In this work, for the first time, we visibly generated a lightning through nanoporous membrane in micro/nanofluidic platform. Firstly, a micro bubble was formed by Joule heating inside a microchannel with an application of electric field (E) of 100 V/cm. Secondly, vapor phase was changed into plasma phase and bright light was emitted at |E| = 500 V/cm. Finally, it was observed that emitted light propagated through the perm-selective nanoporous membrane along with strong cation flux. These findings would have a scientific significance for the visualization of cation trajectories inside the nanoporous membrane.

The MechanoBiology Institute prides itself having some of the fastest and most accurate optical tracking machinery for force sensing by deflection measurements of transparent polydimethyl­siloxane pillars. Optical tracking allows for long term in-vivo observation of dense (up to about one vector per square mm) force maps with low pN accuracy. This translates into hundreds of frames per second recording at better than 4nm localization. The convection of the medium is used to provide local cooling for the region where the pump light penetrates the biological material and the immersion medium itself must feature a low absorption of the tracking wavelengths in order to serve as a coolant.

The refractive index difference of the pillars from this medium provides both the necessary contrast mechanism as well as a noticeable distortion of the recorded optical image of those pillars. Scattering in the specimen itself, small devices brought into the specimen, or the small chamber above the specimen further harm the imaging of the pillars. We believe that – as contrast and artefact are generate by the same mechanism – that these distortions can be minimized but not entirely avoided.

We present some design steps to limit the optical distortion of the images and some image processing insights that allow for the discrimination of the pillar projection and scattering artifacts along the beam path.

As a result the useful range of these observations and the scope of where these observations are accurate are greatly expanded and allow for some simplifications of crucial cellular force sensing experiments.

Conventional optical microscopes are limited by the so-called diffraction limit and can resolve features of around half of the wavelength of illumination λ. Besides, as the light collection capability of a standard microscope objective is limited by its numerical aperture (NA), only relatively large objects can be detected because the scattered light intensity decreases with decreasing size. While a microsphere is known to act as a focusing microlens, when it has a refractive index contrast relative to the fluid medium less than 2:1 and a diameter between several to tens of λ, a highly-focused propagating beam from the shadow-side surface of the microsphere is generated due to constructive interference of the light field. This beam is termed as “photonic nanojet” and has a sub-λ full width at half-maximum (FWHM) transverse dimension and typically is several λ in length. Due to the properties of the photonic nanojet, microspheres can be used to detect nanoparticles and generate images of the objects with super-resolution.

The first part of this presentation introduces the fast detection of gold and fluorescent nanoparticles, down to 20 nm in size, during their motion in water-based medium in a microfluidic device by using a conventional bright-field or fluorescence microscope. A very interesting predicted property of a photonic nanojet is that the presence of a particle, much smaller than λ and positioned within the nanojet, significantly enhances backscattering of the light through the microsphere [1]. In this work, an array of dielectric microspheres is firstly positioned at the bottom of a microfluidic channel. The microspheres based on melamine resin are electrostatically self-assembled in a microwell array template on a glass substrate. These microspheres with high refractive index (n = 1.68) act as microlenses, focusing the illumination light originating from the microscope objective into photonic nanojets, which expose the fluid medium within the microfluidic channel (see Figure 1). When a nanoparticle in the medium is randomly transported through a nanojet, its back-scattered light (for a bare gold nanoparticle) or its fluorescent emission is strongly enhanced and instantaneously detected by video microscopy. The working principle of this technique and the test results on gold and fluorescent nanoparticles are shown in Figure 2, respectively. The experimental intensity is found to be proportional to the area occupied by the nanoparticle in the nanojet and can be used for size-dependent detection of the nano-objects in the medium. The further potential of this technique is also demonstrated by detecting immunocomplexes conjugated to the gold nanoparticles (see Figure 3). In future, this technique, which dynamically exploits the unique properties of a photonic nanojet, could evolve in a general tool to detect objects of environmental or biological importance, such as even smaller nanoparticles, viruses, other biological agents, or single molecules.

