Photonic gas sensors exploiting directly the optical properties of hybrid carbon nanotube localized surface plasmon structures

Chloroform-infiltrated photonic crystal fiber with high-temperature sensitivity

The temperature sensor is the core part of the temperature measurement instrument, and its performance directly determines the temperature measurement accuracy. Photonic crystal fiber (PCF) is a new type of temperature sensor with extremely high potential. In this paper, we propose a high-performance, structurally simple, liquid-filled PCF temperature sensor, which is based on a SMF-PCF-SMF (single mode fiber, SMF) sandwich structure. By adjusting the structural parameters of the the PCF, it is possible to obtain optical properties that are superior to those of ordinary optical fibers. This allows for more obvious responsive changes of the fiber transmission mode under small external temperature changes. By optimizing the basic structure parameters, a new PCF structure with a central air hole is designed, and its temperature sensitivity is -0.04696 nm/°C. When filling the air holes of PCFs with temperature-sensitive liquid materials, the response of the optical field against the temperature fluctuations can be effectively enhanced. The Chloroform solution is used to selectively infiltrate the resulting PCF owing to its large thermo-optical coefficient. After comparing various filling schemes, the calculation results show that the highest temperature sensitivity of -15.8 nm/°C is finally realized. The designed PCF sensor has a simple structure, high-temperature sensitivity, and good linearity showing great application potential.

Recent Advances in Rolling 2D TMDs Nanosheets into 1D TMDs Nanotubes/Nanoscrolls

Transition metal dichalcogenides (TMDs) van der Waals (vdW) 1D heterostructures are recently synthesized from 2D nanosheets, which open up new opportunities for potential applications in electronic and optoelectronic devices. The most recent and promising strategies in regards to forming 1D TMDs nanotubes (NTs) or nanoscrolls (NSs) in this review article as well as their heterostructures that are produced from 2D TMDs are summarized. In order to improve the functionality of ultrathin 1D TMDs that are coaxially combined with boron nitride nanotubes and single-walled carbon nanotubes. 1D heterostructured devices perform better than 2D TMD nanosheets when the two devices are compared. The photovoltaic effect in WS₂ or MoS₂ NTs without a junction may exceed the Shockley-Queisser limit for the above-band-gap photovoltage generation. Photoelectrochemical hydrogen evolution is accelerated when monolayer WS₂ or MoS₂ NSs are incorporated into a heterojunction. In addition, the photovoltaic performance of the WSe₂ /MoS₂  NSs junction is superior to that of the performance of MoS₂ NSs. The summary of the current research about 1D TMDs can be used in a variety of ways, which assists in the development of new types of nanoscale optoelectronic devices. Finally, it also summarizes the current challenges and prospects.

An Omnidirectional Dual-Functional Metasurface with Ultrathin Thickness

Although metasurfaces have received enormous attention and are widely applied in various fields, the realization of multiple functions using a single metasurface is still rarely reported to date. In this work, we propose a novel dual-functional metasurface that can be applied as a mid-infrared narrowband thermal light source in optical gas sensing and a long-wave infrared broadband absorber in photodetection. By actively tailoring the structure and constituent materials of the metasurface, the device yields an absorptivity of over 90% from 8 µm to 14 µm, while it exhibits an emissivity of 97.4% at the center wavelength of 3.56 μm with a full width at half-maximum of 0.41 µm. Notably, the metasurface is insensitive to the incident angle under both TM- and TE-polarized light. The proposed dual-functional metasurface possesses many advantages, including a simple structure, thin thickness, angle and polarization insensitivity, and compatibility with optical devices, which are expected to simplify the existing imaging systems and improve the performance of photodetection equipment.

Environmental Monitoring: A Comprehensive Review on Optical Waveguide and Fiber-Based Sensors

Globally, there is active development of photonic sensors incorporating multidisciplinary research. The ultimate objective is to develop small, low-cost, sensitive, selective, quick, durable, remote-controllable sensors that are resistant to electromagnetic interference. Different photonic sensor designs and advances in photonic frameworks have shown the possibility to realize these capabilities. In this review paper, the latest developments in the field of optical waveguide and fiber-based sensors which can serve for environmental monitoring are discussed. Several important topics such as toxic gas, water quality, indoor environment, and natural disaster monitoring are reviewed.

