Optofluidic Waveguide Research Articles

Hybrid Fibers with Subwavelength-Scale Liquid Core for Highly Sensitive Sensing and Enhanced Nonlinearity.

Interest grows in designing silicon-on-insulator slot waveguides to trap optical fields in subwavelength-scale slots and developing their optofluidic devices. However, it is worth noting that the inherent limitations of the waveguide structures may result in high optical losses and short optical paths, which challenge the device's performance in optofluidics. Incorporating the planar silicon-based slot waveguide concept into a silica-based hollow-core fiber can provide a perfect solution to realize an efficient optofluidic waveguide. Here, we propose a subwavelength-scale liquid-core hybrid fiber (LCHF), where the core is filled with carbon disulfide and surrounded by a silicon ring in a silica background. The waveguide properties and the Stimulated Raman Scattering (SRS) effect in the LCHF are investigated. The fraction of power inside the core of 56.3% allows for improved sensitivity in optical sensing, while the modal Raman gain of 23.60 m-1·W-1 is two times larger than that generated around a nanofiber with the interaction between the evanescent optical field and the surrounding Raman media benzene-methanol, which enables a significant low-threshold SRS effect. Moreover, this in-fiber structure features compactness, robustness, flexibility, ease of implementation in both trace sample consumption and reasonable liquid filling duration, as well as compatibility with optical fiber systems. The detailed analyses of the properties and utilizations of the LCHF suggest a promising in-fiber optofluidic platform, which provides a novel insight into optofluidic devices, optical sensing, nonlinear optics, etc.

Machine learning at the edge for AI-enabled multiplexed pathogen detection

Multiplexed detection of biomarkers in real-time is crucial for sensitive and accurate diagnosis at the point of use. This scenario poses tremendous challenges for detection and identification of signals of varying shape and quality at the edge of the signal-to-noise limit. Here, we demonstrate a robust target identification scheme that utilizes a Deep Neural Network (DNN) for multiplex detection of single particles and molecular biomarkers. The model combines fast wavelet particle detection with Short-Time Fourier Transform analysis, followed by DNN identification on an AI-specific edge device (Google Coral Dev board). The approach is validated using multi-spot optical excitation of Klebsiella Pneumoniae bacterial nucleic acids flowing through an optofluidic waveguide chip that produces fluorescence signals of varying amplitude, duration, and quality. Amplification-free 3× multiplexing in real-time is demonstrated with excellent specificity, sensitivity, and a classification accuracy of 99.8%. These results show that a minimalistic DNN design optimized for mobile devices provides a robust framework for accurate pathogen detection using compact, low-cost diagnostic devices.

Manufacture of Three-Dimensional Optofluidic Spot-Size Converters in Fused Silica Using Hybrid Laser Microfabrication

We propose a hybrid laser microfabrication approach for the manufacture of three-dimensional (3D) optofluidic spot-size converters in fused silica glass by a combination of femtosecond (fs) laser microfabrication and carbon dioxide laser irradiation. Spatially shaped fs laser-assisted chemical etching was first performed to form 3D hollow microchannels in glass, which were composed of embedded straight channels, tapered channels, and vertical channels connected to the glass surface. Then, carbon dioxide laser-induced thermal reflow was carried out for the internal polishing of the whole microchannels and sealing parts of the vertical channels. Finally, 3D optofluidic spot-size converters (SSC) were formed by filling a liquid-core waveguide solution into laser-polished microchannels. With a fabricated SSC structure, the mode spot size of the optofluidic waveguide was expanded from ~8 μm to ~23 μm with a conversion efficiency of ~84.1%. Further measurement of the waveguide-to-waveguide coupling devices in the glass showed that the total insertion loss of two symmetric SSC structures through two ~50 μm-diameter coupling ports was ~6.73 dB at 1310 nm, which was only about half that of non-SSC structures with diameters of ~9 μm at the same coupling distance. The proposed approach holds great potential for developing novel 3D fluid-based photonic devices for mode conversion, optical manipulation, and lab-on-a-chip sensing.

