Plasmonic Structures for Subwavelength Guiding and Enhanced Light-Matter Interactions

In this dissertation we design and analyze nanostructures for subwavelength guiding and enhanced light-matter interactions. We first investigate three-dimensional plasmonic waveguide-cavity structures, built by side-coupling stub resonators that consist of plasmonic coaxial waveguides of finite length, to a plasmonic coaxial waveguide. These structures are capable of guiding and manipulating light in deep-subwavelength volumes. We show that three-dimensional plasmonic coaxial waveguides offer a platform for practical realization of deep-subwavelength optical waveguides. We then introduce compact wavelength-scale slit-based structures for coupling free space light into the fundamental mode of plasmonic coaxial waveguides. We consider single-, double-, and triple-slit structures optimized at the optical communication wavelength and find that, when the slits are at resonance, the coupling to the plasmonic coaxial waveguide increases. We also investigate slit-based outcoupling structures for light extraction from the waveguide into free space. We also numerically design and experimentally test a SERS-active substrate for enhancing the SERS signal of a single layer of graphene in water. The graphene is placed on top of an array of silver-covered nanoholes in a polymer and is covered with water. We report a large enhancement in the SERS signal of the graphene on the patterned plasmonic nanostructure for a 532 nm excitation wavelength. We find that the enhancement is due to the increase in the confinement of electromagnetic fields on the location of graphene that results in enhanced light absorption in graphene at the excitation wavelength. We also find that water droplets increase the density of optical radiative states at the location of graphene, leading to enhanced spontaneous emission rate of graphene. Finally, we introduce a structure for near total absorption in a graphene monolayer at visible wavelengths. The optical interaction of graphene with local fields is enhanced by means of critical coupling. The graphene monolayer is placed on a grating slab without being covered with other structures, so the quality of graphene remains intact. We investigate the enhanced light-graphene interactions in this structure. We use experimental data for the dielectric permittivity of the materials used in the structure. The structure could find applications in the design of efficient nanoscale photodetectors and modulators

Plasmonic Optical Sensors: Performance Analysis and Engineering Towards Biosensing

Surface plasmon resonance (SPR) sensing for quantitative analysis of chemical reactions and biological interactions has become one of the most promising applications of plasmonics. This thesis focuses on performance analysis for plasmonic sensors and implementation of plamonic optical sensors with novel nanofabrication techniques. A universal performance analysis model is established for general two-dimensional plasmonic sensors. This model is based on the fundamental facts of surface plasmon theory. The sensitivity only depends on excitation light wavelength as well as dielectric properties of metal and dielectrics. The expression involves no structure-specified parameters, which validates this formula in broad cases of periodic, quasiperiodic and aperiodic nanostructures. Further analysis reveals the intrinsic relationship between plamonic sensor performance and essential physics of surface plasmon. The analytical results are compared to the sensitivities of previously reported plasmonic sensors in the field. This universal model is a promising qualification criterion for plasmonic sensors. Plasmonic optical sensors are engineered into high-performance on-chip sensors, plasmonic optical fibers and freestanding nanomembranes. (1) Periodic nanohole arrays are patterned on chip by a simple and robust template-transfer approach. A spectral analysis approach is also developed for improving the sensor performance. This sensor is applied to demonstrate the on-chip detection of cardiac troponin-I. (2) Plasmonic optical fibers are constructed by transferring periodic metal nanostructures from patterned templates onto endfaces of optical fibers using an epoxy adhesive. Patterned metal structures are generally extended from nanohole arrays to nanoslit arrays. A special plasmonic fiber is designed to simultaneously implement multimode refractive index sensing with remarkably narrow linewidth and high figure of merit. A real-time immunoassay relying on plasmonic fiber is demonstrated. Plasmonic optical fibers also take advantages of consistent optical responses, excellent stability during fiber bending and capability of spectrum filtering. (3) Large-area freestanding metal nanomembranes are implemented using a novel fabrication approach. The formed transferrable membranes feature high-quality and uniform periodic nanohole arrays. The freestanding nanomembranes exhibit remarkably higher transmission intensity in comparison to the nanohole arrays with same features on the substrate. These three modalities of plasmonic sensors possess different applicability to fulfill various plasmonic sensing tasks in respective scenarios

Sequential trapping of single nanoparticles using a gold plasmonic nanohole array

