Ahmad Ahsan Nawaz

Tunable nanowire patterning using standing surface acoustic waves.

Patterning of nanowires in a controllable, tunable manner is important for the fabrication of functional nanodevices. Here we present a simple approach for tunable nanowire patterning using standing surface acoustic waves (SSAW). This technique allows for the construction of large-scale nanowire arrays with well-controlled patterning geometry and spacing within 5 s. In this approach, SSAWs were generated by interdigital transducers, which induced a periodic alternating current (ac) electric field on the piezoelectric substrate and consequently patterned metallic nanowires in suspension. The patterns could be deposited onto the substrate after the liquid evaporated. By controlling the distribution of the SSAW field, metallic nanowires were assembled into different patterns including parallel and perpendicular arrays. The spacing of the nanowire arrays could be tuned by controlling the frequency of the surface acoustic waves. Additionally, we observed 3D spark-shaped nanowire patterns in the SSAW field. The SSAW-based nanowire-patterning technique presented here possesses several advantages over alternative patterning approaches, including high versatility, tunability, and efficiency, making it promising for device applications.

Standing surface acoustic wave (SSAW)-based microfluidic cytometer

The development of microfluidic chip-based cytometers has become an important area due to their advantages of compact size and low cost. Herein, we demonstrate a sheathless microfluidic cytometer which integrates a standing surface acoustic wave (SSAW)-based microdevice capable of 3D particle/cell focusing with a laser-induced fluorescence (LIF) detection system. Using SSAW, our microfluidic cytometer was able to continuously focus microparticles/cells at the pressure node inside a microchannel. Flow cytometry was successfully demonstrated using this system with a coefficient of variation (CV) of less than 10% at a throughput of ~1000 events s(-1) when calibration beads were used. We also demonstrated that fluorescently labeled human promyelocytic leukemia cells (HL-60) could be effectively focused and detected with our SSAW-based system. This SSAW-based microfluidic cytometer did not require any sheath flows or complex structures, and it allowed for simple operation over a wide range of sample flow rates. Moreover, with the gentle, bio-compatible nature of low-power surface acoustic waves, this technique is expected to be able to preserve the integrity of cells and other bioparticles.

Beam bending via plasmonic lenses.

We have designed and characterized three different types of plasmonic lenses that cannot only focus, but can also bend electromagnetic (EM) waves. The bending effect is achieved by constructing an asymmetric phase front caused by varying phase retardations in EM waves as they pass through a plasmonic lens. With an incident wave normal to the lens surface, light bends up to 8° off the axial direction. The optical wave propagation was numerically investigated using the finite-difference time-domain (FDTD) method. Simulation results show that the proposed plasmonic lenses allow effective beam bending under both normal and tilted incidence. With their relatively large bending range and capability to perform in the far field, the plamsonic lenses described in this article could be valuable in applications such as photonic communication and plasmonic circuits.

An acoustofluidic micromixer via bubble inception and cavitation from microchannel sidewalls.

During the deep reactive ion etching process, the sidewalls of a silicon mold feature rough wavy structures, which can be transferred onto a polydimethylsiloxane (PDMS) microchannel through the soft lithography technique. In this article, we utilized the wavy structures of PDMS microchannel sidewalls to initiate and cavitate bubbles in the presence of acoustic waves. Through bubble cavitation, this acoustofluidic approach demonstrates fast, effective mixing in microfluidics. We characterized its performance by using viscous fluids such as poly(ethylene glycol) (PEG). When two PEG solutions with a resultant viscosity 54.9 times higher than that of water were used, the mixing efficiency was found to be 0.92, indicating excellent, homogeneous mixing. The acoustofluidic micromixer presented here has the advantages of simple fabrication, easy integration, and capability to mix high-viscosity fluids (Reynolds number: ∼0.01) in less than 100 ms.

A droplet-based, optofluidic device for high-throughput, quantitative bioanalysis

Analysis of chemical or biomolecular contents in a tiny amount of specimen presents a significant challenge in many biochemical studies and diagnostic applications. In this work, we present a single-layer, optofluidic device for real-time, high-throughput, quantitative analysis of droplet contents. Our device integrates an optical fiber-based, on-chip detection unit with a droplet-based microfluidic unit. It can quantitatively analyze the contents of individual droplets in real-time. It also achieves a detection throughput of 2000 droplets per second, a detection limit of 20 nM, and an excellent reproducibility in its detection results. In a proof-of-concept study, we demonstrate that our device can be used to perform detection of DNA and its mutations by monitoring the fluorescent signal changes of the target DNA/molecular beacon complex in single droplets. Our approach can be immediately extended to a real-time, high-throughput detection of other biomolecules (such as proteins and viruses) in droplets. With its advantages in throughput, functionality, cost, size, and reliability, the droplet-based optofluidic device presented here can be a valuable tool for many medical diagnostic applications.

