Optofluidic Components Research Articles

Dynamically variable intensity patterns projected using an optofluidic phase-shifter array

We present a concept for the dynamically variable definition of arbitrary intensity distributions using an array of optofluidic phase-shifters. These components consist of cylindrical fluidic surfaces whose interface may be controllably shaped using electrowetting-on-dielectrics actuation. Key to the generation of defined intensity distributions is the ability to calculate the required shape for this phase-shifting interface and to this end we present a novel procedure, to our knowledge, that combines a real-time optimization algorithm with an influence matrix approach to generate the required surface. We illustrate the effectiveness of this approach by reconstructing arbitrary surface profiles with one optofluidic phase-shifting component that is then used to project a desired two-dimensional intensity distribution. Using this approach, we further demonstrate the capability of a two-dimensional phase-shifter array to generate larger dynamically programmable intensity patterns by combining individual intensity distributions.

Thin liquid film as an optical nonlinear-nonlocal medium and memory element in integrated optofluidic reservoir computer

Understanding light–matter interaction lies at the core of our ability to harness physical effects and to translate them into new capabilities realized in modern integrated photonics platforms. Here, we present the design and characterization of optofluidic components in an integrated photonics platform and computationally predict a series of physical effects that rely on thermocapillary-driven interaction between waveguide modes and topography changes of optically thin liquid dielectric film. Our results indicate that this coupling introduces substantial self-induced phase change and transmittance change in a single channel waveguide, transmittance through the Bragg grating waveguide, and nonlocal interaction between adjacent waveguides. We then employ the self-induced effects together with the inherent built-in finite relaxation time of the liquid film, to demonstrate that the light-driven deformation can serve as a reservoir computer capable of performing digital and analog tasks, where the gas–liquid interface operates both as a nonlinear actuator and as an optical memory element.

Flexible Wire Electrode Driving Focus-Tunable Electrowetting Optofluidic Lens

An electrowetting-on-dielectrics (EWOD) liquid lens, based on a flexible wire electrode, is proposed to avoid the defects prone to occur in the large area coating dielectric layer inside the cavity. A liquid lens cavity is formed by winding the flexible conductive wire with a thin dielectric layer (named EWOD wire) along the direction of light transmission in a circular wall uniformly. A tubular double-liquid electrowetting lens with a clear aperture of 7 mm is fabricated to demonstrate the practicality of this fabrication process. It can reach a focal length variation range of (<inline-formula> <tex-math notation="LaTeX">$-\infty $ </tex-math></inline-formula>, &#x2212;30 mm] <inline-formula> <tex-math notation="LaTeX">$\cup $ </tex-math></inline-formula> [69 mm, <inline-formula> <tex-math notation="LaTeX">$+\infty $ </tex-math></inline-formula>) at a voltage of 0&#x2013;190 V_rms and shows the other performance not inferior to those of liquid lenses fabricated by traditional methods. The EWOD wire can be assembled into any shape, performing the potential to produce other optofluidic components.

3D Printing Optofluidic Mach-Zehnder Interferometer on a Fiber Tip for Refractive Index Sensing

An optofluidic polymer Mach-Zehnder interferometer (MZI) fabricated via direct laser writing (DLW) on a fiber tip is proposed and experimentally demonstrated as a refractometer where one of the interference arms is made of the polymer waveguide while the other arm is a hollow cavity with a 1- $\mu \text{m}$ wall thickness and containing two open channels. The analytes can be filled through the channels, forming an optofluidic component of the interferometer. In this letter, two MZI microstructures with different hollow lengths are described and characterized. The experimental results of the two MZIs in air and water show that the MZI with a longer hollow cavity has higher sensitivity. Analytes such as de-ionized (DI) water and glucose solutions with different concentrations have been measured in the MZI with the long hollow cavity. The results show that this kind of sensing device provides a promising platform for refractive index measurements with a sensitivity of 593.75 nm/RIU based on the FSR calculation method.

A multi-parameter miniature photometric analyser for fully automated environmental water monitoring

Nowadays, the increasing demand for constant in-situ monitoring of clean water, wastewater and sea water requires the deployment of automated platforms capable of on-line autonomous sampling, extended unattended operation and of multi-analyte detection. The Micro Loop Injection Analyser (μLIA) system hereby presented relies on integrated additively manufactured opto-fluidic components which allow for increased performance solutions to be implemented cost-effectively and in miniaturised footprints, targeting a wide spectrum of applications which ranges from in-situ environmental monitoring and field water testing to industrial process analysis. Preliminary tests of the μLIA unit have shown good reproducibility and stability of the system and a good performance of its self-calibration routines.

