Whispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM‐modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence‐based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors.

Surface Plasmon Resonance (SPR) fiber sensor research has grown since the first demonstration over 20 year ago into a rich and diverse field with a wide range of optical fiber architectures, plasmonic coatings, and excitation and interrogation methods. Yet, the large diversity of SPR fiber sensor designs has made it difficult to understand the advantages of each approach. Here, we review SPR fiber sensor architectures, covering the latest developments from optical fiber geometries to plasmonic coatings. By developing a systematic approach to fiber-based SPR designs, we identify and discuss future research opportunities based on a performance comparison of the different approaches for sensing applications.

. Whispering gallery mode biosensor operated in the stimulated emission regime, Applied Physics Letters, 2009; 94(3):031101-1.

Whispering gallery mode lasers are of interest for a wide range of applications and especially biological sensing, exploiting the dependence of the resonance wavelengths on the surrounding refractive index. Upon lasing, the Q factors of the resonances are greatly improved, enabling measurements of wavelength shifts with increased accuracy. A way forward to improve the performance of the refractive index sensing mechanism is to reduce the size of the optical resonator, as the refractive index sensitivity is inversely proportional to the resonator dimensions. However, as the lasing threshold is believed to depend on the Q factor among other parameters, and the reduction of the microresonator size results in lower Q, this poses additional challenges for reaching the lasing threshold. In this letter, we demonstrate lasing in 10 μm diameter dye doped polystyrene microspheres in aqueous solution, the smallest polystyrene microsphere lasers ever reported in these conditions. We also investigate the dependence of the lasing threshold on the Q factor by changing the refractive index surrounding the sphere, highlighting a much stronger dependency than initially reported.

A new concept for an optical biosensor based on whispering gallery mode (WGM) excitations in clusters of spherical microresonators is presented. Clusters of microresonators offer the advantage to exhibit specific WGM spectra that can be considered as their fingerprint. Therefore, individual clusters can be traced throughout an experiment even without knowledge of their precise positions. Polyelectrolyte adsorption onto clusters of 10μm polystyrene spheres is monitored in situ. It is shown that the WGMs shift to the same amount as those of a single microresonator and thus sensitivity does not depend on the number of microresonators present in the cluster.

Whispering gallery modes (WGMs) in surface-fixated fluorescent polystyrene microbeads are studied in view of their capability of sensing the formation of biochemical adsorption layers on their outer surface with the well-established biotin-streptavidin specific binding as the model system. Three different methods for analysis of the observed shifts in the WGM wavelength positions are applied and used to quantify the adsorbed mass densities, which are then compared with the results of a comparative surface plasmon resonance (SPR) study.

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We compare the Tollens reaction, a simple chemical coating method, with the more commonly used RF sputtering technique for use in creating surfaces suitable for sensing applications based on surface plasmon resonance. Experiments show that by optimising the chemical process, these two approaches can produce surfaces that exhibit similar surface plasmon resonance (SPR) performance. Thus, the Tollens method is a viable one for coating the interior surfaces of a microstructured optical fibre. As a demonstration, this technique was applied to a microstructured optical fibre, and a 60-nm silver layer is produced along the entire length of a 1-m-long fibre. This is the first step towards the realisation of a SPR sensing device based on microstructured optical fibres.Keywords Surface plasmon resonance . Microstructure optical fibre Microstructured optical fibres (MOF) are an emerging platform for bio-sensing that can offer a large overlap between the guided light and the surrounding environment and that can work with tiny sample volumes (pL scale) [1]. Although to date, these fibres have primarily been used for fluorescence sensing applications [2, 3] which relies on the use of a sandwich assay as bio-sensing scheme, label-free alternatives based on surface plasmon resonance (SPR) have been proposed [4,5]. Such MOF-based SPR sensors have the potential to enable the excitation of multiple SPR resonances due to the curvature of the SPR surface (fibre core) while offering tuneability of the resonance wavelength from the visible to the near IR by modifying the fibre effective refractive index. It has been shown that the detection limit of such MOF SPR sensor should be close to 3×10 −5 RIU, comparable to the performance of the best existing fibres and waveguide-based sensors optimised for aqueous analytes [5].One reason this type of sensor has not been experimentally realised is due to the restricted geometry of MOFs, which in general mandates the use of coating techniques that are capable of coating the internal surfaces of micronscale holes within the fibre. This implies that conventional deposition techniques such as RF sputtering and thermal evaporation cannot be used. Chemical vapour deposition of silver has been proposed as one possible coating method that may work for these fibres [6]. However, this approach involves rather complex organometallic chemistry. A simpler and cheaper alternative is the Tollens reaction [7,8], which is a chemical technique that has been used successfully for the chemical deposition of silver nanoparticles for surface enhanced raman spectroscopy or more recently for the development of an electrochemical sensor inside a MOF [9]. So far, to the best of our knowledge, no systematic research has been conducted on the quality of the chemically deposited silver films that can be achieved with the Tollens reaction, and this is of particular importance for relating to SPR sensing. Within this paper, the influence of the different parameters involved in Tollens reaction are examined and the resultant...