Optofluidics:the interaction between light and flowing liquids in integrated devices

Optofluidics is a rising technology that combines microfluidics and optics.Its goal is to manipulate light and flowing liquids on the micro/nanoscale and exploiting their interaction in optofluidic chips.The fluid flow in the on-chip devices is reconfigurable,non-uniform and usually transports substances being analyzed,offering a new idea in the accurate manipulation of lights and biochemical samples.In this paper,we summarized the light modulation in heterogeneous media by unique fluid dynamic properties such as molecular diffusion,heat conduction,centrifugation effect,light-matter interaction and others.By understanding the novel phenomena due to the interaction of light and flowing liquids,quantities of tunable and reconfigurable optofluidic devices such as waveguides,lenses,and lasers are introduced.Those novel applications bring us firm conviction that optofluidics would provide better solutions to high-efficient and high-quality lab-on-chip systems in terms of biochemical analysis and environment monitoring.

Optofluidics in bio-imaging applications

Bio-imaging generally indicates imaging techniques that acquire biological information from living forms.Recently, the ability to detect, diagnose, and monitor pathological, physiological, and molecular dynamics is in great demand, while scaling down the observing angle, achieving precise alignment, fast actuation, and a miniaturized platform become key elements in next-generation optical imaging systems. Optofluidics, nominally merging optic and microfluidic technologies, is a relatively new research field, and it has drawn great attention since the last decade. Given its abilities to manipulate both optic and fluidic functions/elements in the micro-/nanometer regime, optofluidics shows great potential in bio-imaging to elevate our cognition in the subcellular and/or molecular level. In this paper, we emphasize the development of optofluidics in bio-imaging, from individual components to representative applications in a more modularized, systematic sense. Further, we expound our expectations for the near future of the optofluidic imaging discipline.

Tunable plasmofluidic lens for subwavelength imaging

A tunable plasmofluidic lens consisting of nanoslit arrays on a metal film is proposed for subwavelength imaging in far field at different wavelengths.The nanoslit arrays with constant depths but varying widths could generate desired optical phase retardations based on the propagation property of the surface plasmon polaritons(SPPs)through the metal-dielectric-metal(MDM)nanoslit waveguide.We demonstrate the tunability of the plasmofluidic lens for subwavelength imaging by changing the surrounding dielectric fluid.This work provides a novel approach for developing integrative tunable plasmofluidic lens for a variety of lab-on-chip applications.

Movable electrowetting optofluidic lens for optical axial scanning in microscopy

Optical axial scanning is essential process to obtain 3D information of biological specimens. To realize optical axial scanning without moving, the tunable lens is a solution. However, the conventional tunable lenses usually induce non-uniform magnification and resolution issues. In this paper, we report a movable electrowetting optofluidic lens. Unlike the conventional tunable lens, our proposed optofluidic lens has two liquid-liquid (L-L) interfaces, which can move in the cell by an external voltage. The object distance and image distance are adjusted by shifting the L-L interface position. Therefore, the proposed lens can realize optical axial scanning with uniform magnification and resolution in microscopy. To prove the concept, we fabricate an optofluidic lens and use it in optical axial scanning. The scanning distance is more than 1 mm with uniform magnification and good imaging quality. Widespread application of such a new adaptive zoom lens is foreseeable.

Chirality discerning and monitoring in a metal cladding optofluidic chip

The metabolic process of chiral drugs plays a significant role in clinics and in research on drugs.Here,we experimentally demonstrate by all-optical means that the chiral molecules can be quickly discriminated and monitored with the ultrahighorder modes excited in a metal cladding optofluidic chip,achieving over 5 times sensitivity with a low-dosage sample.We show that the varying concentration of the chiral drugs can be monitored both in cell and animal experiments,presenting a significant difference between chiral enantiomers at the optimal function time and the effect of the reaction.To our knowledge,this approach provides a new way to achieve important chiral discrimination for the pharmacokinetics and the pharmacodynamics and may present opportunities in indicating the health status of humans.

Nonlinear,tunable,and active optical metasurface with liquid film

Optical metamaterials and metasurfaces,which emerged in the course of the last few decades,have revolutionized our understanding of light and light–matter interaction.While solid materials are naturally employed as key building elements for construction of optical metamaterials mainly due to their structural stability,practically no attention was given to study of liquid-made optical two-dimensional(2-D)metasurfaces and the underlying interaction regimes between surface optical modes and liquids.We theoretically demonstrate that surface plasmon polaritons and slab waveguide modes that propagate within a thin liquid dielectric film trigger optical self-induced interaction facilitated by surface tension effects,which leads to the formation of 2-D optical liquid-made lattices/metasurfaces with tunable symmetry and can be leveraged for tuning of lasing modes.Furthermore,we show that the symmetry breaking of the 2-D optical liquid lattice leads to phase transition and tuning of its topological properties,which allows the formation,destruction,and movement of Dirac-points in the k-space.Our results indicate that optical liquid lattices support extremely low lasing threshold relative to solid dielectric films and have the potential to serve as configurable analogous computation platform.

