Showing papers by "Ian M. White published in 2005"

Refractometric sensors based on microsphere resonators

Abstract: We have developed a highly sensitive refractometric sensor based on fused silica microsphere resonators. The spectral position of the whispering gallery mode (WGM) of a sphere shifts in response to the refractive index change in the surrounding medium. The strong light-matter interaction due to the extremely high Q factor associated with the WGM results in a sensitivity of approximately 30nm∕RIU (refractive index units). This, together with the high spectral resolution of our sensor system (∼0.01pm), yields a detection limit of refractive index change on the order of 10−7RIU. Theoretical calculation is also performed and agrees well with the experimental data.

Label-Free Protease Sensors Based on Optical Microsphere Resonators

Abstract: Simple and effective sensors for proteolytic activity are of interest to clinical diagnosis, homeland security, and nanofabrication. We demonstrate a novel label-free sensor for proteolytic activity utilizing an optical microsphere resonator that relies on circulation of the light to enhance the interaction with analytes. As a model, we employ the protease trypsin and a monolayer of BSA as a substrate on the sphere surface. We show a trypsin detection limit of approximately 10−4 Units/ml, corresponding to removal of 0.6% of each BSA molecule on the sphere surface. The detection limit obtained in this work indicates that a microsphere sensor with specificity for a protease of interest can be designed using synthesized peptides and a functionalized microsphere.

Subfemtomole detection of small molecules with microsphere sensors.

Abstract: We investigated the feasibility of using a silica microsphere sensor for detection of small molecules. Using the silica molecules (60 Da) at the sphere's surface as a model system, we measured the spectral shifts in the whispering-gallery modes (WGMs) when the sphere size was decreased by a hydrofluoric acid (HF) solution. The results demonstrate that our sensor is capable of detecting a 4 pm (or 0.01 layer of silica) decrease in sphere radius, corresponding to a change of 0.4 fmol silica molecule. These results suggest that small molecules can be detected in trace quantities at the surface of an optical microsphere sensor.

Tuning whispering gallery modes in optical microspheres with chemical etching.

Abstract: We demonstrate a new method to tune the resonance of whispering gallery modes in a fused silica optical microsphere resonator by removing atomic layers from the sphere surface with low concentrations of hydrofluoric acid Our results show that the WGMs can be tuned over 660 pm (430 GHz), more than one free spectral range of the microsphere resonator, with a tuning precision better than 02 pm (130 MHz) Both atomic force microscope images and a Q-factor measurement performed in air suggest that no additional degradation in Q-factor due to surface roughness is introduced during this etching process

Demonstration of composite microsphere cavity and surface enhanced raman spectroscopy for improved sensitivity

Abstract: Surface enhanced Raman spectroscopy (SERS) has widely been used for material composition analysis because it can provide good selectivity and sensitivity without the labeling process required by fluorescence detection. SERS enhancements on the order of 10 14 have been demonstrated, which can enable the detection of a single molecule. However, further enhancement is necessary to increase the sensitivity of SERS systems, and to make single molecule detection and analysis more practical. In this work, we demonstrate a composite system of silver nanoparticles and an optical microsphere resonator to create an even higher average Raman enhancement than SERS alone. The Raman pump is coupled into the microsphere resonator, where it repeatedly circulates around the surface via total internal reflection in the form of whispering gallery modes (WGMs). Microsphere resonators can have Q-factor values higher than 10 6 , which results in a tremendous local field enhancement. The evanescent field of the WGM interacts with the silver nanoparticles and target analytes, which are adsorbed onto the surface of the microsphere. With this composite system, we demonstrate an increase in Raman enhancement of approximately 300. Engineering improvements to this experimental prototype system may increase the enhancement by an order of magnitude. Further improvements that can leverage the microsphere resonator system to promote stimulated Raman scattering (SRS) may result in a dramatic increase in sensitivity. Ultimately, this composite system will increase the sensitivity of SERS sensor systems, and will bring single molecule detection and analysis systems closer to practical implementation.

Development of label-free microsphere optical resonator bio/chemical sensors

Abstract: Whispering Gallery Modes (WGMs) in microsphere ring resonators enable excellent sensitivity due to the high Q-factor (> 10 6 ) that significantly increases the light-matter interaction. The analytes attached to the sphere surface change the local refractive index, leading to a spectral shift in the resonances of the WGM. A practical microsphere-based sensor must detect minute changes in the refractive index. In addition, the microsphere surface should be functionalized for subsequent binding of bio/chemical molecules. Both functionalization and binding processes should be monitored in order to better anchor the captured molecules and to acquire quantitative and kinetic binding information. In this work, we have carried out a series of experiments towards developing highly sensitive fused silica microsphere based sensors. A fiber prism is used to couple the light from a tunable diode laser to the sphere. A fluidic well is built to allow for injection and withdrawal of samples. The sensor sensitivity of refractive index is characterized by using the mixture of water and alcohol. It is shown that our system is able to detect changes in refractive index as low as 10 -7 . We further monitor the kinetics of layer deposition when the sphere surface is functionalized with silane solution. Finally we monitor the protein binding to and peptide cleavage from the functionalized microsphere. Our results should lead to highly sensitive microsphere bio/chemical sensor arrays with applications in biomedical sciences, environmental monitoring, and drug discovery.

