Low refractive index gas sensing using a Surface Plasmon Resonance fibre device

A series of surface plasmonic fibre devices were fabricated using multiple coatings deposited on a lapped section of a single mode fibre. Coupling from the guided mode to surface plasmons was promoted following UV laser irradiation of the coated region through a phase mask, which generated a surface relief grating structure. The devices showed high spectral sensitivities and strong coupling for low refractive indices as compared to other grating-type fibre devices. The plasmonic devices were used to detect the variation in the refractive indices of alkane gases with measured wavelength and coupling sensitivity to index of 3400 nm RIU-1 and 8300 dB RIU-1, respectively. As a demonstration of the performance of these gas sensors, a minimum concentration of 2% by volume of butane in ethane was achieved.

Exploitation of multilayer coatings for infrared surface plasmon resonance fiber sensors

We demonstrate surface plasmon resonance (SPR) fiber devices based upon ultraviolet inscription of a grating-type structure into both single-layered and multilayered thin films deposited on the flat side of a lapped D-shaped fiber. The single-layered devices were fabricated from germanium, while the multilayered ones comprised layers of germanium, silica, and silver. Some of the devices operated in air with high coupling efficiency in excess of 40 dB and an estimated index sensitivity of Delta lambda/Delta n = 90 mn from 1 to 1.15 index range, while others provided an index sensitivity of Delta lambda/Delta n = 6790 mn for refractive indices from 1.33 to 1.37. (C) 2009 Optical Society of America

Multilayered coated infra-red surface plasmon resonance fibre sensors for aqueous chemical sensing

A series of surface plasmonic fibre devices were fabricated by depositing multiple thin coatings on a lapped section of a standard single mode telecoms fibre forming a D-shaped section and then inscribing a grating-type structure using UV light. The coatings consisted of base coatings of semi-conductor (germanium) and dielectric (silicon dioxide) materials, followed by different metals. These fibre devices showed high spectral refractive index sensitivity with high coupling efficiency in excess of 40 dB for indices in the aqueous regime and below, with estimated index sensitivities of Lambda lambda/Lambda n = 90-800 nm from 1 to 1.15 index range and Lambda lambda/Lambda n = 1200-4000 nm for refractive indices from 1.33 to 1.39. (C) 2009 Elsevier Inc. All rights reserved.

Characterization of infrared surface plasmon resonances generated from a fiber-optical sensor utilizing tilted Bragg gratings

We demonstrate the use of tilted fiber gratings to assist with the generation of infrared surface plasmons on a metal film coating the flat of a D-shaped fiber. The wavelength of the strong (>25 dB) resonance is tunable over similar to 1000 nm by adjusting the polarization state of the light and is highly sensitive to the refractive index of any aqueous medium surrounding the fiber (sensitivity= 3365 nm).

Generation of infrared surface plasmon resonances with high refractive index sensitivity utilizing tilted fiber Bragg gratings

We demonstrate the use of tilted fiber gratings to assist the generation of localized infrared surface plasmons with short propagation lengths and a sensitivity of d lambda/dn = 3365 nm in the aqueous index regime. It was also found that the resonances could be spectrally tuned over 1000 nm at the same spatial region with high coupling efficiency (in excess of 25 dB) by altering the polarization of the light illuminating the device.

Formation and characterization of ultra-sensitive surface plasmon resonance sensor based upon a nano-scale corrugated multi-layered coated D-shaped optical fiber

We present experimental results on the performance of a series of coated, D-shaped optical fiber sensors that display high spectral sensitivities to external refractive index. Sensitivity to the chosen index regime and coupling of the fiber core mode to the surface plasmon resonance (SPR) is enhanced by using specific materials as part of a multi-layered coating. We present strong evidence that this effect is enhanced by post ultraviolet radiation of the lamellar coating that results in the formation of a nano-scale surface relief corrugation structure, which generates an index perturbation within the fiber core that in turn enhances the coupling. We have found reasonable agreement when we modeling the fiber device. It was found that the SPR devices operate in air with high coupling efficiency in excess of 40 dB with spectral sensitivities that outperform a typical long period grating, with one device yielding a wavelength spectral sensitivity of 12000 nm/RIU in the important aqueous index regime. The devices generate SPRs over a very large wavelength range, (visible to 2 mu m) by alternating the polarization state of the illuminating light.

Aptamer-based surface plasmon sensor for thrombin detection

A series of surface plasmonic fibre devices were fabricated using multiple coatings deposited on a lapped section of a single mode fibre and post-fabrication UV laser irradiation processing with a phase mask, producing a surface relief grating structure. These devices showed high spectral sensitivity in the aqueous index regime ranging up to 4000 nm/RIU for wavelength and 800 dB/RIU for intensity. The devices were then coated with human thrombin binding aptamer. Several concentrations of thrombin in buffer solution were made, ranging from 1nM to 1µM. All the concentrations were detectable by the devices demonstrating that sub-nM concentrations may be monitored.