Alternatively, the use of dielectric microspheres on top of the objects can achieve near-field focusing and magnification, which in turn results in the capability to resolve features beyond the diffraction limit. Once the microspheres are placed on top of the sample object, they collect the underlying sample’s near-field nanofeatures and subsequently transform the near-field evanescent waves into far-field propagating waves, creating a magnified image in the far-field, which is collected by a conventional optical microscope. Our previous work demonstrated the potential of the technique by resolving the structure of fluorescently stained centrioles, mitochondria, chromosomes, and the mitochondrial encoded protein expression in a mouse liver cell line [2]. This work indicated that the development of the photonic nanojet is essential to the super-resolution imaging capability of a microsphere, however, the exact link remained unclear. In our recent study, the role of the photonic nanojet for super-resolution imaging is discussed in a quantitative way [3]. A numerical study of the light propagation through microspheres of different size using the finite element method (FEM) is performed. This allows characterizing the photonic nanojet at the rear-surface of a microsphere and relating the microsphere’s theoretical magnification factor to the light focusing capability of the photonic nanojet (see Figure 4). Furthermore, a systematic experimental study is performed, using microspheres with different sizes to image linear test nanostructures. The experimental magnification factor and the point spread function that is analytically determined from the images allow evaluating the resolution of the optical system, which is shown to directly correlate with the calculated properties of a microsphere’s photonic nanojet. In conclusion, due to these physical insights, dielectric microspheres will be increasingly used in the future, providing a straightforward and robust tool to be integrated with a conventional microscope for super-resolution optical microscopy. Recent study shows that this technique can be used for scanning microscopy, this will be briefly introduced in the talk.

Due to the demonstrated performance and future potential of using dielectric microspheres integrated with microfluidics for optical detection and imaging, this talk will be of high importance to the interdisciplinary research community formed by the attendees of Optofluidics2017.

The forming process of the lens is conducted in an automated CNC (Computer numerical control) machining process for designed aspherical lenses [1]. Even though the aspherical lens surface, such as freeform, can be manufactured by CNC method, the processes involved in making lenses are often expensive buying the CNC tools. In this paper, we present a simple method for making aspherical lenses in a PDMS mold by a 3D printer. A fabrication method has been developed to cast the Norland optical adhesive 65 (NOA65) film using a polydimethylsiloxane (PDMS) mold defined by a 3D printer. Therefore, we can duplicate the same shape lenses.

In Figure 1, we connect a plunger into a plastic syringe with the end cut-off. A 3D printed model [2] , which has a circular hole on a rectangular plate, is pasted on the flange extender. After the 3D printed model is immersed within PDMS, we push the plunger and inject the air into the PDMS. The captive bubble has the aspherical shape because the large density difference between the air and PDMS.

In the situation of fluid statics, we consider two cases: PDMS sessile drop on the solid substrate and sessile bubble on the 3D printed mold. We assume the tangent lines of the sessile bubble and the sessile drop have same angle with the horizontal line by choosing the proper 3D printed mold for the bubble and the proper substrate for the drop. Because the net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body, the bubble and the drop have the similar shapes when the bubble and the drop have the same surface tension. Here, we introduce the axisymmetric drop shape analysis for sessile drops [3] to predict the shape of the sessile bubble. Figure 2 shows the simulation results of Young-Laplace analytical approximation for capillary constant: 0 mm^-2(black line) and 0.4 mm^-2 (red line), with 180⁰ contact angle apex radius: (a) 2 mm, (b) 2.5 mm, (c) 3 mm. The shape becomes more aspherical when the apex radius increases.

Once the PDMS is hardened after a half day, we separate the PDMS mold from the 3D printed mold in Figure 3(a). In Figure 3(b), we fill the NOA 65 into the bubble space of the PDMS mold and put a microscopic slide on the mold. Then, we can use ultra-violet (UV) light to cure adhesive, and take out the lens to be a plano-convex lens. Because the adhesive will be hardened in about thirty minutes, we can prepare the PDMS molds with different bubble shapes in a laboratory and made the lenses in a very short time when we need to use them.

Figure 4 shows the different NOA 65 lenses from the molds under injecting different air volumes into the PDMS. In summary, smooth lenses with different diameters radius of curvatures can be fabricated using a mold made by a 3D printer.

A brand new design of slot-waveguide is proposed using a subwavelength hollow-core fiber, which enables light to be confined in an extremely small hollow-core with long interaction length. The slot-waveguide fiber (SWF) can be used in various applications such as modulators, directional couplers and multimode interference devices, and it has specific advantages in ultra-low volume optofluidic sensing compared with other waveguide structures. The guiding properties and optimization of the design to maximize its sensitive will be discussed.

Optomechanical resonators have been the subject of extensive research in a variety of fields, such as advanced sensing, communication and novel quantum technologies. We present our work towards the development of nano-optomechanical semiconductor disks as ultrasensitive mass sensors. In particular, we focus on one family of mechanical modes: the radial breathing modes. With micrometer radius disks, these modes possess high mechanical Q even in liquid (>10), low mass (pg) and high mechanical frequency (GHz) (see Figure 1). In this work, we develop novel analytical and numerical models in order to predict their capabilities as sensors (see Figure 2). Nano-optomechanical disks appear as probes of rheological information of unprecedented sensitivity and speed. Minimum mass detection of 14·10^-24 g, density changes of 2·10^-7 kg/m³ and viscosity changes of 5·10^-9 Pa·s, for 1s integration time, are extrapolated from our measurements in liquids (see Figure 3). While putting miniature disk fluidic sensors on a firm ground, our recent investigations also provide a solid picture of nano-optomechanical dissipation in liquids.

