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: I. Concepts and implementations

We review recent developments and current status of liquid-core optical waveguides in optofluidics with emphasis on suitability for creating fully planar optofluidic labs-on-a-chip. In this first of two contributions, we give an overview of the different waveguide types that are being considered for effectively combining micro and nanofluidics with integrated optics. The large number of approaches is separated into conventional index-guided waveguides and more recent implementations using wave interference. The underlying principle for waveguiding and the current status are described for each type. We then focus on reviewing recent work on microfabricated liquid-core antiresonant reflecting optical (ARROW) waveguides, including the development of intersecting 2D waveguide networks and optical fluorescence and Raman detection with planar beam geometry. Single molecule detection capability and addition of electrical control for electrokinetic manipulation and analysis of single bioparticles are demonstrated. The demonstrated performance of liquid-core ARROWs is representative of the potential of integrated waveguides for on-chip detection with ultrahigh sensitivity, and points the way towards the next generation of high-performance, low-cost and portable biomedical instruments.     

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.     

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.     

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).     

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.     

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.     

MEMS on silicon for integrated optic metrology and communication systems

 Integrated optic micro-electromechanical systems (IO-MEMS) based on silicon-, Al₂O₃-SiO₂ and TiO₂-SiO₂ waveguides, doped with Ti, Cr, and Er on silicon substrates allow to generate complex metrology and optical communication systems. They exhibit low loss across a wide spectral range, occupy small space, and exhibit high functionality at low production cost. The waveguides are deposited by CVD and patterned by anisotropic plasma etching. The micro-electromechanical structures are formed by standard micro-machining processes and anodic bonding of silicon-glass. An integrated optical pressure sensor using the interferometer principle, a gas monitor for the near and middle infrared using elevated silicon single mode waveguides, an electrostatic-tuned Ti:/Cr:-sapphire laser, and a UV-VIS-NIR-spectrometer including integrated broad banded light sources represent metrology systems. Optical communication systems are described like a waveguide grating based wavelength demultiplexer, an optical transceiver using self aligned detector and emitter as well as a tapered fibre coupler, and an integrated optical amplifier. Received: 10 July 2001/Accepted: 15 August 2001 This work was funded by Deutsche Forschungsgemeinschaft, the Ministry of Research and Technology and the City of Hamburg. This paper was presented at the Workshop “Optical MEMS and Integrated Optics” in June 2001.     

Nanohole arrays in metal films as optofluidic elements: progress and potential

Subwavelength holes in metal films exhibit coupled optical phenomena specific to structure geometry, incident light and properties of the near-surface medium. As optofluidic components, nanohole arrays in metal films present several opportunities. This review provides an overview of the unique optical characteristics of such arrays, with emphasis on their application in the micro and nano-fluidic environment. The majority of contributions in this area have focused on sensor applications, and the results of nanohole array based chemical and biomolecular sensors are reviewed here. Also relevant to on-chip analysis, various field and spectroscopic enhancements achieved with nanohole arrays are discussed. The general benefits and limitations of nanohole arrays for analytical applications are discussed in the context of existing tools. Beyond sensing, particle trapping and other potential optofluidic applications of nanohole arrays are discussed.     

Microfluidic device with dual-channel fluorescence acquisition for quantification/identification of cancer cells

This paper presents an optofluidic device for cell discrimination with two independent interrogation regions. Pumping light is coupled to the device, and cell fluorescence is extracted from the two interrogation zones by using optical fibers embedded in the optofluidic chip. To test the reliability of this device, AU-565 cells—expressing EpCAM and HER2 receptors—and RAMOS cells were mixed in a controlled manner, confined inside a hydrodynamic focused flow in the microfluidic chip and detected individually so that they could be discriminated as positive (signal reception from fluorescently labeled antibodies from the AU-565 cells) or negative events (RAMOS cells). A correlation analysis of the two signals reduces the influence of noise on the overall data.     

Research progresses of SOI optical waveguide devices and integrated optical switch matrix

1Introduction Dense wavelength division multiplexing(DWDM)is the most powerful method to upgrade the transmission capacity of the optical fiber transmission system.In DWDM system,the enormous available bandwidth is utilized by multiplexing laser beams with different wavelength in one single fiber.With DWDM technology,parallel transmission of data streams with different format leads to high speed information communication,thereby reduces the requirement for the high speed electronic device…     

Refractive-index measurements using an integrated Mach-Zehnder interferometer

In this paper a refractometer system with an integrated optical sensor is presented. The sensitive element is a single or multichannel Mach-Zehnder interferometer (MZI) fabricated on silicon using waveguides made from SiO₂ and SiON. The refractometer allows for real-time monitoring of refractive indices of gases and liquids in an index range between 1 and 1.49 using the silicon MZI as a low-cost disposable part.     

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.     

Recent advances in theory and applications of substrate‐integrated waveguides: A review

The use of substrate integrated waveguides (SIW) for microwave and millimeter wave integrated components has increased dramatically over the last decade. They mimic the performance of conventional metallic waveguides and they are fabricated using printed circuit boards using the top and bottom metallization with two rows of vias forming the side walls. This creates a low profile, compact, and light weight alternative to conventional metallic waveguides, and they allow a direct interconnection with printed circuit boards and active components. This article reviews the fundamental theory, documents the research that has been performed over the past decade, and summarizes progress up to the recent state‐of‐the‐art including novel SIW structures for passive circuits and antennas as well as new applications for reconfigurable and printed circuits using SIW technology. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:129–145, 2016.     

