Interdigital concept in photonic sensors based on an array of lossy mode resonances.

Multi-parameter detection is key in the domain of sensors. Here it is demonstrated that an indium tin oxide (ITO) nanocoating can be used to generate multiple lossy mode resonances (LMRs) in the optical spectrum. To achieve this, a nanocoating with a gradient in thickness is generated on the surface of a planar waveguide, permitting broadening of the LMR because the position of an LMR in the optical spectrum is directly related to the nanocoating thickness. The nanocoating with a gradient in thickness contributes multiple LMRs, each one centred at a different wavelength. With a further etching or deposition using a mask, a pattern of deposited and non-deposited regions can be created, resulting in isolation of the LMRs by preventing LMR overlap. This enables tracking of each central wavelength separately, which can be tuned through control of the gradient or nanocoating pattern. The array of LMR-based sensors is a photonics analogue to the interdigital concept in electronics, enabling multiple resonances to be used for multiparameter sensing.

Dually nanocoated planar waveguides towards multi-parameter sensing.

The incidence of light on the edge of a glass coverslip for a microscope slide, deposited with a thin film on both faces, permits exciting two resonances in each polarisation state of the input light, TE and TM. This dually nanocoated waveguide can be used for detecting simultaneously two different parameters on the basis of a further deposition of suitable materials on each face. As an example, the possibility of detecting temperature and humidity by using polydimethylsiloxane and agarose coatings, respectively, was demonstrated, which opens the path for the development of other dual-parameter sensors, and for even more parameters in cases in which each face of the coverslip is patterned. Moreover, the device was optimised in order to position two resonances in the near infrared (NIR) and two resonances in the visible region, with sensitivities of 0.34 nm/°C and 0.23 nm/%RH in the visible region and 1.16 nm/°C and 0.34 nm/%RH in the NIR, respectively, demonstrating the possibility of using the device in both spectral ranges and opening the path for the development of sensors based on multiple resonances, each one related to a different parameter to be detected.

Author Correction: Lossy mode resonance sensors based on lateral light incidence in nanocoated planar waveguides.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Generation of lossy mode resonances with different nanocoatings deposited on coverslips.

The generation of lossy mode resonances (LMRs) with a setup based on lateral incidence of light in coverslips is a simple platform that can be used for sensing. Here the versatility of this platform is proved by studying the deposition of different coating materials. The devices were characterized with both SEM and AFM microscopy, as well as ellipsometry, which allowed obtaining the main parameters of the coatings (thickness, refractive index and extinction coefficient) and relating them with the different sensitivities to refractive index attained with each material. In this way it was possible to confirm and complete the basic rules observed with lossy mode resonance based optical fiber sensors towards the design of simpler and more compact applications in domains such as chemical sensors or biosensors.

Lossy mode resonance sensors based on lateral light incidence in nanocoated planar waveguides.

The deposition of an indium oxide (In2O3) thin film on conventional planar waveguides (a coverslip and a glass slide) allows generating lossy mode resonances (LMR) by lateral incidence of light on the waveguide and by registering the optical spectrum in a spectrometer. This novel sensing system becomes an alternative to optical fibre, the substrate where LMR-based sensors have been developed so far, since it is easier to handle and more robust. An additional advantage is that cost effective waveguides, such as slides or coverslips, can be used in a platform that resembles surface plasmon resonance-based sensors in the Kretschmann configuration but without the need for a coupling prism and with the advantage of being able to generate TE and TM LMR resonances with metallic oxide or polymer thin films. The results are corroborated with simulations, which provide in-depth understanding of the phenomena involved in the sensing system. As a proof-of-concept for the optical platform, two refractometers were developed, one with low sensitivity and for a wide range of refractive indices, and the other with higher sensitivity but for a narrower refractive index range. The sensors presented here open up the path for the development of LMR-based chemical sensors, environmental sensors, biosensors, or even the generation of other optical phenomena with the deposition of multilayer structures, gratings or nanostructures, which is much easier in a planar waveguide than in an optical fibre.

Fluorescent Sensors for the Detection of Heavy Metal Ions in Aqueous Media.

