R-3 Repository :: Browsing by Author "Halas, Naomi J"

Browsing by Author "Halas, Naomi J"

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Aluminum Nanocrystals as a Plasmonic Photocatalyst for Hydrogen Dissociation
(2016-03-31) Zhang, Chao; Halas, Naomi J
Hydrogen dissociation is a critical step in many hydrogenation reactions central to industrial chemical production and pollutant removal. This step typically utilizes the favorable band structure of precious metal catalysts like platinum and palladium to achieve high efficiency under mild conditions. Here we demonstrate that aluminum nanocrystals (Al NCs), when illuminated, can be used as a photocatalyst for hydrogen dissociation at room temperature and atmospheric pressure, despite the high activation barrier toward hydrogen adsorption and dissociation. We show that hot electron transfer from Al NCs to the antibonding orbitals of hydrogen molecules facilitates their dissociation. Hot electrons generated from surface plasmon decay and from direct photoexcitation of the interband transitions of Al both contribute to this process. Our results pave the way for the use of aluminum, an earth-abundant, nonprecious metal, for photocatalysis.
Aluminum Plasmonics for Detection and Spectroscopy
(2019-01-02) Cerjan, Ben Witte; Halas, Naomi J
Increasing interest in reducing the size, cost, and limit of molecular detection has led to the use of plasmonic structures to increase signal intensity and reduce detection limits dramatically. Of particular interest is vibrational spectroscopy, a powerful mid-infrared technique for measuring the presence of molecular bonds. Vibrational spectroscopy has traditionally been hindered by its inability to directly quantify the number of molecules sampled and by it's confinement to a laboratory bench, as the equipment necessary is bulky and impractical for field studies or embedding in small instrumentation. In this thesis, I present plasmonic structures and integrated devices designed to alleviate these issues and help enable miniaturized infrared spectroscopy.  The first half of this thesis is devoted to demonstrating how aluminum plasmonic antennas can be used for quantitative surface-enhanced infrared absorption (SEIRA). Based on the self-terminating aluminum oxide layer on the surface of the antenna, a self-calibrating metric for determining the number of molecules adsorbed within the hot-spot of the antenna is proposed. This metric is based on the ratio in signal strength between the target molecule of interest and the Al2O3 reference vibration. In order to measure both the target vibration of interest as well as the Al2O3 reference, I used asymmetric antennas with plasmonic resonances at both spectral positions. Experimental measurements demonstrate that this combination of a specifically-tailored antenna design with a mechanism for determining the relative strength of the targeted molecular vibrational resonance is capable of quantitative evaluations of the number of molecules sampled.   The second half of this work is on the design and fabrication of an integrated optoelectronic spectrometer using plasmonic aluminum gratings on p-doped silicon. By using intra-band transitions in the silicon substrate, a photoresponse can be measured as a change in resistance through the device. An array of plasmonic gratings act as both filter elements and electrical contacts, each with a distinct response spectrum. By measuring the response from the gratings under an unknown illumination spectrum and using the previously determined photoresponse spectra the input spectrum can be reconstructed. Standard least-squares reconstruction algorithms are limited by the number of gratings, however the achievable resolution of the spectrometer can be improved by adjusting the reconstruction algorithm using techniques from the compressive sensing literature. Incorporating an L1 constraint on our reconstruction algorithm to select for sparsity enables sub-Nyquist resolution in our reconstructed spectrum. The spectrometer is also used for contact-free molecular detection, demonstrating the capability to function as a "nanophotonic nose". Finally, simulations are performed to demonstrate the ultimate achievable resolution of such a spectrometer with a larger number of gratings.
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Catalyzing the Future of Nanomaterials Synthesis
(2023-02-01) Solti, David; Halas, Naomi J
Plasmonic nanomaterials have been the subject of an increasing amount of research interest due to the unique light-harvesting properties of localized surface plasmon resonances. However, the common noble metals used – Au, Ag, and Cu – have high costs and low abundance, making larger scale applications prohibitive. Research into alternative plasmonic materials focuses on the more bountiful Al and Mg as supplements to existing materials. However, synthesis of Al and Mg nanocrystals has historically been challenging, as they are highly oxophilic, requiring O2 and H2O-free environments. Furthermore, the reduction potentials of these metals are so negative that they have traditionally been used as reducing agents within chemical reactions. The first part of this thesis presents work that focused on developing our understanding of Al nanocrystal growth and expanding our repertoire of particles shaped through use of transition-metal catalysts. To this end, initial studies focused on spectroscopic evidence of several Ti-based cyclopentadienyl molecules acting not only as catalysts for the reduction of Al precursor species to Al0, but also as capping agents present on the surface of our grown crystals. This has allowed us to propose a mechanism for the growth of Al{100} and {111}-terminated nanoparticles, guiding future catalyst design. Additionally, extensive work born of our knowledge on catalyst-binding yielded growth of reduced-dimensionality Al particles. Through targeted catalyst design, we were able to grow crystalline Al nanoparticles from bars to nanowires, and even into two-dimensional nanoparticles such as twinned 2D Al nanodiamonds. This is accomplished through modification of the binding strength between the Al precursor and reduction catalyst. We then probe the ability of the Al nanowires as waveguides through extensive optical and electronic characterization. The wires hybridize with plasmonic Au films of various thicknesses and reveal more insight into the physical processes of plasmon hybridization. The wires also demonstrate remote excitation and detection of coupled visible-light emitting particles, paving the way for future air-stable plasmonic waveguiding devices. Concurrently, the last chapter in this thesis focuses on using our Al nanomaterials as catalysts themselves in organic chemical solution-phase reactions. Al NCs converted light into electrons through the excitation of ballistic hot electrons, which were ejected into the surrounding solvent. This process formed solvated electrons, characterized by a long-lived lifetime well beyond the ultrafast timescale of hot carriers. Their reactivity was harnessed with a spin-trap and a radical cyclization reaction. These plasmonically-generated solvated electrons offered far more control over other traditional methods of generation and gave rise to reaction selectivities typically not expressed by the constituents. Finally, our solvated electrons have quantum yields of over 1.1%, demonstrating a functional avenue for synthetic chemists to use for reduction reactions.