Low-loss optofluidic waveguides in fused silica enabled by spatially shaped femtosecond laser assisted etching combined with carbon dioxide laser irradiation

We demonstrate the fabrication of low-loss optofluidic waveguides encapsulated in fused silica glass using femtosecond laser microfabrication followed by carbon dioxide laser irradiation. Spatially-shaped femtosecond laser-assisted chemical etching is first used to fabricate microchannels with circular cross-sections and a string of open extra-access ports in the glass. Further, the carbon dioxide laser direct writing on the glass surface is used to create a thermal reflow effect of etched glass microstructures for simultaneously polishing all internal surfaces of channels and sealing the extra-access ports. With this effect, the inner surface roughness of the etched microchannels can be reduced to ∼ 40 nm. Finally, a single-mode microfluidic optical waveguide with a propagation loss of ∼ 0.78 dB/cm at 1310 nm is obtained inside the glass by filling a mixture solution of decane and liquid paraffin into a laser-polished microchannel.

Reconfigurable Size‐Sorting of Micronanoparticles in Chalcogenide Waveguide Array

Optical tweezers are considered as a revolution, allowing for manipulating particles ranging in scale from a few hundred nanometers (nm) to several micrometers (μm). Near-field optical force allows effective trapping of a broad range of entities, from atom to living cells. Yet, there are formidable challengings for existing on-chip photonic trapping techniques to simultaneously handle at will multiple entities with cross-scales (i.e., from nm to μm). Herein, optical transportation and trapping of polystyrene particles with the different diameters are demonstrated in optofluidic nanophotonic sawtooth waveguide array (ONSWA) made of chalcogenide alloys Sb2Se3. The chalcogenide ONSWA produces sawtooth-like light fields that can be actively modulated via the phase transition of Sb2Se3. Particularly, the chalcogenide ONSWA can stably trap the polystyrene particles with the diameters of 500 nm and 1 μm for both the amorphous and crystalline states, respectively. It is experimentally demonstrated that the phase transition of Sb2Se3 from amorphous to crystalline and vice versa can be achieved in nanoseconds. Using the technique of nanosecond laser-induced phase transition of Sb2Se3, a dynamically reconfigurable size-sorting of objects on the identical ONSWA is proposed.

Optofluidic Particle Manipulation Platform with Nanomembrane.

We demonstrate a method for fabricating and utilizing an optofluidic particle manipulator on a silicon chip that features a 300 nm thick silicon dioxide membrane as part of a microfluidic channel. The fabrication method is based on etching silicon channels and converting the walls to silicon dioxide through thermal oxidation. Channels are encapsulated by a sacrificial polymer which fills the length of the fluid channel by way of spontaneous capillary action. The sacrificial material is then used as a mold for the formation of a nanoscale, solid-state, silicon dioxide membrane. The hollow channel is primarily used for fluid and particle transport but is capable of transmitting light over short distances and utilizes radiation pressure for particle trapping applications. The optofluidic platform features solid-core ridge waveguides which can direct light on and off of the silicon chip and intersect liquid channels. Optical loss values are characterized for liquid and solid-core structures and at interfaces. Estimates are provided for the optical power needed to trap particles of various sizes.

Real-time and high-sensitivity refractive index sensing with an arched optofluidic waveguide

Refractive index (RI) sensing plays an important role in analytical chemistry, medical diagnosis, and environmental monitoring. The optofluidic technique is considered to be an ideal tool for RI sensor configuration for its high integration, high sensitivity, and low cost. However, it remains challenging to achieve RI measurement in real time with high sensitivity and low detection limit (DL) simultaneously. In this work, we design and fabricate a RI sensor with an arched optofluidic waveguide by monitoring the power loss of the light passing through the waveguide, which is sandwiched by the air-cladding and the liquid-cladding under test, we achieve RI detection of the sample in real time and with high sensitivity. Furthermore, both numerical simulation and experimental investigation show that our RI sensor can be designed with different geometric parameters to cover multiple RI ranges with high sensitivities for different applications. Experimental results illustrate that our sensor is capable to achieve a superior sensitivity better than -19.2 mW/RIU and a detection limit of 5.21×10-8 RIU in a wide linear dynamic range from 1.333 to 1.392, providing a promising solution for real-time and high-sensitivity RI sensing.

