Browsing by Subject "Plasmonics"

Active plasmonic nanostructures with tunable resonances promise to enable smart materials with multiple functionalities, on-chip spectral-based imaging and low-power optoelectronic devices. A variety of tunable materials have been integrated with plasmonic structures, however, the tuning range in the visible regime has been limited and small on/off ratios are typical for dynamically switchable devices. An all optical tuning mechanism is desirable for on-chip optical computing applications. Furthermore, plasmonic structures are traditionally fabricated on rigid substrates, restricting their application in novel environments such as in wearable technology.

In this dissertation, I explore the mechanisms behind dynamic tuning of plasmon resonances, as well as demonstrate all-optical tuning through multiple cycles by incorporating photochromic molecules into plasmonic nanopatch antennas. Exposure to ultraviolet (UV) light switches the molecules into a photoactive state enabling dynamic control with on/off ratios up to 9.2 dB and a tuning figure of merit up to 1.43, defined as the ratio between the spectral shift and the initial line width of the plasmonic resonance. Moreover, the physical mechanisms underlying the large spectral shifts are elucidated by studying over 40 individual nanoantennas with fundamental resonances from 550 to 720 nm revealing good agreement with finite-element simulations.

To fully explore the tuning capabilities, the molecules are incorporated into plasmonic metasurface absorbers based on the same geometry as the single nanoantennas. The increased interaction between film-coupled nanocubes and resonant dipoles in the photochromic molecules gives rise to strong coupling. The coupling strength can be quantified by the Rabi-splitting of the plasmon resonance at ~300 meV, well into the ultrastrong coupling regime.

Additionally, fluorescent emitters are incorporated into the tunable absorber platform to give dynamic control over their emission intensity. I use optical spectroscopy to investigate the capabilities of tunable plasmonic nanocavities coupled to dipolar photochromic molecules. By incorporating emission sources, active control over the peak photoluminescence (PL) wavelength and emission intensity is demonstrated with PL spectroscopy.

Beyond wavelength tuning of the plasmon resonance, design and characterization is performed towards the development of a pyroelectric photodetector that can be implemented on a flexible substrate, giving it the ability to be conformed to new shapes on demand. Photodetection in the NIR with responsivities up to 500 mV/W is demonstrated. A detailed plan is given for the next steps required to fully realize visible to short-wave infrared (SWIR) pyroelectric photodetection with a cost-effective, scalable fabrication process. This, in addition to real-time control over the plasmon resonance, opens new application spaces for photonic devices that integrate plasmonic nanoparticles and actively tunable materials.

Solid-state quantum emitters embedded in carefully engineered nanostructures could enable a new generation of quantum information and sensing technologies, including networked processors for quantum computing and precise monitors of temperature and strain at the nanoscale. The primary goal when designing these nanostructures is to utilize the Purcell effect to improve the emission rate, directionality and brightness of quantum emitters, as long decay times, nondirectional emission and weak fluorescence limit their applications. One particularly promising emitter is the silicon vacancy (SiV) in diamond, which offers excellent photostability and minimal spectral diffusion, in addition to coherent emission at its zero-phonon line (ZPL) comprising 80% of its total fluorescence. In this dissertation, up to 121-fold enhancement of the spontaneous emission rate of SiVs coupled to plasmonic nanogap cavities is demonstrated. The vacancy centers are implanted into a monolithic diamond thin film, which is then etched to nanometer-scale thickness, an approach with a clear path towards wafer-scale fabrication. A novel approach to creating film-coupled nanogap metasurfaces was developed to support this research and consists of transferring EBL-fabricated nanoparticles by using a PDMS stamp. Up to seven orders of magnitude of enhancement of nonlinear frequency conversion was also observed in diamond thin films coupled to these metasurfaces. Furthermore, a robust mechanism for actively tuning the nanocavity absorption resonance by integrating sub-10-nm films of the phase-change material vanadium dioxide. This platform opens up opportunities for on-chip quantum networks and nanoscale sensors based on nanocavity-coupled SiVs with the potential for in-situ frequency conversion to outcouple to photonic circuits and reconfigurable properties by incorporating VO2 thin films.

