Tamm modes, strongly confined between a metal and a distributed Bragg reflector (DBR), offer the advantage of being accessible with conventional excitation sources, without requiring coupling elements such as prisms. Here, we experimentally observe higher-order Tamm modes in such structures by inserting a cavity between the metal and the Bragg reflector. These modes exhibit stronger energy localization and higher quality factors than conventional Tamm modes, making them attractive candidates for future optical sensing applications. To explore their potential for refractive index sensing while maintaining the excitation conditions of higher-order Tamm modes, we numerically investigate a nanostructuring of both the gold layer and the cavity. This approach preserves the high reflectivity of the gold layer, ensuring the excitation of Tamm modes, while enabling access of the analyte to the confined field region, thereby providing a promising proof-of-concept for future liquid sensing applications.

In this work, a single-port surface acoustic wave (SAW) resonator with a Co40Fe40B20/SiO2/(X+90 degrees) ST-cut quartz multilayer structure is investigated as a magnetic field sensor. The Love wave resonator design is aimed at achieving a high magnetic field sensitivity, together with a temperature compensation. Two versions of sensitive layers are studied: Co40Fe40B20 (100 nm) and Co40Fe40B20(100 nm)/ SiO2(3 nm)/Co40Fe40B20(100 nm). For the latter, the 3-nm-thick SiO2 layer is inserted between two 100-nmthick amorphous Co40Fe40B20 layers, with the objective of controlling the magnetic properties of the magnetoelastic layer. A quartz crystal is selected as the piezoelectric substrate and a 375-nm-thick SiO2 layer is used as insulating medium. This thickness is chosen to achieve insensitivity to temperature variations of the Love wave velocity at 433 MHz industrial, scientific, and medical (ISM) band, when no magnetic field is applied. Optimized structures are fabricated and characterized, demonstrating a good agreement between experimental results and the simulation. The structure exhibits high magnetic field sensitivity of -6900 ppm/mT (corresponding to 3 Hz/nT) along the hard axis with the 200 nm total thickness of Co40Fe40B20 bi-layer. A substantial 26-fold increase in magnetic field sensitivity is achieved compared to that of the 100-nm-thick Co40Fe40B20.

This paper presents an analytical study of bound states in the continuum (BICs) in a photonic step-ladder waveguide structure. We demonstrate that BICs can arise in a cavity formed by one vertical and two horizontal waveguides inserted between two semi-infinite leads. Using the Green’s function method, we derive exact analytical expressions for the system’s eigenmodes, transmission, reflection, and the conditions required for BIC formation under both Neumann and Dirichlet boundary conditions. We show that when the horizontal waveguide lengths are commensurate, BICs are generated independently of the vertical guide length, allowing resonance control through geometric design. Depending on the vertical guide length, the system operates in either weak or strong coupling regimes, producing Friedrich-Wintgen BICs (FW-BICs). Breaking the symmetry of the structure leads to electromagnetically induced transparency (EIT) or reflection (EIR) resonances with sharp transmission peaks and high quality factors, making the proposed design promising for highly sensitive photonic sensing applications.

Magnetic field sensors combining surface acoustic waves (SAW) and magnetostrictive thin films have shown promising sensitivity in laboratory studies, but their translation into real-world applications has remained limited. In this work, we report the first practical deployment of a shear-horizontal SAW magnetic sensor for the characterization of ferromagnetic electrical steels, which are key materials for the fabrication of high-efficiency electrical machines. The proposed sensor design is built on a Quartz substrate and features a nanostructured TbCo₂/FeCo film (140nm) overlaid with a 700nm SiO₂ layer that enhances shear-wave confinement and protects the surface. The sensor chip is then integrated directly onto the surface of a ferromagnetic lamella. Using a compact radar-based RF circuit (AD8302 IC), we achieve phase-sensitive detection with a sensitivity of ~91V/T and a noise spectral density of 83 nV/√Hz. Both static and dynamic magnetic losses were measured at the lamella surface, showing excellent agreement with Hall sensor benchmarks. This study establishes the first proof of concept for the application of SAW magnetostrictive sensors, demonstrating their potential as a powerful tool for the in-situ characterization and monitoring of ferromagnetic materials in electrical engineering.