Chemical reaction monitoring using tunable optofluidic Y-branch waveguides developed with counter-flow

Chemical reaction monitoring using tunable optofluidic Y-branch waveguides developed with counter-flow

High-efficient subwavelength-scale optofluidic waveguides with tapered microstructured optical fibers.

Microstructured optical fibers (MOFs) have attracted intensive research interest in fiber-based optofluidics owing to their ability to have high-efficient light-microfluid interactions over a long distance. However, there lacks an exquisite design guidance for the utilization of MOFs in subwavelength-scale optofluidics. Here we propose a tapered hollow-core MOF structure with both light and fluid confined inside the central hole and investigate its optofluidic guiding properties by varying the diameter using the full vector finite element method. The basic optical modal properties, the effective sensitivity, and the nonlinearity characteristics are studied. Our miniature optofluidic waveguide achieves a maximum fraction of power inside the core at 99.7%, an ultra-small effective mode area of 0.38 µm2, an ultra-low confinement loss, and a controllable group velocity dispersion. It can serve as a promising platform in the subwavelength-scale optical devices for optical sensing and nonlinear optics.

Ultrasensitive detection of SARS-CoV-2 RNA and antigen using single-molecule optofluidic chip.

Nucleic acids and proteins are the two most important target types used in molecular diagnostics. In many instances, simultaneous sensitive and accurate detection of both biomarkers from the same sample would be desirable, but standard detection methods are highly optimized for one type and not cross-compatible. Here, we report the simultaneous multiplexed detection of SARS-CoV-2 RNAs and antigens with single molecule sensitivity. Both analytes are isolated and labeled using a single bead-based solid-phase extraction protocol, followed by fluorescence detection on a multi-channel optofluidic waveguide chip. Direct amplification-free detection of both biomarkers from nasopharyngeal swab samples is demonstrated with single molecule detection sensitivity, opening the door for ultrasensitive dual-target analysis in infectious disease diagnosis, oncology, and other applications.

Fabrication of single-mode circular optofluidic waveguides in fused silica using femtosecond laser microfabrication

We demonstrate fabrication of single-mode circular optofluidic waveguides in fused silica glass substrates using femtosecond (fs) laser assisted chemical etching based on slit beam shaping. The fabrication procedure consists of four steps: slit-assisted fs laser direct writing of fused silica, selective chemical etching, polydimethylsiloxane film bonding, and vacuum-assisted liquid filling. The combination of slit beam shaping and high numerical aperture objectives ensures very narrowly modified lines with nearly circular cross-sections during laser direct writing. Introduction of a string of extra-access ports allows production of uniform circular microchannels with the diameters of ~10 µm and the lengths at the centimeter scale due to the improvement of wet chemical etching process. By vacuum-assisted filling a mixture of liquid paraffin and decane into a microchannel structure, a single-mode circular optofluidic waveguide embedded in glass can be obtained.

Optofluidic Flow-Through Biosensor Sensitivity - Model and Experiment.

We present a model and simulation for predicting the detected signal of a fluorescence-based optical biosensor built from optofluidic waveguides. Typical applications include flow experiments to determine pathogen concentrations in a biological sample after tagging relevant DNA or RNA sequences. An overview of the biosensor geometry and fabrication processes is presented. The basis for the predictive model is also outlined. The model is then compared to experimental results for three different biosensor designs. The model is shown to have similar signal statistics as physical tests, illustrating utility as a pre-fabrication design tool and as a predictor of detection sensitivity.

An aerogel-based photocatalytic microreactor driven by light guiding for degradation of toxic pollutants

