Mengqian Lu

Dynamic Tuning of Plasmon–Exciton Coupling in Arrays of Nanodisk–J‐aggregate Complexes

Y.B.Z and B.K.J contributed equally to this work. This research was supported by the Air Force Office of Scientific Research (FA9550-08-1-0349), the National Science Foundation (ECCS-0801922, ECCS-0609128, and ECCS-0609128), and the Penn State Center for Nanoscale Science (MRSEC). Components of this work were conducted at the Pennsylvania State University node of the NSF-funded National Nanotechnology Infrastructure Network. Y.B.Z. recognizes the support from KAUST Scholar Award and the Founders Prize and Grant of the American Academy of Mechanics. The authors thank I-Kao Chiang, Aitan Lawit and Thomas R. Walker for helpful discussions.

Light-driven tunable dual-band plasmonic absorber using liquid-crystal-coated asymmetric nanodisk array

We experimentally demonstrated a light-driven reconfigurable near perfect plasmonic absorber working at dual frequencies in infrared range. By employing nanodisks with different sizes in certain arrangement, near perfect absorption of incident electromagnetic waves can be achieved for different working frequencies due to the resonance between the incident light and the nanodisk of different sizes. We showed that optically induced changes in the dielectric constant of the adjacent liquid crystal layer is an effective means to tune the absorption bands of an asymmetric gold nanodisk array. Our liquid crystal based infrared plasmonic absorber can be tuned by using visible light in real time. A tunable range of 25 nm has been confirmed by both simulation and experiment.

Selectively manipulable acoustic-powered microswimmers

Selective actuation of a single microswimmer from within a diverse group would be a first step toward collaborative guided action by a group of swimmers. Here we describe a new class of microswimmer that accomplishes this goal. Our swimmer design overcomes the commonly-held design paradigm that microswimmers must use non-reciprocal motion to achieve propulsion; instead, the swimmer is propelled by oscillatory motion of an air bubble trapped within the swimmers polymer body. This oscillatory motion is driven by the application of a low-power acoustic field, which is biocompatible with biological samples and with the ambient liquid. This acoustically-powered microswimmer accomplishes controllable and rapid translational and rotational motion, even in highly viscous liquids (with viscosity 6,000 times higher than that of water). And by using a group of swimmers each with a unique bubble size (and resulting unique resonance frequencies), selective actuation of a single swimmer from among the group can be readily achieved.

Scalable Manufacturing of Plasmonic Nanodisk Dimers and Cusp Nanostructures using Salting-out Quenching Method and Colloidal Lithography

Localization of large electric fields in plasmonic nanostructures enables various processes such as single-molecule detection, higher harmonic light generation, and control of molecular fluorescence and absorption. High-throughput, simple nanofabrication techniques are essential for implementing plasmonic nanostructures with large electric fields for practical applications. In this article we demonstrate a scalable, rapid, and inexpensive fabrication method based on the salting-out quenching technique and colloidal lithography for the fabrication of two types of nanostructures with large electric field: nanodisk dimers and cusp nanostructures. Our technique relies on fabricating polystyrene doublets from single beads by controlled aggregation and later using them as soft masks to fabricate metal nanodisk dimers and nanocusp structures. Both of these structures have a well-defined geometry for the localization of large electric fields comparable to structures fabricated by conventional nanofabrication techniques. We also show that various parameters in the fabrication process can be adjusted to tune the geometry of the final structures and control their plasmonic properties. With advantages in throughput, cost, and geometric tunability, our fabrication method can be valuable in many applications that require plasmonic nanostructures with large electric fields.

Shape-controlled synthesis of hybrid nanomaterials via three-dimensional hydrodynamic focusing.

