Electrically tunable optofluidic light switch for reconfigurable solar lighting

Static and Dynamic Optical Analysis of Micro Wrinkle Formation on a Liquid Surface

A spatially periodic voltage was used to create a dielectrophoresis induced periodic micro wrinkle deformation on the surface of a liquid film. Optical Coherence Tomography provided the equilibrium wrinkle profile at submicron accuracy. The dynamic wrinkle amplitude was derived from optical diffraction analysis during sub-millisecond wrinkle formation and decay, after abruptly increasing or reducing the voltage, respectively. The decay time constant closely followed the film thickness dependence expected for surface tension driven viscous levelling. Modelling of the system using numerical solution of the Stokes flow equations with electrostatic forcing predicted that wrinkle formation was faster than decay, in accord with observations.

Amplitude Holographic Interference-Based Microfluidic Colorimetry at the Micrometer Scale

Quantitative molecular analysis is usually based on spectrophotometric methods using colorimetric assay. Conventional methods, however, rely on the direct uniform absorption of the sample under test, and the detection sensitivity is strictly limited by the length of the absorption cell at the millimeter scale. Here, we report a new methodology for colorimetric assay based on the amplitude holographic interference (AHI) caused by nonuniform absorption of light, with detection sensitivity at the micrometer scale. In our method, the curved surface of the microfluidics results in a phase profile with a high diffraction efficiency, and the nonuniform absorption of samples exactly matches with the amplitude modulation in the holographic interference. The signal intensity is affected by not only direct sample absorption but also the sequential optical interference behind the liquid level. Both single- and multiple-wavelength colorimetric analyses of the Griess-Saltzman dye (GSD) were carried out using this method, and we found that the sensitivity can be improved by approximately 2-fold in comparison to the conventional method. This interference-based method would be a useful tool for the colorimetric assay of chemical samples in highly integrated systems with better performance.

Optofluidic differential colorimetry for rapid nitrite determination

Nitrite detection plays a very important role in environmental monitoring and for industrial purposes. The commonly used colorimetric analysis requires the measurement of a system's calibration curve by asynchronously preparing and detecting a dozen standard samples, leading to time-consuming, slow and cumbersome procedures. Here, we present a differential colorimetry method that determines the nitrite level based on the paired chromaticity gradient, formed by coupling the colour reaction into the microfluidic network. The two gradients reshape each other and contain enough information for the quantitative analysis of the sample being tested, without the need for a calibration curve. The independence of the two gradients of the absorbance change caused by the detecting system and water quality results in a high stability and anti-interference performance, with the assistance of its self-correcting ability. This differential colorimetry method requires little time and energy consumption as only one sample is needed. Standard nitrite solutions of 0.50 mM and 0.33 mM have been determined with an error of 1.16% and 0.50%, respectively. These measurements are advantageous in terms of greater stability by up to 10 times and accuracy by 6 times, compared with the calibration curve approaches. It is foreseeable that this differential colorimetry method will find a wide range of applications in the field of chemical detection.

Electrowetting-driven solar indoor lighting (e-SIL): an optofluidic approach towards sustainable buildings

Optofluidics is an emerging research field that combines the two disciplines of microfluidics and optics. By using microfluidic technologies for light control, optofluidic devices can offer several advantages over solid-type optical components, including optical-grade smoothness at the fluidic interface and a high degree of optical tunability without bulky and complex mechanical moving parts. These features have made optofluidic devices more versatile and reconfigurable to improve their optical performances. In this paper, we present a novel optofluidic sunlight manipulation technology for solar indoor lighting using the electrowetting principle. Rooftop sunlight is collected by a solar concentrator and guided to individual rooms along an optical fiber (waveguide) on the bottom of which tunable liquid prisms are linearly integrated. In the light-off mode, electrowetting controls the apex angle of the prisms to be φ = 0°. Under this condition, incoming sunlight experiences total internal reflection and thus keeps propagating along the optical fiber without leaking to the prism bottom for indoor lighting. In contrast, when liquid prisms are controlled to have the angle at φ > 0°, incoming sunlight is partially transmitted to the bottom surface of the arrayed prisms to contribute to interior illumination. Simulation studies validate that our electrowetting-driven solar indoor lighting (e-SIL) system is capable of variably tuning the lighting power from 0% to 98.6% of the input solar power by controlling the prism angle and varying the refractive index of prism materials. For experimental studies, we fabricated an array of 5 prisms filled with silicone oil and water. Using a fiber illuminator as a white light source that includes visible light with various incident angles, we have demonstrated two important lighting functions, (1) light on/off and (2) illumination power control. Lighting performance can be further enhanced by lowering the aspect ratio of the prism as well as increasing the number of prisms. The e-SIL technology based on tunable liquid prisms offers a new approach towards sustainable buildings that are able to reduce their electricity usage as well as provide a healthy and comfortable indoor environment under illumination of natural sunlight.