The chemical industry depends on heterogeneous thermocatalytic processes to satisfy the ever-increasing demand for fuels and fertilizers. High temperatures and high pressures are generally required to accelerate chemical transformations and operate practical rates. These harsh conditions, however, lead to huge energy consumption and other side effects, such as the lifetime of catalysts and parasitic formation of by-products. Light is used as an alternative energy input to drive chemical reactions on semiconducting photocatalysts, but the slow reaction rates and insufficient control of product selectivity hinder wide adaptation of photocatalysis. Plasmonic metal nanoparticles have been recently proposed as a new type of catalysts with photoactivities. As already been widely used in thermocatalytic reactions, the strong light absorption capability from excitation of localized surface plasmon resonance (LSPR) of plasmonic catalysts could combine light and thermal energy to work cooperatively in enhancing rates of chemical reactions. This dissertation summarizes our efforts aiming to design plasmonic catalysts with high efficiency and high product selectivity. The catalytic properties of synthesized rhodium (Rh) and ruthenium (Ru) catalysts are investigated in two model reactions, carbon dioxide (CO2) hydrogenation and ammonia (NH3) synthesis.

Chapter 2 describes the development of slow-injection polyol methods to synthesize monodispersed Rh nanocubes with tunable size and resonant energy. The wide size tunability of slow-injection methods allows for the red-shift of resonant wavelength of small Rh nanostructures, which are in the deep ultraviolet (UV) region, to more accessible and practical near-UV and visible regions by increasing the size of Rh nanocubes.

Chapter 3 focuses on the product selectivity of plasmonic Rh nanocubes in CO2 hydrogenation. Rh nanocubes supported on aluminum oxide (Al2O3) nanoparticles equally produce methane (CH4) and carbon monoxide (CO) in pure thermal conditions. Under illumination of UV and blue light, the rate of CH4 production is significantly enhanced, and almost exclusive CH4 production is observed. This photo-selectivity can be attributed to selective activation of specific reaction intermediate by photo-generated hot electrons among competing reaction pathways.

Chapter 4 describes the effects of catalyst support and morphology of plasmonic Rh nanostructures on the catalytic activities in plasmon-enhanced CO2 hydrogenation. Significant improvements of reaction rates are observed by switching to reducible titanium oxide (TiO2) support and shrinking the size of Rh nanostructures. The enhancement of reaction rates by light can be partially attributed to local heating of catalyst bed.

Chapter 5 focuses on the catalytic activities of Ru-based catalysts for NH3 synthesis under light illumination. Photo-enhanced NH3 production, which highly depends on the size, support, and promoter of catalysts, is observed.

Chapter 6 discusses conclusion and future directions of this project. Molecular level insights of plasmon-enhanced catalysis are highly desired for both fundamental research and practical applications.

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.

Nanometer-scale metallic structures have been widely and intensively studied over the last decade because of their remarkable plasmonic properties that can enhance local electromagnetic (EM) fields. However, most plasmonic applications are restricted to the visible and near infrared photon energies due to the limitations of the surface plasmon resonance energies of the most commonly used plasmonic metals: Au and Ag. Plasmonic applications in ultraviolet (UV) are of great interest because Raman scattering sections are larger and do not overlap fluorescence spectra. UV plasmonics also benefit from high spatial resolution and low penetration depth. However, an appropriate UV plasmonic material must be identified.

We proposed and demonstrated that gallium is a highly-promising and compelling material for UV nanoplasmonics through synthesis of size-controlled nanoparticle arrays, EM modeling of local field enhancement, ellipsometric and spatial characterization of the arrays, and analytical measurement of UV- enhanced Raman and fluorescence spectra. Self-assembled arrays of hemispherical gallium nanoparticles deposited by molecular beam epitaxy on a sapphire support are characterized with spatial and ellipsometric measurements. Spin-casting a thin film of crystal violet upon these nanoparticles permitted the demonstration of surface-enhanced Raman spectra, fluorescence, and molecular photodegradation following excitation by a HeCd laser operating at 325 nm (UV). Measured local Raman enhancement factors exceeding 107 demonstrated the potential of gallium nanoparticle arrays for plasmonically-enhanced ultraviolet detection and remediation.

