Browsing by Subject "Metasurface"

Fluorescence-based methodologies have been used extensively for biosensing and to analyze molecular dynamics and interactions. An emerging, promising diagnostic tool are fluorescence-based microarrays due to their high throughput, small sample volume and multiplexing capabilities. However, their low fluorescence output has limited their implementation for in vitro diagnostics applications in a point-of-care (POC) setting. Here, by integration of a sandwich immunoassay microarray within a plasmonic nanogap metasurface, we demonstrate strongly enhanced fluorescence enabling readout by a fluorescence microarray even at low sensitivities. We have named this plasmonic architecture the plasmonically enhanced D4 (PED4) assay. The immunoassay consists of inkjet-printed capture and fluorescently labeled detection antibodies on a polymer brush which is grown on a gold film. Colloidally synthesized silver nanocubes (SNCs) are placed on top of the brush through a polyelectrolyte layer and interacts with the underlying gold film creating a nanogap plasmonic structure supporting local electromagnetic field enhancements of ~100-fold. By varying the thickness of the brush between 5 and 20 nm, a 151-fold increase in fluorescence and a 14-fold improvement in the limit-of-detection (LOD) is observed for the cardiac biomarker B-type natriuretic peptide (BNP) compared to the unenhanced assay, paving the way for a new generation of point-of-care clinical diagnostics.

To move the PED4 towards a single step point of care test (POCT), SNCs are conjugated with a secondary antibody that attaches specifically to the detection antibody. This allows SNCs to deposit on the surface without the need of a charged polyelectrolyte layer. In addition, multiplexing capabilities are demonstrated in this plasmonic platform where NT-proBNP, Galectin-3, and NGAL are simultaneously detected and fluorescently enhanced. Microfluidics integration and use of a low-cost detector is also demonstrated.

Metamaterials are artificially engineered materials or structures that exhibit exotic properties that are not found in nature. They have been serving as a primary approach to fully control the behavior of electromagnetic waves, acoustic waves and elastic waves in recent years, and is at present a highly active research area. Metasurfaces, as the 2D version of metamaterials, have opened up unprecedented possibilities for controlling waves at will, offering a solution of molding wave propagation within a thin sheet of structures. Most metasurface designs are based on the so-called generalized Snell's Law (GSL) which achieves their functionalities by engineering the local phase shift in the unit cells. However, the efficiency of phase-gradient metasurfaces is fundamentally limited by the impedance mismatch and local porer flow mismatch between incident field and reflected/transmitted field, so that part of the energy is scattered into unwanted higher-order diffracted modes, which hinders the applicability in various scenarios. In this work, we approach these issues by exploiting acoustic bianisotropy (Willis coupling for acoustics) as an additional degree of freedom to control waves. We have explored highly efficient wavefront engineering in airborne acoustics, from manipulating simple plane waves and cylindrical harmonics to more complicated fields and finally, arbitrary wavefronts. Then we extended the application of bianisotropic metasurfaces to general impedance matching problems and demonstrated wavefront engineering in underwater acoustics with two examples: an aberration-layer penetration metasurface and a 3D acoustic tweezer.

This dissertation provides a summary of the work undertaken to achieve highly efficient and functional wavefront engineering devices, and briefly outlines some objectives for future work. Firstly, we designed an acoustic bianisotropic unit cell with full control over its scattering properties and demonstrated bianisotropic metasurfaces that overcome the fundamental limits of phase-gradient based metasurfaces. Second, we mapped the approach from Cartesian coordinates into cylindrical coordinates and demonstrated the generation of a pure field with high angular momentum. Third, we introduced surface waves to help power redistribution along the metasurface and achieved highly-efficient beam splitting and reflection. Forth, we further introduced the power-flow conformal metasurface to meet the power balance requirements for an arbitrary perfect wavefront transformation. Then we extended the application of bianisotropic metasurfaces and proposed a general impedance matching strategy, and demonstrated the idea with a case of aberration-layer penetration in water. Last but not least, by shaping the wavefront of underwater ultrasound, a 3D acoustic tweezer is demonstrated for manipulating a wide range of particles in a contact-less manner.

Nonlinear generation of optical fields has enabled many exciting breakthroughs in light science such as nonlinear imaging, all-optical switching and supercontinuum generation. The intrinsic nonlinear response from bulk materials is extremely weak and phase matching conditions need to be satisfied for efficient generation. Plasmonic structures have proven to be a promising platform to investigate nonlinear optics due to the capability to enhance and localize electromagnetic fields within subwavelength volumes beyond the diffraction limit. However, the origin of the large nonlinear response observed in the plasmonic structures is not fully understood and the investigations of nonlinear processes involving multiple excitation wavelengths are limited. Furthermore, simultaneous enhancement and precise manipulation of multiple nonlinear optical processes have not been experimentally demonstrated.

In this dissertation, I describe a specific film-coupled plasmonic nanogap cavity structure consisting of arrays of nanoparticles separated from a metallic ground plane by an ultrathin dielectric layer. Polarization-dependent, dual-band and spatially-overlapped resonances are obtained with arrays of rectangle nanoparticles where the two resonances can be tuned independently. Highly efficient third harmonic generation (THG) is achieved by integrating dielectric materials in plasmonic nanogap cavities, resulting in more than six orders of magnitude enhancement in the THG response compared with a bare gold film. Utilizing comprehensive spectral analysis and finite-element simulation, it is concluded that the main contributing nonlinear source is dielectric material in the gap. Furthermore, I demonstrate simultaneous enhancement of three nonlinear responses from THG, sum frequency generation (SFG) and four wave mixing (FWM) by integrating 1-7 nm Al2O3 layer in the nanocavities formed by a gold ground plane and silver nanorectangles. Enhancement up to 106-fold for both THG and FWM and 104-fold for SFG is achieved when the excitation wavelengths overlap with the resonance wavelengths from transverse and longitudinal modes of the nanorectangles. Precise control of the relative strength of these nonlinear responses is demonstrated either actively by varying the ratio between excitation powers or passively by changing the Al2O3 gap thickness. Moreover, a metasurface-based efficient frequency mixer is realized utilizing diamond and a novel polymer transfer process is employed for creating nanoparticle arrays. This new insight into the nonlinear response in ultrathin gaps between metals is expected to be promising for both the fundamental understanding of nonlinear optics at the deep nanoscale and efficient on-chip nonlinear devices such as ultrafast optical switching and entangled photon sources. The capability to precisely manipulate nonlinear optical processes at the nanoscale could find important applications for nonlinear imaging and quantum communication.