Highly sensitive force measurements in an optically generated, harmonic hydrodynamic trap

The use of optical tweezers to measure forces acting upon microscopic particles has revolutionised fields from material science to cell biology.However,despite optical control capabilities,this technology is highly constrained by the material properties of the probe,and its use may be limited due to concerns about the effect on biological processes.Here we present a novel,optically controlled trapping method based on light-induced hydrodynamic flows.Specifically,we leverage optical control capabilities to convert a translationally invariant topological defect of a flow field into an attractor for colloids in an effectively one-dimensional harmonic,yet freely rotatable system.Circumventing the need to stabilise particle dynamics along an unstable axis,this novel trap closely resembles the isotropic dynamics of optical tweezers.Using magnetic beads,we explicitly show the existence of a linear force-extension relationship that can be used to detect femtoNewton-range forces with sensitivity close to the thermal limit.Our force measurements remove the need for laser-particle contact,while also lifting material constraints,which renders them a particu-larly interesting tool for the life sciences and engineering.

The engineered eyeball, a tunable imaging system using soft-matter micro-optics

We demonstrate a tunable imaging system based on the functionality of the mammalian eye using soft-matter micro-optical components.Inspired by the structure of the eye,as well as by the means through which nature tunes its optical behavior,we show that the technologies of microsystems engineering and micro-optics may be used to realize a technical imaging system whose biomimetic functionality is entirely distinct from that of conventional optics.The engineered eyeball integrates a deformable elastomeric refractive structure whose shape is mechanically controlled through application of strain using liquid crystal elastomer(LCE)actuators;two forms of tunable iris,one based on optofluidics and the other on LCEs with embedded heaters;a fixed lens arrangement;and a commercial imaging sensor chip.The complete microsystem,optimized to yield optical characteristics close to those of the human eye,represents the first fully functional,soft-matter-based tunable single-aperture eye-like imager.

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.

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 1.5mmlong slit aperture,with controllably variable discrete widths down to 45 mm,may be scanned across a length of 1.5mmwith 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 mmwith 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.

Wafer-scale fabricated thermo-pneumatically tunable microlenses

We have developed a self-contained,liquid tunable microlens based on polyacrylate membranes integrated with compact on-chip thermo-pneumatic actuation fabricated using full-wafer processing.Silicone oil is used as the optical liquid,which is pushed or pulled into the lens cavity via an extended microfluidic channel structure without any pumps,valves or other mechanical means.The heat load generated by the thermal actuator is physically isolated from the lens chamber.The back focal length may be tuned from infinity to 4 mm with a maximum power consumption of 300 mW.The principal application is fine tuning of the back focal length,for which tuning time constants as small as 100 ms are suitable.

Cascading optofluidic phase modulators for performance enhancement in refractive adaptive optics

We discuss the implementation and performance of an adaptive optics(AO)system that uses two cascaded deformable phase plates(DPPs),which are transparent optofluidic phase modulators,mimicking the common woofer/tweeter-type astronomical AO systems.One of the DPPs has 25 electrodes forming a keystone pattern best suited for the correction of low-order and radially symmetric modes;the second device has 37 hexagonally packed electrodes better suited for high-order correction.We also present simulation results and experimental validation for a new open-loop control strategy enabling simultaneous control of both DPPs,which ensures optimum correction for both large-amplitude low-order,and complex combinations of low-and high-order aberrations.The resulting system can reproduce Zernike modes up to the sixth radial order with stroke and fidelity up to twice better than what is attainable with either of the DPPs individually.The performance of the new AO configuration is also verified in a custom-developed fluorescence microscope with sensorless aberration correction.

In-channel integration of designable microoptical devices using flat scaffold-supported femtosecond-laser microfabrication for coupling-free optofluidic cell counting

The high-precision integration of three-dimensional(3D)microoptical components into microfluidics in a customizable manner is crucial for optical sensing,fluorescence analysis,and cell detection in optofluidic applications;however,it remains challenging for current microfabrication technologies.This paper reports the in-channel integration of flexible two-dimensional(2D)and 3D polymer microoptical devices into glass microfluidics by developing a novel technique:flat scaffold-supported hybrid femtosecond laser microfabrication(FSS-HFLM).The scaffold with an optimal thickness of 1–5 μm is fabricated on the lower internal surface of a microfluidic channel to improve the integration of high-precision microoptical devices on the scaffold by eliminating any undulated internal channel surface caused by wet etching.As a proof of demonstration,two types of typical microoptical devices,namely,2D Fresnel zone plates(FZPs)and 3D refractive microlens arrays(MLAs),are integrated.These devices exhibit multicolor focal spots,elongated(>three times)focal length and imaging of the characters‘RIKEN’in a liquid channel.The resulting optofluidic chips are further used for coupling-free white-light cell counting with a success rate as high as 93%.An optofluidic system with two MLAs and a W-filter is also designed and fabricated for more advanced cell filtering/counting applications.