Efficient utilization of light in photocatalytic chemical processes requires careful optimization of the photocatalytic reactor layout to maximize the interaction between the incident light, photocatalyst and reactant molecules. Herein, we report a new type of photocatalytic flow microreactor with an integrated light guide, formed by a channel fabricated inside a hydrophobic composite aerogel monolith made of silica and titania (TiO2). The liquid-filled channel simultaneously acts as a reaction vessel and as a liquid-core optofluidic waveguide, distributing the incident light over the whole reaction volume. Anatase TiO2 nanoparticles embedded in the channel walls then serve as a photocatalyst that can efficiently interact with both the guided light and the reactant solution along the channel length. Composite aerogels were synthesized with TiO2 content between 1 and 50 wt %, retaining their interconnected mesoporous network, low refractive index, and waveguide propagation losses below −3.9 dB/cm over this range of compositions. Using photocatalytic degradation of phenol – an organic compound with harmful environmental effects – as a model chemical reaction, the performance of the microreactor was systematically investigated. Reactant conversion was observed to increase with increasing incident light power, decreasing reactant flow rate and increasing mass fraction of TiO2 in the composite. An analytical model of the reactor/light guide system was developed that predicted successfully the scaling of the reactant conversion with the incident light power and reactant flow rate. The presented concept of aerogel-based optofluidic photocatalytic microreactors is readily scalable and possesses great potential for carrying out other photocatalytic reactions in both polar and non-polar solvents.

High-Aspect-Ratio SU-8-Based Optofluidic Device for Ammonia Detection in Cell Culture Media.

Miniaturization of sensing technology has led to the development of multifunctional micro total analysis systems (μTAS) that benefit from microfluidic technology. Optical sensing is one of the most commonly used sensing approaches integrated into μTAS devices and features high sensitivity and low detection limits. Different materials have been used for the fabrication of μTAS devices, each having their advantages and disadvantages. Herein, a high-aspect-ratio optofluidic waveguide fabricated from SU-8 is presented for the first time. The suitable optical properties and chemical inertness of SU-8 provide a durable device made by a flexible and cost-efficient fabrication process. The optofluidic device was used for colorimetric ammonia (NH3) sensing with a dynamic range of 3-70 μM, a detection limit of 2.5 μM, a response time of 8 min, and close to 10 times better analytical performance compared to using a standard microplate reader. The μTAS device was capable of monitoring NH3 accumulating in the cell culture media of prostatic epithelial cell (BPH-1) culture.

Dual-optofluidic waveguide in-line fiber biosensor for real-time label-free detection of interferon-gamma with temperature compensation.

Temperature cross-sensitivity is a long-standing challenge for most of the in-line fiber optofluidic waveguide biosensors. In this paper, we propose a dual-optofluidic waveguide antiresonant reflecting optical waveguide (ARROW) biosensor for the detection of interferon-gamma (IFN-γ) concentration with temperature compensation. Two Fabry-Perot resonators infiltrated with IFN-γ and NaCl were formed in a hollow core fiber, which generate two resonance dips based on the ARROW model. The optical biosensor for the detection of interferon-gamma (IFN-γ) has been a key research interest in recent years because IFN-γ is an important early biomarker for many serious human diseases. Based on the dual-optofluidic waveguide ARROW biosensor, the IFN-γ concentration can be measured through the modulation of the resonance condition of the ARROW, while the temperature fluctuation can be eliminated due to same thermo-optic coefficients of two infiltration liquids. The experimental results show that the response of the ARROW biosensor can be amplified significantly with the signal-enhanced streptavidin, and the limit of detection of 0.5 ng/ml can be achieved for the IFN-γ concentration. More importantly, the influence of the temperature could be compensated through the referenced resonance dip. The proposed fiber biosensor has a great potential for the real-time detection of IFN-γ concentrations in the fields of health monitoring, cancer prevention, biological engineering, etc.

A Tunable Optical Bragg Grating Filter Based on the Droplet Sagging Effect on a Superhydrophobic Nanopillar Array.

Nanostructures have been widely applied on superhydrophobic surfaces for controlling the wetting states of liquid microdroplets. Many modern optic devices including sensors are also integrated with micro- or nanostructures for function enhancement. However, it is rarely reported that both microfluidics and optics are compatibly integrated in the same nanostructures. In this paper, a novel microfluidic-controlled tunable filter composed of an array of periodic micro/nanopillars on top of a planar waveguide is proposed and numerically simulated, in which the periodic pillars endow both the Bragg grating and the superhydrophobic functions. The tunability of grating is achieved by controlling the sagging depth of a liquid droplet into the periodic pillars. Simulation results show that a narrow bandwidth of 0.4 nm and a wide wavelength tuning range over 25 nm can be achieved by such a microfluidic-based tunable optofluidic waveguide Bragg grating filter. Moreover, this proposed scheme can be easily modified as a refractive index sensor with a sensitivity of 103 nm per refractive index unit.

