Optofluidic waveguides: II. Fabrication and structures

We review fabrication methods and common structures for optofluidic waveguides, defined as structures capable of optical confinement and transmission through fluid filled cores. Cited structures include those based on total internal reflection, metallic coatings, and interference based confinement. Configurations include optical fibers and waveguides fabricated on flat substrates (integrated waveguides). Some examples of optofluidic waveguides that are included in this review are Photonic Crystal Fibers (PCFs) and two-dimensional photonic crystal arrays, Bragg fibers and waveguides, and Anti Resonant Reflecting Optical Waveguides (ARROWs). An emphasis is placed on integrated ARROWs fabricated using a thin-film deposition process, which illustrates how optofluidic waveguides can be combined with other microfluidic elements in the creation of lab-on-a-chip devices.     

Optofluidic waveguides: II. Fabrication and structures

We review fabrication methods and common structures for optofluidic waveguides, defined as structures capable of optical confinement and transmission through fluid filled cores. Cited structures include those based on total internal reflection, metallic coatings, and interference based confinement. Configurations include optical fibers and waveguides fabricated on flat substrates (integrated waveguides). Some examples of optofluidic waveguides that are included in this review are Photonic Crystal Fibers (PCFs) and two-dimensional photonic crystal arrays, Bragg fibers and waveguides, and Anti Resonant Reflecting Optical Waveguides (ARROWs). An emphasis is placed on integrated ARROWs fabricated using a thin-film deposition process, which illustrates how optofluidic waveguides can be combined with other microfluidic elements in the creation of lab-on-a-chip devices.     

Optofluidics technology based on colloids and their assemblies

Optofluidic technology is believed to provide a breakthrough for the currently underlying problems in microfluidics and photonics/optics by complementary integration of fluidics and photonics. The key aspect of the optofluidics technology is based on the use of fluidics for tuning the optical properties and addressing various functional materials inside of microfluidic channels which have build-in photonic structures. Through the optofluidic integrations, fluidics enhances the controllability and tunability of optical systems. In particular, colloidal dispersion gives novel properties such as photonic band-gaps and enhanced Raman spectrum that conventional optofluidic devices cannot exhibit. In this paper, the state of the art of the colloidal dispersions is reviewed especially for optofluidic applications. From isolated singlet colloidal particles to colloidal clusters, their self-organized assemblies lead to optical manipulation of the photonic/optical properties and responses. Finally, we will discuss the prospects of the integrated optofluidics technology based on colloidal systems.     

Two optofluidic devices for the refractive index measurement of small volume of fluids

Two simple optofluidic devices based on a microprism and a refraction channel, respectively, are proposed for measuring the refractive index of fluids. The microprism chip consists of an optical waveguide channel and a single triangular chamber filled with the test fluid, while the refraction channel chip consists of a single turning channel which functions as a liquid-core/solid-cladding optical waveguide. In both chips, the refractive index of the test fluid is calculated from a CCD image of the refracted ray in accordance with simple geometrical optics principles. The experimental results show that the refraction channel chip provides a more accurate measurement performance than the microprism chip; particularly for colloidal samples. However, the refraction channel chip is suitable only for the measurement of fluids with a refractive index greater than that of the chip substrate. Overall, the results presented in this study show that both devices provide a simple, low cost and effective means of determining the refractive index of a wide range of common test fluids with nano-liter volume.     

Optofluidic sensor system with Ge PIN photodetector for CMOS-compatible sensing

Vertical optofluidic biosensors based on refractive index sensing promise highest sensitivities at smallest area footprint. Nevertheless, when it comes to large-scale fabrication and application of such sensors, cheap and robust platforms for sample preparation and supply are needed—not to mention the expected ease of use in application. We present an optofluidic sensor system using a cyclic olefin copolymer microfluidic chip as carrier and feeding supply for a complementary metal–oxide–semiconductor compatibly fabricated Ge PIN photodetector. Whereas typically only passive components of a sensor are located within the microfluidic channel, here the active device is directly exposed to the fluid, enabling top-illumination. The capability for detecting different refractive indices was verified by different fluids with subsequent recording of the optical responsivity. All components excel in their capability to be transferred to large-scale fabrication and further integration of microfluidic and sensing systems. The photodetector itself is intended to serve as a platform for further sophisticated collinear sensing approaches.     