The use of multiple optomechanical cavities is essential to further improve their sensing capabilities, as it enlarges the sensing area while keeping their individual assets. Here we present new collective configurations where optomechanical disk resonators, each supporting its own localized optical and mechanical mode, are placed in a cascaded configuration and unidirectionally coupled through a common optical waveguide (see Figure 4). In collective configurations, overcoming fabrication imperfections and allowing spectral alignment of resonators is essential. Here we present a new simple and scalable tuning method to achieve this in a permanent manner. The method introduces an approach of cavity-enhanced photoelectrochemical (PEC) etching in a fluid. This resonant process is highly selective and allows controlling the resonator size with pm precision, well below the material’s interatomic distance. The technique is illustrated by finely aligning up to five resonators in liquid and two in air (see Figure 5). This technique opens the way of fabricating large networks of identical resonators. As an example of a possible application, we finally demonstrate the all-optical light-mediated locking of multiple spatially distant optomechanical oscillators (see Figure 6). We inject light simultaneously in all the resonators using a single laser, eventually locking their very high-frequency mechanical oscillations.

Coupled photonic cavities that give rise to symmetric and anti-symmetric resonant modes are well known in photonics. The resonant frequencies/wavelengths of such modes vary as functions of coupling strength. Integrated with nanoelectromechanical systems (NEMS) actuators, we have demonstrated various tunable photonic resonators, for example those tuned through varying their coupling gaps as well as through laterally shifting their center-to-center offsets. We demonstrated experimentally that large resonance wavelength shifts up to a few tens of nanometers could be achieved with small movements of the NEMS actuators. Alternatively, such coupled photonic cavities can also be configured as sensors, since nanoscale displacements can induce large resonance frequency shifts. As an example, we demonstrate here a resonant magnetic field sensor based on coupled nanophotonic cavities with one cavity fixed to the substrate and the other movable suspended by NEMS springs. An external magnetic field generates a Lorenz force, which deflects the movable cavity in this coupled-cavity system thereby changing the cavity coupling strength. Subsequently, this induces changes in the resonant frequencies of both symmetric and anti-symmetric supermodes. Through the detection of this frequency variation, the external magnetic field strength can be detected. Due to the small mode volume, high Q factor, and high optomechanical coupling coefficient, this mechanism may potentially lead to new highly sensitive, ultra-small sensors with wider bandwidths and thus faster response compared with existing MEMS magnetometers.

This paper reports a new design of optofluidic Fabry–Pérot (FP) micro cavity that combines the highest reported quality factor for an on-chip FP resonator that exceeds 3600, and the highest reported sensitivity for an on-chip volume refractometer based on a FP cavity that is about 1000 nm/RIU.

For using the optical resonator as a refractometer, it is convenient to have sharp and highly selective resonance peaks for accurate measurements; thereby the quality factor (Q) of the resonator is preferred to be high. The highest reported Q factor reported by other groups is only 400 [1]. This limitation comes from using straight mirror for the FP, which causes high diffraction loss due to beam expansion after few round trips. Our group has previously reported a cavity employing curved mirrors and a micro-tube in-between holding the analyte [2]. The curvature of the mirrors and the micro-tube achieved better light confinement and hence high Q factors up to 2,800. On the other hand, the sensitivity was only 428 nm/RIU since the analyte doesn’t fill the entire cavity. The highest reported sensitivity by literature was 907 nm/RIU in case of the analyte occupying the whole cavity [1].

In this work, a novel structure for FP micro-cavity is reported, achieving both high Q factor resonator and high sensitivity refractometer. The proposed structure is schematically depicted in Fig. 1. It employs cylindrical Bragg mirrors forming the FP cavity to confine light in the in-plane direction, while an external cylindrical lens - implemented by a fiber rod lens (FRL) - is used to confine light in the out-of-plane direction before it enters the FP cavity and after it exits to be efficiently collected by the collecting fiber. The cavities are fabricated from silicon by Deep Reactive Ion Etching (DRIE) process, and then capped by a PDMS cover. The FRLs are placed later after micro fabrication. A photo of the chip combining several cavities with different lengths is shown in Fig. 2. The analyte is passed between the mirrors enabling its detection from the shift of resonance peaks of the transmission spectra. The spectra are obtained by recording the output from an optical detector while varying the input light wavelength from a tunable laser in the near infrared band. The spectrum of a cavity of 318 µm physical length filled with DI-water is presented in Fig. 3 showing the narrow line widths of the resonance peaks. The peak has a linewidth of 0.44 nm, which provides a Q factor of 3649.