Plasmonic nanostructures and metamaterials have found many applications as small-scale sources of controllable emission. Of particular interest is utilizing these types of structures as potential coherent radiation sources. Plasmonic Film-coupled Nanoparticles(FCNP), or nanopatch antennas, are good candidates for low-threshold, room-temperature nanolasing that can be predicted analytically. In this dissertation, I present results from multiphysical numerical models used to validate the predictions of a recent analytical theory, using optical pump intensity, population inversion, and pump photon count as metrics of lasing threshold. I show that a single cylindrical nanopatch antenna made of silver with an embedded fluorescent dye is capable of lasing at a threshold on the order of $10^4$ W/cm$^2$. I go beyond the hypotheses of the theoretical model by investigating the impact of spectrally non-separated absorption and emission transitions through the influence of lasing signal/absorption line and pump/emission line interactions. Furthermore, I tighten the model constraints and analytical predictions to facilitate experimental verification and ultimately demonstrate predicted lasing behavior. Thresholds on the order of $10^5$ W/m$^2$ are verified from fabricated experimental samples through spectral and coherence measurements of emission as a function of incident optical pump intensity from single film-coupled nanocubes with a variety of embedded dyes corresponding favorable geometric and material parameters. Agreement between analytically predicted thresholds and experimentally measured thresholds validates the previously developed theory and demonstrates the utility of the single film-coupled nanoparticle platform for lasing.

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.

A patterned metasurface can strongly scatter incident light, functioning as an extremely low-profile lens, filter, reflector or other optical devices. Nonlinear metasurfaces‒combine the properties of natural nonlinear medium with novel features such as negative refractive index, magneto-electric coupling‒provide novel nonlinear features not available in nature. Compared to conventional optical components that often extend many wavelengths in size, nonlinear metasurfaces are flexible and extremely compact.

Characterization of a nonlinear metasurface is challenging, not only due to its inherent anisotropy, but also because of the rich wave interactions available. This thesis presents an overview of the work by the author in modeling and application of nonlinear metasurfaces. Analytical methods - transfer matrix method and surface homogenization method - for characterizing nonlinear metasurfaces are presented. A generalized transfer matrix method formalism for four wave mixing is derived, and applied to analyze nonlinear interface, film, and metallo-dielectric stack. Various channels of plasmonic and Fabry-perot enhancement are investigated. A homogenized description of nonlinear metasurfaces is presented. The homogenization procedure is based on the nonlinear generalized sheet transition conditions (GSTCs), where an optically thin nonlinear metasurface is modeled as a layer of dipoles radiating at fundamental and nonlinear frequencies. By inverting the nonlinear GSTCs, a retrieval procedure is developed to retrieve the nonlinear parameters of the nonlinear metasurface. As an example, we investigate a nonlinear metasurface which presents nonlinear magnetoelectric coupling in near infrared regime. The method is expected to apply to any patterned metasurface whose thickness is much smaller than the wavelengths of operation, with inclusions of arbitrary geometry and material composition, across the electromagnetic spectrum.

The second part presents the applications of nonlinear metasurfaces. First, we show that the third-harmonic generation (THG) can be drastically enhanced by the nonlinear metasurfaces – film-coupled nanostripes. The large THG enhancement is experimentally and theoretically demonstrated. With numerical simulations, we present methods to clarify the origin of the THG from the metasurface. Second, the enhanced two-photon photochormism is investigated by integrating spiropyrans with film-coupled nanocubes. This metasurface platform couples almost 100% energy at resonance, and induces isomerization of spiropyrans to merocyanines. Due to the large Purcell enhancement introduced by the film-coupled nanocubes, fluorescence lifetime measurements on the merocyanine form reveal large enhancements on spontaneous emission rate, as well as high quantum efficiency. We show that this metasurface platform is capable of storing information, supports reading and writing with ultra-low power, offering new possibilities in optical data storage.

Recent advances in nanotechnology have led to the application of nanoparticles in a wide variety of fields. In particular, anisotropic nanoparticles have shown great potential for surface-enhanced Raman scattering (SERS) detection due to their unique optical properties. Gold nanostars are a type of anisotropic nanoparticle with one of the highest SERS enhancement factors in a non-aggregated state. By utilizing the distinct characteristics of gold nanostars, new plasmonic materials for sensing, diagnostics, and therapy can be synthesized. The work described herein is divided into two main themes. The first half demonstrates the development and application of a novel label-free inverse molecular sentinel (iMS) nanoprobe for detection of microRNA biomarkers related to cancer progression as well as those related to gene expression in plants. This work also describes the initial proof-of-concept for a SERS-based electrowetting-on-dielectric (EWD) digital microfluidic platform as a diagnostic platform requiring samples of nanoliter volume. The second half demonstrates the utility of plasmonic nanoparticles for SERS imaging as well as photothermal therapy (PTT) and photodynamic therapy (PDT).