Micro-light distribution system via optofluidic cascading prisms

This article reports a micro-light distribution system realized by altering the reflective indices of two optofluidic cascading prisms. Different micron scale light distribution configurations can be tuned via the imposed flow rates of the microfluidic mixers. The variable optical interface’s reflectivity of the cascading prisms is based on the tuning of refractive indices of micromixed fluids within the cascading prisms. The microscale light distribution is achieved via total internal reflection and partial refraction occurs at the fluid–solid optical interfaces. 1 × 3 light switching denotes one optical inlet while the light can be guided via any one of the three optical outlets. The 1 × 3 light switching capability is demonstrated. The light splitting capability to achieve different proportion of light power distribution is also demonstrated and characterized. The optofluidic cascading prisms are integrated with micromixer, prolonging the working life of the optical compartment considerably as the fluids are only consumed when optical tuning is required. The proposed technique eliminates the disadvantage of optofluidic compartments based on liquid–liquid interfaces as liquid–liquid interface possess weaker mechanical stability than solid–liquid interface. The proposed design also does not require continuous supply of fluids as in optofluidic compartments based on liquid–liquid interfaces. The optofluidic cascading prisms can be cascaded further to form a complex planar light distribution system for seamless light distribution in lab-on-a-chip excitation and sensing applications.     

Tunable optofluidic devices

The emerging field of optofluidics provides exciting opportunities for the realization of tunable optofluidic devices (TODs) using a large variety of physical mechanisms. This is because microfluidics is a promising technology for achieving a high degree of tunability—a capability that is not available in many of the current optical devices. In addition, microfluidics holds a great potential for rapid prototyping, miniaturization and integration. TODs already find commercial applications in various fields such as display and imaging, and are expected to become a key player in future optical systems for biology, medicine, communication and information processing. We review the recent progress in the field and discuss potential future directions.     

Memory and Network Architecture Interaction in an Optically Interconnected Distributed Shared Memory System

This paper develops a performance model of an optically interconnected parallel computer system operating in a distributed shared memory environment. The performance model is developed to reflect the impact of low level optical media access protocol and optical device switching latency on high level system performance. This enables the model to predict the performance impact of supporting distributed shared memory with different address allocation schemes and media access protocols. The passive star-coupled photonic network operates through wavelength division multiple access. Two media access protocols are examined for this WDM network, both are designed to operate in a multiple-channel multiple-access environment and require each node to possess a wavelength tunable transmitter and a fixed (or slow tunable) receiver. A semi-Markov model has been developed to study the interaction of the distributed shared memory architecture and the two access protocols of the photonic network. This analytical model has been validated by extensive simulation. The model is then used to examine the system performance with varying numbers of nodes and wavelength channels and varying, memory and channel access times.     

基于集成光波导的超快光子微积分运算研究进展

为了克服电子计算的速率瓶颈,采用全光计算可以有效释放光子的巨大带宽资源,同时全光计算在全光通信网络中有着举足轻重的作用,集成光波导器件以其尺寸小、质量轻、功率代价小等优势已经成为最受关注的光子计算芯片资源之一。光子微积分运算是指在光域中直接对输入信号进行微积分数学运算。本文回顾了几种 常见的硅基光波导器件用于光子微积分运算的实现方案,包括高阶光子微分运算、分数阶微分运算、高阶常系数微分方程求解、可重构的一阶常系数微分方程求解,分别采用的硅基集成光子器件包括级联马赫增德尔干涉仪、掺杂型马赫增德尔干涉仪、级联微环谐振器和掺杂型微环谐振器。本文指出利用集成光波导器件来实现光子微积分器势必会成为光子微积分运算的重要发展方向。     

Development and characterisation of integrated microfluidics on waveguide-based photonic platforms fabricated from hybrid materials

This article reports on a detailed investigation of sol–gel processed hybrid organic–inorganic materials for use in lab-on-a-chip (LoC) applications. A particular focus on this research was the implementation of integrated microfluidic circuitry in waveguide-based photonic sensing platforms. This objective is not possible using other fabrication technologies that are typically used for microfluidic platforms. Significant results on the surface characterisation of hybrid sol–gel processed materials have been obtained which highlight the ability to tune the hydrophilicity of the materials by careful adjustment of material constituents and processing conditions. A proof-of-principle microfluidic platform was designed and a fabrication process was established which addressed requirements for refractive index tuning (essential for waveguiding), bonding of a transparent cover layer to the device, optimized sol–gel deposition process, and a photolithography process to form the microchannels. Characterisation of fluid flow in the resulting microchannels revealed volumetric flow rates between 0.012 and 0.018 μl/min which is characteristic of capillary-driven fluid flow. As proof of the integration of optical and microfluidic functionality, a microchannel was fabricated crossing an optical waveguide which demonstrated that the presence of optical waveguides does not significantly disrupt capillary-driven fluid flow. These results represent the first comprehensive evaluation of photocurable hybrid sol–gel materials for use in waveguide-based photonic platforms for lab-on-a-chip applications.     