Engineering Aluminum Nanocomposites for Sensing, Photocatalysis and Photothermal Conversion
(2019-12-18) Tian, Shu; Halas, Naomi J
As the field of plasmonics continues to expand, researchers are seeking for other possibilities beyond noble metals, and an alternative plasmonic material that has tremendous potential is aluminum (Al). This thesis focuses on the study of both fundamental and practical aspects of surface plasmon excitations in Al nanostructures. Al, with its low cost, abundance, and better optical tunability compared to noble metals, has demonstrated its potential in sensing, photocatalysis, optoelectronics, and many other areas. The world of Al plasmonics has been greatly expanded with the development of Al nanocrystals (NCs) synthesis. The Al NCs have a native oxide layer which provides more possibilities of chemical bonding. We have demonstrated the potential use of Al NC aggregates as a plasmonic substrate for surface-enhanced Raman spectroscopy (SERS). The native oxide layer serves as a valuable linker between molecules and substrate, prohibiting non-specific adsorption on the Al NC surface. Al NC aggregates, as synthesized, are SERS substrates that enable the first quantitative label-free detection of ssDNA with no modification to either the ssDNA or the substrate surface. Besides the external field enhancement, the internal field induced hot carrier generation is also investigated. Al NCs generate hot carriers at plasmon resonance, as well as interband transitions. However, the lifetime of hot carriers is on the order of picoseconds before they decay into heat. Instead, we developed Al@TiO2 core-shell nanoparticles as antenna-reactor with efficient hot carrier generation and excellent photocatalytic performance. Analysis of the Al-doped TiO2 interlayer in Al@TiO2 core-shell heterostructure greatly extends our knowledge on the interface at the nanoscale. Unlike the native oxide layer of Al NCs, this interlayer does not block the hot carrier transfer pathway. Instead, it enables direct contact between Al nanoantenna and TiO2 reactor, where the aligned Fermi energy level allows almost barrierless charge transfer. We demonstrate experimentally that Al@TiO2 nanoparticles can drive the photoreduction of 4-nitrophenol. By comparing wavelength-dependent results with the simulated hot carrier generation, we conclude that the photocatalytic reactivity is generated from plasmonic Al nanoantenna under visible, even near IR illumination. The combination of Al and TiO2 presented in this thesis is a new demonstration of antenna-reactor geometry for plasmon-induced photocatalysis with low cost and promising large-scale industrial applications. We further investigated the optical and photothermal properties of small Al NCs with a plasmon resonance in UV region. The color of UV absorbing solution is almost colorless due to little interaction with visible light. However, the color of Al NCs solution is observed to change from almost colorless to totally black with increasing concentration. The simulation results indicate that besides the dipolar plasmon resonance in UV, the Al NCs also serves as a pure absorber in the visible to near IR spectral region. This is because of the larger imaginary part of dielectric function of Al in the visible range, which makes Al NCs a great candidate in photothermal applications. In order to investigate the photothermal performance of Al NCs in aqueous environment, a silica layer is coated with controlled thickness to improve their water stability. The photothermal conversion measurements shows the temperature increase both at the laser spot and in bulk, demonstrating the absorber nature of Al@SiO2 nanoparticles. The photothermal conversion efficiency reaches 54.67% under 800 nm laser illumination, which make Al@SiO2 a low cost but efficient candidate for solar applications. In summary, we have investigated the plasmonic properties of Al in three aspects: hot spots induced near field enhancement, hot carrier generation followed by photocatalysis, and the photothermal conversion. These observations and results, both experimental and theoretical, have demonstrated that Al NCs, along with its nanocomposites, are promising candidates for many different areas of plasmonics.