The properties of free-standing epoxy polymer multi-mode optical waveguides

The paper reports on the fabrication and characterisation of free-standing multimode optical epoxy polymer waveguides consisting of a core made of EpoCore and EpoClad polymer cladding and cover protection layers. The 50 × 50 μm² rectangular waveguides are intended for short-reach optical interconnection and optimised for an operating wavelength of 850 nm. The waveguides of the proposed shapes were fabricated by a standard photolithography process on a silicon substrate provided with a Poly(vinyl alcohol) thin layer. The free-standing structure was then achieved by peeling the deposited EpoClad/EpoCore/EpoClad structures of that substrate. The optical scattering losses of the created planar waveguides, measured by the fibre probe technique at 632.8 and 964 nm, were 0.30 dB cm⁻¹ at 632.8 nm and 0.17 dB cm⁻¹ at 964 nm. Propagation optical loss measurements for rectangular waveguides were performed by the cut-back method and the best samples had optical losses below 0.55 dB cm⁻¹ at 850 and 1310 nm.

     

LIGA-fabricated compact mm-wave linear accelerator cavities

 Millimeter-wave rf cavities for use in linear accelerators, free-electron lasers, and mm-wave undulators are under development at Argonne National Laboratory. Typical cavity dimensions are in the 1000 μm range, and the overall length of the accelerator structure, which consists of 30–100 cavities, is about 50–100 mm. An accuracy of 0.2% in the cavity dimensions is necessary in order to achieve a high Q-factor of the cavity. To achieve this, these structures are being fabricated using deep X-ray lithography, electroforming, and assembly (LIGA). The first prototype cavity structures are designed for 108 GHz and 2π/3-mode operation. Input and output couplers are integrated with the cavity structures. The cavities are fabricated on copper substrates by electroforming copper into 1 mm-thick PMMA resists patterned by deep x-ray lithography and polishing the copper down to the desired thickness. These are fabricated separately and subsequently assembled with precision spacing and alignment using microspheres, optical fibers, or microfabricated spacers/alignment pieces. Details of the fabrication process, alignment, and assembly work are presented in here. Received: 25 August 1997/Accepted: 20 October 1997     

Surface enhanced Raman spectroscopy and its application to molecular and cellular analysis

In this paper, we review the state-of-the-art in surface-enhanced Raman scattering (SERS) based optical detection techniques with an application focus on cancer diagnostics. As we describe herein, SERS has several analytical, biological and engineering advantages over other methods including extremely high sensitivity, inherent molecular specificity of unlabeled targets, and narrow spectral bands. We review advances in both in vitro and in vivo applications of SERS and examine how technical issues with the technology are being addressed. A special technology focus is given to emerging optofluidic devices which aim to merge microfluidic and optical detection technologies into simple packages. We conclude with a brief discussion of some of the emerging challenges in the field and some of the approaches that are likely to enhance their application. Y. S. Huh and A. J. Chung contributed equally.     

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.     

An integrated microfluidic signal generator using multiphase droplet grating

An integrated and reconfigurable optofluidic signal generator based on multiphase droplet grating is demonstrated in this paper. The chip is fabricated with an inexpensive, optically clear and non-toxic silicone elastomer-polydimethylsiloxane (PDMS) by conventional soft lithography. Droplet grating is formed by a stream of plugs which are generated through a typical microfluidic T-junction. Since the refractive indices of the two immiscible liquids are different, the alternative mobility of the plug results in the periodical change of the reflectivity at the fluid/PDMS interface. The real-time tunability in the frequency and amplitude of the signal can be realized by varying the flow rates of the liquids. In experiments, both rectangle and triangle signals are displayed and the signal frequency ranges from 1 to 525 Hz. This signal generator can be easily integrated into other microfluidic networks to create versatile functionalities. Furthermore, we present coding functions based on the signal generator on a chip. Such a signal generator has great potential as a signal source or a part of functionalities for lab-on-a-chip applications.     

InP-based micromachined Mach-Zehnder interferometer stress sensors

 In the field of III–V-based compounds, new functionalities can be reached by integrating micromechanical structures with electro-optical functions, in order to fabricate Micro Opto Electro Mechanical Systems (MOEMS). A possible application is an InP-based integrated optical stress sensor. Such a system is based on a partly suspended waveguide that can be strained under the effect of an external stress. The photoelastic effect induces a phase shift that can be converted into an intensity shift of the signal if the device is configurated as a Mach-Zehnder interferometer. This system can be integrated monolithically with the optical source and the photodetector. The mechanical, photoelastic and optical properties of this structure has been simulated in order to configurate the alloy composition of the epitaxial layers and the geometry of the device. Micromachining processes have been developed in order to realize InP-based suspended microstructures by sacrificial layer etching and bulk micromachining. Preliminary results showed that the optical behaviour of the waveguides is close to the theoretical analysis. Characterisation of the complete interferometer is underway in our group. Received: 13 December 1996/Accepted: 18 December 1996