Single-Shot Characterization of Enzymatic Reaction Constants Km and kcat by an Acoustic-Driven, Bubble-Based Fast Micromixer

In this work we present an acoustofluidic approach for rapid, single-shot characterization of enzymatic reaction constants K(m) and k(cat). The acoustofluidic design involves a bubble anchored in a horseshoe structure which can be stimulated by a piezoelectric transducer to generate vortices in the fluid. The enzyme and substrate can thus be mixed rapidly, within 100 ms, by the vortices to yield the product. Enzymatic reaction constants K(m) and k(cat) can then be obtained from the reaction rate curves for different concentrations of substrate while holding the enzyme concentration constant. We studied the enzymatic reaction for β-galactosidase and its substrate (resorufin-β-D-galactopyranoside) and found K(m) and k(cat) to be 333 ± 130 μM and 64 ± 8 s(-1), respectively, which are in agreement with published data. Our approach is valuable for studying the kinetics of high-speed enzymatic reactions and other chemical reactions.

Optofluidic tunable microlens by manipulating the liquid meniscus using a flared microfluidic structure

We have designed, demonstrated, and characterized a simple, novel in-plane tunable optofluidic microlens. The microlens is realized by utilizing the interface properties between two different fluids: CaCl(2)solution and air. A constant contact angle of ∼90° is the pivotal factor resulting in the outward bowing and convex shape of the CaCl(2) solution-air interface. The contact angle at the CaCl(2) solution-air interface is maintained by a flared structure in the polydimethylsiloxane channel. The resulting bowing interface, coupled with the refractive index difference between the two fluids, results in effective in-plane focusing. The versatility of such a design is confirmed by characterizing the intensity of a traced beam experimentally and comparing the observed focal points with those obtained via ray-tracing simulations. With the radius of curvature conveniently controlled via fluid injection, the resulting microlens has a readily tunable focal length. This ease of operation, outstandingly low fluid usage, large range tunable focal length, and in-plane focusing ability make this lens suitable for many potential lab-on-a-chip applications such as particle manipulation, flow cytometry, and in-plane optical trapping.

High contrast modulation of plasmonic signals using nanoscale dual-frequency liquid crystals.

We have designed and simulated a dual-frequency liquid crystal (DFLC) based plasmonic signal modulator capable of achieving over 15 dB modulation depth. The voltage-controlled DFLC is combined with a groove and slit configuration and its operation is discussed. Using the finite-difference time domain (FDTD) method, simulations were conducted to discover the groove-slit separation distance that enabled a practically useful modulation depth for the two states of the DFLC. Moreover, we have shown that significant improvement in modulation depth can be achieved by addition of a second groove to the design structure. Additionally, a performance analysis indicates a switching energy on the order of femtojoules and a switching speed on the order of 100 microseconds. Results of this investigation can be useful for the future design, simulation, and fabrication of DFLC-based plasmonic signal modulating devices, which have application in electro-optical and all-optical information systems.

Nanoscale super-resolution imaging via a metal–dielectric metamaterial lens system

We have proposed a method for super-resolution imaging using an interlayer cascaded structure comprising two metamaterial lenses. The metamaterial lenses are designed using the effective medium theory. The lens structures consist of two different planar dielectric films alternated with similar thin metallic films, making a diverging and converging lens. With this two-lens system, an image is formed at the output surface of the lens with subwavelength resolution. We have shown, through numerical simulations and an analytical approach, that an image with resolution nine times smaller than the light wavelength (365 nm) is achievable with this metamaterial lens system. The loss during transmission through the lens system is smaller compared with the hyperlens configuration with a similar design.

Mechanically Tuning the Localized Surface Plasmon Resonances of Gold Nanostructure Arrays

We report on chemical etching of ordered Au nanostructure arrays to continuously tune their localized surface plasmon resonances (LSPR). Real-time extinction spectra were recorded from both Au nanodisks and nanospheres immobilized on glass substrates when immersed in Au etchant. The time-dependent LSPR frequencies, intensities, and bandwidths were studied theoretically with discrete dipole approximations and the Mie solution, and they were correlated with the evolution of the etched Au nanostructures’ morphology (as examined by atomic force microscopy). Since this chemical etching method can conveniently and accurately tune LSPR, it offers precise control of plasmonic properties and can be useful in applications such as surface-enhanced Raman spectroscopy and molecular resonance spectroscopy.