Optofluidic marine phosphate detection with enhanced absorption using a Fabry-Pérot resonator

Real-time detection of phosphate has significant meaning in marine environmental monitoring and forecasting the occurrence of harmful algal blooms. Conventional monitoring instruments are dependent on artificial sampling and laboratory analysis. They have various shortcomings for real-time applications because of the large equipment size and high production cost, with low target selectivity and the requirement of time-consuming procedures to obtain the detection results. We propose an optofluidic miniaturized analysis chip combined with micro-resonators to achieve real-time phosphate detection. The quantitative water-soluble components are controlled by the flow rate of the phosphate solution, chromogenic agent A (ascorbic acid solution) and chromogenic agent B (12% ammonium molybdate solution, 80% concentrated sulfuric acid and 8% antimony potassium tartrate solution with a volume ratio of 80 : 18 : 2). Subsequently, an on-chip Fabry-Pérot microcavity is formed with a pair of aligned coated fiber facets. With the help of optical feedback, the absorption of phosphate can be enhanced, which can avoid the disadvantages of the macroscale absorption cells in traditional instruments. It can also overcome the difficulties of traditional instruments in terms of size, parallel processing of numerous samples and real-time monitoring, etc. The absorption cell length is shortened to 300 μm with a detection limit of 0.1 μmol L^-1. The time required for detection is shortened from 20 min to 6 seconds. Predictably, microsensors based on optofluidic technology will have potential in the field of marine environmental monitoring.

Optofluidic bioanalysis: fundamentals and applications

Over the past decade, optofluidics has established itself as a new and dynamic research field for exciting developments at the interface of photonics, microfluidics, and the life sciences. The strong desire for developing miniaturized bioanalytic devices and instruments, in particular, has led to novel and powerful approaches to integrating optical elements and biological fluids on the same chip-scale system. Here, we review the state-of-the-art in optofluidic research with emphasis on applications in bioanalysis and a focus on waveguide-based approaches that represent the most advanced level of integration between optics and fluidics. We discuss recent work in photonically reconfigurable devices and various application areas. We show how optofluidic approaches have been pushing the performance limits in bioanalysis, e.g. in terms of sensitivity and portability, satisfying many of the key requirements for point-of-care devices. This illustrates how the requirements for bianalysis instruments are increasingly being met by the symbiotic integration of novel photonic capabilities in a miniaturized system.

A Study of Dip-Coatable, High-Capacitance Ion Gel Dielectrics for 3D EWOD Device Fabrication

We present a dip-coatable, high-capacitance ion gel dielectric for scalable fabrication of three-dimensional (3D) electrowetting-on-dielectric (EWOD) devices such as an n × n liquid prism array. Due to the formation of a nanometer-thick electric double layer (EDL) capacitor, an ion gel dielectric offers two to three orders higher specific capacitance (c ≈ 10 μF/cm²) than that of conventional dielectrics such as SiO₂. However, the previous spin-coating method used for gel layer deposition poses several issues for 3D EWOD device fabrication, particularly when assembling multiple modules. Not only does the spin-coating process require multiple repetitions per module, but the ion gel layer also comes in risks of damage or contamination due to handling errors caused during assembly. In addition, it was observed that the chemical formulation previously used for the spin-coating method causes the surface defects on the dip-coated gel layers and thus leads to poor EWOD performance. In this paper, we alternatively propose a dip-coating method with modified gel solutions to obtain defect-free, functional ion gel layers without the issues arising from the spin-coating method for 3D device fabrication. A dip-coating approach offers a single-step coating solution with the benefits of simplicity, scalability, and high throughput for deposition of high-capacitance gel layers on non-planar EWOD devices. An ion gel solution was prepared by combining the [EMIM][TFSI] ionic liquid and the [P(VDF-HFP)] copolymer at various wt % ratios in acetone solvent. Experimental studies were conducted to fully understand the effects of chemical composition ratios in the gel solution and how varying thicknesses of ion gel and Teflon layers affects EWOD performance. The effectiveness and potentiality of dip-coatable gel layers for 3D EWOD devices have been demonstrated through fabricating 5 × 1 arrayed liquid prisms using a single-step dip-coating method. Each prism module has been individually controlled to achieve spatial beam steering without the need for bulky mechanical moving parts.