Cost-Effective Fiber Optic Solutions for Biosensing

In the last years, optical fiber sensors have proven to be a reliable and versatile biosensing tool. Optical fiber biosensors (OFBs) are analytical devices that use optical fibers as transducers, with the advantages of being easily coated and biofunctionalized, allowing the monitorization of all functionalization and detection in real-time, as well as being small in size and geometrically flexible, thus allowing device miniaturization and portability for point-of-care (POC) testing. Knowing the potential of such biosensing tools, this paper reviews the reported OFBs which are, at the moment, the most cost-effective. Different fiber configurations are highlighted, namely, end-face reflected, unclad, D- and U-shaped, tips, ball resonators, tapered, light-diffusing, and specialty fibers. Packaging techniques to enhance OFBs' application in the medical field, namely for implementing in subcutaneous, percutaneous, and endoscopic operations as well as in wearable structures, are presented and discussed. Interrogation approaches of OFBs using smartphones' hardware are a great way to obtain cost-effective sensing approaches. In this review paper, different architectures of such interrogation methods and their respective applications are presented. Finally, the application of OFBs in monitoring three crucial fields of human life and wellbeing are reported: detection of cancer biomarkers, detection of cardiovascular biomarkers, and environmental monitoring.

Optical properties and application potential of a hybrid cavity compound grating structure

In this paper, we propose a new type of metal-insulator-metal (MIM) hybrid cavity compound grating micro-structure array, which can achieve dual narrowband super-absorption in the near-infrared window. The thin plasmonic microstructure effectively modulates coupling and hybridization effects between surface plasmon polaritons of different transmission resonance cavities to form designable dual narrowband resonance states to achieve near-infrared operation proving manipulation of the optical characteristics in the near-infrared light field. Furthermore, we conduct an in-depth theoretical exploration of the structure's unique properties, such as its high-quality factor, low noise, super-absorption, precise control, and the physical mechanism of its excellent performance in ambient refractive index sensing and detection. This study provides developmental insights for the miniaturization, easy modulation, and multi-function development of surface plasmon superabsorbers while broadening their application in near-infrared environment refractive index detection. The proposed microstructure is also suitable for integration with optical elements.

A Review: Application and Implementation of Optic Fibre Sensors for Gas Detection

At the present time, there are major concerns regarding global warming and the possible catastrophic influence of greenhouse gases on climate change has spurred the research community to investigate and develop new gas-sensing methods and devices for remote and continuous sensing. Furthermore, there are a myriad of workplaces, such as petrochemical and pharmacological industries, where reliable remote gas tests are needed so that operatives have a safe working environment. The authors have concentrated their efforts on optical fibre sensing of gases, as we became aware of their increasing range of applications. Optical fibre gas sensors are capable of remote sensing, working in various environments, and have the potential to outperform conventional metal oxide semiconductor (MOS) gas sensors. Researchers are studying a number of configurations and mechanisms to detect specific gases and ways to enhance their performances. Evidence is growing that optical fibre gas sensors are superior in a number of ways, and are likely to replace MOS gas sensors in some application areas. All sensors use a transducer to produce chemical selectivity by means of an overlay coating material that yields a binding reaction. A number of different structural designs have been, and are, under investigation. Examples include tilted Bragg gratings and long period gratings embedded in optical fibres, as well as surface plasmon resonance and intra-cavity absorption. The authors believe that a review of optical fibre gas sensing is now timely and appropriate, as it will assist current researchers and encourage research into new photonic methods and techniques.

Simultaneous Measurements of Refractive Index and Methane Concentration through Electromagnetic Fano Resonance Coupling in All-Dielectric Metasurface

Dual-parameter measurements of refractive index and methane concentration based on electromagnetic Fano resonance are proposed. Two independent Fano resonances can be produced through electric dipole and toroidal dipole resonance in an all-dielectric metasurface separately. The linear relationship between the spectral peak-shifts and the parameters to be measured will be obtained directly. The refractive index (RI) sensitivity and gas sensitivity are 1305.6 nm/refractive index unit (RIU), −0.295 nm/% for one resonance peak (dip1), and 456.6 nm/RIU, −0.61 nm/% for another resonance peak (dip2). Such a metasurface has simpler structure and higher sensitivity, which is beneficial for environmental gas monitoring or multi-parameter measurements.

