Optical Properties and Behavior of Whispering Gallery Mode Resonators in Complex Microsphere Configurations: Insights for Sensing and Information Processing Applications

Whispering gallery mode (WGM) resonators are garnering significant attention due to their unique characteristics and remarkable properties. When integrated with optical sensing and processing technology, WGM resonators offer numerous advantages, including compact size, high sensitivity, rapid response, and tunability. This paper comprehensively investigates the optical properties and behavior of WGMs in complex microsphere resonator configurations. The findings underscore the potential of WGMs in sensing applications and their role in advancing future optical information processing. The study explores the impact of configuration, size, excitation, polarization, and coupling effects on the WGMs properties. The paper provides crucial insights and valuable guidance for designing and optimizing microsphere resonator systems, enabling their realization for practical applications.Comment: 11 pages, 13 figure

Glassy Materials Based Microdevices

Microtechnology has changed our world since the last century, when silicon microelectronics revolutionized sensor, control and communication areas, with applications extending from domotics to automotive, and from security to biomedicine. The present century, however, is also seeing an accelerating pace of innovation in glassy materials; as an example, glass-ceramics, which successfully combine the properties of an amorphous matrix with those of micro- or nano-crystals, offer a very high flexibility of design to chemists, physicists and engineers, who can conceive and implement advanced microdevices. In a very similar way, the synthesis of glassy polymers in a very wide range of chemical structures offers unprecedented potential of applications. The contemporary availability of microfabrication technologies, such as direct laser writing or 3D printing, which add to the most common processes (deposition, lithography and etching), facilitates the development of novel or advanced microdevices based on glassy materials. Biochemical and biomedical sensors, especially with the lab-on-a-chip target, are one of the most evident proofs of the success of this material platform. Other applications have also emerged in environment, food, and chemical industries. The present Special Issue of Micromachines aims at reviewing the current state-of-the-art and presenting perspectives of further development. Contributions related to the technologies, glassy materials, design and fabrication processes, characterization, and, eventually, applications are welcome

Cascaded Photoenhancement: Implications for Photonic Chemical and Biological Sensors

Our analysis shows that coupling of gold nanoparticles to microspheres will evoke a cascading effect from the respective photoenhancement mechanisms. We refer to this amplification process as cascaded photoenhancement, and the resulting cavity amplification of surface-enhanced Raman scattering (SERS) and fluorescence as CASERS and CAF, respectively. Calculations, based on modal analysis of scattering and absorption by compound spheres, presented herein indicate that the absorption cross sections of metal nanoparticles immobilized onto dielectric microspheres can be greatly enhanced by cavity resonances in the microspheres without significant degradation of the resonators. Gain factors associated with CSP of 10(exp 3) - 10(exp 4) are predicted for realistic experimental conditions using homogenous microspheres. Cascaded surface photoenhancement thus has the potential of dramatically increasing the sensitivities of fluorescence and vibrational spectroscopies

