}}, isbn = {{9781943580811}}, keywords = {{tet_topic_waveguide}}, title = {{{Towards Semiconductor-Superconductor-Crystal Hybrid Integration for Quantum Photonics}}}, doi = {{10.1364/quantum.2020.qth7a.8}}, year = {{2020}}, } @article{3740, abstract = {{Oblique propagation of semi-guided waves across slab waveguide structures with bent corners is investigated. A critical angle can be defined beyond which all radiation losses are suppressed. Additionally an increase of the curvature radius of the bends also leads to low-loss configurations for incidence angles below that critical angle. A combination of two bent corner systems represents a step-like structure, behaving like a Fabry-Perot interferometer, with two partial reflectors separated by the vertical height between the horizontal slabs. We numerically analyse typical high-index-contrast Si/SiO2 structures for their reflectance and transmittance properties. When increasing the curvature radius the resonant effect becomes less relevant such that full transmittance is reached with less critical conditions on the vertical distance or the incidence angle. For practical interest 3-D problems are considered, where the structures are excited by the fundamental mode of a wide, shallow rib waveguide. High transmittance levels can be observed also for these 3-D configurations depending on the width of the rib.}}, author = {{Ebers, Lena and Hammer, Manfred and Förstner, Jens}}, journal = {{Optics Express}}, keywords = {{tet_topic_waveguide}}, number = {{14}}, pages = {{18621--18632}}, publisher = {{OSA Publishing}}, title = {{{Oblique incidence of semi-guided planar waves on slab waveguide steps: effects of rounded edges}}}, doi = {{10.1364/OE.26.018621}}, volume = {{26}}, year = {{2018}}, } @inbook{3743, abstract = {{Frequently, optical integrated circuits combine elements (waveguide channels, cavities), the simulation of which is well established through mature numerical eigenproblem solvers. It remains to predict the interaction of these modes. We address this task by a general, “Hybrid” variant (HCMT) of Coupled Mode Theory. Using methods from finite-element numerics, the properties of a circuit are approximated by superpositions of eigen-solutions for its constituents, leading to quantitative, computationally cheap, and easily interpretable models.}}, author = {{Hammer, Manfred}}, booktitle = {{Recent Trends in Computational Photonics}}, editor = {{Agrawal, Arti}}, isbn = {{978-3-319-55438-9}}, keywords = {{tet_topic_waveguide, tet_topic_numerics}}, pages = {{77--105}}, publisher = {{Springer}}, title = {{{Guided Wave Interaction in Photonic Integrated Circuits — A Hybrid Analytical/Numerical Approach to Coupled Mode Theory}}}, volume = {{204}}, year = {{2017}}, } @article{3828, abstract = {{The 3D implementation of a hybrid analytical/numerical variant of the coupled-mode theory is discussed. Eigenmodes of the constituting dielectric channels are computed numerically. The frequency-domain coupled-mode models then combine these into fully vectorial approximations for the optical electromagnetic fields of the composite structure. Following a discretization of amplitude functions by 1D finite elements, pro- cedures from the realm of finite-element numerics are applied to establish systems of linear equations for the then- discrete modal amplitudes. Examples substantiate the functioning of the technique and allow for some numerical assessment. The full 3D simulations are highly efficient in memory consumption, moderately demanding in com- putational time, and, in regimes of low radiative losses, sufficiently accurate for practical design. Our results include the perturbation of guided modes by changes of the refractive indices, the interaction of waves in parallel, horizontally or vertically coupled straight waveguides, and a series of crossings of potentially overlapping channels with fairly arbitrary relative positions and orientations.