Fractal Nanoparticle Plasmonics and Rotational Spectroscopy as an Analytical Tool for Photocatalysis
(2017-09-08) Gottheim, Samuel E; Halas, Naomi J
When photons impinge on a metal nanoparticle surface the conduction band electrons are excited and oscillate coherently, generating what is known as surface plasmons. The optical properties of plasmonic nanostructures can be engineered to their specific applications by tuning their size, shape, and material. This thesis presents two advancements in the field of plasmonics, the first is a fundamental advancement in the design of multiply resonant plasmonic nanoparticles through simple design principles, and the second is a demonstration of the analytical power that rotational spectroscopy can bring to the study of gas phase plasmonic photocatalysis. Initially this thesis will focus on the role of self-similarity for the design of plasmon resonance lineshapes. Fractal-like nanoparticles and films have long been known to possess a remarkably broadband optical response and are potential nanoscale components for realizing spectrum-spanning optical effects. By computing and fabricating simple Cayley tree nanostructures of increasing fractal order N, we are able to identify the principle behind how the multi-modal plasmon spectrum of this system develops as the fractal order is increased. With increasing N, the fractal structure acquires an increasing number of modes with certain degeneracies: these modes correspond to plasmon oscillations on the different length scales inside a fractal. As a result, fractals with large N exhibit broad, multi-peaked spectra from plasmons with large degeneracy numbers. The Cayley tree serves as an example of a more general, fractal-based route for the design of structures and media with highly complex optical lineshapes. In the latter half of this thesis, frequency modulated rotational spectroscopy is presented as an alternative methodology to study plasmonic photocatalysis. This technique is compelling for its unambiguous recognition specificity and extremely sensitive (ppm or better) real-time quantitative measurement of molecular species, validated by the accurate measurement of the natural abundance of molecular isotopomers. The decomposition of the toxic industrial compound carbonyl sulfide (OCS) and the production of carbon monoxide (CO) by exciting chemically synthesized aluminum nanocrystals serves as a model experiment to showcase the power of this technique. Two measures of reaction efficiency, collision efficiency and external quantum efficiency, enabled by this technique are quantitatively described.
Hot Carrier Generation in Metallic Nanostructures: Mechanisms and Novel Devices
(2016-04-21) Zheng, Bob; Halas, Naomi J
Hot carrier generation in metallic nanostructures offers a potential route to circumventing thermodynamic efficiencies of traditional light-harvesting devices and structures. However, previous experimental realizations of hot electron devices have shown low photo-conversion efficiencies. Several theoretical works have sought to understand the fundamental processes behind hot carrier generation and explore routes toward increasing the carrier generation efficiency. In this thesis, we discriminate between hot carrier generation from interband transitions and surface plasmons by comparing photocurrent generation in Schottky and ohmic devices. By comparing the functional form of the two types of photocurrent generation, we show that hot carrier generation in metallic nanostructures obeys the field intensity inside the metallic nanostructure, paving the way towards more efficient plasmon-induced hot carrier devices. Next, I focus on plasmonic photodetectors for the mid-IR spectral region, a technologically and scientifically important spectral region where molecular vibrational resonances exist. Despite the significance of the mid-IR, the low energy of mid-IR photons poses significant challenges for efficient photodetection and light emission. We circumvent the limitations of traditional mid-IR photodetectors by exploiting hot carrier generation in metals and demonstrate a novel uncooled CMOS-compatible photodetector for the middle wave infrared (mid-IR) spectral region.
Hot-carrier-mediated Chemical Processes in Plasmonic Photocatalysis
(2019-04-09) Zhou, Linan; Halas, Naomi J
Plasmonic nanomaterials, featured with high optical cross-section resulted from the induction of the collective oscillation of free electrons in metallic nanostructures, known as localized surface plasmon resonance, by the alternative electromagnetic wave in light, is emerging as a new promising photocatalyst. Hot carriers derived from the non-radiative decay of LSPR are capable in activating chemical bond, in an intrinsically different mechanism from the conventional thermal-driven means, and provide the possibility in achieving chemical transformation in milder conditions with sustainable energy. When further combined with catalytically active materials in a synergic way to form the antenna-reactor complexes, the versatility and efficiency of plasmonic photocatalysts are greatly boosted. In this thesis, I will present four plasmonic photocatalysts, classified into two categories, for three reactions to show the stepwise understanding of the structure-property-function relationship in plasmonic photocatalysts and subsequent improvement in the design of photocatalysts. The first part, including chapters 3 and 4, involves applying monometallic plasmonic nanomaterials in H2 activation. Au and Al nanomaterials, though being inert towards H2 activation if driven thermally, are demonstrated to be active in hydrogen dissociation under light excitation. They both exhibit linear intensity dependence in photocatalytic H2-D2 exchange reaction and H-H bond activated by the electronic transition of initial hot carriers is proposed to be the dominated mechanism. In contrast, Cu nanoparticles exhibit an S-shape intensity dependence in photocatalytic H2-D2 exchange reaction with a more-than-1 external quantum yield of light-to-chemical conversion. The hot carrier multiplication resulted from thermalization of hot carriers through electron-electron scattering plays a crucial role in the Cu system. The rate-determining step (RDS) is believed to be associative desorption of HD, different from the dissociative adsorption of H2/D2 on Au and Al surface, making the transition barrier of hot carriers low and the thermalized hot carriers effective. Next, I designed a new antenna-reactor structure, surface alloy, to incorporate materials with the favorable electronic structure for activation of specific molecules into plasmonic nanomaterials with intent to achieve better usage of hot carriers. Cu-Ru surface alloy was prepared and shows highly efficient photocatalytic activity towards ammonia decomposition reaction, making it feasible for studying the effect of plasmon-mediated hot carriers on the activation barrier of chemical reactions. By carefully tuning the loading ratio of Cu and Ru, I further synthesized single-atom-alloy plasmonic photocatalyst composed of a Cu core antenna with atomically dispersed Ru sites reactor on the surface. This antenna-reactor complex exhibits outstanding coke resistance in methane dry reforming reaction under illumination. Both of the hot carriers and single-atom structure were demonstrated to be essential for the observed stability. This thesis increases the knowledge in the mechanism of hot-carrier-mediated chemical reaction and guides the design of new generation of plasmonic photocatalysts.