Pillared Graphene Structures Supported by Vertically Aligned Carbon Nanotubes as the Potential Recognition Element for DNA Biosensors

The development of electrochemical biosensors is an important challenge in modern biomedicine since they allow detecting femto- and pico-molar concentrations of molecules. During this study, pillared graphene structures supported by vertically aligned carbon nanotubes (VACNT-graphene) are examined as the potential recognition element of DNA biosensors. Using mathematical modeling methods, the atomic supercells of different (VACNT-graphene) configurations and the energy profiles of its growth are found. Regarding the VACNT(12,6)-graphene doped with DNA nitrogenous bases, calculated band structure and conductivity parameters are used. The obtained results show the presence of adenine, cytosine, thymine, and guanine on the surface of VACNT(12,6)-graphene significantly changes its conductivity so the considered object could be the prospective element for DNA biosensing.

Nanomaterial-Based CO2 Sensors

The detection of carbon dioxide (CO2) is critical for environmental monitoring, chemical safety control, and many industrial applications. The manifold application fields as well as the huge range of CO2 concentration to be measured make CO2 sensing a challenging task. Thus, the ability to reliably and quantitatively detect carbon dioxide requires vastly improved materials and approaches that can work under different environmental conditions. Due to their unique favorable chemical, optical, physical, and electrical properties, nanomaterials are considered state-of-the-art sensing materials. This mini-review documents the advancement of nanomaterial-based CO2 sensors in the last two decades and discusses their strengths, weaknesses, and major applications. The use of nanomaterials for CO2 sensing offers several improvements in terms of selectivity, sensitivity, response time, and detection, demonstrating the advantage of using nanomaterials for developing high-performance CO2 sensors. Anticipated future trends in the area of nanomaterial-based CO2 sensors are also discussed in light of the existing limitations.

Binary Gas Analyzer Based on a Single Gold Nanoparticle Photothermal Response

Although thermal conductivity gas analyzers are ubiquitous in industry, shrinking the sensing unit to a microscopic scale is rarely achieved. Since heat transfer between a metal nanoparticle and its ambient gas changes the temperature, refractive index, and density of the gaseous surrounding, one may tackle the problem using a single nanoparticle's photothermal effect. Upon heating by a 532 nm laser, a single gold nanoparticle transfers heat to the surrounding gas environment, which results in a change in the photothermal polarization of a 633 nm probe laser. The amplitude of the photothermal signal correlates directly with the concentration of binary gas mixture. In He/Ar, He/N₂, He/air, and H₂/Ar binary gas mixtures, the signal is linearly proportional to the He and H₂ molar concentrations up to about 10%. The photothermal response comes from the microscopic gaseous environment of a single gold nanoparticle, extending from the nanoparticle roughly to the length of the gas molecule's mean free path. This study points to a way of sensing binary gas composition in a microscopic volume using a single metal nanoparticle.

Vertically-Aligned Multi-Walled Carbon Nano Tube Pillars with Various Diameters under Compression: Pristine and NbTiN Coated

In this paper, the compressive stress of pristine and coated vertically-aligned (VA) multi-walled (MW) carbon nanotube (CNT) pillars were investigated using flat-punch nano-indentation. VA-MWCNT pillars of various diameters (30-150 µm) grown by low-pressure chemical vapor deposition on silicon wafer. A conformal brittle coating of niobium-titanium-nitride with high superconductivity temperature was deposited on the VA-MWCNT pillars using atomic layer deposition. The coating together with the pillars could form a superconductive vertical interconnect. The indentation tests showed foam-like behavior of pristine CNTs and ceramic-like fracture of conformal coated CNTs. The compressive strength and the elastic modulus for pristine CNTs could be divided into three regimes of linear elastic, oscillatory plateau, and exponential densification. The elastic modulus of pristine CNTs increased for a smaller pillar diameter. The response of the coated VA-MWCNTs depended on the diffusion depth of the coating in the pillar and their elastic modulus increased with pillar diameter due to the higher sidewall area. Tuning the material properties by conformal coating on various diameter pillars enhanced the mechanical performance and the vertical interconnect access (via) reliability. The results could be useful for quantum computing applications that require high-density superconducting vertical interconnects and reliable operation at reduced temperatures.

Linewidth reduction effect of a cavity-coupled dual-passband plasmonic filter

We propose a novel cavity-coupled MIM nano-hole array structure that exhibits a tunable dual passband in the near-infrared regime. When compared with the traditional single metal film, the designed structure provides a coupling effect between Gspp and SPP to significantly reduce the linewidths of the two transmission peaks. We also reveal the physical origin of the positive and negative influence of the cavity effect on the transmission of high-frequency and low-frequency peaks. This work supplies a new modulation theory for plasmonic devices based on the EOT phenomenon and has a wide application prospect in the fields of infrared sensor, plasmonic filter, and hyperspectral imaging.

