Ultimate resolution for refractometric sensing with whispering gallery mode microcavities

Beyond Q: The Importance of the Resonance Amplitude for Photonic Sensors

Resonant photonic sensors are enjoying much attention based on the worldwide drive toward personalized healthcare diagnostics and the need to better monitor the environment. Recent developments exploiting novel concepts such as metasurfaces, bound states in the continuum, and topological sensing have added to the interest in this topic. The drive toward increasingly higher quality (Q)-factors, combined with the requirement for low costs, makes it critical to understand the impact of realistic limitations such as losses on photonic sensors. Traditionally, it is assumed that the reduction in the Q-factor sufficiently accounts for the presence of loss. Here, we highlight that this assumption is overly simplistic, and we show that losses have a stronger impact on the resonance amplitude than on the Q-factor. We note that the effect of the resonance amplitude has been largely ignored in the literature, and there is no physical model clearly describing the relationship between the limit of detection (LOD), Q-factor, and resonance amplitude. We have, therefore, developed a novel, ab initio analytical model, where we derive the complete figure of merit for resonant photonic sensors and determine their LOD. In addition to highlighting the importance of the optical losses and the resonance amplitude, we show that, counter-intuitively, optimization of the LOD is not achieved by maximization of the Q-factor but by counterbalancing the Q-factor and amplitude. We validate the model experimentally, put it into context, and show that it is essential for applying novel sensing concepts in realistic scenarios.

Optical fiber laser refractometer based on an open microcavity Mach-Zehnder interferometer with an ultra-low detection limit

A fiber laser refractometer based on an open microcavity Mach-Zehnder interferometer (OMZI) is proposed. The open microcavity is constructed by embedding a segment single-mode fiber (SMF) into two multi-mode fiber (MMF) joints with lateral offset for liquid sample, which has the advantages of micro sensing element and easy fabrication. The transmission characteristics of the MMF-assisted OMZI are investigated by simulating and manufacturing the OMZIs with different microcavity lengths and offset distances. By inserting the MMF-assisted OMZI into the erbium-doped fiber ring laser (FRL) cavity, the lasing wavelength can be used to detect the refractive index (RI) change of the medium in the microcavity. Experimental results show a high sensitivity of -2953.444 nm/RIU within the measurement range of 1.33302∼1.33402. More importantly, with the combination of OMZI and FRL, the proposed fiber laser refractometer realizes ultra-low detection limit (DL) and high-quality factor Q, which are two orders of magnitude better than that of previous reports.

Precisely ordered Ge quantum dots on a patterned Si microring for enhanced light-emission

Semiconductor microcavities can greatly enhance the light-emission of embedded quantum dots (QDs). Here, a new route toward the microcavity-QD system by fabricating microcavities followed by growing ordered QDs on a patterned microresonator is proposed, which keeps QDs from being etched. Self-assembled Ge QDs prefer to form at the rims of Si microrings or microdisks. The Ge QDs on the pit- or groove-patterned microring resonator (MRR) show better size uniformity and position accuracy. These features are explained by the evolutions of surface morphology and surface chemical potential distribution. Sharp photoluminescence peaks in the telecommunication band with the quality factors in the range of 450-850 from groove-patterned MRR are observed at 295 K due to efficient overlap between Ge QDs and resonant modes. Our schemes shed light on the exactly site-controlled growth of QDs on micro- and nano-structures, which further facilitates the investigation of light-matter interactions.

Ultrahigh Resolution Thickness Measurement Technique Based on a Hollow Core Optical Fiber Structure

An ultrahigh resolution thickness measurement sensor was proposed based on a single mode–hollow core–single mode (SMF–HCF–SMF) fiber structure by coating a thin layer of material on the HCF surface. Theoretical analysis shows that the SMF–HCF–SMF fiber structure can measure coating thickness down to sub-nanometers. An experimental study was carried out by coating a thin layer of graphene oxide (GO) on the HCF surface of the fabricated SMF–HCF–SMF fiber structure. The experimental results show that the fiber sensor structure can detect a thin layer with a thickness down to 0.21 nanometers, which agrees well with the simulation results. The proposed sensing technology has the advantages of simple configuration, ease of fabrication, low cost, high resolution, and good repeatability, which offer great potential for practical thickness measurement applications.