Optofluidic Dye Lasers Based on Holey Fibers: Modeling and Performance Analysis

Holey optical fibers with air-filled openings can be turned into unconventional optofluidic components by selectively introducing a suitable liquid into their internal holes. Enhanced interaction between the light guided by the fiber and molecular species suspended in the confined liquid then forms the basis for designing unique optofluidic devices—filters, sensors, or lasers—whose operating characteristics are extremely sensitive to minute changes in the liquid composition. We present a comprehensive mathematical analysis of dye lasers based on suspended-core and hollow-core fibers, with aqueous solution of Rhodamine B dye injected into the internal holes of the fibers acting as the gain medium. Our model consists of coupled first-order differential equations that characterize the population of lasing levels and the variation of pump and signal laser powers along the fiber length in the steady state. We obtain optimum operating conditions of the proposed holey fiber dye lasers, such as the optimum fiber length and dye concentration, and compare their performance to conventional dye jet lasers. We conclude that the fiber variant of dye laser using the hollow-core fiber is superior in terms of low lasing threshold and high slope efficiency. The presented theoretical framework can find applications in designing practical holey fiber dye lasers serving as fiber-based alternatives of conventional dye jet lasers as well as novel biological/chemical sensors and bio-lasers.

Design considerations of a hollow microneedle-optofluidic biosensing platform incorporating enzyme-linked assays

A hollow metallic microneedle is integrated with microfluidics and photonic components to form a microneedle-optofluidic biosensor suitable for therapeutic drug monitoring (TDM) in biological fluids, like interstitial fluid, that can be collected in a painless and minimally-invasive manner. The microneedle inner lumen surface is bio-functionalized to trap and bind target analytes on-site in a sample volume as small as 0.6 nl, and houses an enzyme-linked assay on its 0.06 mm2 wall. The optofluidic components are designed to rapidly quantify target analytes present in the sample and collected in the microneedle using a simple and sensitive absorbance scheme. This contribution describes how the biosensor components were optimized to detect in vitro streptavidin-horseradish peroxidase (Sav-HRP) as a model analyte over a large detection range (0–7.21 µM) and a very low limit of detection (60.2 nM). This biosensor utilizes the lowest analyte volume reported for TDM with microneedle technology, and presents significant avenues to improve current TDM methods for patients, by potentially eliminating blood draws for several drug candidates.

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.

Microsecond switching of plasmonic nanorods in an all-fiber optofluidic component

As information technologies move from electron-to photon-based systems, the need to rapidly modulate light is of paramount importance. Here, we study the evolution of the electric-field-induced ali ...

A Perspective on the Rise of Optofluidics and the Future

In the recent past, the field of optofluidics has thrived from the immense efforts of researchers from diverse communities. The concept of optofluidics combines optics and microfluidics to exploit novel properties and functionalities. In the very beginning, the unique properties of liquid, such as mobility, fungibility and deformability, initiated the motivation to develop optical elements or functions using fluid interfaces. Later on, the advancements of microelectromechanical system (MEMS) and microfluidic technologies enabled the realization of optofluidic components through the precise manipulation of fluids at microscale thus making it possible to streamline complex fabrication processes. The optofluidic system aims to fully integrate optical functions on a single chip instead of using external bulky optics, which can consequently lower the cost of system, downsize the system and make it promising for point-of-care diagnosis. This perspective gives an overview of the recent developments in the field of optofluidics. Firstly, the fundamental optofluidic components will be discussed and are categorized according to their basic working mechanisms, followed by the discussions on the functional instrumentations of the optofluidic components, as well as the current commercialization aspects of optofluidics. The paper concludes with the critical challenges that might hamper the transformation of optofluidic technologies from lab-based procedures to practical usages and commercialization.

Direct birefringence and transmission modulation via dynamic alignment of P3HT nanofibers in an advanced opto-fluidic component

Poly-3-hexylthiophene (P3HT) nanofibers are semiconducting high-aspect ratio nanostructures with anisotropic absorption and birefringence properties found at different regions of the optical spectr ...

Variable optofluidic slit aperture.

The shape of liquid interfaces can be precisely controlled using electrowetting, an actuation mechanism which has been widely used for tunable optofluidic micro-optical components such as lenses or irises. We have expanded the considerable flexibility inherent in electrowetting actuation to realize a variable optofluidic slit, a tunable and reconfigurable two-dimensional aperture with no mechanically moving parts. This optofluidic slit is formed by precisely controlled movement of the liquid interfaces of two highly opaque ink droplets. The 1.5 mm long slit aperture, with controllably variable discrete widths down to 45 µm, may be scanned across a length of 1.5 mm with switching times between adjacent slit positions of less than 120 ms. In addition, for a fixed slit aperture position, the width may be tuned to a minimum of 3 µm with high uniformity and linearity over the entire slit length. This compact, purely fluidic device offers an electrically controlled aperture tuning range not achievable with extant mechanical alternatives of a similar size.

Chemical-assisted femtosecond laser writing of lab-in-fibers.