We have used a gold nanohole array to trap single polystyrene nanoparticles, with a mean diameter of 30 nm, into separated hot spots located at connecting nanoslot regions. A high trap stiffness of approximately 0.85  fN/(nm·mW) at a low-incident laser intensity of ∼0.51  mW/μm2 at 980 nm was obtained. The experimental results were compared to the simulated trapping force, and a reasonable match was achieved. This plasmonic array is useful for lab-on-a-chip applications and has particular appeal for trapping multiple nanoparticles with predefined separations or arranged in patterns in order to study interactions between them

Optofluidic plasmonic onchip nanosensor array for biodetection

Thesis (Ph.D.)--Boston UniversitySurface plasmon resonance (SPR) sensing has been demonstrated in the past decade to be the gold standard technique for biochemical interaction analysis, and plays an important role in drug discovery and biomedical research. The technique circumvents the need of fluorescence/radioactive tagging or enzymatic detection, enables ultrasensitive remote sensing, and quantitatively monitors bio-interaction in real time. Although SPR has these attractive features that can satisfy most research/clinic requirements, there still exist problems that limit its applications. First, the reflection geometry of the prism coupling scheme adds limitations for high throughput screening application. Additionally, SPR instrumentations are bulky and not suitable for point-of-care settings. Moreover, the SPR sensor is embedded in conventional micro-fluidic cells, in which the sensor performance is limited by inefficient analyte transport. Suspended plasmonic nanohole array (PNA) offers an opportunity to overcome these limitations. A collinear excitation/collection coupling scheme combined with the small footprint of PNA provides unique platform for multiplexing and system minimization. The suspended nanohole structure also offers a unique configuration to integrate nano-photonics with nano-fluidics. This thesis focuses on developing a lab-on-a-chip PNA platform for point-of-care bio-detection. To achieve this, we first demonstrate that the figure-of-merit of our PNA sensor surpasses that of the prism coupled SPR. We also show that the ultrasensitive label-free PNA sensor is able to directly detect intact viruses from biological media at clinically relevant concentrations with little sample preparation. We then present a plasmonic microarray with over one million PNA sensors on a microscope slide for high throughput screening applications. A dual-color filter imaging method is introduced to increase the accuracy, reliability, and signal-to-noise ratio in a highly multiplexed manner. Finally, we present a nanoplasmonic-nanofluidic platform enabling active delivery of analyte to the sensor. Sensor response time is reduced by an order of magnitude compared to the conventional flow scheme. A dynamic range spanning 5 orders of magnitude from 10^3 to 10^7 particles/mL is shown on this platform corresponding to analyte concentration sufficient for clinical applications. The proposed approach opens up opportunities of a lab-on-a-chip bio-detection system for drug screening, disease diagnostic as well as clinic studies

Scalable trapping of single nanosized extracellular vesicles using plasmonics

Heterogeneous nanoscale particles released by cells known as extracellular vesicles (EVs) are actively investigated for early disease detection1, monitoring2, and advanced therapeutics3. Due to their extremely small size, the stable trapping of nano-sized EVs using diffraction-limited optical tweezers4 has been met with challenges. Plasmon-enhanced optical trapping can confine light to the nanoscale to generate tight trapping potentials. Unfortunately, a long-standing challenge is that plasmonic tweezers have limited throughput and cannot provide rapid delivery and trapping of particles at plasmonic hotspots while precluding the intrinsic plasmon-induced photothermal heating effect at the same time. We report our original geometry-induced electrohydrodynamic tweezers (GET) that generate multiple electrohydrodynamic potentials for the parallelized transport and trapping of single EVs in parallel within seconds while enhancing the imaging of single trapped EVs. We show that the integration of nanoscale plasmonic cavities at the center of each GET trap results in the parallel placement of single EVs near plasmonic cavities enabling instantaneous plasmon-enhanced optical trapping upon laser illumination without any detrimental heating effect for the first time. These non-invasive scalable hybrid nanotweezers open new horizons for high-throughput tether-free plasmon-enhanced single EV trapping and spectroscopy. Other potential areas of impact include nanoplastics characterization, and scalable hybrid integration for quantum photonics.Comment: 21 pages, 5 figure

Reversible optical tuning of GeSbTe phase-change metasurface spectral filters for mid-wave infrared imaging.