Hybrid and Molecular Plasmonics for Strongly Coupled Nanosystems and Photoelectrochemical Devices
(2015-12-04) Schlather, Andrea E; Halas, Naomi J; Nordlander, Peter; Landes, Christy F
The field of nanophotonics has realized rapid growth over the past several decades, as novel nanoscale materials are consistently being developed and researched for a wide variety of promising light-driven applications. Plasmonics is a particularly fascinating subset of nanomaterials research, owing to the unique ability of metallic nanostructures to interact with light from areas larger than their physical size, effectively focusing it to dimensions below the diffraction limit. This interaction, called a localized surface plasmon, arises when the electric field of light induces a coherent oscillation of the conduction electrons in the metal. Tuning the geometry and near-field environment of metallic nanostructures allows for controlled light scattering and absorption across a broad spectral range. Moreover, the strong spatial confinement of electric fields near the metal surface can remarkably enhance a host of molecular processes, motivating the development of plasmonic nanomaterials for single molecule sensing, photocatalysis and photoelectrochemical devices. This thesis will focus on two sets of interactions between plasmonic metal nanostructures and molecules in their local environment. Using spectroscopic and electrochemical techniques, the experimental far-field responses are correlated to the calculated near-field properties of the metal nanostructures. This is followed by a demonstration that molecules themselves may sustain plasmon resonances through active electrochemical charging. In the first part of this thesis, the near-field coupling of plasmons and molecular excitons are studied at the single-particle level. Polarization-dependent hyperspectral dark field microspectroscopy is used to probe the far-field scattering response of plasmonic dimers, which is influenced by strong near-field coupling to molecular J-aggregates located in the dimer junction. The coupling strengths are quantified and a rigorous theoretical investigation reveals that the plexcitonic coupling is dependent on the intensity of the plasmonic field enhancement in the dimer junction, which can be tuned by varying the polarization of incident light. These nanostructures represent a class of hybrid plasmonic materials that show reversible, all-optical spectral modulation, a necessary feature for a number of applications ranging from ultrafast optical switches to tactical obscurants. The next part of this thesis investigates the factors influencing the efficiency and kinetics of plasmonic hot carrier- driven redox reactions in an photoelectrochemical cell. Multi-layered Au-SiO2-Au nanoparticles, called nanomatroyshkas (NMs), serve as nano-electrodes due to their interesting optical properties. Strong light absorption by the NM electrodes leads to the formation of energetic hot electron-hole pairs that can be utilized to drive chemical reactions of surface adsorbates. The photooxidation of citrate by plasmonic hot holes and subsequent reduction of water by hot electrons on the surface of the NM electrodes is studied as a function of excitation wavelength, electrode potential and incident laser power. A qualitative system for optimizing plasmon-enabled photoelectrochemical reactions is presented by considering the interplay between plasmonic absorption and the energy alignment of hot carriers with molecules at the metal-solution interface. This result is an important step toward the ultimate goal of designing optimized nanomaterials for efficient photoelectrochemical devices. In the last part of this thesis, a new class of carbon-based plasmonic materials is proposed by reconsidering our understanding of the nature of optical transitions in charged polycyclic aromatic hydrocarbons (PAHs). 2D- graphene doped with charge carriers has been shown to support a surface plasmon resonance that is optically- and electrically-tunable in the mid-infrared (IR) and terahertz regimes. This study confirms theoretical predictions that spatial confinement of graphene to its smallest dimensions, that is, to individual PAH molecules containing only a few tens of atoms, can result in visible optical transitions when the molecules are charged with single electrons. A custom spectroelectrochemical setup is built to study the visible resonances of a series of PAH molecules in their reduced state, sustained by the absence of water or oxygen in a non-aqueous electrochemical cell. Time-dependent density functional theory (TDDFT) calculations provide insight into the origins of the broad, intensely-absorbing experimental optical spectra, which are concluded to be a superposition of light-induced electronic and vibronic transitions that are dipolar in nature. While efforts to fully understand these polarizable molecular transitions are ongoing, a number of research possibilities and potential applications arise from the addition of PAH molecules to the nanophotonics toolbox.