Generation of infrared surface plasmon resonances with high refractive index sensitivity utilizing tilted fiber Bragg gratings

We demonstrate the use of tilted fiber gratings to assist the generation of localized infrared surface Plasmons with short propagation lengths and a sensitivity of dλ/dn = 3365 nm in the aqueous index regime. It was also found that the resonances could be spectrally tuned over 1000 nm at the same spatial region with high coupling efficiency (in excess of 25 dB) by altering the polarization of the light illuminating the device. © 2007 Optical Society of America.

An ultra-sensitive localised surface plasmon resonance fibre device for environmental sensing based upon a structured bi-metal coating

We demonstrate a bi-metal coating (platinum and gold or silver) localised surface plasmon resonance fibre device that produces an index spectral sensitivity of over 11,000 nm/RIU, yielding an index resolution of 5×10-6in the aqueous index regime, consisting of a structured multi-layered thin film on D-shaped fibre. © 2014 SPIE.

Surface plasmon resonance generation utilising gratings for biochemical sensing

We demonstrate the use of gratings to assist in the generation of surface plasmon resonances resulting in a device having a high index resolution of 3×10-5 in the aqueous index regime.

Highly sensitive, localized surface plasmon resonance fiber device for environmental sensing, based upon a structured bi-metal array of nano-wires

We demonstrate a bi-metal coated (platinum and gold or silver), localized surface plasmon resonance fiber sensor with an index sensitivity exceeding 11,900 nm/RIU, yielding an index resolution of 2 × 10-5 in the aqueous index regime. This is one of the highest index sensitivities achieved with an optical fiber sensor. The coatings consist of arrays of bi-metal nano-wires (typically 36 nm in radius and 20 μm in length), supported by a silicon dioxide thin film on a thin substrate of germanium, the nano-wires being perpendicular to the longitudinal axis of the D-shaped fiber.

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

We investigate the modification of the optical properties of carbon nanotubes (CNTs) resulting from a chemical reaction triggered by the presence of a specific compound (gaseous carbon dioxide (CO2)) and show this mechanism has important consequences for chemical sensing. CNTs have attracted significant research interest because they can be functionalized for a particular chemical, yielding a specific physical response which suggests many potential applications in the fields of nanotechnology and sensing. So far, however, utilizing their optical properties for this purpose has proven to be challenging. We demonstrate the use of localized surface plasmons generated on a nanostructured thin film, resembling a large array of nano-wires, to detect changes in the optical properties of the CNTs. Chemical selectivity is demonstrated using CO2 in gaseous form at room temperature. The demonstrated methodology results additionally in a new, electrically passive, optical sensing configuration that opens up the possibilities of using CNTs as sensors in hazardous/explosive environments.

Surface plasmon resonance-induced photocatalysis by Au nanoparticles decorated mesoporous g-C3N4 nanosheets under direct sunlight irradiation

In recent years, surface plasmon-induced photocatalytic materials with tunable mesoporous framework have attracted considerable attention in energy conversion and environmental remediation. Herein we report a novel Au nanoparticles decorated mesoporous graphitic carbon nitride (Au/mp-g-C3N4) nanosheets via a template-free and green in situ photo-reduction method. The synthesized Au/mp-g-C3N4 nanosheets exhibit a strong absorption edge in visible and near-IR region owing to the surface plasmon resonance effect of Au nanoparticles. More attractively, Au/mp-g-C3N4 exhibited much higher photocatalytic activity than that of pure mesoporous and bulk g-C3N4 for the degradation of rhodamine B under sunlight irradiation. Furthermore, the photocurrent and photoluminescence studies demonstrated that the deposition of Au nanoparticles on the surface of mesoporous g-C3N4 could effectively inhibit the recombination of photogenerated charge carriers leading to the enhanced photocatalytic activity. More importantly, the synthesized Au/mp-g-C3N4 nanosheets possess high reusability. Hence, Au/mp-g-C3N4 could be promising photoactive material for energy and environmental applications.

Localized surface plasmon fiber device coated with carbon nanotubes for the specific detection of CO2