Development of accessible strategies for efficient detection of nucleic acid biomarkers is a major unmet need for applications ranging from cancer screening to agricultural biotechnology and biofuel development. MicroRNAs (miRNAs) have great promise as a new important class of biomarkers for early detection of various cancers; however, these small molecules have not been adopted into early diagnostics for clinical practice because of challenges adapting complex laboratory techniques into accessible clinical tests. In a blinded study, the surface-enhanced Raman scattering (SERS)-based plasmonics-active nanoprobes described herein, referred to as inverse molecular sentinels (iMS), demonstrated diagnostic accuracy for in vitro identification of endoscopic biopsy samples as tumor, Barrett’s esophagus or normal tissue via miRNA detection. The iMS nanoprobe technology can be designed to detect a wide range of nucleic acids for a variety of applications. In addition to medical applications, the knowledge over gene expression dynamics and location in plants is crucial for applications ranging from basic biological research to agricultural biotechnology. However, current methods are unable to provide in vivo dynamic detection of genomic targets in plants, due to the complex sample preparation needed by current methods for nucleic acids detection, which disrupt spatial and temporal resolution. We have developed a multimodal technique utilizing iMS nanoprobes for in vivo imaging and biosensing of microRNA biotargets within whole plants. This work lays the foundations for in vivo functional imaging of RNA biotargets in plants with previously unmet spatial and temporal resolution.

The prevalence of cancer has increasingly become a significant threat to human health and as such, there exists a strong need for developing novel methods for early detection and effective therapy. Gold nanostars (AuNS) with tip-enhanced plasmonics have become one of the most promising platforms in photothermal therapy (PTT) as they exhibit superior photon-to-heat conversion efficiency and can be delivered specifically to tumors. We have demonstrated that AuNS are endocytosed into multiple cancer cell lines irrespective of receptor status or drug resistance and allow for the effective photothermal ablation of tumor cells. Additionally, we demonstrate a unique in vitro preclinical model that mimics the tumor structures assumed by inflammatory breast cancer (IBC) in vivo. IBC has a unique presentation of diffuse tumor cell clusters called tumor emboli. AuNS are able to penetrate the tumor embolic core in 3D culture, allowing effective photothermal ablation of the IBC tumor emboli.

Additionally, we have furthered the development of the gold nanostar treatment platform by developing a theranostic nanoconstruct that consist of Raman-labeled gold nanostars coated with a silica shell that is loaded with photosensitizer molecules for PDT. The outer surface of the nanoconstruct was functionalized for targeting to allow for specific treatment of folate positive breast cancer. SERS detection and PDT are performed at different wavelengths, so there is no interference between the diagnostic and therapeutic modalities. Singlet oxygen generation (a measure of PDT effectiveness) was demonstrated from the drug-loaded nanocomposites. In vitro testing demonstrated the effectiveness of the nanoconstruct for targeted PDT.