Imaging through opaque media and photodetection using engineered nanoparticles and nanostructures
(2018-04-20) Tanzid, Mehbuba; Halas, Naomi J
Extending our capabilities to image through opaque media can be highly significant for a wide range of applications: from bioimaging to seeing through fog or rain. We consider narrowband resonant nanoparticles, as well as broadband scatterer and absorber nanoparticles-constituent media, to characterize the properties of image transmission through complex media. Metallic nanoparticles can create narrowband opaque media possessing plasmon resonances with strongly frequency-dependent absorption and scattering cross-sections. We show that perceived image quality through plasmonic media, as well as the spatial resolution of the image both depend on the scattering and absorption cross-sections of the constituent nanoparticles. A nonlinear dependence of image quality is observed on optical density by varying optical pathlength and nanoparticle concentration. This approach should prove useful for evaluating object visualization through media consisting of subwavelength nanostructures as well as in the assessment of plasmonic optical imaging systems. We also examined the image transmission through opaque media consisting of broadband scattering and absorbing nanoparticles. We show both experimentally and computationally that image resolution through scattering media can be enhanced with the addition of absorbers where scattered photons having a longer pathlength are absorbed preferentially compared to ballistic photons. The increase in ballistic-to-scattered photon ratio enhances the image resolution. However, this image enhancement varies significantly depending on the absorption and scattering coefficients and the anisotropy factor of the scattering medium. We demonstrate that the addition of absorbers to a forward-scattering medium (e.g., biological tissue) has a negligible effect on image quality as compared to an isotropic scattering medium. However, we show that a medium which is forward-scattering in visible wavelength can be converted to an isotropic-scattering medium in near-infrared (NIR) wavelength depending on the size of the scatterers. For the same scattering medium, substantial image resolution enhancement is achieved in the NIR wavelength regime compared to visible wavelength. This work leads to an additional control for absorption-induced image resolution enhancement in scattering media by varying imaging wavelength, especially in the NIR wavelength window ideal for various imaging applications. Subsequently, we developed an NIR plasmonic photodetector which could provide us with a room temperature Si-based cost-effective narrowband imaging sensor. To achieve the enhanced responsivity of this photodetector, we used plasmonic nanostructures on highly-doped p-type Si substrate which combines two different detection mechanisms: plasmonic hot carrier generation and free carrier absorption (FCA) in highly doped Si. Using Au and Pd gratings on p-type Si substrate, we obtained >1 A/W responsivity at a low bias of 275 mV and 93 mV, respectively making the plasmonic narrowband photodetector performance comparable to the commercially available non-Si-based photodetectors.
Lifetime Characterization of Molecular Plasmons
(2017-04-19) Chapkin, Kyle David; Halas, Naomi J
Recent theoretical and experimental work has shown that polycyclic aromatic hydrocarbons (PAHs), a sub-nanometer, hydrogen passivated graphene-like system, can support a collective electron resonance, like a plasmon, with the addition or removal of at least a single electron. Here we more directly probe the plasmonic nature of these systems by taking excited state lifetime measurements of three molecular plasmon systems: the anion states of anthanthrene, benzo[ghi]perylene, and perylene. These systems exhibit, at minimum, bi-exponential ultrafast decay dynamics, both on picosecond timescales (orders of magnitude faster than typical single electron molecular excitations). The two components of the decay are attributed to the molecular plasmon dephasing and the vibrational relaxation of the molecule. One candidate, benzo[ghi]perylene, also exhibits an incomplete ground state recovery, shown to be a long-term lifetime, and attributed to lower excited state fluorescence. In total, this work explores the ultrafast dynamics of the molecular plasmon system and illuminates the distinction of molecular plasmons from single excitation systems, and emphasizes their similarity to other plasmonic materials, like metals and graphene.
Light Transport in Nanomaterial Systems
(2017-04-13) Hogan, Nathaniel J; Halas, Naomi J
What happens as light traverses a medium composed of both traditional materials and many ($10^5-10^{12}$ $cm^{-3}$) nanoparticles? These types of systems are present in many active areas of research in the nanotechnology sphere. Examples include nanoparticles in aqueous and non-aqueous solvents during chemical synthesis or for solar energy harvesting applictions; nanoparticles embedded in homogeneous and non-homogeneous solids for photocatalysis; nanoparticles in biological tissue for medical appplications, and more. Because nanoparticles composed of a certain material can have optical properties very different from the bulk material, these types of systems also display unique optical properties. In this thesis I outline an approach to solving light transport in nanomaterial systems based on the Monte-Carlo method. This method is shown to be optimal for nanomaterial systems where the extinction coefficient is composed of relatively equal contributions of scattering and absorption. Furthermore, I show that this computational tool can be utilized to solve problems in a wide variety of fields. In plasmonic photocatalysis, where mixtures of nanoparticles are driven resonantly to efficiently catalyze chemical reactions, this method elucidates the photothermal contribution. Experimental results combined with calculations suggest that the photocatalysis of a novel antenna-reactor complex composed of an Al core and a Cu$_2$O shell is primarily from hot-electron injection. Calculations involving taking optical images of objects through mixtures of nanoparticles explain the phenomenon that absorptive particles can enhance image quality and resolution of images taken through a scattering medium. Previous reports on this effect were limited in their explanation. We show that the reduced scattering coefficient is not sufficient to explain the phenomenon. Rather, all of the optical parameters must be known independently. The addition of absorptive particles increases image quality be selectively removing photons with the longest path-length through the system. These photons are the most likely to cause image distortion, having undergone multiple scattering events, having lost the original information of the image. Simulations of light transport through highly concentrationed solutions of nanoshells (1$\times$10$^9$-1$\times$10$^{11}$ NP/ml) show a localization and efficiency of absorbed light that explains previous results obtained in light-triggered release of DNA from nanoparticle surfaces. The strong temperature gradients obtained from these calculations help clarify previous results, which showed DNA release below the dehybridization temperature with CW laser irradiation. Further studies motivated by these calculations elucidate two regimes in light-triggered release with NIR radiation. CW radiation causes dehybridization of DNA due to melting, whereas ultrafast radiation causes Au-S bond breakage. Although previous studies have shown Au-S bond breakage for 400 nm ultrafast irradiation, this work is the first to explicitly show this mechanism for 800 nm radiation. Light transport calculations coupled to thermodynamic calculations show a clear damage threshhold of the nanoshells below which DNA release is optimal. This method of solving light transport for small nanomaterial systems is flexible, relatively easy to implement, and remarkably efficient with even modest computational resources.