Tunable optofluidic microlens through active pressure control of an air–liquid interface

We demonstrate a tunable in-plane optofluidic microlens with a 9× light intensity enhancement at the focal point. The microlens is formed by a combination of a tunable divergent air–liquid interface and a static polydimethylsiloxane lens, and is fabricated using standard soft lithography procedures. When liquid flows through a straight channel with a side opening (air reservoir) on the sidewall, the sealed air in the side opening bends into the liquid, forming an air–liquid interface. The curvature of this air–liquid interface can be conveniently and predictably controlled by adjusting the flow rate of the liquid stream in the straight channel. This change in the interface curvature generates a tunable divergence in the incident light beam, in turn tuning the overall focal length of the microlens. The tunability and performance of the lens are experimentally examined, and the experimental data match well with the results from a ray-tracing simulation. Our method features simple fabrication, easy operation, continuous and rapid tuning, and a large tunable range, making it an attractive option for use in lab-on-a-chip devices, particularly in microscopic imaging, cell sorting, and optical trapping/manipulating of microparticles.     

Toward the commercialization of optofluidics

Optofluidics is a marriage between the field of optics and microfluidics. This field aims at providing practical solutions with the integration of optical tools into lab-on-chip systems. Often, this results in opportunities for commercialization due to the advancement offered after the integration. Although numerous novel functions and properties have been demonstrated with the combination of optics and microfluidics, the market has witnessed only few transferals of optofluidic technologies from academic laboratories. This stemmed from a lack of a “killer applications” despite several decades of development. Therefore, it is necessary to have a retrospective review on this topic, particularly on the basic optofluidic components, to analyze what might be the hurdles to stop the market uptake of optofluidic devices. Specifically, this review paper is focused on discussion of optofluidic components in terms of fabrication standardization, device and operational cost and practicability for end users. It is believed that these factors play important roles in the market uptake of a novel technology. We then provide perspectives on how to align the development of optofluidics with the requirements imposed by the industry.     

Integrated optical planar waveguide components

 Planar optical waveguides for applications in communication networks can be fabricated using conventional chip-manufacturing techniques. We present a planar optical waveguide technology that is based on a silicon-oxynitride (SiON) core and silicon-oxide cladding layers. In addition to more compact, conventional optical devices, it also enables enhanced optical functions such as dynamically reconfigurable planar integrated optical devices. Examples of adaptive devices realized in this technology include finite and infinite impulse response (FIR and IIR) filters. Received: 13 February 2002/Accepted: 28 February 2002 In realizing the SiON waveguide technology and the adaptive optical filter functions with the subsystem control, the dedicated work of the waveguide process technology, the photonic device technology, and the engineering services teams at IBM's Zurich Research Laboratory were instrumental and are gratefully acknowledged. For the concept-level optical-packaging work we thank Optospeed SA. This paper was presented at the Workshop “Optical MEMS and Integrated Optics” in June 2001.     

Liquid�Cliquid fluorescent waveguides using microfluidic-drifting-induced hydrodynamic focusing

We demonstrate fluorescent liquid-core/liquid-cladding (L ²) waveguides focused in three-dimensional (3-D) space based on a 3-D hydrodynamic focusing technique. In the proposed system, the core and vertical cladding streams are passed through a curved 90° corner in a microfluidic channel, leading to the formation of a pair of counter rotating vortices known as the Dean vortex. As a result, the core fluid is completely confined within the cladding fluid and does not touch the top and bottom poly(dimethylsiloxane) (PDMS) surfaces of the microfluidic channel. Because the core stream was not in contact with the PDMS channel, whose refractive index contrast and optical smoothness with the core fluid are lower than that between the core and the cladding fluids, the 3-D focused L ² waveguide exhibited a higher captured fraction (η) and lower propagation loss when compared to conventional two-dimensional (2-D) focused L ² waveguides. Because the proposed 3-D focused L ² waveguides can be generated in planar PDMS microfluidic devices, such optofluidic waveguides can be integrated with precise alignment together with other in-plane microfluidic and optical components to achieve micro-total analysis systems (μ-TAS).     