Probing the dynamic structural changes of DNA using ultrafast laser pulse in graphene-based optofluidic device

The ultrafast monitoring of deoxyribonucleic acid(DNA)dynamic structural changes is an emerging and rapidly growing research topic in biotechnology.The existing optical spectroscopy used to identify different dynamical DNA structures lacks quick response while requiring large consumption of samples and bulky instrumental facilities.It is highly demanded to develop an ultrafast technique that monitors DNA structural changes with the external stimulus or cancer-related disease scenarios.Here,we demonstrate a novel photonic integrated graphene-optofluidic device to monitor DNA structural changes with the ultrafast response time.Our approach is featured with an effective and straightforward design of decoding the electronic structure change of graphene induced by its interactions with DNAs in different conformations using ultrafast nanosecond pulse laser and achieving refractive index sensitivity of~3×10^(−5) RIU.This innovative technique for the first time allows us to perform ultrafast monitoring of the conformational changes of special DNA molecules structures,including G-quadruplex formation by K+ions and i-motif formation by the low pH stimulus.The graphene-optofluidic device as presented here provides a new class of label-free,ultrafast,ultrasensitive,compact,and cost-effective optical biosensors for medical and healthcare applications.

Recent Progress in Fiber Optofluidic Lasing and Sensing

Fiber optofluidic laser(FOFL)integrates optical fiber microcavity and microfluidic channel and provides many unique advantages for sensing applications.FOFLs not only inherit the advantages of lasers such as high sensitivity,high signal-to-noise ratio,and narrow linewidth,but also hold the unique features of optical fiber,including ease of integration,high repeatability,and low cost.With the development of new fiber structures and fabrication technologies,FOFLs become an important branch of optical fiber sensors,especially for application in biochemical detection.In this paper,the recent progress on FOFL is reviewed.We focuse mainly on the optical fiber resonators,gain medium,and the emerging sen sing applicatio ns.The prospects for FOFL are also discussed.We believe that the FOFL sensor provides a promising technology for biomedical analysis and environmental monitoring.

Secondary structure changes of ox-LDL by photoirradiation in an optofluidic resonator

Atherosclerotic cardio-cerebral vascular disease is the most common disease that threatens human health.Many researches indicated that oxidatively modified low-density lipoprotein[ox-LDL]is a key pathogenic factor of atherosclero-sis.Here,we report the change of the secondary structure of ox-LDL caused by photoirradiation in an optofluidic resonator.The content ratios of amphipathic o-helices and β-sheets of ox-LDL are changed under laser beam ilumination,resulting in an increasing binding rate of ox-LDL and ox-LDL antibodies.Our findings may provide a potential way for clinical athero-sclerosis treatment and prompt recovery rate of atherosclerotic cardio-cerebral vascular disease by optical technology and immunotherapy.

Optofluidic Refractive Index Sensor Based on Partial Reflection

We demonstrate a novel optofluidic refractive index （RI） sensor with high sensitivity and wide dynamic range based on partial reflection. Benefited from the divergent incident light and the output fibers with different tilting angles, we have achieved highly sensitive RI sensing in a wide range from 1.33 to 1.37. To investigate the effectiveness of this sensor, we perform a measurement of RI with a resolution of ca. 5.0× 10^-5 refractive index unit （RIU） for ethylene glycol solutions. Also, we have measured a series of liquid solutions by using different output fibers, achieving a resolution of ca. 0.52mg/mL for cane surge. The optofluidic RI sensor takes advantage of the high sensitivity, wide dynamic range, small footprint, and low sample consumption, as well as the efficient fluidic sample delivery, making it useful for applications in the food industry.

Detection of low-concentration EGFR with a highly sensitive optofluidic resonator

A hollow-core metal-cladding waveguide（HCMW） optofluidic resonator that works based on a free-space coupling technique is designed. An HCMW can excite ultra-high-order modes（UOMs） at the coupled angle, which can be used as an optofluidic resonator to detect alterations of the epidermal growth factor receptor（EGFR）concentration. Theoretical analysis shows that the UOMs excited in the HCMW have a highly sensitive response to the refractive index（RI） variation of the guiding layer. An EGFR solution with a 0.2 ng/mL alteration is detected, and the RI variation caused by the concentration alteration is about 2.5 × 10^（-3）.