Tunable narrowband spectral filtering across arbitrary optical wavebands is highly desirable in a plethora of applications, from chemical sensing and hyperspectral imaging to infrared astronomy. Yet, the ability to reconfigure the optical properties, with full reversibility, of a solid-state large-area narrowband filter remains elusive. Existing solutions require either moving parts, have slow response times, or provide limited spectral coverage. Here, we demonstrate a 1-inch diameter continuously tunable, fully reversible, all-solid-state, narrowband phase-change metasurface filter based on a GeSbTe-225 (GST)-embedded plasmonic nanohole array. The passband of the presented device is ∼74nm with ∼70% transmittance and operates across the 3-5 µm thermal imaging waveband. Continuous, reconfigurable tuning is achieved by exploiting intermediate GST phases via optical switching with a single nanosecond laser pulse, and material stability is verified through multiple switching cycles. We further demonstrate multispectral thermal imaging in the mid-wave infrared using our active phase-change metasurfaces. Our results pave the way for highly functional, reduced power, compact hyperspectral imaging systems and customizable optical filters for real-world system integration

Efficient Microparticle Trapping with Plasmonic Annular Apertures Arrays

In this work, we demonstrate trapping of microparticles using a plasmonic tweezers based on arrays of annular apertures. The transmission spectra and the E- field distribution are simulated to calibrate the arrays. Theoretically, we observe sharp peaks in the transmission spectra for dipole resonance modes and these are redshifted as the size of the annular aperture is reduced. We also expect an absorption peak at approximately 1,115 um for the localised plasmon resonance. Using a laser frequency between the two resonances, multiple plasmonic hotspots are created and used to trap and transport micron and submicron particles. Experimentally, we demonstrate trapping of individual 0.5 um and 1 um polystyrene particles and particle transportation over the surface of the annular apertures using less than 1.5 mW/um2 incident laser intensity at 980 nm

Recent Progress in Plasmonic Colour Filters for Image Sensor and Multispectral Applications

Using nanostructured thin metal films as colour filters offers several important advantages, in particular high tunability across the entire visible spectrum and some of the infrared region, and also compatibility with conventional CMOS processes. Since 2003, the field of plasmonic colour filters has evolved rapidly and several different designs and materials, or combination of materials, have been proposed and studied. In this paper we present a simulation study for a single- step lithographically patterned multilayer structure able to provide competitive transmission efficiencies above 40% and contemporary FWHM of the order of 30 nm across the visible spectrum. The total thickness of the proposed filters is less than 200 nm and is constant for every wavelength, unlike e.g. resonant cavity-based filters such as Fabry-Perot that require a variable stack of several layers according to the working frequency, and their passband characteristics are entirely controlled by changing the lithographic pattern. It will also be shown that a key to obtaining narrow-band optical response lies in the dielectric environment of a nanostructure and that it is not necessary to have a symmetric structure to ensure good coupling between the SPPs at the top and bottom interfaces. Moreover, an analytical method to evaluate the periodicity, given a specific structure and a desirable working wavelength, will be proposed and its accuracy demonstrated. This method conveniently eliminate the need to optimize the design of a filter numerically, i.e. by running several time-consuming simulations with different periodicities. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Nanophotonics for bacterial detection and antimicrobial susceptibility testing

Photonic biosensors are a major topic of research that continues to make exciting advances. Technology has now improved sufficiently for photonics to enter the realm of microbiology and to allow for the detection of individual bacteria. Here, we discuss the different nanophotonic modalities used in this context and highlight the opportunities they offer for studying bacteria. We critically review examples from the recent literature, starting with an overview of photonic devices for the detection of bacteria, followed by a specific analysis of photonic antimicrobial susceptibility tests. We show that the intrinsic advantage of matching the optical probed volume to that of a single, or a few, bacterial cell, affords improved sensitivity while providing additional insight into single-cell properties. We illustrate our argument by comparing traditional culture-based methods, which we term macroscopic, to microscopic free-space optics and nanoscopic guided-wave optics techniques. Particular attention is devoted to this last class by discussing structures such as photonic crystal cavities, plasmonic nanostructures and interferometric configurations. These structures and associated measurement modalities are assessed in terms of limit of detection, response time and ease of implementation. Existing challenges and issues yet to be addressed will be examined and critically discussed