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Metamaterials for Deep and Vacuum Ultraviolet Light Generation and Manipulation
(2020-12-04) Semmlinger, Michael; Halas, Naomi J
Deep (DUV, 200 – 280 nm) and vacuum ultraviolet (VUV, 100 – 200 nm) light has many important applications ranging from photodissociation to lithography. However, the generation and manipulation of electromagnetic radiation in this wavelength regime remains challenging. Popular sources like excimer lasers are bulky and many traditional optical materials suffer from strong absorption in this short wavelength regime. In this thesis, I experimentally demonstrate how nonlinear metasurfaces provide an alternative approach to simultaneously generate and control such radiation. Metasurfaces are composed of highly engineered subwavelength nanostructures, called meta-atoms, that give them the exceptional ability to manipulate the amplitude, phase, and polarization of the light they are interacting with. The ability of meta-atoms to strongly confine the local electric field, enables enhancement of nonlinear processes like second (SHG) and third harmonic generation (THG). This allows the conversion of longer wavelength radiation to the DUV and VUV regime via a compact device. In addition, unlike nonlinear crystals, nonlinear metasurfaces do not require phase matching due to their subwavelength interaction length. Local phase control via meta-atoms makes it possible to manipulate the output wavefront of a metasurface. In this way, nonlinear metasurfaces can be used not just for the generation but also the manipulation of DUV and VUV light. This thesis consists of three main parts. The first part presents a plasmonic metasurface consisting of gold nanostructures on top of an indium tin oxide (ITO) thin film that were designed to exhibit a toroidal resonance at the pump wavelength of 785 nm. The nanostructures can generate an electric field enhancement pattern that reaches into the underlying ITO layer, where it enhances THG to generate DUV light at 262 nm. The nonlinear signal from the toroidal metasurface is about five times larger than that of a dimer metasurface fabricated for comparison with the same amount of gold per unit area and underlying ITO layer. The second part presents an all-dielectric metasurface consisting of titanium dioxide (TiO2) nanostructures designed to facilitate an anapole resonance around the pump wavelength of 555 nm. An anapole resonance is caused by an interference between an electric and a toroidal dipole mode giving rise to exceptional electric field enhancement. Here, this is utilized to enhance THG and produce VUV light at 185 nm. Notably, the observed nonlinear signal from the nonlinear metasurface is around 180 times stronger compared to an unpatterned TiO2 substrate of the same thickness. In the third part, multifunctional nonlinear metasurfaces for VUV generation and manipulation are demonstrated. They consist of zinc oxide (ZnO) nanotriangles that show a magnetic dipole resonance around the pump wavelength of 394 nm that was designed to boost SHG. Geometric rotation of individual nanotriangles allows for local phase manipulation via the geometric phase, when excited with circularly polarized light. In this way, nonlinear metasurfaces for both focusing and beam steering of VUV light are demonstrated.
Molecular Plasmonics
(2017-10-17) Lauchner, Adam Clarence; Halas, Naomi J
Plasmons, coherent oscillations of conduction band electrons, have been well characterized in many different material systems including thin metal films, metal and metallodielectric nanoparticles, semiconductors, and graphene. Graphene's unique band structure provides a direct mechanism with which to tune the plasmon resonance energy with relatively small changes to its carrier . It has recently been shown that this remarkable electronic plasmon tunability persists as graphene is spatially confined to nanoribbons and nanoislands, down even to the molecular scale where Polycyclic Aromatic Hydrocarbon (PAH) molecules support molecular plasmons sensitive to the addition or removal of a single electron. We have previously demonstrated that molecular plasmons in PAHs exhibit behavior consistent with other plasmonic systems, notably the geometric and environmental tunability of the resonant wavelengths. This thesis reports on the further characterization of molecular plasmons and the use of their charge-sensitive response in electrochromic devices. Unlike larger graphene nanostructures, the PAH absorption spectra reveal rich features due to the coupling of the molecular plasmons with molecular vibrations. We investigate this surprising plasmon-phonon coupling with direct comparisons between experimental results and theoretical models accounting for the vibrational coupling. We proceed to demonstrate a series of electrochromic devices, color-changing glass, based on PAH plasmon resonances that can be reversibly switched between colorless and vivid colors dependent upon the chosen molecules. Finally, we explore the ultrafast dynamics of the molecular plasmon system and contrast this behavior with single-particle excitations. As the smallest examples of graphene and as readily available chemical species, PAHs provide an ideal platform for investigation of molecular plasmonics and are also well suited to large-scale applications since they are industrially available in large quantities and high purity.