基于光纤布拉格光栅的微波光子信号处理

由于有效利用了光子技术的优点,微波光子技术克服了传统微波系统中的一些瓶颈,从而提高已有系统性能,甚至开发出了全新的系统应用。很多光子器件已经被用在微波光子系统中,光纤布拉格光栅（Fiber Bragg grating, FBG）就是其中一种非常重要的全光纤器件。由于具有灵活的频谱响应特性、损耗低、质量轻、结构紧凑、以及与其他光纤器件耦合性好等独特的优势,光纤布拉格光栅已经成为了微波光子信号处理系统中的关键组件之一。本文主要介绍了近年来光纤布拉格光栅在微波光子信号处理应用中的最新进展,重点讨论的主要应用包括微波光子滤波器,微波任意波形产生,微波频谱感知以及光纤光栅传感器实时解调。最后,本文还讨论了在微波光子系统中应用光纤布拉格光栅的局限性及可能的解决方案。     

Miniaturized flow cytometer with 3D hydrodynamic particle focusing and integrated optical elements applying silicon photodiodes

In this study, the design, realization and measurement results of a novel optofluidic system capable of performing absorbance-based flow cytometric analysis is presented. This miniaturized laboratory platform, fabricated using SU-8 on a silicon substrate, comprises integrated polymer-based waveguides for light guiding and a biconcave cylindrical lens for incident light focusing. The optical structures are detached from the microfluidic sample channel resulting in a significant increase in optical sensitivity. This allows the application of standard solid-state laser and standard silicon-based photodiodes operated by lock-in-amplification resulting in a highly practical and effective detection system. The easy-to-fabricate single-layer microfluidic structure enables independently adjustable 3D hydrodynamic sample focusing to an arbitrary position in the channel. To confirm the fluid dynamics and raytracing simulations and to characterize the system, different sets of microparticles and T-lymphocyte cells (Jurkat cell line) for vital staining were investigated by detecting the extinction (axial light loss) signal. The analytical classification via signal peak height/width demonstrates the high sensitivity and sample discrimination capability of this compact low-cost/low-power microflow cytometer.     

Intel research expands Moore's law

Intel's research agenda includes 90-nanometer fabrication processes and work in extreme ultraviolet lithography that will help extend Moore's law. It also embraces disruptive technologies exemplified in devices such as micro-electromechanical-systems microradiators, smart antennas, and radiofrequency components for analog switches, resonators, and filters; ad hoc sensor networks with wireless communications; and photonic devices such as optical switches and cheap tunable lasers. To implement these advances, Intel has created a network of university-based labs that group the corporation's scientists with academic researchers to form multidisciplinary teams. These lablets leverage industry and academic synergy to nurture off-the-roadmap ideas and technologies and provide a proving ground for testing their viability.     

Dielectrophoretically assembled particles: feasibility for optofluidic systems

This work presents the dielectrophoretic manipulation of sub-micron particles suspended in water and the investigation of their optical responses using a microfluidic system. The particles are made of silica and have different diameters of 600, 450, and 250 nm. Experiments show a very interesting feature of the curved microelectrodes, in which the particles are pushed toward or away from the microchannel centerline depending on their levitation heights, which is further analyzed by numerical simulations. In doing so, applying an AC signal of 12 V_p–p and 5 MHz across the microelectrodes along with a flow rate of 1 μl/min within the microchannel leads to the formation of a tunable band of particles along the centerline. Experiments show that the 250 nm particles guide the longitudinal light along the microchannel due to their small scattering. This arrangement is employed to study the feasibility of developing an optofluidic system, which can be potentially used for the formation of particles-core/liquid-cladding optical waveguides.     

Fabrication of the plastic component for photonic crystal using micro injection molding

In this decade, many techniques have been introduced to fabricate photonic crystal in optical applications. Most of the processes used to fabricate the photonic crystal are time consuming and not cost effective. This study demonstrates an efficient method to fabricate photonic crystals. A polymer-based photonic crystal slab has been developed by embedding mixture with a high dielectric constant. Photonic crystals have patterned structures in which periodicity of dielectric properties can manipulate electromagnetic waves. The operation wavelength is about half of the characteristic dimension. Technique of injection molding is applied to make polymer parts with the photonic crystal pattern. Then mixture of barium titanate powder and epoxy is embedded on the patterned structure of the polymer part. The contrast of dielectric coefficients between mixture and polymer can constitute a structure with some photonic band gap. By means of polymer processing, mass production of photonic crystal devices like optical switch, optical waveguide, optical filter and so forth can be realized in a cost effective way.     

Interaction of guided light in rib polymer waveguides with dielectrophoretically controlled nanoparticles

This work demonstrates an optofluidic system, where dielectrophoretically controlled suspended nanoparticles are used to manipulate the properties of an optical waveguide. This optofluidic device is composed of a multimode polymeric rib waveguide and a microfluidic channel as its upper cladding. This channel integrates dielectrophoretic (DEP) microelectrodes and is infiltrated with suspended silica and tungsten trioxide nanoparticles. By applying electrical signals with various intensities and frequencies to the DEP microelectrodes, the nanoparticles can be concentrated close to the waveguide surface significantly altering the optical properties in this region. Depending on the particle refractive indices, concentrations, positions and dimensions, the light remains confined or is scattered into the surrounding media in the microfluidic channel.