Multilayer dielectric thin films are fundamental components in modern microelectronic and photonic devices where thermal management is critical. This work presents a robust methodological framework for accurately determining the cross-plane effective thermal conductivity (κeff,⊥) of such complex systems using the 3-omega method. We use a five-layer Si3N4/SiO2 stack fabricated by plasma-enhanced chemical vapor deposition as a case study. The analysis combines experimental data from multiple heater geometries with a 3D finite element method based inverse analysis. We first demonstrate, through a frequency-dependent sensitivity analysis, that a direct multi-parameter fit for intrinsic layer properties is an ill-posed problem. This analysis provides a clear, quantitative justification for adopting a simpler and more robust effective medium model (EMA). The validity and application boundaries of the EMA are then rigorously established through a numerical study on a series of "virtual samples." Finally, applying this validated framework to our experimental data, the thermal conductivity of a fused silica substrate was determined to be 1.287 ± 0.030 W/(m K), and the effective thermal conductivity of the 1288 nm thick stack was reliably determined to be 0.621 ± 0.008 W/(m K). This work provides not only a key thermophysical property for Si3N4/SiO2 multilayers but also a comprehensive and validated workflow for reliably characterizing complex thin film systems where standard analytical solutions fail.

We propose the design of a comb-shaped cavity that can support several BICs when attached to a waveguide. The cavity consists of a finite comb made of one stub of length d_3 placed between two stubs of length d_2 via two guides of length d_1 and the whole comb is attached vertically along a waveguide. In order to classify these BICs, we compare their frequencies with those of an infinite periodic comb in the presence of a stub defect of length d_3 . Therefore, we show that beyond the resonances associated with the defect stub in the gaps, there exist two types of BICs: (i) BICs induced by the upper guides of the finite system, which fall in the wide passband of the infinite system and (ii) BICs induced by the whole finite comb which coincide with the flat band domain. These latter BICs appear as anti-symmetric and symmetric modes, giving rise to a doubly-degenerate BIC (D-BIC) when they fall at the same frequency. By deviating from the BIC condition, we show that these two types of BICs transform into quasi-BICs in the shape of electromagnetically induced reflection (EIR) or electromagnetic induced transparency (EIT) resonances. The analytical results obtained by the Green’s function method are confirmed by experiments using coaxial cables operating in the radio-frequency regime. This investigation contributes to an enhanced theoretical and experimental comprehension of BICs in stubbed structures, offering valuable insights for prospective applications.

Flow control aims at modifying a natural flow state to reach an other flow state considered as advantageous. In this paper, active feedback flow separation control is investigated with two different closed-loop control strategies, involving a reference signal tracking architecture. Firstly, a data-driven control law, leading to a linear (integral) controller is employed. Secondly, a phenomenological/model-driven approach, leading to a non-linear positive (integral) control strategy is investigated. While the former benefits of a tuning simplicity, the latter prevents undesirable effects and formally guarantees closed-loop stability. Both control approaches were validated through wind tunnel experiments of flow separation control over a movable NACA 4412 plain flap. These control laws were designed with respect to hot-film measurements, performed over the flap for different deflection angles. Both control approaches proved efficient in avoiding flow separation. The main contribution of this work stands in providing practitioners, simple but yet efficient control design methods for the flow separation phenomena. Equivalently important, a complete validation campaign data-set is also provided.

In this work, we propose a sensor based on Tamm plasmonic resonance; the structure is composed of gold nanoribbons deposited on a Distributed Bragg Reflector (DBR) (SiO 2 /Si 3 N 4 ) 6 .We have enhanced the sensitivity of our sensor from 40 nm/RIU to 200 nm/RIU for a refractive index change of 1% by replacing the last layer of Si 3 N 4 in contact with gold with porous Si 3 N 4 with a porosity of p = 40%.