Multiparticle Optical and Thermal Effects in Illuminated Solutions of Plasmonic Nanoparticles
(2014-10-22) Hogan, Nathaniel J; Halas, Naomi J; Nordlander, Peter J; Hafner, Jason
Plasmonic nanoparticles are found in a number of applications as efficient converters of optical energy into heat, e.g. cancer therapy of nanoparticle-laden tumors. More recently, aqueous solutions of plasmonic nanoparticles have proven the ability to produce steam with relatively high efficiencies upon solar illumination. We show in this report through modeling of the light transport in nanoparticle solutions that this effect originates in the optical properties of the nanoparticles. Strong optical scattering leads to multiparticle interactions that can concentrate light resulting in large temperature increases in the focused region. This model can be extended to all systems of dense nanoparticles in which light to heat conversion is crucial, e.g. photothermal cancer therapy and materials processing.
Nanophotonics-enabled Solar Membrane Distillation for Off-grid Water Purification
(2017-04-21) Dongare, Pratiksha D.; Halas, Naomi J
With more than a billion people lacking accessible drinking water, there is a critical need to convert non-potable sources such as seawater to water suitable for human use. However, energy requirements of desalination plants account for half their operating costs, so alternative, lower-energy approaches are equally critical. Membrane distillation (MD) has shown potential due to its low operating temperature and pressure requirements, but the requirement of heating the input water makes it energy intensive. Here we demonstrate nanophotonics-enabled solar membrane distillation (NESMD), where highly localized photothermal heating induced by solar illumination alone drives the distillation process, entirely eliminating the requirement of heating the input water. Unlike MD, NESMD can be scaled to larger systems and shows increased efficiencies with decreased input flow velocities. Along with its increased efficiency at higher ambient temperatures, these properties all point to NESMD as a promising solution for household- or community- scale desalination.
Nonlinear Nanophotonic Systems for Harmonic Generation, Parametric Amplification, Optical Processing and Single-Molecule Detection
(2015-02-19) Zhang, Yu; Halas, Naomi J; Nordlander, Peter J; Link, Stephan
Metallic nanoparticles support collective oscillations of conduction-band electrons, in response to light incidences. Such phenomenon is called localized surface plasmons, which confine large electromagnetic fields in sub-wavelength dimensions, enabling the light manipulation at the nanoscale. Plasmonic nanoparticles have established many promising applications, such as infrared photodetections, photothermal generation steam, chemical photocatalysis, cancer therapy and surface-enhanced spectroscopy. More interesting, plasmonic nanostructures could generate strong nonlinear-optical effects by relatively low excitation powers, and have been widely used in different processes like second-harmonic generations (SHG), difference-frequency generation (DFG), third-harmonic generation (THG), optical four-wave mixing (FWM) and surface-enhanced Raman scattering (SERS). This thesis will focused on two types of second-order and two types of third-order nonlinear-optical processes, enhanced by artificial plasmonic nanostructures. Firstly, the second-harmonic generation on a single nanocup is studied, and the signal is demonstrated to have increasing intensity as the 3D symmetry of the nanocup is reduced. Then, optical four-wave mixing is generated on a plasmonic nanocluster which supports a coherent oscillation of two Fano resonances. The electric fields from both Fano resonances add coherently resulting in strong fields and correspondingly large signals. This nanocluster has a large color-conversion efficiency, and could be used for building blocks of optical processors that convert two input colors into a third color. Later, one specific application of four-wave mixing, the coherent anti-Stokes Raman scattering (CARS) is studied. By exploiting the unique light harvesting properties of a Fano resonance of a specially designed nano-quadrumer, the surface-enhanced CARS (SECARS) technique amplifies the Raman signals of molecules on the quadrumer by about 100 billion times. This enables the accurate identification of a single molecule with less than 20 atoms. Finally, a plasmon-enhanced optical parametric amplifier (OPA) is designed: A BaTiO3 nanosphere is used as the nonlinear OPA medium; A nanoshell wrapping this nanosphere is used as a triply resonant cavity for all the pump, signal and idler beams; The generated idler beam has a wide tuning range in the near-infrared by changing the delay between the narrowband pump beam and broadband signal beam. This surface-plasmon-enhanced OPA could be an efficient light source working in the infrared regime, with large wavelength tunabilities and nanoscale dimensions easily integrated into the next-generation optoelectronic devices.