Switchable 3D optofluidic Y-branch waveguides tuned by Dean flows

Optical branch waveguides are one of the most important optical elements and have been widely exploited for optical communication systems. However, prevailing devices are typically solid and have limit in tunability. Liquid optical devices have attracted more interest for the advantage of tunability of liquid media, but their signals suffer serious leakage if the refractive index (RI) of liquid is smaller than that of solid channels. This paper demonstrates the tunable three-dimensional (3D) optofluidic Y-branch waveguides in plannar microchannels by simply introducing Dean flow. This device can reconfigure 3D Y-branch profiles and separate the intensity of light as tunable ratio from 0 to 1 by adjusting the flow rates with low loss. Different from the prevailing 2D liquid counterparts, the 3D configuration offer much more freedom in the selection of liquid media as liquid's RI can be totally independent to the solid channel structure. The transmission loss through the device is estimated to 0.97 db when the splitting angle is 10°, which shows the light is confined better in the 3D liquid structures than traditional 2D liquid counterparts. The Y-branch waveguides show potential in applications of integrated optofluidic devices.

Tunable focusing properties using optofluidic Fresnel zone plates

A Fresnel zone plate (FZP) is a unique diffractive optical device and widely used in integrated optical systems such as interferometers and antennas. A traditional FZP utilizes solid materials and cannot be modulated in real time for desired focusing properties. This paper reports a tunable optofluidic FZP based on a solid-liquid hybrid structure. This FZP consists of two parts including a fast microfluidic mixer, which can adjust the refractive index of liquids from 1.332 to 1.432, and subsequently an optical FZP with a solid-liquid combination. Simulations and experiments successfully showed the real-time tunability of the focusing properties such as peak intensity, focal spot sizes and focal lengths. The focal spot size can be modulated from 16 μm to 80 μm at λ₀ = 532 nm in experiments with focal length changes of approximately 700 μm. Moreover, it can be easily switched between focusing, defocusing and collimation. The dispersion with different wavelengths was also investigated, showing that these types of focusing properties are quite different from a traditional optofluidic lens by refraction or reflection. It is foreseeable that such a hybrid FZP may find wider applications in lab-on-a-chip systems and optical devices.

A label-free infrared opto-fluidic method for real-time determination of flow rate and concentration with temperature cross-sensitivity compensation

The ability to accurately measure the flow rate, concentration, and temperature in real-time in micro total analysis systems (μTAS) is crucial when improving their practical sensing capabilities within extremely small volumes. Our label-free infrared (1500-1600 nm) opto-fluidic method, presented in this study, utilizes a cantilever-based flow meter integrated with two parallel optical fiber Fabry-Perot interferometers (FPIs). The first FPI serves as an ultra-sensitive flow meter and includes a Fiber Bragg Grating (FBG) tip for localized temperature sensing. The second FPI has a fabricated photopolymer micro-tip for highly sensitive refractive index (RI) determination. In this work, we performed 3-D simulation analysis to characterize cantilever deflection as well as temperature distribution and its effect on the RI. The experimental results from temperature cross-sensitivity analysis lead to real-time measurement resolutions of 5 nL min^-1, 1 × 10^-6 RIU and 0.05 °C, for the flow rate, refractive index, and temperature, respectively.

Optofluidic chlorophyll lasers

Chlorophylls are essential for photosynthesis and also one of the most abundant pigments on earth. Using an optofluidic ring resonator of extremely high Q-factors (>10(7)), we investigated the unique characteristics and underlying mechanism of chlorophyll lasers. Chlorophyll lasers with dual lasing bands at 680 nm and 730 nm were observed for the first time in isolated chlorophyll a (Chla). Particularly, a laser at the 730 nm band was realized in 0.1 mM Chla with a lasing threshold of only 8 μJ mm(-2). Additionally, we observed lasing competition between the two lasing bands. The presence of laser emission at the 680 nm band can lead to quenching or significant reduction of laser emission at the 730 nm band, effectively increasing the lasing threshold for the 730 nm band. Further concentration-dependent studies, along with theoretical analysis, elucidated the mechanism that determines when and why the laser emission band appears at one of the two bands, or concomitantly at both bands. Finally, Chla was exploited as the donor in fluorescence resonance energy transfer to extend the laser emission to the near infrared regime with an unprecedented wavelength shift as large as 380 nm. Our work will open a door to the development of novel biocompatible and biodegradable chlorophyll-based lasers for various applications such as miniaturized tunable coherent light sources and in vitro/in vivo biosensing. It will also provide important insight into the chlorophyll fluorescence and photosynthesis processes inside plants.