Functional lasing microcapillaries for surface-specific sensing

Lasing-based sensors have several advantages over fluorescent devices, specifically related to the high light intensity and narrow mode linewidth that can improve the speed and accuracy of the sensor performance. In this work, a microcapillary-based lasing sensor is demonstrated, in which the lasing wavelengths are sensitive to the surface binding of specific materials. In order to achieve this, we utilized lasing into the "star" and "triangle" modes of a conventional microcapillary and tracked the mode positions after the deposition of a polyelectrolyte tri-layer and the subsequent amide binding of carboxy-functionalized polystyrene microspheres. While the lasing mode spectrum becomes increasingly complicated by the addition of the surface layers, careful mode selection can be used to monitor the layer-by-layer surface binding in a mechanically and optically robust device. For polystyrene microspheres, the detection limits were 9.75 nM based upon the lasing mode shift, which compares favorably with fluorescence-based devices. The methods presented in this work could readily be extended to other surface binding schemes and lasing wavelengths, showing that capillary microlasers could be used for many potential applications that capitalize on stable lasing-based detection methods.

Sensitive refractive index sensor based on an assembly-free fiber multi-mode interferometer fabricated by femtosecond laser

We propose and demonstrate a highly sensitive refractive index (RI) sensor based on a novel fiber-optic multi-mode interferometer (MMI), which is formed with a femtosecond-laser-induced in-core negative refractive index modified line in a standard single mode fiber. The proposed MMI structure is directly written with femtosecond laser in one step, which removes the splicing process needed in conventional MMI fabrication and also significantly improves the robustness. This device exhibits a high sensitivity to surrounding refractive index, with a maximum sensitivity up to 10675.9 nm/RIU at the RI range of 1.4484-1.4513. The distinct advantages of high sensitivity, compact, robust and assembly-free all-fiber structure make it attractive for real physical, chemical and biological sensing.

Refractometric micro-sensor using a mirrored capillary resonator

We report on a flow-through optical sensor consisting of a microcapillary with mirrored channels. Illuminating the structure from the side results in a complicated spectral interference pattern due to the different cavities formed between the inner and outer capillary walls. Using a Fourier transform technique to isolate the desired channel modes and measure their resonance shift, we obtain a refractometric detection limit of (6.3 ± 1.1) x 10^-6 RIU near a center wavelength of 600 nm. This simple device demonstrates experimental refractometric sensitivities up to (5.6 ± 0.2) x 10² nm/RIU in the visible spectrum, and it is calculated to reach 1540 nm/RIU with a detection limit of 2.3 x 10^-6 RIU at a wavelength of 1.55 µm. These values are comparable to or exceed some of the best Fabry-Perot sensors reported to date. Furthermore, the device can function as a gas or liquid sensor or even as a pressure sensor owing to its high refractometric sensitivity and simple operation.

Lasing of whispering gallery modes in optofluidic microcapillaries

This paper demonstrates lasing of the whispering gallery modes in polymer coated optofluidic capillaries and their application to refractive index sensing. The laser gain medium used here is fluorescent Nile Red dye, which is embedded inside the high refractive index polymer coating. We investigate the refractometric sensing properties of these devices for different coating thicknesses, revealing that the high Q factors required to achieve low lasing thresholds can only be realized for relatively thick polymer coatings (in this case ≥ 800 nm). Lasing capillaries therefore tend to have a lower refractive index sensitivity, compared to non-lasing capillaries which can have a thinner polymer coating, due to the stronger WGM confinement within the polymer layer. However we find that the large improvement in signal-to-noise ratio realized for lasing capillaries more than compensates for the decreased sensitivity and results in an order-of-magnitude improvement in the detection limit for refractive index sensing.

Tuning whispering gallery lasing modes from polymer fibers under tensile strain

Wavelength tuning of whispering gallery lasing modes has been observed from Rhodamine-B-doped polymer fibers under tensile strain. Good quality whispering gallery lasing modes are produced from both solid and hollow fibers by transverse optical pumping. The lasing modes are shifted linearly toward the shorter wavelength side when the fiber is elongated in the axial direction. Compared with solid fiber, the lasing modes of hollow fiber can be tuned over the entire gain spectrum with a tuning range of ∼5  nm. It is found that the tuning of the lasing modes of hollow fiber is reversible.