If electronics are ever to be completely replaced by optics, a significant possibility in the wake of the fiber revolution, it is likely that nonlinear materials will play a central and enabling role. Indeed, nonlinear optics is the study of the mechanisms through which light can change the nature and properties of matter and, as a corollary, how one beam or color of light can manipulate another or even itself within such a material. However, of the many barriers preventing such a lofty goal, the narrow and limited range of properties supported by nonlinear materials, and natural materials in general, stands at the forefront. Many industries have turned instead to artificial and composite materials, with homogenizable metamaterials representing a recent extension of such composites into the electromagnetic domain. In particular, the inclusion of nonlinear elements has caused metamaterials research to spill over into the field of nonlinear optics. Through careful design of their constituent elements, nonlinear metamaterials are capable of supporting an unprecedented range of interactions, promising nonlinear devices of novel design and scale. In this context, I cast the basic properties of nonlinear metamaterials in the conventional formalism of nonlinear optics. Using alternately transfer matrices and coupled mode theory, I develop two complementary methods for characterizing and designing metamaterials with arbitrary nonlinear properties. Subsequently, I apply these methods in numerical studies of several canonical metamaterials, demonstrating enhanced electric and magnetic nonlinearities, as well as predicting the existence of nonlinear magnetoelectric and off-diagonal nonlinear tensors. I then introduce simultaneous design of the linear and nonlinear properties in the context of phase matching, outlining five different metamaterial phase matching methods, with special emphasis on the phase matching of counter propagating waves in mirrorless parametric amplifiers and oscillators. By applying this set of tools and knowledge to microwave metamaterials, I experimentally confirm several novel nonlinear phenomena. Most notably, I construct a backward wave nonlinear medium from varactor-loaded split ring resonators loaded in a rectangular waveguide, capable of generating second-harmonic opposite to conventional nonlinear materials with a conversion efficiency as high as 1.5\%. In addition, I confirm nonlinear magnetoelectric coupling in two dual gap varactor-loaded split ring resonator metamaterials through measurement of the amplitude and phase of the second-harmonic generated in the forward and backward directions from a thin slab. I then use the presence of simultaneous nonlinearities in such metamaterials to observe nonlinear interference, manifest as unidirectional difference frequency generation with contrasts of 6 and 12 dB in the forward and backward directions, respectively. Finally, I apply these principles and intuition to several plasmonic platforms with the goal of achieving similar enhancements and configurations at optical frequencies. Using the example of fluorescence enhancement in optical patch antennas, I develop a semi-classical numerical model for the calculation of field-induced enhancements to both excitation and spontaneous emission rates of an embedded fluorophore, showing qualitative agreement with experimental results, with enhancement factors of more than 30,000. Throughout these series of works, I emphasize the indispensability of effective design and retrieval tools in understanding and optimizing both metamaterials and plasmonic systems. Ultimately, when weighed against the disadvantages in fabrication and optical losses, the results presented here provide a context for the application of nonlinear metamaterials within three distinct areas where a competitive advantage over conventional materials might be obtained: fundamental science demonstrations, linear and nonlinear anisotropy engineering, and extremely compact resonant all-optical devices.

Two-dimensional transition metal dichalcogenides (TMDCs), with direct bandgaps in the visible to near-infrared wavelength, offer a tantalizing platform for making optoelectronic devices with enhanced and novel functionalities. In this dissertation, we explore the valley dynamics and various excitonic states in monolayer TMDCs, as well as demonstrate tunable and enhanced exciton emission using plasmonic nanocavities. First, we probe the origin of the excitonic and localized states in monolayer WSe2 using polarization-resolved PL spectroscopy at temperatures from 10 K to room temperature. Next, Kerr rotation experiments are used to investigate the temporal and spatial valley dynamics of monolayer TMDCs using femtosecond pump and probe pulses.

Despite these remarkable optical properties, atomically thin TMDC monolayer suffer from intrinsically weak light absorption (~3 %) and low photoluminescence (PL) quantum yield (~0.4 %). Furthermore, among the complex excitonic states of monolayer TMDCs, the B exciton emission is inherently weak compared to the dominant A exciton emission. Thus, we demonstrate a tunable plasmonic nanocavity where emitters are sandwiched in a sub-10-nm dielectric gap between a metallic film and colloidally synthesized silver nanocubes. When emitters with an intrinsic long lifetime are embedded in the gap region, the spontaneous emission rate enhancements can be exceeding 1,000 times while the structure maintains a high quantum efficiency (>50 %) and directional emission. Incorporating semiconductor quantum dots into the plasmonic cavity enable ultrafast spontaneous emission with emission rates exceeding 90 GHz. Finally, when MoS2 monolayers are integrated into this plasmonic nanocavity with tunable plasmon resonances, we observe a 1,200-fold enhancement for the A exciton emission and a 6,100-fold enhancement for the B exciton emission. Moreover, we show a strong modification of the PL emission peaks, which exhibits a strong correlation between the emission wavelengths and the nanocavity resonance. Manipulating the optical properties of these 2D materials using tunable plasmon resonances is promising for the design of novel optical devices with precisely tailored responses, which is critical for optimizing the performance of future optoelectronic and nanophotonic devices.