Ethanol Gas Sensitivity Sensor Based on Roughened POF Taper of Modified Polypyrrole Films

The three polypyrrole (PPy) films with different mixture ratios, namely PPy1, PPy2, and PPy3, were synthesized by chemical oxidation with pyrrole and ferric chloride (FeCl₃). The roughened plastic optical fiber (POF) taper assembled PPy films (POF-PPy1, POF-PPy2, and POF-PPy3) were facilely prepared and bentU shape structure for testing ethanol gas at room temperature. The morphologies of the PPy films and the roughened POF taper were studied using electron microscopy. The effect of the three PPy films on the gas response was investigated and the results showed that the POF-PPy2 exhibited a high sensitivity of 5.08 × 10^-5 dB/ppm. The detection limit of the sensor was 140 ppm and its response and recovery times were 5 s and 8 s, respectively. The results also showed that as the bending radius decreased, the response and recovery times gradually shortened, while the output power increased. In addition, the proposed sensor has advantages of a low cost and simple structure.

A Review: Evolution and Diversity of Optical Fibre Plasmonic Sensors

The purpose of this review is to bring to the attention of the wider research community how two quite different optical sensory techniques were integrated resulting in a sensor device of exceptional sensitivity with wide ranging capability. Both authors have collaborated over a 20 year period, each researching initially surface plasmon resonance (SPR) and optical fibre Bragg grating devices. Our individual research, funded in part by EPSRC and industry into these two areas, converged, resulting in a device that combined the ultra-sensitive working platform of SPR behavior with that of fibre Bragg grating development, which provided a simple method for SPR excitation. During this period, they developed a new approach to the fabrication of nano-structured metal coatings for plasmonic devices and demonstrated on fibre optic platform, which has created an ultra-sensitive optical sensing platform. Both authors believe that the convergence of these two areas will create opportunities in detection and sensing yet to be realised. Furthermore, giving the reader "sign-post" research articles to help to construct models to design sensors and to understand their experimental results.

Carbon-Based Nanomaterials for Plasmonic Sensors: A Review

The surface plasmon resonance (SPR) technique is a remarkable tool, with applications in almost every area of science and technology. Sensing is the foremost and majorly explored application of SPR technique. The last few decades have seen a surge in SPR sensor research related to sensitivity enhancement and innovative target materials for specificity. Nanotechnological advances have augmented the SPR sensor research tremendously by employing nanomaterials in the design of SPR-based sensors, owing to their manifold properties. Carbon-based nanomaterials, like graphene and its derivatives (graphene oxide (GO)), (reduced graphene oxide (rGO)), carbon nanotubes (CNTs), and their nanocomposites, have revolutionized the field of sensing due to their extraordinary properties, such as large surface area, easy synthesis, tunable optical properties, and strong compatible adsorption of biomolecules. In SPR based sensors carbon-based nanomaterials have been used to act as a plasmonic layer, as the sensitivity enhancement material, and to provide the large surface area and compatibility for immobilizing various biomolecules, such as enzymes, DNA, antibodies, and antigens, in the design of the sensing layer. In this review, we report the role of carbon-based nanomaterials in SPR-based sensors, their current developments, and challenges.

Cavity-driven hybrid plasmonic ultra-narrow bandpass filter

We propose a novel compound grating structure that exhibits a tunable ultra-narrowband transmission in the near infrared regime. The thin microstructure can realize a steep wave form through a Fano-like resonance by coupling different propagation-type SPP modes and with a narrow line width formed by the energy band gap. Additionally, the out-of-band suppression is remarkably enhanced. It effectively solves the constraint relationship between high transmittance, narrow line width, and weak side peak of the plasmonic filter, and the structure is suitable for integration with detectors in the near infrared regime.

High-Performance Photodiode Based on Atomically Thin WSe2 /MoS2 Nanoscroll Integration

Self-assembled structures of 2D materials with novel physical and chemical properties, such as the good electrical and optoelectrical performance in nanoscrolls, have attracted a lot of attention. However, high photoresponse speed as well as high responsivity cannot be achieved simultaneously in the nanoscrolls. Here, a photodiode consisting of single MoS₂ nanoscrolls and a p-type WSe₂ is demonstrated and shows excellent photovoltaic characteristics with a large open-circuit voltage of 0.18 V and high current intensity. Benefiting from the heterostructure, the dark current is suppressed resulting in an increased ratio of photocurrent to dark current (two orders of magnitude higher than the single MoS₂ nanoscroll device). Furthermore, it yields high responsivity of 0.3 A W^-1 (corresponding high external quantum efficiency of ≈75%) and fast response time of 5 ms, simultaneously. The response speed is increased by three orders of magnitude over the single MoS₂ nanoscroll device. In addition, broadband photoresponse up to near-infrared could be achieved. This atomically thin WSe₂ /MoS₂ nanoscroll integration not only overcomes the disadvantage of MoS₂ nanoscrolls, but also demonstrates a single nanoscroll-based heterostructure with high performance, promising its potential in the future optoelectronic applications.

