We present the design and experimental validation of a compact refractive index sensor based on a multimode-fluorine-doped-multimode (MFM) fiber structure. The sensor exploits modal interference effects in the visible spectrum by incorporating a FG105LCA multimode fiber segment with a fluorine-doped cladding. By varying the length and diameter of the central fiber segment, we achieve a significant shift in the attenuation peaks, enabling a high-resolution and compact design. Experimental results demonstrate a maximum sensitivity of 137.07 nm/RIU and a sensitivity of the wavelength shift to the refractive index up to 41.66 nm/RIU. Furthermore, a miniaturized reflective configuration using a 15 mm segment is proposed, offering both performance and integration advantages.

Long-period fiber gratings (LPFGs) are an important structure in the field of optical fiber sensors, recognized for their ability to couple light between core and cladding modes, enabling sensitivity to the surrounding medium. This characteristic has facilitated their application in biosensing, chemical sensing, and environmental monitoring. Over the years, several strategies have been employed to enhance the sensitivity of LPFGs, including cladding diameter reduction, operation at the dispersion turning points (DTP), and the mode transition phenomenon induced by thin films. However, challenges such as non-linear spectral shifts have limited their practical implementation. In this work, we present a detailed experimental and numerical investigation of LPFGs modified with a gold thin film positioned between the cladding and a high-refractive-index TiO2 layer. This configuration demonstrates improved linearity in the wavelength shifts observed at DTPs and during mode transitions. Experimental results reveal that increasing the gold layer thickness from 15 nm to 30 nm significantly enhances linearity, reducing the quadratic fitting parameter by nearly 50-fold while maintaining high sensitivity. These findings provide a pathway toward more robust and precise LPFG-based sensors with improved spectral response towards practical applications.

This letter reports the development of an integrated optoelectronic sensor for refractive index (RI) measurement, based on a zinc oxide metal-semiconductor-metal photodetector with electrohydrodynamic (EHD)-printed interdigitated electrodes and a polydimethylsiloxane (PDMS) waveguide placed on the sensor surface. The sensor detects variations in photocurrent caused by changes in the evanescent field of the PDMS waveguide when an analyte is deposited on it. A higher analyte RI results in decreased photocurrent. This work reports a promising application of conventional ultraviolet photodetectors for RI sensing. The use of scalable techniques, such as EHD printing and sputtering, together with direct integration of the PDMS waveguide, highlights the potential of the device as a compact and versatile sensing platform.

Surface plasmon resonance (SPR) and lossy mode resonance (LMR) are prominent sensing mechanisms utilized across various fields. The Kretschmann configuration is commonly employed for SPR, while LMR is favored in planar waveguides or optical fibers due to high incidence angles. Recently, hyperbolic mode resonance (HMR) has emerged as a hybrid approach, combining metallic and dielectric thin films. This study explores the impact of incidence angle on HMR using the Kretschmann configuration. Four samples with varying gold (Au) and tin dioxide (SnO2) layer thicknesses were fabricated and characterized using Atomic Force Microscopy (AFM). Experimental setups employed the Kretschmann configuration for reflectance spectrum analysis. Results indicate enhanced sensitivity and figure of merit (FoM) with an additional SnO2 layer compared to the case without SnO2. Particularly with a 36 nm Au thickness the sensitivity doubles and the FoM improves by 16 %. Numerical simulations validate these findings, confirming the optimized performance of HMR for specific layer thicknesses and incidence angles.

Lossy mode resonances (LMRs) have been widely employed for the development of sensors in the last years. However, the theoretical frameworks for LMRs are scarce and difficult to systematize, hampering the development of this technology. In this work, we propose a new systemic model for assessing LMRs in arbitrary waveguide configurations, based solely on modal analysis of the unperturbed waveguide and the waveguide with a thin film optimized for LMR generation. The model is first developed for a generic waveguide, and leveraged to design, for the first time, LMRs in a silicon nitride photonic wire waveguide. It is furthermore demonstrated that the model only requires a few modes to reliably describe LMRs in D-shaped fibers, reducing the computational cost of simulating them. Therefore, the suggested model is valid for both high and low contrast waveguides, and it is considered it provides new insights about LMRs, which will help in the design of new LMR-based devices and its extension to novel platforms.