Due to the risks that water contamination implies for human health and environmental protection, monitoring the quality of water is a major concern of the present era. Therefore, in recent years several efforts have been dedicated to the development of fast, sensitive, and selective sensors for the detection of heavy metal ions. In particular, fluorescent sensors have gained in popularity due to their interesting features, such as high specificity, sensitivity, and reversibility. Thus, this review is devoted to the recent advances in fluorescent sensors for the monitoring of these contaminants, and special focus is placed on those devices based on fluorescent aptasensors, quantum dots, and organic dyes.

Fabrication of Long Period Gratings by Periodically Removing the Coating of Cladding-Etched Single Mode Optical Fiber Towards Optical Fiber Sensor Development.

Here, we present a novel method to fabricate long period gratings using standard single mode optical fibers (SMF). These optical devices were fabricated in a three-step process, which consisted of etching the SMF, then coating it with a thin-film and, the final step, which involved removing sections of the coating periodically by laser ablation. Tin dioxide was chosen as the material for this study and it was sputtered using a pulsed DC sputtering system. Theoretical simulations were performed in order to select the appropriate parameters for the experiments. The responses of two different devices to different external refractive indices was studied, and the maximum sensitivity obtained was 6430 nm/RIU for external refractive indices ranging from 1.37 to 1.39.

Micro and Nanostructured Materials for the Development of Optical Fibre Sensors.

The measurement of chemical and biomedical parameters can take advantage of the features exclusively offered by optical fibre: passive nature, electromagnetic immunity and chemical stability are some of the most relevant ones. The small dimensions of the fibre generally require that the sensing material be loaded into a supporting matrix whose morphology is adjusted at a nanometric scale. Thanks to the advances in nanotechnology new deposition methods have been developed: they allow reagents from different chemical nature to be embedded into films with a thickness always below a few microns that also show a relevant aspect ratio to ensure a high transduction interface. This review reveals some of the main techniques that are currently been employed to develop this kind of sensors, describing in detail both the resulting supporting matrices as well as the sensing materials used. The main objective is to offer a general view of the state of the art to expose the main challenges and chances that this technology is facing currently.

Humidity Sensor Based on Bragg Gratings Developed on the End Facet of an Optical Fiber by Sputtering of One Single Material.

The refractive index of sputtered indium oxide nanocoatings has been altered just by changing the sputtering parameters, such as pressure. These induced changes have been exploited for the generation of a grating on the end facet of an optical fiber towards the development of wavelength-modulated optical fiber humidity sensors. A theoretical analysis has also been performed in order to study the different parameters involved in the fabrication of this optical structure and how they would affect the sensitivity of these devices. Experimental and theoretical results are in good agreement. A sensitivity of 150 pm/%RH was obtained for relative humidity changes from 20% to 60%. This kind of humidity sensors shows a maximum hysteresis of 1.3% relative humidity.

Recent Developments in Fiber Optics Humidity Sensors.

A wide range of applications such as health, human comfort, agriculture, food processing and storage, and electronic manufacturing, among others, require fast and accurate measurement of humidity. Sensors based on optical fibers present several advantages over electronic sensors and great research efforts have been made in recent years in this field. The present paper reports the current trends of optical fiber humidity sensors. The evolution of optical structures developed towards humidity sensing, as well as the novel materials used for this purpose, will be analyzed. Well-known optical structures, such as long-period fiber gratings or fiber Bragg gratings, are still being studied towards an enhancement of their sensitivity. Sensors based on lossy mode resonances constitute a platform that combines high sensitivity with low complexity, both in terms of their fabrication process and the equipment required. Novel structures, such as resonators, are being studied in order to improve the resolution of humidity sensors. Moreover, recent research on polymer optical fibers suggests that the sensitivity of this kind of sensor has not yet reached its limit. Therefore, there is still room for improvement in terms of sensitivity and resolution.

Sensitivity optimization with cladding-etched long period fiber gratings at the dispersion turning point.

This work presents a refractive index sensor based on a long period fiber grating (LPFG) made in a reduced cladding fiber whose low order cladding modes have the turning point at large wavelengths. The combination of these parameters results in an improved sensitivity of 8734 nm/refractive index unit (RIU) for the LP0,3 mode in the 1400-1650 wavelength range. This value is similar to that obtained with thin-film coated LPFGs, which permits to avoid the coating deposition step. The numerical simulations are in agreement with the experimental results.

Analysis of lossy mode resonances on thin-film coated cladding removed plastic fiber.