Tunable Perfect Narrow-Band Absorber Based on a Metal-Dielectric-Metal Structure

In this paper, a metal-dielectric-metal structure based on a Fabry–Perot cavity was proposed, which can provide near 100% perfect narrow-band absorption. The lossy ultrathin silver film was used as the top layer spaced by a lossless silicon oxide layer from the bottom silver mirror. We demonstrated a narrow bandwidth of 20 nm with 99.37% maximum absorption and the absorption peaks can be tuned by altering the thickness of the middle SiO2 layer. In addition, we established a deep understanding of the physics mechanism, which provides a new perspective in designing such a narrow-band perfect absorber. The proposed absorber can be easily fabricated by the mature thin film technology independent of any nano structure, which make it an appropriate candidate for photodetectors, sensing, and spectroscopy.

Low-dimensional nano-patterned surface fabricated by direct-write UV-chemically induced geometric inscription technique

We investigate a nano-patterning process which creates reproducible periodic surface topological features that range in size from ∼100  μm to ∼20  μm. Specifically, we have fabricated multi-layered thin films consisting of germanium/silicon strata on a planar substrate, with each layer having nanometers thickness. The material processing exploits focused 244 nm ultra-violet laser light and an opto-mechanical setup typically applied to the inscription of fiber gratings, and is based upon the well-known material compaction interaction of ultra-violet light with germanium oxides. We show this process can be extended to create arrays of metal nano-antennas by adding a metal overlay to the thin film. This results in arrays with dimensions that span nanometer- to centimeter-length scales. Also, each nano-antenna consists of "nano-blocks." Experimental data are presented that show the UV irradiance dosage used to create these metal nanostructures on D-shaped optical fibers has a direct relationship to their transmission spectral characteristics as plasmonic devices.

Chemical Sensing Applications of Carbon Nanotube-Deposited Optical Fibre Sensors

Carbon nanotubes are unique one-dimensional materials which can experience a modification in their optical properties as the chemical composition of their ambient environment varies. One of the ways to interrogate these variations in optical properties is through the use of optical fibres. As such, their integration with optical fibre technology would potentially allow for the development of devices for various chemical sensing applications.

Structurally tunable plasmonic absorption bands in a self-assembled nano-hole array

In this paper, we demonstrate a theoretical and experimental study on a nano-hole array that can realize perfect absorption in the visible and near-infrared regions. The absorption spectrum can be easily controlled by adjusting the structural parameters including the radius and period of the nano-hole, and the maximal absorption can reach 99.0% in theory. In order to clarify the physical mechanism of the absorber, we start from the extraordinary optical transmission supported by the nano-hole array in a thin metallic film coated on a glass substrate, and then analyse the perfect absorption in the metal-insulator-metal structure. The surface plasmon modes supported by the nano-hole array are completely clarified and both the FDTD simulation and waveguide theory are used to help us understand the physical mechanism, which can provide a new perspective in designing this kind of perfect absorber. In addition, the nano-hole array can be fabricated by simple and low-cost nanosphere lithography, which makes it a more appropriate candidate for spectroscopy, photovoltaics, photodetectors, sensing, and surface enhanced Raman scattering.

Tailoring terahertz surface plasmon wave through free-standing multi-walled carbon nanotubes metasurface

Surface plasmons have a fundamental role in the dynamics of photon-electron interactions and in optical metamaterials. Terahertz (THz) time-domain spectroscopy was used to characterize the complex dielectric constant, index of refraction, and conductivity of super-aligned, free-standing, multi-walled carbon nanotube films over the range 0.2-2.5 THz. These complex parameters were in excellent agreement with Maxwell-Garnett and Drude-Lorentz models. In addition, surface plasmon excitations in engineered, subwavelength, multi-walled carbon nanotube metasurfaces were examined. The observed surface plasmon resonances, reproduced by simulation, could be changed over the THz frequency range by altering the lattice constant of the arrays. The THz transmission was enhanced at the resonance peak. Overall, the results indicate potential applications for THz metasurfaces based on super-aligned, free-standing multi-walled carbon nanotubes.