Communication network route tracing

Abstract: A method for tracing a communication route through a network coupling a first device with a second device is provided. A first signal is transferred from the first device to the second device to cause a plurality of intermediate devices to report first information concerning the first signal. The first information is processed to identify the intermediate devices as defining the communication route. The intermediate devices are configured so that a second signal, when transferred from the first device toward the second device, causes a subset of the intermediate devices to report second information concerning the second signal. The second signal is transferred from the first device toward the second device. The second information is processed to identify the subset of the intermediate devices as a portion of a sequence of the intermediate devices defining the communication route.

Refractometric characterization of microsphere resonator based optical sensors

Abstract: We have carried out the refractometric characterization on the whispering gallery mode (WGM) of a fused silica microsphere as the first step towards developing novel bio/chemical sensors with high sensitivity and high multiplexing capability. The WGM spectral position shifts when the refractive index of the surrounding medium is varied by adding ethanol to water. It is found that a sensitivity of 30 nm/RIU (refractive index units) and a refractive index detection limit on the order of 10-7 RIU can be achieved. Theoretical calculation is also performed and agrees well with the experimental data

Service-aware transport network: opportunities and challenges

Abstract: In this paper we propose a new concept of future service-aware transport networks that are built on converged wireless-wired transport networks, enable service layer convergence, and also support application layer convergence. This concept is derived from the emerging trend of business convergence among the telecommunication, cable, and entertainment segments, and technology convergence among the telecommunications, computer, and software industries. We also identify QoS as a technical backbone to future service-aware transport networks, and from the perspective of an integrated carrier having both wireless and wired assets, discuss related research opportunities and challenges, especially those involved in optical networks. We finally present an adaptive apparatus for high availability, capacity efficiency, and QoS-guaranteed protection and restoration for optical networks. This paper is intended to promote academic and industrial attention to some core technical challenges that lie ahead for the telecommunication industry and to spur strategically important research and perhaps also some standards activities.

Aqueous mercuric ion detection with microsphere optical resonator sensors

Abstract: We have developed a novel optical sensor based on fused silica microsphere resonators for mercuric ion Hg(II) detection in an aqueous environment. The whispering gallery mode (WGM) of the sphere is the surface mode. Therefore, its spectral position shifts in response to the binding of Hg(II) to the sphere surface. In addition, the WGM features very high Q-factor (>10). This light circulation effect significantly increases the light interaction with the analytes on the sphere surface, resulting in an enhanced sensor sensitivity. In our experiment, we add Hg(II) to water in which the sensor is immersed. The sensorgram is obtained by monitoring the WGM position in real time. Our experimental results show that a detection limit of 50 ppb is achievable when Hg(II) ions bind to the thiol group on the sphere surface with nearly 1:3 stoichiometry. Controlled experiments with Zn(II) are also performed, and virtually no spectral shift in the WGM is observed. INTRODUCTION Optical microsphere resonators [1-14] have been demonstrated as an excellent technical solution for bio/chemical sensors. Several label-free applications have been demonstrated, including the detection of protein and DNA molecules [9,12-13], as well as refractometric measurements of solutions [10-11]. Microspheres are attractive as sensors because of their small size, low optical power requirement, and high light-matter interaction. Light of resonant wavelengths propagates around the surface of the microsphere cavity via total internal reflection in the form of whispering gallery modes (WGM). Microspheres with a Q-factor of greater than 10 have been demonstrated [14]. This high degree of optical confinement results in excellent light matter interaction, as photons may circulate the sphere hundreds to thousands of times on average. Thus, a microsphere with sub-millimeter dimensions can have an interaction length on the order of tens to hundreds of centimeters, while only consuming a sample volume on the order of microliters. The light-matter interaction at the surface of the sphere is a result of the evanescent field of the WGM. As the light propagates around the surface of the sphere, an evanescent field protrudes into the medium beyond the sphere surface, as illustrated in Figure 1. As a result, the spectral position of the WGM is sensitive to the refractive index beyond the surface of the sphere. A change in refractive index of the surrounding solution will cause a shift in the spectral position of the WGM, as will the binding or removal of molecules at the sphere surface. Therefore, quantitative bio/chemical sensing can be performed by measuring the spectral shift of a WGM. We utilize these capabilities of optical microsphere resonators to demonstrate the detection of the presence of mercuric ions (Hg(II)) in water. This design provides the advantages of simplicity, lower cost, faster measurement time, and compactness, as compared to the standard method of cold vapor atomic fluorescence detection [15]. Other simple optical techniques, including surface plasmon resonance (SPR) [16-19], have already been demonstrated for Hg(II) detection. Detection limits of 0.1 ppm and nanomolar have been achieved respectively by a commercialized SPReeta system and by a lab SPR system with the assistance of split-field photodiode detection technology [18-19]. However, due to its small size, low sample consumption, and high light-matter interaction, the microsphere resonator may ultimately prove to be a better concept for heavy metal detection. 0 r0 Captured molecule

Optical wavelength conversion for phase-modulated optical signals

Abstract: An optical communication system comprises a first optical interface, an optical processing system, and a second optical interface. The first optical interface receives an optical signal that is phase-modulated and has a first wavelength. The optical processing system converts the first wavelength of the optical signal to a second wavelength that is different from the first wavelength without converting the optical signal into an electrical format. The second optical interface transfers the optical signal that is phase-modulated and has the second wavelength.