The lab-on-chip (LOC) platform has presented a powerful opportunity to improve functionalization, parallelization, and miniaturization on planar or multilevel geometries that has not been possible with fiber optic technology. A migration of such LOC devices into the optical fiber platform would therefore open the revolutionary prospect of creating novel lab-in-fiber (LIF) systems on the basis of an efficient optical transport highway for multifunctional sensing. For the LIF, the core optical waveguide inherently offers a facile means to interconnect numerous types of sensing elements along the optical fiber, presenting a radical opportunity for optimizing the packaging and densification of diverse components in convenient geometries beyond that available with conventional LOCs. In this paper, three-dimensional patterning inside the optical fiber by femtosecond laser writing, together with selective chemical etching, is presented as a powerful tool to form refractive index structures such as optical waveguides and gratings as well as to open buried microfluidic channels and optical resonators inside the flexible and robust glass fiber. In this approach, optically smooth surfaces (~12 nm rms) are introduced for the first time inside the fiber cladding that precisely conform to planar nanograting structures when formed by aberration-free focusing with an oil-immersion lens across the cylindrical fiber wall. This process has enabled optofluidic components to be precisely embedded within the fiber to be probed by either the single-mode fiber core waveguide or the laser-formed optical circuits. We establish cladding waveguides, X-couplers, fiber Bragg gratings, microholes, mirrors, optofluidic resonators, and microfluidic reservoirs that define the building blocks for facile interconnection of inline core-waveguide devices with cladding optofluidics. With these components, more advanced, integrated, and multiplexed fiber microsystems are presented demonstrating fluorescence detection, Fabry-Perot interferometric refractometry, and simultaneous sensing of refractive index, temperature, and bending strain. The flexible writing technique and multiplexed sensors described here open powerful prospects to migrate the benefits of LOCs into a more flexible and miniature LIF platform for highly functional and distributed sensing capabilities. The waveguide backbone of the LIF inherently provides an efficient exchange of information, combining sensing data that are attractive in telecom networks, smart catheters for medical procedures, compact sensors for security and defense, shape sensors, and low-cost health care products.

All-Fiber Optofluidic Component to Combine Light and Fluid

A component is presented that combines light and fluid in an optical fiber arrangement for optofluidics. The component couples light from a standard telecom fiber (STF) to the solid-core of a microstructured fiber, and delivers fluid from a capillary to the holes of the same microstructured fiber. The light is then made to interact longitudinally with the delivered fluid in a hollow-core fiber or capillary. The component is all-spliced, hermetic and allows for fluid flow without interrupting the optical beam. Light is brought from the STF to the solid-core/fluid interface with a loss <;0.1 dB.

Three-dimensional optofluidic waveguides in hydrophobic silica aerogels via supercritical fluid processing

Optofluidic components enable flexible routing and transformations of light beams in integrated lab-on-a-chip systems with the use of carefully shaped fluid parcels. For structural integrity reasons, the working fluid is typically contained within a solid-material chip. One of the outstanding challenges in optofluidics is the preparation and processing of optofluidic waveguides. These require solid cladding materials that are sufficiently strong to contain the fluid while possessing optical properties that allow efficient confinement of light within fluidic channels. Here, we report on a new technique to obtain liquid-core optofluidic waveguides based on total internal reflection of light in three-dimensional water-filled channels embedded in hydrophobic silica aerogel. To form the channels, we employ a fiber made of cage-like silicon-oxygen compound – trifluoropropyl polyhedral oligomeric silsesquioxane (trifluoropropyl POSS) – which has high solubility in supercritical CO2 (scCO2). A U-shaped fiber made of trifluoropropyl POSS is obtained by melt/freeze processing of POSS powder and subsequently placed in a silicate sol. After gelation of the sol and aging of the gel, scCO2 extraction is used to dry the wet gel and extract the POSS fiber, yielding a dry silica aerogel with a U-shaped empty channel inside it. Finally, the silanol groups at the surface of the aerogel are reacted with hexamethyldisilazane (HMDS) in the presence of scCO2 to render the aerogel surface hydrophobic and the channel is filled with water. We demonstrate efficient waveguiding by coupling light into the water-filled channel and monitoring the channel output. The presented procedure opens up new possibilities for creating complex three-dimensional networks of liquid channels in aerogels for optofluidic applications.

LIGHT MODULATION ENABLED BY LIQUID CRYSTAL MICROFLOWS

In a previous paper,1 we proposed an optofluidic modulator based on peristaltic flow. A microchannel made of polydimethylsiloxane is filled with nematic liquid crystal molecules that align homeotropically in the steady state. Once we apply a periodic peristaltic flow through mechanical deformation of the channel, liquid crystal molecules tend to realign with the gradient velocity field and thus change their optical properties. In this paper, we focus on the channel deformation with the finite elements program Comsol®, and we highlight the limitations in frequency of such optofluidic components.

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.