Gas sensors play a critical role in numerous human activities. Their necessity continues to grow across diverse fields as technological advancements drive demand for precision agriculture and bioengineering among other applications. Among existing sensor technologies, optical gas sensors stand out due to their ability to operate remotely in high-risk environments while remaining unaffected by electromagnetic interference. Resonance-based optical sensors offer targeted gas detection through the functionalization of their sensitive surfaces. This work focuses on reviewing the state of the art in resonance-based optical gas sensors (ROGS), addressing their fundamental principles, recent advances in fabrication processes, waveguide designs, and materials employed both for resonance generation and as sensitive coatings. Additionally, the review examines achieved sensitivity, emerging applications, and key developments in the field, including those efforts on improving ROGS performances by means of artificial intelligence (AI) techniques. The study encompasses optical sensors leveraging Surface Plasmon Resonance (SPR), Lossy Mode Resonance (LMR), and Hyperbolic Mode Resonance (HMR)—the latter representing a notable breakthrough in recent years as a particular case of Bloch Surface Waves (BSWs).

Polymeric optical waveguides represent an essential component in photonic technology thanks to their ability to guide light through controlled structures, enabling applications in telecommunications, sensors, and integrated devices. With the development of new materials and increasingly versatile manufacturing methods, these structures are being integrated into various systems at a rapid pace, while their dimensions are constantly being reduced. This article explores the main fabrication methods for polymeric optical waveguides, such as traditional and maskless photolithography, laser ablation, hot embossing, nanoimprint lithography, the Mosquito method, inkjet printing, aerosol jet printing, and electrohydrodynamic (EHD) printing. The operating principle of each method, the equipment and materials used, and their advantages, limitations, and practical applications are evaluated, in addition to the propagation losses and characterization of the waveguides obtained with each method.

Achieving a high figure of merit alongside exceptional sensitivity is critical for advancing optical biosensors capable of ultra-low limits of detection. In this work, we investigate the performance of D-shaped single-mode fiber devices coated with 1D photonic crystal stacks. These nano-assembled layers, composed of alternating high- and low-refractive-index (RI) materials, enable the excitation of Bloch surface waves (BSWs). By systematically varying the RI contrast between the layers, we demonstrate that while a moderate increase in RI contrast improves sensitivity and narrows the full width at half maximum (FWHM) of the BSW resonance, the sensitivity tends to stabilized as we further increase the RI contrast, while the FWHM still can be pushed towards a higher FoM. Real-world applicability is validated through the detection of immunoglobulin G (IgG) at record-low concentrations of 70 aM, leveraging the solution as a versatile, operando, high-performance biosensing platform.

From an optical perspective, depending on the relationship between the real (n) and imaginary (k) parts of its refractive index, three broad categories of materials can be distinguished: metals (k >> n ), dielectrics (n >> k ), and materials in which n approximate to k (termed here excitonic materials). The modes and optical resonances that appear in a thin film bounded by two dielectrics with similar refractive index, what we call here a double interface structure, have been widely studied in the case of metals, but not for dielectrics, or materials with n approximate to k. In this work, we propose a new approach, based on employing the phase matching condition to correlate the resonances that appear in the wavelength versus incident angle color maps of the reflected power with the modal analysis of the cross section of the structure. This analysis is performed, using an attenuated total reflection (ATR) setup, for thin film materials that belong to each of the mentioned categories: a metal (gold, Au), a dielectric (titanium dioxide, TiO2), and a material with n approximate to k (chromium, Cr). The theoretical analysis is supported with experimental results. It is demonstrated that this method enables to identify any resonance at any wavelength or incident angle, being valid for all three types of materials. Therefore, it is considered the suggested approach will help the research in these materials and in the double interface structure in the optics and photonics field.

Long Period Fiber Gratings (LPFGs) offer a versatile platform for sensing applications due to their ability to couple core and cladding modes through periodic refractive index modulation, which makes the device sensitive to changes in the surrounding refractive index (SRI). The sensitivity can be enhanced through mechanisms such as the dispersion turning point and mode transition. However, the performance is non-linear. To avoid this, here we explore the less-examined relationship between mode transition and lossy mode resonances (LMRs), both of which occur within the same thin-film thickness range. By depositing a gold layer followed by a high refractive index TiO2 layer, we obtained a highly sensitive and linear performance, while aiming at minimizing the bandwidth of the attenuation bands. Two LPFGs with different modulation indices were fabricated and their spectral response to varying TiO2 thicknesses was monitored. The results demonstrated that LPFGs can achieve a sensitivity to the thin film thickness variation of 9 nm/nm, similar to LMR-based sensors, and a linear evolution as a function of thickness, with the possibility to improve the bandwidth. This improvement could boost the applicability of LPFGs in domains such as environmental sensing and biosensing, with the potential for further improvement through thin film refinement and optimization of LPFG parameters.