We explored the potential of a carbon nanotube (CNT) coating working in conjunction with a recently developed localized surface plasmon (LSP) device (based upon a nanostructured thin film consisting of of nano-wires of platinum) with ultra-high sensitivity to changes in the surrounding index. The uncoated LSP sensor’s transmission resonances exhibited a refractive index sensitivity of Δλ/Δn ~ -6200nm/RIU and ΔΙ/Δn ~5900dB/RIU, which is the highest reported spectral sensitivity of a fiber optic sensor to bulk index changes within the gas regime. The complete device provides the first demonstration of the chemically specific gas sensing capabilities of CNTs utilizing their optical characteristics. This is proven by investigating the spectral response of the sensor before and after the adhesion of CNTs to alkane gases along with carbon dioxide. The device shows a distinctive spectral response in the presence of gaseous CO2 over and above what is expected from general changes in the bulk refractive index. This fiber device yielded a limit of detection of 150ppm for CO2 at a pressure of one atmosphere. Additionally the adhered CNTs actually reduce sensitivity of the device to changes in bulk refractive index of the surrounding medium. The polarization properties of the LSP sensor resonances are also investigated and it is shown that there is a reduction in the overall azimuthal polarization after the CNTs are applied. These optical devices offer a way of exploiting optically the chemical selectivity of carbon nanotubes, thus providing the potential for real-world applications in gas sensing in many inflammable and explosive environments. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Optical fibre sensors with applications in gas and biological sensing

This thesis describes the study of various grating based optical fibre sensors for applications in refractive index sensing. The sensitivity of these sensors has been studied and in some cases enhanced using novel techniques. The major areas of development are as follows. The sensitivity of long period gratings (LPGs) to surrounding medium refractive index (SRI) for various periods was investigated. The most sensitive period of LPG was found to be around 160 µm and this was due to the core mode coupling to a single cladding mode but phase matching at two wavelength locations, creating two attenuation peaks, close to the waveguide dispersion turning point. Large angle tilted fibre gratings (TFGs) have similar behaviour to LPGs, in that they couple to the co-propagating cladding modes. The tilted structure of the index modulation within the core of the fibre gives rise to a polarisation dependency, differing the large angle TFG from a LPG. Since the large angle TFG couple to the cladding mode they are SRI sensitive, the sensitivity to SRI can be further increased through cladding etching using HF acid. The thinning of the cladding layer caused a reordering of the cladding modes and shifted to more SRI sensitive cladding modes as the investigation discovered. In a SRI range of 1.36 to 1.40 a sensitivity of 506.9 nm/URI was achieved for the etched large angle TFG, which is greater than the dual resonance LPG. UV inscribed LPGs were coated with sol-gel materials with high RIs. The high RI of the coating caused an increase in cladding mode effective index which in turn caused an increase in the LPG sensitivity to SRI. LPGs of various periods of LPG were coated with sol-gel TiO2 and the optimal thickness was found to vary for each period. By coating of the already highly SRI sensitive 160µm period LPG (which is a dual resonance) with a sol-gel TiO2, the SRI sensitivity was further increased with a peak value of 1458 nm/URI, which was an almost 3 fold increase compared to the uncoated LPG. LPGs were also inscribed using a femtosecond laser which produced a highly focused index change which was no uniform throughout the core of the optical fibre. The inscription technique gave rise to a large polarisation sensitivity and the ability to couple to multiple azimuthal cladding mode sets, not seen with uniform UV inscribed gratings. Through coupling of the core mode to multiple sets of cladding modes, attenuation peaks with opposite wavelength shifts for increasing SRI was observed. Through combining this opposite wavelength shifts, a SRI sensitivity was achieved greater than any single observed attenuations peak. The maximum SRI achieved was 1680 nm/URI for a femtosecond inscribed LPG of period 400 µm. Three different types of surface plasmon resonance (SPR) sensors with a multilayer metal top coating were investigated in D shape optical fibre. The sensors could be separated into two types, utilized a pre UV inscribed tilted Bragg grating and the other employed a post UV exposure to generate surface relief grating structure. This surface perturbation aided the out coupling of light from the core but also changed the sensing mechanism from SPR to localised surface plasmon resonance (LSPR). This greatly increased the SRI sensitivity, compared to the SPR sensors; with the gold coated top layer surface relief sensor producing the largest SRI sensitivity of 2111.5nm/URI was achieved. While, the platinum and silver coated top layer surface relief sensors also gave high SRI sensitivities but also the ability to produce resonances in air (not previously seen with the SPR sensors). These properties were employed in two applications. The silver and platinum surface relief devices were used as gas sensors and were shown to be capable of detecting the minute RI change of different gases. The calculated maximum sensitivities produced were 1882.1dB/URI and 1493.5nm/URI for silver and platinum, respectively. Using a DFB laser and power meter a cheap alternative approach was investigated which showed the ability of the sensors to distinguish between different gases and flow rates of those gases. The gold surface relief sensor was coated in a with a bio compound called an aptamer and it was able to detect various concentrations of a biological compound called Thrombin, ranging from 1mM to as low as 10fM. A solution of 2M NaCl was found to give the best stripping results for Thrombin from the aptamer and showed the reusability of the sensor. The association and disassociation constants were calculated to be 1.0638×106Ms-1 and 0.2482s-1, respectively, showing the high affinity of the Aptamer to thrombin. This supports existing working stating that aptamers could be alternative to enzymes for chemical detection and also helps to explain the low detection limit of the gold surface relief sensor.