A Whispering Gallery Mode Microlaser Biosensor

A biological sensor, commonly referred to simply as a biosensor, is a transducing device that allows quantitative information about specific interactions, analytes or other biological parameters to be monitored and recorded. The development of biosensors that are low-cost, reliable and simple to use stand to facilitate fundamental breakthroughs and revolutionize current medial diagnostic methods. Notably, there remains an unmet need for developing in-vivo biosensors, allowing insights to be directly gained from the precise location of biological interactions within the human body. Over the last two decades, whispering gallery modes (WGM) within microresonators have emerged as a promising technology for developing highly sensitive and selective biosensors, among many other applications. However, significant work remains to allow WGM sensors to make the transition from primarily being used within purely research environments to real-world applications. Specifically, one of the key limiting factors is the requirement of an external phase-matched coupling scheme (such as a tapered or angle polished optical fiber, prism or waveguide) to excite the WGMs, despite these devices displaying tremendous sensing performance. One way to lift this dependency on complex interrogation schemes is introduce a gain medium, such as a fluorescent dye or coating the resonator with quantum dots for example, thereby rendering it active and allowing remote excitation and collection of the WGM spectrum. Using active WGM resonators has allows the creation of novel sensing opportunities such as tagging, tracking and monitoring forces from insides living cells. Applications like these could not have been realized using external phase-matched coupling schemes. The biosensing platform presented here is based on combining WGM within active microspherical resonators with microstructured optical fibers (MOF). The MOF enables both the excitation and collection method for the WGM spectrum while simultaneously providing a robust and easy to manipulate dip sensing architecture that has the potential to address the unmet need for real time labelfree in-vivo sensing by combining with a catheter. The platform is investigated fundamentally as well as experimentally, beginning with the development of an analytical model that is able to generate the WGM spectrum of active microspherical resonators. This provides the opportunity to pinpoint the optimal choice of resonator to be used for undertaking refractive index based biosensing. Specifically by being able to extract the quality (Q) factor, a measure of the resonance linewidth, and refractive index sensitivity from the WGM spectrum, the optimal combination of resonator parameters (diameter and resonator refractive index) can be identified for optimizing the resonators sensing performance. Further, the availability, biocompatibility and cost, as well as fabrication requirements can be also considered when selecting the ideal resonator. Next, the inherently lower Q-factors observed in active resonators compared to their passive counterparts (i.e. resonators without a gain medium) is examined using a combination of theoretical, experimental and imaging methods. Through this examination process, the inherent asphericity of the resonator is identified as being the limiting factor on the Q-factor of active resonators, with its effect most notably being observed for measurements made in the far field. Experimentally, the first demonstration of this platform operating as a biosensor is presented by monitoring the well-documented specific interaction of Biotin/neutravidin in pure solutions. Including identifying ways to improve sensing performance and lower the detection limit, such as operating the resonator above its lasing threshold. Although, it is noted that in its current form, this platform is best suited for the monitoring of protein, preferably occurring in higher concentrations, until further improvements to the sensing performance can be implemented. However, the robust design coupled with its ability to provide access to previously difficult to obtain locations provides an insight into its potential future application capabilities. Finally, the extension of the platform to operating in complex samples, namely undiluted human serum, is outlined. By self-referencing the platform, through the addition of a second, almost identical resonator (only varying in its surface functionalization) into one of the remaining vacant holes on the tip of the fiber, the effects of non-specific binding as well as changes in local environmental conditions (i.e. temperature fluctuations), can be eliminated.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 201

Whispering gallery modes in quantum dot-embedded dielectric microspheres for tagless remote refractometric sensing

This thesis presents the development of a refractometric sensor based on quantum dot-embedded polystyrene microspheres. The technique uses optical resonances within a microsphere, known as Whispering-Gallery Modes (WGMs), which produce narrow spectral peaks. The basic theory of WGMs is reviewed and specifically discussed for biosensing application. The spectral shifts of WGM peaks are sensitive to changes in the local refractive index. In the experiments, two-photon excited luminescence from the quantum dots couples into several WGMs within the microresonator. By optimizing the detection area, the spectral visibility of the WGMs is improved. The spectral shifts are measured as the surrounding index of refraction changes. The experimental sensitivity is about five times greater than that predicted by Mie theory. The sensor element is based on commercially available dielectric microspheres with a diameter about 10 μm. Thus, the technique is more economic and suitable for sensing applications, compared to microspheres of 100 μm in size which can only be made in the laboratory

Whispering-Gallery Modes in Quantum Dot Embedded Microspheres for Sensing Applications

New methods of biological analyte sensing are needed for development of miniature biosensors that are highly sensitive and require minimal sample preparation. One novel technique employs optical resonances known as Whispering Gallery Modes (WGMs). These modes arise from total internal reflection of light at the internal surface of a high index microsphere within a low index medium and produce an evanescent field that extends into the surrounding medium. The WGMs produce multiple narrow spectral peaks that shift position with variations in the local index of refraction sampled by the evanescent tail of the WGMs. To excite these WGMs, we embed quantum dots (QDs) in the periphery of polystyrene microspheres to serve as local light sources. By coupling emission from the QDs to the WGMs, the sensors can be excited and interrogated remotely and, by monitoring the shift of multiple resonance modes, may provide higher sensitivity and accuracy compared with similar techniques. The high refractometric sensitivity of the WGMs offers potential for trace detection of molecules adsorbed onto or bound to the microsphere sensor elements. The sensitivity of these sensors is demonstrated by monitoring the wavelength shift of multiple resonant modes as bulk index of refraction is changed. The potential for targeted biosensing is explored through addition of a protein that adsorbs to the microsphere surface, thrombin. Microsensor response in all cases demonstrated increased sensitivity over theoretical predictions. Models based on Mie theory and continuity of the radial functions across the sphere-media interface were used to model the location, Q-factor, and sensitivity of the WGMs in microspheres by considering the embedded QDs as a high index outer layer. This model was used, along with estimates of the QD-layer index and penetration depth, to relate the locations and sensitivities of the modes to our experimental results with strong agreement between the two. In all, these microspheres demonstrate feasibility for use as remote microsensors with sensitivities rivaling current techniques