Whispering gallery mode sensors

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

Material candidates for optical frequency comb generation in microspheres

This paper evaluates the opportunities for using materials other than silica for optical frequency comb generation in whispering gallery mode microsphere resonators. Different materials are shown to satisfy the requirement of dispersion compensation in interesting spectral regions such as the visible or mid-infrared and for smaller microspheres. This paper also analyses the prospects of comb generation in microspheres within aqueous solution for potential use in applications such as biosensing. It is predicted that to achieve comb generation with microspheres in aqueous solution the visible low-loss wavelength window of water needs to be exploited. This is because efficient comb generation necessitates ultra-high Q-factors, which are only possible for cavities with low absorption of the evanescent field outside the cavity. This paper explores the figure of merit for nonlinear interaction efficiency and the potential for dispersion compensation at unique wavelengths for a host of microsphere materials and dimensions and in different surroundings.

Refractometric sensitivity and thermal stabilization of fluorescent core microcapillary sensors: theory and experiment

Fluorescent-core microcapillaries (FCMs) present a robust basis for the application of optical whispering gallery modes toward refractometric sensing. An important question concerns whether these devices can be rendered insensitive to local temperature fluctuations, which may otherwise limit their refractometric detection limits, mainly as a result of thermorefractive effects. Here, we first use a standard cylindrical cavity formalism to develop the refractometric and thermally limited detection limits for the FCM structure. We then measure the thermal response of a real device with different analytes in the channel and compare the result to the theory. Good stability against temperature fluctuations was obtained for an ethanol solvent, with a near-zero observed thermal shift for the transverse magnetic modes. Similarly good results could in principle be obtained for any other solvent (e.g., water), if the thickness of the fluorescent layer can be sufficiently well controlled.

Protein biosensing with fluorescent microcapillaries

Capillaries with a high-index fluorescent coating represent a new type of whispering-gallery-mode (WGM) microcavity sensor. By coating silicon quantum dots (Si-QDs) onto the channel wall of a microcapillary, a cylindrical microcavity forms in which the optical confinement arises from the index contrast at the interface between the QD layer and the glass capillary wall. However, the ability to functionalize the QD layer for biosensing applications is an open question, since the layer consists of a mixture of Si-QDs embedded in a glassy SiOx matrix. Here, we employ a polyelectrolyte (PE) multilayer approach to functionalize the microcapillary inner surface and demonstrate the potential of this refractive index sensing platform for label-free biosensing applications, using biotin-neutravidin as a specific interaction model.

Laser emission from the whispering gallery modes of a graded index fiber

Whispering gallery mode (WGM) laser emission has been observed from rhodamine B doped polymer optical graded index (GI) fiber by transverse pumping with a frequency doubled Q-switched Nd:YAG laser. The propagation and confinement of these modes were also observed. A variation in the free spectral range from 0.29 to 1.24 nm is obtained along the length due to the confinement of WGMs in the GI fiber.

Fluorescent polymer coated capillaries as optofluidic refractometric sensors

A capillary microresonator platform for refractometric sensing is demonstrated by coating the interior of thick-walled silica capillaries with a sub-wavelength layer of high refractive index, dye-doped polymer. No intermediate processing, such as etching or tapering, of the capillary is required. Side illumination and detection of the polymer layer reveals a fluorescence spectrum that is periodically modulated by whispering gallery mode resonances within the layer. Using a Fourier technique to calculate the spectral resonance shifts, the fabricated capillary resonators exhibited refractometric sensitivities up to approximately 30 nm/RIU upon flowing aqueous glucose through them. These sensors could be readily integrated with existing biological and chemical separation platforms such as capillary electrophoresis and gas chromatography where such thick walled capillaries are routinely used with polymer coatings. A review of the modelling required to calculate whispering gallery eigenmodes of such inverted cylindrical resonators is also presented.

Synthesis and operation of fluorescent-core microcavities for refractometric sensing

This paper discusses fluorescent core microcavity-based sensors that can operate in a microfluidic analysis setup. These structures are based on the formation of a fluorescent quantum-dot (QD) coating on the channel surface of a conventional microcapillary. Silicon QDs are especially attractive for this application, owing in part to their negligible toxicity compared to the II-VI and II-VI compound QDs, which are legislatively controlled substances in many countries. While the ensemble emission spectrum is broad and featureless, an Si-QD film on the channel wall of a capillary features a set of sharp, narrow peaks in the fluorescence spectrum, corresponding to the electromagnetic resonances for light trapped within the film. The peak wavelength of these resonances is sensitive to the external medium, thus permitting the device to function as a refractometric sensor in which the QDs never come into physical contact with the analyte. The experimental methods associated with the fabrication of the fluorescent-core microcapillaries are discussed in detail, as well as the analysis methods. Finally, a comparison is made between these structures and the more widely investigated liquid-core optical ring resonators, in terms of microfluidic sensing capabilities.