Electrical Double Layer-Induced Ion Surface Accumulation for Ultrasensitive Refractive Index Sensing with Nanostructured Porous Silicon Interferometers

Herein, we provide the first experimental evidence on the use of electrical double layer (EDL)-induced accumulation of charged ions (using both Na⁺ and K⁺ ions in water as the model) onto a negatively charged nanostructured surface (e.g., thermally growth SiO₂)-Ion Surface Accumulation, ISA-as a means of improving performance of nanostructured porous silicon (PSi) interferometers for optical refractometric applications. Nanostructured PSi interferometers are very promising optical platforms for refractive index sensing due to PSi huge specific surface (hundreds of m² per gram) and low preparation cost (less than $0.01 per 8 in. silicon wafer), though they have shown poor resolution ( R) and detection limit (DL) (on the order of 10^-4-10^-5 RIU) compared to other plasmonic and photonic platforms ( R and DL on the order of 10^-7-10^-8 RIU). This can be ascribed to both low sensitivity and high noise floor of PSi interferometers when bulk refractive index variation of the solution infiltrating the nanopores either approaches or is below 10^-4 RIU. Electrical double layer-induced ion surface accumulation (EDL-ISA) on oxidized PSi interferometers allows the interferometer output signal (spectral interferogram) to be impressively amplified at bulk refractive index variation below 10^-4 RIU, increasing, in turn, sensitivity up to 2 orders of magnitude and allowing reliable measurement of refractive index variations to be carried out with both DL and R of 10^-7 RIU. This represents a 250-fold-improvement (at least) with respect to the state-of-the-art literature on PSi refractometers and pushes PSi interferometer performance to that of state-of-the-art ultrasensitive photonics/plasmonics refractive index platforms.

Differential loss magnetic field sensor using a ferrofluid encapsulated D-shaped optical fiber

A ferrofluid immersed, D-shape optical fiber exhibits differential loss up to 12 dB with respect to an azimuthally rotating magnetic field placed around its longitudinal axis, manifested in its measured transmission power. Investigating the magneto induced refractive index and loss changes by using ferrofluid overlaid diffractive elements a differential loss mechanism is revealed, associated with the relative light polarization direction and the magnetic field application direction. The results were used for performing modal profile simulations of ferrofluid immersed D-shape optical fiber. It is demonstrated that such an optical system can act as a magnetic field sensor with field angle and intensity sensing capabilities.

Silicon photonic dual-gas sensor for H2 and CO2 detection

We report a silicon photonic dual-gas sensor based on a wavelength-multiplexed microring resonator array for simultaneous detection of H₂ and CO₂ gases. The sensor uses Pd as the sensing layer for H₂ gas and a novel functional material based on the Polyhexamethylene Biguanide (PHMB) polymer for CO₂ gas sensing. Gas sensing experiments showed that the PHMB-functionalized microring exhibited high sensitivity to CO₂ gas and excellent selectivity against H₂. However, the Pd-functionalized microring was found to exhibit sensitivity to both H₂ and CO₂ gases, rendering it ineffective for detecting H₂ in a gas mixture containing CO₂. We show that the dual-gas sensing scheme can allow for accurate measurement of H₂ concentration in the presence of CO₂ by accounting for the cross-sensitivity of Pd to the latter.

Ultrasensitive plasmonic sensing in air using optical fibre spectral combs

Surface plasmon polaritons (SPP) can be excited on metal-coated optical fibres, enabling the accurate monitoring of refractive index changes. Configurations reported so far mainly operate in liquids but not in air because of a mismatch between permittivities of guided light modes and the surrounding medium. Here we demonstrate a plasmonic optical fibre platform that overcomes this limitation. The underpinning of our work is a grating architecture-a gold-coated highly tilted Bragg grating-that excites a spectral comb of narrowband-cladding modes with effective indices near 1.0 and below. Using conventional spectral interrogation, we measure shifts of the SPP-matched resonances in response to static atmospheric pressure changes. A dynamic experiment conducted using a laser lined-up with an SPP-matched resonance demonstrates the ability to detect an acoustic wave with a resolution of 10^-8 refractive index unit (RIU). We believe that this configuration opens research directions for highly sensitive plasmonic sensing in gas.