Mixtures of ethanol and DI-water with different ratios are used as analytes with different refractive indices to exploit the device as a micro-opto-fluidic refractometer, which are plotted in Fig. 4. The sensitivity is obtained from the slope of the linear plot in Fig. 5 of the resonance wavelength shift versus the difference in refractive index between the analytes and the DI-water, which is taken as a reference. The cavity used has a physical length of 256 µm and the obtained sensitivity is 1000 nm/RIU.

Optical techniques offer a powerful tool for both medical and biological applications. Many of these devices can achieve results without requiring biochemical reactions. Devices can use absorption, reflection, luminescence or fluorescence.

Waveguides can use  absorption in an evanescent wave, using a coating layer on the surface. Coating layers can be used which trap specific bacteria. The absorption increases with increasing levels of trapped bacteria. Oxygen saturation is measured through the absorption of two wavelengths. This simple technique is widely used in medical applications and can also be miniaturised to be integrated into catheter. Emission of light, whether through fluorescence or luminescence is also widely used in measurements. Markers can be used to attach to the cells which emit light when illuminated. Impregnated polymers can be used to generate fluorescence which is controlled  by the element of interest. An example of the is an oxygen sensor. This device measures oxygen partial pressure and can be applied to blood and tissue. It is also suitable for implantation. Parameter such as blood sugar can also be measured optically. Most of these devices require optical activation, but this can be achieved using LEDs integrated into the device.

Optical techniques can be used to measure a wide range of body parameters and also bacteria and viruses.

This paper will present the different optical approaches, and the types of structures which can be made. A description will also be given for the fabrication technologies and finally examples will be given of devices for the different application.

Drinking water quality is critical for all countries while a more challenging problem in many developing countries. In the worldwide well accepted standard for drinking water quality, setup by U.S. Environmental Protection Agency (EPA), a single harmful bacterium cannot be allowed in tap water. However, the EPA recommended standard testing method is still based on the conventional approach of antibody-bacterium conjugation. An immunological detection method, such as ELISA, operated in a biological lab, needs a skillful operator with a cycle taking about 24 hours. It certainly cannot meet the requirements of today’s fast changing living environment. Especially currently we are very concern about food, water and air poisoning and contamination. Thus rapid identification of bacterial species in single bacterium level becomes very crucial.

The measurement of physical parameters, the length, width, the ratio of them and the refractive index of bacteria species were reported by Liu et al. with the approach of refractometry [1]. Our presentation reports latest efforts in investigation of optical scattering patterns of single bacterium of three species, namely E. coli, Shigella flexneri, and Bacillus subtilis under acoustic focusing. These optical scattering patterns were obtained with a customized a prototype system including a customized laser illumination module and a scattering pattern recording module. Representative scattering patterns of a single bacterium flowing through a microfluidic channel are displayed in Fig. 1. As their average physical dimensions still have some differences though having overlapping, their scattering patterns also have differences. But low signal to noise ratio and size non-uniformity of bacteria of the same species, as well as waterborne impurity microbial from 0.8 mm to 3 mm make the optical patterns not reliable for bacterial identification.

We have also measured four strains of E. coli with their scattering patterns are displayed in Fig. 2 which show almost identical scattering patterns. Thus, it is concluded that bacteria species and strains cannot be identified reliably from only physical parameters mentioned above. These parameters can enrich our knowledge to bacteria species in different time points of their life cycle, but not enough to distinguish them in single bacterium level.

To address this problem, we have proposed a fluo-scattering detection system with a schematic drawing is shown in Fig. 3. The approach combines the fluorescence detection of immuno-labeling of bacterium with specific antibody, like what illustrated in Fig. 4, with the optical scattering module to increase the identification efficiency. Fig. 5 shows our preliminary result of differentiation of E. coli (O114) labeled with SyBr gold and 2 mm polystyrene beads. While Fig. 6 shows differentiation of Bacillus spore with unknown waterborne impurity microbial.

With more fluorescence detection channels are built in the fluo-scattering detection system, it has high potential to be able to differentiation 3 to 4 species/strains of bacteria.

REFERENCES:

• Y. Liu, L. K. Chin, W. Ser, T. C. Ayi, P. H. Yap, T. Bourouina and Y. Leprince-Wang, “An optofluidic imaging to measure the biophysical signature of single waterborne bacterium,” Lab on a Chip, Vol. 14, pp. 4237-4243 (2014).

ACKNOWLEDGEMENT:

This work was supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Programme (1102-IRIS-05-02 and 1102-IRIS-05-05).