A new type of microphotoreactor with integrated optofluidic waveguide based on solid-air nanoporous aerogels.

In this study, we developed a new type of microphotoreactor based on an optofluidic waveguide with aqueous liquid core fabricated inside a nanoporous aerogel. To this end, we synthesized a hydrophobic silica aerogel monolith with a density of 0.22 g cm−3 and a low refractive index of 1.06 that—from the optical point of view—effectively behaves like solid air. Subsequently, we drilled an L-shaped channel within the monolith that confined both the aqueous core liquid and the guided light, the latter property arising due to total internal reflection of light from the liquid–aerogel interface. We characterized the efficiency of light guiding in liquid-filled channel and—using the light delivered by waveguiding—we carried out photochemical reactions in the channel filled with aqueous solutions of methylene blue dye. We demonstrated that methylene blue could be efficiently degraded in the optofluidic photoreactor, with conversion increasing with increasing power of the incident light. The presented optofluidic microphotoreactor represents a versatile platform employing light guiding concept of conventional optical fibres for performing photochemical reactions.

Real-time refractive-index sensing by using liquid core/liquid cladding optofluidic waveguide

In this study, an optofluidic refractive index (RI) sensor is realized by employing the liquid core/liquid cladding (L2) waveguide through measuring the output power of the waveguide. It is shown that the sensor is capable of measuring the RI changes with a dynamic range of 0.07 RIU from 1.342 to 1.411 which can cover a wide range of applications in the Lab-on-a-Chip technology. The maximum resolution of the sensor is 7 × 10−6 RIU.

Chemical reaction monitoring via the light focusing in optofluidic waveguides

This paper studies the light focusing phenomenon in optofluidic waveguides and uses it to monitor chemical reactions. Firstly, the relationship between the light focusing pattern and its contributing factors is investigated experimentally. Next, a characterization experiment is conducted to validate the use of light focusing pattern as an indicator of diffusion properties. The sensitivity and the limit-of-detection (LOD) are measured to be 1.54 μm/(μm2/s) and 3.93 × 10−12 m2/s in the over-mixed region, respectively. Then, the sucrose hydrolysis reaction is monitored using the proposed optofluidic method as a demonstration. The initial hydrolysis rate of this reaction is measured to be 19.62 μM/min, which agrees reasonably well with the reported value. Lastly, this method is extended to determine the diffusion coefficient of binary solutions. The diffusion coefficients of ethylene glycol and glycerol in water are measured to be 5.56 ± 0.12 × 10-10 and 7.01 ± 0.20 × 10-10 m2/s, respectively. This study demonstrates a new method for potential integrated biochemical sensing and paves the way for a broad range of sensing applications in microreactors, chemical synthesis, and quantification of biomolecular interactions.

Superhydrophobic waveguide: Liquid-core air-cladding waveguide platform for optofluidics

In this paper, we present an optofluidic waveguide platform consisting of liquid as a core material and air as cladding, enabled by using a superhydrophobic channel featured with hydrophobized high-aspect-ratio sharp-tip nanostructures. The contact of the liquid core with the superhydrophobic channel wall is minimized with an air layer retained between them so that the effective refractive index of the cladding layer is close to that of air. Thus, when light is introduced through the core liquid having a higher refractive index than that of the cladding air at the incident angle parallel to the channel direction less than a critical angle, it is reflected at the liquid-gas interface by the total internal reflection. When the cladding layer is filled with water (i.e., Wenzel state), the waveguide losses for the incident angles of 0 and 10° were ∼3.9 and ∼6.8 dB/cm, respectively. In contrast, when the cladding layer is retained with air (i.e., Cassie-Baxter state), the waveguide losses for the same incident angles were as low as ∼0.1 and ∼1.8 dB/cm, respectively. The significantly lowered waveguide losses at the Cassie-Baxter state indicate that superhydrophobic channels can provide the effective waveguide platform for optofluidics, exploiting the air layer as the cladding material.