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

All-Optical Nanopositioning of High-Q Silica Microspheres

A tunable, all-optical, coupling method has been realized for a high-\textit{Q} silica microsphere and an optical waveguide. By means of a novel optical nanopositioning method, induced thermal expansion of an asymmetric microsphere stem for laser powers up to 171~mW has been observed and used to fine tune the microsphere-waveguide coupling. Microcavity displacements ranging from (0.612~$\pm$~0.13) -- (1.5 $\pm$ 0.13) $\mu$m and nanometer scale sensitivities varying from (2.81 $\pm$ 0.08) -- (7.39 $\pm$ 0.17) nm/mW, with an apparent linear dependency of coupling distance on stem laser heating, were obtained. Using this method, the coupling was altered such that different coupling regimes could be explored for particular samples. This tunable coupling method, in principle, could be incorporated into lab-on-a-chip microresonator systems, photonic molecule systems, and other nanopositioning frameworks.Comment: 6pages,4figure

Method for predicting whispering gallery mode spectra of spherical microresonators

A full three-dimensional Finite-Difference Time-Domain (FDTD)-based toolkit is developed to simulate the whispering gallery modes of a microsphere in the vicinity of a dipole source. This provides a guide for experiments that rely on efficient coupling to the modes of microspheres. The resultant spectra are compared to those of analytic models used in the field. In contrast to the analytic models, the FDTD method is able to collect flux from a variety of possible collection regions, such as a disk-shaped region. The customizability of the technique allows one to consider a variety of mode excitation scenarios, which are particularly useful for investigating novel properties of optical resonators, and are valuable in assessing the viability of a resonator for biosensing.Comment: Published 10 Apr 2015 in Opt. Express Vol. 23, Issue 8, pp. 9924-9937; The FDTD toolkit supercomputer scripts are hosted at: http://sourceforge.net/projects/npps/files/FDTD_WGM_Simulator

Ultra high-Q WGM microspheres from ZBLAN for the mid-IR band

The advantages of high-quality-factor whispering gallery mode microresonators can be applied to develop novel photonic devices for the mid-IR range. ZBLAN (glass based on heavy metal fluorides) is one of the most promising materials to be used for this purpose due to low optical losses in the mid-IR. We developed original fabrication method based on melting of commercially available ZBLAN-based optical fiber to produce high-Q ZBLAN microspheres with the diameters of 250 to 350 $\mu$m. We effectively excited whispering gallery modes in these microspheres and demonstrated high quality factor both at 1.55 $\mu$m and 2.64 $\mu$m. Intrinsic quality factor at telecom wavelength was shown $(5.4\pm0.4)\cdot10^8$ which is defined by the material losses in ZBLAN. In the mid-IR at 2.64 $\mu$m we demonstrated record quality factor in ZBLAN exceeding $10^8$ which is comparable to the highest values of the Q-factor among all materials in the mid-IR

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

A tellurite glass optical microbubble resonator

We present a method for making microbubble whispering gallery resonators (WGRs) from tellurite, which is a soft glass, using a CO2 laser. The customized fabrication process permits us to process glasses with low melting points into microbubbles with loaded quality factors as high as 2.3 × 106. The advantage of soft glasses is that they provide a wide range of refractive index, thermo-optical, and optomechanical properties. The temperature and air pressure dependent optical characteristics of both passive and active tellurite microbubbles are investigated. For passive tellurite microbubbles, the measured temperature and air pressure sensitivities are 4.9 GHz/K and 7.1 GHz/bar, respectively. The large thermal tuning rate is due to the large thermal expansion coefficient of 1.9 × 10−5 K−1 of the tellurite microbubble. In the active Yb3+-Er3+ co-doped tellurite microbubbles, C-band single-mode lasing with a threshold of 1.66 mW is observed with a 980 nm pump and a maximum wavelength tuning range of 1.53 nm is obtained. The sensitivity of the laser output frequency to pressure changes is 6.5 GHz/bar. The microbubbles fabricated using this method have a low eccentricity and uniform wall thickness, as determined from electron microscope images and the optical spectra. The compound glass microbubbles described herein have the potential for a wide range of applications, including sensing, nonlinear optics, tunable microcavity lasers, and integrated photonics

A taper-fused microspherical laser source

We report on the realization of an integrated lasing device consisting of a microsphere optical resonator fused to a tapered optical fiber. A microsphere fabricated from Er: Yb-codoped phosphate glass is heated above its glass transition temperature of 375degC by pumping it at 977 nm with 70 mW via a tapered optical fiber. The onset of thermal stress in the glass at a maximum pumping power results in the sphere melting and fusing to the taper coupler, without inhibition of whispering gallery mode lasing. A taper-fused microsphere laser with ~4.5 muW of lasing power at 1593 nm is demonstrated