We present three different nano-resonant structures (nanoposts, nanoholes etc.) fabricated on either bulk substrate or micron size tip of optical fiber and one graphene oxide coated glass substrate for gas detection in visible or mid-infrared region of electromagnetic spectrum. Nanostructures provide an efficient way to control and manipulate light at nanoscale paving the way for the development of reliable, sensitive, selective and miniaturized gas sensing technologies. Moreover, the inherent light guiding property of optical fiber over long distances, their microscopic cross-section, their efficient integration capabilities with gas absorption coatings and mechanical flexibility make them suitable for remote sensing applications. The three nanostructure-based gas sensing techniques are based on the detection of surface plasmon resonance (SPR) wavelength shifts, guided mode resonance (GMR) wavelength shifts, and Rayleigh anomaly (RA) mode intensity variations. The SPR and GMR based sensors operate in the visible region of light spectrum. Later, we also integrate a heater with the GMR-based fiber-tip sensor to realize a reusable gas sensor having tunable sensor recovery time. The RA-based sensor is realized by solvent-casting of chalcogenide glass to work as mid-infrared optical resonator. Further, we utilize the dynamic variations in infrared values of graphene oxide in response to gas to realize a gas sensor.

First, we present a high-sensitivity gas sensor based on plasmonic crystal incorporating a thin layer of graphene oxide. The presented plasmonic crystal is formed by an array of polymeric nanoposts with gold disks at the top and perforated nanoholes in a gold thin film at the bottom. The thin coating of graphene oxide assembled on the top surface of mushroom plasmonic nanostructures works as the gas absorbent material for the sensor. The optical response of the plasmonic nanostructure is altered due to different concentrations of gas absorbed in the graphene oxide coating. By coating the surface of multiple identical plasmonic crystals with different thicknesses of graphene oxide layer, the effective refractive index of the graphene oxide layer on each plasmonic crystal will be differently modulated when responding to a specific gas. This allows identifying various gas species using the principal component analysis-based pattern recognition algorithm. The present plasmonic nanostructure offers a promising approach to detect various volatile organic compounds.

Second, we report a simple yet efficient method of transferring nanopatterns to optical fiber tip. We have also demonstrated a TiO2 coated GMR structure which is sensitive to changes in surrounding refractive index and provides shifts in its resonant wavelength. The GMR sensor at the fiber tip is also demonstrated to work as a gas sensor by coating it with a thin layer of graphene oxide. This simplified and rapid nanostructuring at fiber tip can contribute to remote sensing applications through the insertion of the nanopatterned fiber tips into aqueous and gaseous analytes in regions otherwise inaccessible.

Third, we present the first heater integrated nanostructured optical fiber of 200 à ¯à ¿à ½m diameter to realize a high-sensitivity and reusable fiber-optic gas sensor. In our GMR-enabled fiber-optic gas sensor, resonance shifts upon the adsorption of the analytes on the graphene oxide (GO) coated sensor surface. For repeated use of this sensor, a regeneration of the sensor surface is required by a complete desorption of the analyte molecules from the GO layer. In our presented design, this has been achieved by the integration of a controllable heater at the fiber tip.

Fourth, we present a straightforward analysis based on the maximum and minimum envelopes of the reflection spectra to dynamically investigate the changes in complex refractive index of graphene oxide in response to gases. The performance of graphene oxide -based gas sensors is strongly influenced by the variations in optical properties of graphene oxide when exposed to gas. The presented method does not require any complex dispersion model as compared to ellipsometry. Accordingly, the technique we employ can be leveraged to reliably evaluate the optical performance of any graphene oxide-based gas sensors in a simpler manner, when compared to ellipsometry. Furthermore, the accuracy of the derived values of complex refractive index of the graphene oxide layer has been confirmed by comparing with literature.

Finally, we report the development of a first of a kind planar resonant structure that enhances the mid-IR absorption by the analyte adsorbed on its surface, enabling highly sensitive and selective label-free detection of gas and/or biomarkers. Chalcogenide glasses (As2S3) are promising for infrared photonics owing to their transparency in visible to far infrared, where various biomolecules and gases have their characteristic absorption lines, arising from rotational-vibrational transitions. Here we present the proposed design of a nanoscale tunable planar mid-IR optical resonator, realized by solvent-casting of As2S3. Our technique of preparing nanostructure having resonance at mid-IR enables the realization of mid-IR bio as well as gas sensors.