In this letter, a refractive index (RI) sensor was developed and experimentally validated using an interferometric design that integrates single-mode fiber (SMF), no-core multimode fiber, and another segment of SMF. The single-mode-multimode-single-mode structure was employed as a filter and inserted into an erbium-doped fiber ring laser to enhance the sensor's detection accuracy. Experimental findings show that the sensor provides an outstanding linear response, with an RI sensitivity of 96.639 nm/RIU over a measurement range of 1.3468-1.4061, along with strong stability.

This letter describes the fabrication of sensor devices based on lossy mode resonance (LMR) and hyperbolic mode resonance (HMR) using for the first time as generating materials of the optical resonances both, yttrium iron garnet (Y3Fe5O12) and strontium titanate (SrTiO3) with a film thickness of 739.2 and 158.7 nm for Y3Fe5O12 (YIG) and SrTiO3, respectively. First-order resonances were observed at the visible region of the electromagnetic spectrum for both materials, LMR and HMR phenomena. RF sputtering deposition was used to fabricate metal oxide thin films on coverslips in a planar waveguide configuration, the Au metallic thin films were deposited by pulsed dc sputtering in a magnetron system from Moorfield. All devices were characterized under different surrounding medium refractive index. Sensitivities achieved values of 5862 and 5865 nm/RIU (refractive index unit) for HMR versions of Y3Fe5O12 and SrTiO3r, respectively. In addition, the response of the sensors to relative humidity and different ethanol concentrations was evaluated. The best results correspond to the Y3Fe3O3-based sensor, with a sensitivity of 0.2 nm/ppm and a limit of detection (LOD) of 183 ppm for ethanol, and 64 nm/%RH for RH, with an LOD of 2.23%RH, and because its resonance does not vanish unlike SrTiO3.

This work demonstrates the generation of a Lossy Mode Resonance (LMR) on a printed waveguide and the potential use of the structure for refractive index detection. The printed waveguide was fabricated using NOA 61 as the ink, a needle of 50 mu m internal diameter and an electrohydrodynamic (EHD) printing machine. The waveguide is coupled to the input/output fibers using a micro-positioning system. A SnO2 thin film was deposited onto the waveguide using a DC sputtering machine. The LMR spectrum generated is obtained while the sensor is in contact with ultrapure water. The shift of the LMR spectrum was analyzed as a function of exposure to different refractive indices. The sensitivity, FWHM, and FOM of the evanescent-based printed optical sensor were determined.

While the transition from optical fiber to planar waveguide substrates in LMR-based sensors has allowed for the development of more robust, cost-effective, and easily manufactured platforms, it has also presented challenges in optimizing critical sensor parameters such as resolution and sensitivity in biosensing. In this work, we introduce the application of gold nanoparticles (AuNP) to enhance the sensitivity of a Lossy Mode Resonance (LMR)-based biosensor for the detection of vascular endothelial growth factor (VEGF) protein. The sensor was developed by depositing a nanometric TiO2 film on a planar waveguide, and its performance was assessed using three detection approaches: label-free, sandwich assay, and AuNP-labeled sandwich assay. The integration of AuNP significantly improved sensitivity, enabling detection at concentrations as low as 0.1 ng mL-1, surpassing the sensitivity of traditional label-free LMR-based sensors. These results demonstrate the potential of AuNP-enhanced LMR sensors for detecting low concentrations of biomarkers with high specificity and sensitivity, positioning them as promising tools for biosensing applications.

This study presents a novel sensor design utilizing a long-period fiber grating (LPFG) deposited with a TiO2 nanocoating via atomic layer deposition. The study combines theoretical simulations and experimental validation to optimize the grating period and modulation index to operate in the mode transition with a quasi-lossy mode resonance (LMR) behavior, i.e., the LPFG attenuation bands shift similarly to LMRs. This enables the achievement of a remarkable sensitivity of 78 nm/nm, allowing for the detection of sub-angstrom variations in film thickness, which is critical for applications in semiconductor manufacturing. Our setup facilitates continuous monitoring of the transmission spectrum, enabling real-time adjustments during deposition to maximize sensitivity. As proof of concept for the applicability of the sensor as a refractive index sensor, we demonstrated exceptional sensitivity for nitrogen detection, achieving around 10,000 nm/RIU, with a figure of merit of 200. This marks one the highest sensitivities reported for optical fiber gas sensors and suggests this technology could revolutionize the field duet to its simplicity in terms of sensor design.