}}, author = {{Hammer, Manfred and Alhaddad, Samer and Förstner, Jens}}, issn = {{0740-3224}}, journal = {{Journal of the Optical Society of America B}}, keywords = {{tet_topic_waveguide, tet_topic_numerics}}, number = {{3}}, pages = {{613--624}}, publisher = {{The Optical Society}}, title = {{{Hybrid coupled-mode modeling in 3D: perturbed and coupled channels, and waveguide crossings}}}, doi = {{10.1364/josab.34.000613}}, volume = {{34}}, year = {{2017}}, } @article{3830, abstract = {{The modal properties of curved dielectric slab waveguides are investigated. We consider quasi-confined, attenuated modes that propagate at oblique angles with respect to the axis through the center of curvature. Our analytical model describes the transition from scalar 2-D TE/TM bend modes to lossless spiral waves at near-axis propagation angles, with a continuum of vectorial attenuated spiral modes in between. Modal solutions are characterized in terms of directional wavenumbers and attenuation constants. Examples for vectorial mode profiles illustrate the effects of oblique wave propagation along the curved slab segments. For the regime of lossless spiral waves, the relation with the guided modes of corresponding dielectric tubes is demonstrated.}}, author = {{Ebers, Lena and Hammer, Manfred and Förstner, Jens}}, issn = {{0306-8919}}, journal = {{Optical and Quantum Electronics}}, keywords = {{tet_topic_waveguide}}, number = {{4}}, pages = {{49:176}}, publisher = {{Springer Nature}}, title = {{{Spiral modes supported by circular dielectric tubes and tube segments}}}, doi = {{10.1007/s11082-017-1011-x}}, volume = {{49}}, year = {{2017}}, } @inproceedings{3543, author = {{Hildebrandt, Andre and Alhaddad, Samer and Hammer, Manfred and Förstner, Jens}}, booktitle = {{Integrated Optics: Devices, Materials, and Technologies XX}}, editor = {{Broquin, Jean-Emmanuel and Nunzi Conti, Gualtiero}}, keywords = {{tet_topic_waveguide}}, publisher = {{SPIE}}, title = {{{Oblique incidence of semi-guided waves on step-like folds in planar dielectric slabs: Lossless vertical interconnects in 3D integrated photonic circuits}}}, doi = {{10.1117/12.2214460}}, year = {{2016}}, } @inproceedings{3934, abstract = {{Typical optical integrated circuits combine elements, like straight and curved waveguides, or cavities, the simulation and design of which is well established through numerical eigenproblem-solvers. It remains to predict the interaction of these modes. We address this task by a ”Hybrid” variant (HCMT) of Coupled Mode Theory. Using methods from finite-element numerics, the optical properties of a circuit are approximated by superpositions of eigen-solutions for its constituents, leading to quantitative, low-dimensional, and interpretable models in the frequency domain. Spectral scans are complemented by the direct computation of supermode properties (spectral positions and linewidths, coupling-induced phase shifts). This contribution outlines the theoretical background, and discusses briefly limitations and implementational details, with the help of an example of a 2-D coupled-resonator-optical-waveguide configuration.}}, author = {{Hammer, Manfred}}, booktitle = {{Integrated Optics: Devices, Materials, and Technologies XX}}, editor = {{Broquin, Jean-Emmanuel and Nunzi Conti, Gualtiero}}, keywords = {{tet_topic_waveguide, tet_topic_numerics}}, location = {{San Francisco, USA}}, number = {{9750}}, pages = {{975018--975018--8 }}, publisher = {{SPIE}}, title = {{{Wave interaction in photonic integrated circuits: Hybrid analytical / numerical coupled mode modeling}}}, doi = {{10.1117/12.2214331}}, year = {{2016}}, } @article{3845, abstract = {{The device principle of a prism-based on-chip spectrometer for TE polarization is introduced. The spectrometer exploits the modal dispersion in planar waveguides in a layout with slab regions having two different thicknesses of the guiding layer. The set-up uses parabolic mirrors, for the collimation of light of the input waveguide and focusing of the light to the receiver waveguides, which relies on total internal reflection at the interface between two such regions. These regions are connected adiabatically to prevent unwanted mode conversion and loss at the edges of the prism. The structure can be fabricated with two wet etching steps. The paper presents basic theory and a general approach for device optimization. The latter is illustrated with a numerical example assuming SiON technology.}}, author = {{Civitci, F. and Hammer, Manfred and Hoekstra, H.J.W.M.}}, issn = {{0030-4018}}, journal = {{Optics Communications}}, keywords = {{tet_topic_waveguide}}, pages = {{29--37}}, publisher = {{Elsevier BV}}, title = {{{Planar prism spectrometer based on adiabatically connected waveguiding slabs}}}, doi = {{10.1016/j.optcom.2015.11.066}}, volume = {{365}}, year = {{2015}}, }