Reconfigurable liquid-core/liquid-cladding optical waveguides with dielectrophoresis-driven virtual microchannels on an electromicrofluidic platform

An electrically reconfigurable liquid-core/liquid-cladding (L(2)) optical waveguide with core liquid γ-butyrolactone (GBL, ncore = 1.4341, εcore = 39) and silicone oil (ncladding = 1.401, εcladding = 2.5) as cladding liquid is accomplished using dielectrophoresis (DEP) that attracts and deforms the core liquid with the greater permittivity to occupy the region of strong electric field provided by Teflon-coated ITO electrodes between parallel glass plates. Instead of continuously flowing core and cladding liquids along a physical microchannel, the DEP-formed L(2) optical waveguide guides light in a stationary virtual microchannel that requires liquids of limited volume without constant supply and creates stable liquid/liquid interfaces for efficient light guidance in a simply fabricated microfluidic device. We designed and examined (1) stationary and (2) moving L(2) optical waveguides on the parallel-plate electromicrofluidic platform. In the stationary L-shaped waveguide, light was guided in a GBL virtual microchannel core for a total of 27.85 mm via a 90° bend (radius 5 mm) before exiting from the light outlet of cross-sectional area 100 μm × 100 μm. For the stationary spiral waveguide, light was guided in a GBL core containing Rhodamine 6G (R6G, 1 mM) and through a series of 90° bends with decreasing radii from 5 mm to 2.5 mm. With the stationary straight waveguide, the propagation loss was measured to be 2.09 dB cm(-1) in GBL with R6G (0.01 mM). The moving L-shaped waveguide was implemented on a versatile electromicrofluidic platform on which electrowetting and DEP were employed to generate a precise GBL droplet and form a waveguide core. On sequentially applying appropriate voltage to one of three parallel L-shaped driving electrodes, the GBL waveguide core was shifted; the guided light was switched at a speed of up to 0.929 mm s(-1) (switching period 70 ms, switching rate 14.3 Hz) when an adequate electric signal (173.1 VRMS, 100 kHz) was applied.

Optofluidic devices with integrated solid-state nanopores

This review (with 90 refs.) covers the state of the art in optofluidic devices with integrated solid-state nanopores for use in detection and sensing. Following an introduction into principles of optofluidics and solid-state nanopore technology, we discuss features of solid-state nanopore based assays using optofluidics. This includes the incorporation of solid-state nanopores into optofluidic platforms based on liquid-core anti-resonant reflecting optical waveguides (ARROWs), methods for their fabrication, aspects of single particle detection and particle manipulation. We then describe the new functionalities provided by solid-state nanopores integrated into optofluidic chips, in particular acting as smart gates for correlated electro-optical detection and discrimination of nanoparticles. This enables the identification of viruses and λ-DNA, particle trajectory simulations, enhancing sensitivity by tuning the shape of nanopores. The review concludes with a summary and an outlook.

Tunable self-imaging effect using hybrid optofluidic waveguides

Multimode interference (MMI) is a typical self-imaging phenomenon and has been widely exploited for optical devices like couplers and splitters. Usually, it utilizes solid waveguides only and thus has very limited tunability in self-imaging properties. This paper reports our original work on tunable MMI using a hybrid optofluidic waveguide. It is generated by the diffusion between miscible flows in a microchannel and consists of two parts: gradient-index liquid-liquid waveguide for light modulation and step-index liquid-solid waveguide for image formation. Simulation and experiments have shown successful realization of the real-time tuning of self-imaging properties. For detail, the image point (focal spot) width could be modulated from 7 μm to 16 μm, and the self-imaging period changes in the range of 500 μm by varying the fluid properties. It is foreseeable that such a hybrid optofluidic waveguide may find wider applications in lab-on-a-chip systems and optical devices.