This work presents the fabrication of hyperbolic mode resonance-based optical sensors by means of sputtered copper oxide (CuO) and zinc oxide (ZnO), and the study of their performance for gas sensing purposes. Two sensors were fabricated in a planar waveguide configuration with an intermediate gold thin film, and resonances were observed in the visible region of the electromagnetic spectrum. Both materials were analyzed with X-ray diffraction techniques, and their response was characterized by different concentrations of a group of gases comprised of nitric oxide, acetylene (C2H2), ethanol, carbon dioxide, and relative humidity. The best performance corresponds to the CuO sensor for C2H2 gas, presenting a sensitivity of 1.11 nm/parts per million (ppm) and a limit of detection of 12.6 ppb, with response and recovery times of 70 and 68 s, respectively. ZnO-based sensors allowed for a comprehensive study of ethanol in a range of thousands of ppm, while CuO-based sensors showed exceptional sensitivity for most gases in the range of a few ppm. All measurements were performed at room temperature.

This work introduces a novel single-package optical sensing device for multiple gas sensing, which is suitable for breath analysis applications. It is fabricated on a coverslip substrate via a sputtering technique and uses a planar waveguide configuration with lateral incidence of light. It features three sequentially ordered strips of different materials, which serve to increase the multivariate nature of the response of the device to different gases. For the proof-of-concept, the selected materials are indium tin oxide (ITO), tin oxide (SnO2), and chromium oxide III (Cr2O3), while the selected gases are nitric oxide (NO), acetylene (C2H2), and ammonia (NH3). The sensing mechanism is based on the hyperbolic mode resonance (HMR) effect, with the first-order resonance obtained for each strip located in the near infrared region. The multivariate response of the resonances and the correlation with the concentration of each gas allow training a machine learning (ML) model based on a nonlinear autoregressive neural network, enabling the accurate prediction of the concentration of each gas. The obtained limit of detection for all the gases was in the order of a few parts per billion. This innovative approach coined as the multivariate optical resonances spectroscopy demonstrates the potential of HMR-based optical sensors in combination with ML techniques for ultra-sensitive multi-gas detection applications using a single device.

The current work describes and compares the performance of hyperbolic mode resonance (HMR)-based sensors for the detection of acetone at parts per billion (ppb) concentrations using ensemble machine learning (EML) techniques. A pair of HMR based-sensors with resonances located in the visible (VIS) and mid infrared (MIR) regions were obtained in order to train a set of ensemble machine learning models. The response of the detection system formed by both devices in the VIS and MIR regions, with the help of the EML system, allowed the limit of detection (LoD) of the sensors to be reduced by an order of magnitude. It is the first time that HMR-based sensors are shown in practical applications, at the same time that their performance is improved using EML techniques. This opens new avenues for the use of this type of HMR-based sensors for the detection of other substances, in addition to improving the performance of any optoelectronic sensor using EML techniques.

In this letter, we study the environmental sensing capabilities of a single-mode-multimode-single-mode (SMS) fiber in a simple low-cost configuration. SMS fibers exhibit sensitivity to temperature, humidity, refractive index, and strain, making them suitable for numerous applications in telecommunications, environmental monitoring, and more. Experimental results demonstrate that the sensor achieves a maximum temperature sensitivity of 4.53 nm/degrees C. In addition, SMS fibers can also work as humidity sensors by absorbing or releasing moisture, leading to variations in the refractive index. Monitoring these changes allows for precise humidity measurements, with a sensitivity of 0.1548 nm/%RH. Moreover, SMS fibers show a refractive index sensitivity of 39.65 nm/RIU and strain sensitivities as high as 1.062 nm/mu epsilon, indicating good performance.

This letter demonstrates the fabrication of a temperature optical sensor by printing the corresponding sensitive optical waveguide directly onto a flexible flat substrate. The printed waveguide was carried out using a coaxial needle and an electrohydrodynamic (EHD) machine. The fluorescent organic compound, rhodamine B, was used for doping the core of the printed waveguide as temperature sensible dye. The optical sensitive waveguide manufactured is compact, ensuring coupling with the input and output optical fibers. The response of the printed optical sensor was evaluated to temperature variations by measurement of both, the peak intensity and the wavelength of the fluorescence spectra. The experimental characteristic and sensitivity of the sensor were obtained.