Magnetostrictive thin films, exhibit significant utility as functionalization materials for Surface acoustic wave sensors designed for magnetic field measurements. This research investigates the influence of annealing under vacuum and magnetic field at various temperatures on the magnetic properties of the FeCo/TbCo2 thin film and therefore on the sensitivity of shear and Rayleigh acoustic waveguides functionalized with these magnetic layers. The observed effects include a reduction in magnetic anisotropy field and an increase in magnetostriction. XPS and TEM coupled with EDS microanalysis provide insights into the variations in the properties of the nanostructured magnetic film. To safeguard the magnetic film, a thin layer of SiO2 is deposited on top, serving as a protective shield. The inclusion of this layer amplifies sensitivity to applied magnetic fields for modes with shear polarization while reducing sensitivity for Rayleigh mode. In ST-cut Quartz, the shear waves become waveguided upon the incorporation of magnetic thin films, with further enhancement achieved by introducing a SiO2 layer. Rayleigh waves are evanescent modes, the penetration depth is in the order of wavelength, and the addition of SiO2 decrease the wave confinement and therefore the sensitivity. The findings are supported by a theoretical model and experimentally validated results. This research investigates the influence of vacuum and magnetic field annealing at various temperatures over a 4 h duration on the magnetic properties of the FeCo/TbCo2 thin film and therefore on the sensitivity of acoustic shear and Rayleigh wave magnetic field sensors functionalized with these magnetic layers. image

In this article, we introduce a gas sensor concept consisting of a nanostructured gold grating coupled with a distributed Bragg reflector (DBR). This coupling makes it possible to obtain plasmonic Tamm states, where excitation is possible at normal incidence and does not require the use of the Kretschmann configuration. Through parameters optimization of the gold nanostructured grating, we achieved well-defined and localized Tamm resonances between the gold nanostructured grating and the distributed Bragg reflector composed of SiO2/Si3N4 . To exploit the spatial confinement of the energy of the Tamm states in order to measure the change in refractive index, we propose three configurations, in which we substitute the last nitride layer in the Bragg reflector with porous materials. In the first configuration, we use a porous nitride portion with 30% porosity, producing sensitivity S=170 nm/RIU and figure of merit FOM=27.1 RIU-1 . In the second configuration, we use a porous nitride layer with a porosity of 30% ( S=175 nm/RIU and FOM=27.3 RIU-1 ). Finally, the third configuration adopts a 66% porous silicon layer ( S=268 nm/RIU and FOM=43.6 RIU-1 ). These values show the significant potential for sensing applications.

Bound states in the continuum (BICs) are zero-width (infinite lifetime) trapped eigenmodes that remain confined in the system even though they coexist with a continuum of extended states. The resulting high-frequency resonances may have significant applications in photonic integrated circuits, filtering, sensing, and laser. In this paper, we demonstrate that a simple design based on a photonic triple-stub cavity can display both Fabry-Pérot (FP) and Friedrich-Wintgen (FW) BICs, and their occurrence is very dependent on the way the cavity is attached to the outside medium by one or two ports. We first consider a symmetric cavity where a stub of length d3 is surrounded by two stubs of length d2, and all stubs are separated by segments of length d1. When the cavity is inserted between two ports, we demonstrate theoretically and validate experimentally the existence of symmetric BICs (S-BIC) and antisymmetric BICs (AS-BIC) of FP type under commensurability conditions between the lengths d1, d2, and d3. The S-BICs and AS-BICs may cross each other, giving rise to a doubly degenerate BIC. By breaking the symmetry of the cavity, AS-BICs and S-BICs can couple together and realize a FW-type BIC where one resonance remains with zero width while the other broadens into a bright mode. By considering two additional configurations where the triple-stub cavity is attached with one or two ports from only one side, additional BICs can be induced inside the structure. By slightly detuning from the BIC condition, the latter transforms into either an electromagnetic-induced transparency/reflection or Fano resonance. Finally, such a triple-stub cavity can be designed to realize near-perfect absorption for some frequencies. All the analytical results, obtained from the Green's function method, have been confirmed experimentally in the radiofrequency domain using coaxial cables. Published by the American Physical Society 2024

We numerically explore optical Tamm states (OTS) supported by a photonic structure composed of a nanostructured metallic layer on top of a distributed Bragg reflector (DBR). Several polarizations, incidences and patterning are assessed to map OTS and their properties. We then gain magnetic control of the OTS by adding a cobalt layer below the metal pattern and switching its magnetization. This control, widely used in plasmonics, takes advantage of the Transverse Magneto-Optical Kerr Effect (TMOKE). The simulated TMOKE signal of this structure has an amplitude of the order of 10 -3 and, compared to conventional magnetoplasmonic structures, provides high energy confinement between the metal stripes. In addition to the opening of the metallic layer that allows better access of the analyte to the sensitive area, this paves the way for higher sensitivities in bio- and chemical sensing applications.