A tunable submicro-optofluidic polymer filter based on guided-mode resonance

Optical filters with reconfigurable spectral properties are highly desirable in a wide range of applications. We propose and experimentally demonstrate a tunable submicro-optofluidic polymer guided-mode resonance (PGMR) filter. The device is composed of a periodic grating sandwiched between a high index waveguide layer and a low index capping layer, which integrates submicro-fluidic channel arrays and a PGMR filter elegantly. A finite difference time domain (FDTD) method is employed to understand the spectral properties and determine appropriate device parameters. We fabricated the polymer guided-mode resonance filter with a method combining two-beam interference lithography, floating nanofilm transfer and thermal bonding techniques. Experimental results show that our tunable submicro-optofluidic PGMR filters can provide a broad spectral tuning range (13.181 nm), a narrow bandwidth (<2.504 nm), and a high reflection efficiency (>85%) in the visible region. Such submicro-optofluidic PGMR filters are highly compatible with existing nano/microfluidic technologies and would be valuable for the integrated flexible optical system.

Insect-Human Hybrid Eye (IHHE): an adaptive optofluidic lens combining the structural characteristics of insect and human eyes

Insect compound eyes and human camera eyes are two exquisite optical systems created by nature. The compound eye boasts an angle of view (AOV) up to 180° thanks to its hemispherical arrangement of hundreds of prime microscale lenses. The camera eye, on the other hand, can change shape to focus on objects at various depths, yet accepts light within a smaller AOV. Imitations of either imaging system have been abundant but with limited success. Here, we describe a reconfigurable polymeric optofluidic device that combines the architectural merits of both vision mechanisms, featuring a large AOV (up to 120°) with adaptive focusing capabilities (from 0 to 275 diopter (D)). This device consists of bi-layered microfluidics: an array of millimeter-sized fluidic lenses is integrated into the top layer and arranged on an elastomeric membrane embedded within the bottom layer. The membrane can be deformed from a planar surface into a series of dome-shaped geometries, rearranging individual fluidic lenses in desired curvilinear layouts. Meanwhile, each fluidic lens can vary its radius of curvature for a monocular depth sensation. Such a design presents a new perspective of tunable optofluidics for a broad range of applications, such as robotic vision and medical laparoendoscopy, where adaptive focalization with a large viewing angle is a clear advantage.

Elastomer-based opto-thermo-mechanical actuation for autonomous, self-powered light level control

We present an autonomous, self-powered solar light controller based on functional integration of a flexible cantilever light guide and a paraffin wax-based optothermal actuator. The controller utilizes the optothermally induced volume increase in the elastomer-encapsulated paraffin wax to produce pneumatic force, which subsequently actuates the cantilever light guide to control the level of frustrated total internal reflection. In its linear response regime, it demonstrated 33% reduction in light intensity fluctuation in terms of the root-mean-square value.

Multiphase optofluidics on an electro-microfluidic platform powered by electrowetting and dielectrophoresis

For diverse material phases used on an electro-microfluidic (EMF) platform, exploiting the electro-optical properties of matter in varied phases is essential to reap the benefits of the optofluidic capabilities of that platform. Materials in the four fundamental phases--solid-phase dielectric layer, liquid-phase droplet, gas-phase bubble, and plasma-phase bubble microplasma--have been investigated to offer electrically tunable optical characteristics for the manipulation of fluids on an EMF platform. Here we present an overview of the basic driving mechanisms for electrowetting and dielectrophoresis on the EMF platform. Three optofluidic examples occurring in multiple phases are described: solid optofluidics--liquid and light regulation by electrowetting on a solid polymer dispersed liquid crystal (PDLC) dielectric layer; liquid optofluidics--transmittance and reflectance modulation with formation of particle chains in a liquid droplet; and gas and plasma optofluidics--ignition and manipulation of a bubble microplasma by liquid dielectrophoresis. By combining the various materials possessing diverse electro-optical characteristics in separate phases, the EMF platform becomes an ideal platform for integrated optofluidics.

Wavelength-tunable light shaping with cholesteric liquid crystal microlenses

The ability to guide light on the mesoscopic scale is important both scientifically and technologically. Especially relevant is the development of wavelength-tunable light-shaping microdevices. Here we demonstrate the use of cholesteric liquid crystal polygonal textures organized as an array of microlenses for this purpose. The beam shaping is controlled by tuning the wavelength of the incident light in the visible spectrum. By taking advantage of the self-organization property of liquid crystals, the structure of the lens and its optical response are tailored by changing the annealing time of the single layer material during a completely integrated one-step process. The intrinsic helical organization of the layer is the cause of the light shaping and not the shape of the surface as for conventional lenses. A new concept of light manipulation using the structure chirality of liquid crystals is demonstrated, which concerns soft matter photonic circuits to mould the light.