In this work, the modal transition induced by lossy mode resonances has been analyzed as a function of wavelength for thin-film coated cladding removed fibers. The wavelength dependence of the modal structure allows us to explain the resonance phenomenon. The numerical data obtained were calculated with a method based on the exact calculation of core modes. Theoretical simulations have been compared with experimental results showing good agreement.

Temperature sensor based on a hybrid ITO-silica resonant cavity.

Integrated optical devices comprised of multiple material systems are able to achieve unique performance characteristics, enabling applications in sensing and in telecommunications. Due to ease of fabrication, the majority of previous work has focused on polymer-dielectric or polymer-semiconductor systems. However, the environmental stability of polymers is limited. In the present work, a hybrid device comprised of an indium tin oxide (ITO) coating on a silicon dioxide toroidal resonant cavity is fabricated. Finite element method simulations of the optical field in the multi-material device are performed, and the optical mode profile is significantly altered by the high index film. The quality factor is also measured and is material loss limited. Additionally, its performance as a temperature sensor is characterized. Due to the high thermo-optic coefficient of ITO and the localization of the optical field in the ITO layer, the hybrid temperature sensor demonstrates a nearly 3-fold improvement in performance over the conventional silica device.

Refractometric sensors based on multimode interference in a thin-film coated single-mode-multimode-single-mode structure with reflection configuration.

Thin-film coated single-mode-multimode-single-mode (SMS) structures have been analyzed both theoretically and experimentally with the aim of detecting different refractive indices. By adequate selection of the thickness of the thin film and of the diameter of the multimode segment in the SMS structure, a seven-fold improvement can be obtained in the sensitivity of the device to the surrounding medium refractive index, achieving a maximum sensitivity of 1199.18 nm/refractive index unit for the range of refractive indices from 1.321 to 1.382. Using layer-by-layer self-assembly for deposition, both on the cladding and on the tip of the multimode segment, allows the reflected power to increase, which avoids the application of a mirror on the tip of the multimode segment.

Mode transition in complex refractive index coated single-mode-multimode-single-mode structure.

By coating a single-mode-multimode-single-mode (SMS) structure with a high refractive index thin-film it is possible to obtain a transition of modes for specific combinations of thin-film thickness, thin-film refractive index and surrounding medium refractive index, which permits to develop devices with a high sensitivity to specific parameters. In order to gain a better knowledge of the phenomenon the experimental results are corroborated numerically with the Transfer-Matrix-Method. The influence of losses in the thin-film has also been studied. The results obtained prove that a thin-film coated SMS structure is a simple and cost-effective platform for development of sensors and optical filters.

Tunable electro-optic wavelength filter based on lossy-guided mode resonances.

In this work an optical fiber tunable filter based on lossy guided-mode resonances (LGMR) is proposed. It consists of a multilayer structure deposited onto the surface of a plastic cladding removed multimode fiber. The first layer is used to generate the LGMR and to work as the first electrode as well; the second one to tune the filter and the outer layer forms the other electrode. The fabricated filter has demonstrated a good sensitivity to the applied voltage showing a change of the LGMR wavelength of 0.4 nm/V. Among other applications, this filter is intended to be used as electro-optic wavelength filter or modulator.

Fiber-optic pH-sensors in long-period fiber gratings using electrostatic self-assembly.

A novel pH sensor based on the deposition of electrostatic self-assembled polyallylamine hydrochloride and polyacrylic acid on the cladding of a long-period fiber grating has been designed. The overlay thickness is of the order of magnitude of the light wavelength used to interrogate the sensor. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the position of the attenuation bands as a function of the overlay thickness. A study of the role that pH plays in the thickness of the sensitive overlay was carried out. Experimentally, a wavelength shift of the attenuation bands of up to 85 nm was obtained in the 4-7 range of pH units. The results are reproducible, which indicates the possibilities of a multiple-use sensor.

Spectral evolution with incremental nanocoating of long period fiber gratings.

The incremental deposition of a thin overlay on the cladding of a long-period fiber grating (LPFG) induces important resonance wavelength shifts in the transmission spectrum. The phenomenon is proved theoretically with a vectorial method based on hybrid modes and coupled mode theory, and experimentally with electrostatic self-assembly monolayer process. The phenomenon is repeated periodically for specific overlay thickness values with the particularity that the shape of the resonance wavelength shift depends on the thickness of the overlay. The main applications are the design of wide optical filters and multiparameter sensing devices.