Shape-controlled synthesis of nanomaterials through a simple, continuous, and low-cost method is essential to nanomaterials research toward practical applications. Hydrodynamic focusing, with its advantages of simplicity, low-cost, and precise control over reaction conditions, has been used for nanomaterial synthesis. While most studies have focused on improving the uniformity and size control, few have addressed the potential of tuning the shape of the synthesized nanomaterials. Here we demonstrate a facile method to synthesize hybrid materials by three-dimensional hydrodynamic focusing (3D-HF). While keeping the flow rates of the reagents constant and changing only the flow rate of the buffer solution, the molar ratio of two reactants (i.e., tetrathiafulvalene (TTF) and HAuCl4) within the reaction zone varies. The synthesized TTF–Au hybrid materials possess very different and predictable morphologies. The reaction conditions at different buffer flow rates are studied through computational simulation, and the formation mechanisms of different structures are discussed. This simple one-step method to achieve continuous shape-tunable synthesis highlights the potential of 3D-HF in nanomaterials research.

Acoustofluidic chemical waveform generator and switch.

Eliciting a cellular response to a changing chemical microenvironment is central to many biological processes including gene expression, cell migration, differentiation, apoptosis, and intercellular signaling. The nature and scope of the response is highly dependent upon the spatiotemporal characteristics of the stimulus. To date, studies that investigate this phenomenon have been limited to digital (or step) chemical stimulation with little control over the temporal counterparts. Here, we demonstrate an acoustofluidic (i.e., fusion of acoustics and microfluidics) approach for generating programmable chemical waveforms that permits continuous modulation of the signal characteristics including the amplitude (i.e., sample concentration), shape, frequency, and duty cycle, with frequencies reaching up to 30 Hz. Furthermore, we show fast switching between multiple distinct stimuli, wherein the waveform of each stimulus is independently controlled. Using our device, we characterized the frequency-dependent activation and internalization of the β2-adrenergic receptor (β2-AR), a prototypic G-protein coupled receptor (GPCR), using epinephrine. The acoustofluidic-based programmable chemical waveform generation and switching method presented herein is expected to be a powerful tool for the investigation and characterization of the kinetics and other dynamic properties of many biological and biochemical processes.

Microfluidic hydrodynamic focusing for synthesis of nanomaterials

Microfluidics expands the synthetic space such as heat transfer, mass transport, and reagent consumption to conditions not easily achievable in conventional batch processes. Hydrodynamic focusing in particular enables the generation and study of complex engineered nanostructures and new materials systems. In this review, we present an overview of recent progress in the synthesis of nanostructures and microfibers using microfluidic hydrodynamic focusing techniques. Emphasis is placed on distinct designs of flow focusing methods and their associated mechanisms, as well as their applications in material synthesis, determination of reaction kinetics, and study of synthetic mechanisms.

Self-Powered Glucose-Responsive Micropumps

A self-powered polymeric micropump based on boronate chemistry is described. The pump is triggered by the presence of glucose in ambient conditions and induces convective fluid flows, with pumping velocity proportional to the glucose concentration. The pumping is due to buoyancy convection that originates from reaction-associated heat flux, as verified from experiments and finite difference modeling. As predicted, the fluid flow increases with increasing height of the chamber. In addition, pumping velocity is enhanced on replacing glucose with mannitol because of the enhanced exothermicity associated with the reaction of the latter.

Beam aperture modifier and beam deflector using gradient-index photonic crystals

We designed and simulated a beam aperture modifier and a beam deflector using two-dimensional parabolic gradient-index (GRIN) photonic crystals (PCs). The GRIN PCs are composed of dielectric columns with graded radii along the direction transverse to propagation. Both finite-difference time-domain methods and gradient optics analytical solutions were used to characterize the change in beam width and propagation direction. Multifunctional GRIN PCs combining both beam aperture modification and beam deflection were also designed and simulated. These GRIN PC based designs can be used as optical connectors and bidirectional waveguide couplers in applications such as miniaturized photonic integrated circuits.

Acoustofluidic relay: sequential trapping and transporting of microparticles via acoustically excited oscillating bubbles.

We report an on-chip acoustofluidic method for sequential trapping and transporting of microparticles via acoustically oscillating bubbles. The size and location of bubbles were precisely controlled by lithography. When the acoustic waves were turned off, particles followed the streamlines dictated by laminar flow. When the acoustic waves were turned on, particles were attracted to and trapped in a vortex near the surface of bubble. Therefore, particles could move across the microfluidic channel with programmed trajectories. Additionally, a theoretical model based on acoustic radiation force and drag force due to acoustic microstreaming was established to help design this particle-trapping and -transporting system.