Optical Properties of Plasmonic Heterodimers and Nanoantennas for Surface-Enhanced Infrared Absorption
(2014-11-24) Brown, Lisa V; Halas, Naomi J; Nordlander, Peter; Link, Stephan
Electromagnetic interactions in nanoscale systems are a driving force of research in the field of nanophotonics. The basic properties of these systems set the groundwork for understanding complex optical phenomena useful for the development of new technological devices. Metallic nanospheres are among the most simple and canonical structures that produce plasmon resonances, which are collective oscillations of valence electrons excited by the electric field component of an incident light wave. Much like electron orbitals in molecules, plasmon resonances hybridize to form new modes when two or more structures combine. If a broad and narrow mode overlap in energy, they will interfere to produce a Fano resonance indicated by an asymmetric line shape in the extinction spectrum. Hybridized modes can also focus light into subwavelength volumes with intensities several orders of magnitude greater than that of the incident beam. These strong near-field enhancements can be used to detect extremely small quantities of molecules in a variety of chemical sensing methods, even at the single-molecule level. The goals of this thesis are to explore the optical properties of asymmetric nanoparticle systems and to design antennas with strong near-field enhancements for infrared molecular spectroscopy. The first part will discuss plasmonic heterodimers composed of Au nanoparticles differing in size and shape. These simple geometries give rise to a remarkably rich set of properties. For incident polarization parallel to the dimer axis, the hybrid plasmon modes produce a Fano resonance and demonstrate avoided crossing behavior. For incident polarization perpendicular to the dimer axis, the structure exhibits an optical nanodiode effect, where the scattering profile changes depending on the direction of the incident beam. The second part of this thesis will introduce two Au nanoantenna designs having strong near-field intensities in the mid-infrared range. Zeptomole quantities of molecules are detected through Surface-Enhanced Infrared Absorption (SEIRA), in which a Fano resonance exists between the antenna plasmon mode and the molecular vibration of interest. The first structure, called a cross antenna, consists of four nanorods oriented perpendicularly with a common junction, such that all polarizations of light are simultaneously absorbed. The second structure, called a fan antenna, incorporates a semicircular portion on the outer end of each rod that increases both the overall scattering cross section and the near-field enhancement at the junction. Further enhancement is achieved by placing the antenna above a Au mirror to maximize constructive interference between the incident and scattered light. Using these designs, we demonstrate enhanced detection of several classes of analytes, and we approach the limit of sensitivity for conventional spectroscopic methods in combination with standard lithographic techniques. Such findings are essential for gauging the conditions required for single-molecule infrared spectroscopy and for furthering the development of near-field chemical sensing.
Photonic Metamaterials for Color Devices and Deep UV Second Harmonic Generation
(2018-03-15) Semmlinger, Michael; Halas, Naomi J
Photonic Metamaterials are novel materials that consist of subwavelength optical resonators called meta-atmos. They have attracted much attention, due to their ability to control and confine light. In particular, they have promising applications in color generation and nonlinear optics. Here, I give one example for each of these two applications. Chapter one presents an actively tunable full-spectrum device. An array of plasmonic aluminum particles is integrated into a stretchable polymer substrate. Stretching the substrate in either of its two dimensions causes a change in the array period, and therefore changes the associated scattering color. Using a two-dimensional stretching approach, I demonstrate full-spectrum tuning, as well as image switching. In chapter two I present an all-dielectric metamaterial consisting of an array of zinc oxide (ZnO) nanodisks. The material shows a magnetic dipole resonance around 400nm. When pumped at resonance, the associated field enhancement can be used to generate the second harmonic frequency. This serves as a first demonstration for a simple device to generate deep UV light.
Plasmon Resonances in Metallic Nanostructures for Photodetection and Signal Modulation
(2014-08-15) Wang, Yumin; Nordlander, Peter J.; Halas, Naomi J; Link, Stephan
Metallic nanostructures can strongly interact with the light and exhibit fascinating optical properties due to inherent collective oscillations of electrons in metals, also known as plasmon resonances. Since the plasmons are capable of confining light into a small regime and meanwhile significantly enhancing its field intensity, the metallic nanostructures can be widely used for light harvesting and manipulation. By placing gold gratings on top of a silicon substrate, hot electrons created from plasmon decay can efficiently go across the Schottky barrier and be harvested by the silicon, leading to a substantial photocurrent. This yields a good photodetector which not only possesses a narrowband photoresponse due to the plasmon resonances but also has the ability to work at wide frequency range even below the bandgap of the silicon. Moreover, instead of the top-gratings structures, embedding gold nanostructures into the semiconductors will effectively increase the photoresponsivity. Theoretical calculation shows that the embedment can lead to an increase in the surface area of the Schottky barrier and at the meantime broaden the directional range of the emitted hot electrons able to transport across the Schottky barrier. More importantly, the vertical Schottky barrier is found to be the predominant area where photoemission take places. Aside from creating hot electrons, the plasmons can also influence the performance of the photodetection by facilitating the generation of electron-hole pairs directly in the semiconductors. Here, the aluminum gratings are demonstrated to serve as good color filters when they are integrated with metal-semiconductor-metal photodiodes. The interference of plasmon near-field and incident field could either block or assist the light going through the aluminum gratings to hit the photodiodes. As for light manipulation, the metallic nanostructures act just like optical nanoantennas whose photoresponse can be modulated by loading optical materials in them. The corresponding modulation process can be described in terms of optical nanocircuitry in which various materials are represented by capacitors, inductors, and resistors. With the help of the optical nanocircuitry theory, optical nanofilters become convenient and straightforward to design and build. In addition, substrate also can strongly modify the optical response of the nanoantenna. It has been proven that a conductive substrate will blueshift and reduce the original plasmon resonances and meanwhile bring in a new charge transfer mode appearing at low energy level. Given that plasmon resonances can effectively harvest light and modulate optical signal, they may have promising applications in sensing, imaging and communication systems in the near future.
Plasmonic Heterodimers: Antenna-Reactor Effect and Optical Forces
(2018-08-31) Zhang, Chao; Halas, Naomi J
Surface plasmon is the collective oscillation of free electrons in metallic nanoparticles. When two plasmonic nanoparticles are brought close together as a plasmonic dimer, their near field can interact with each other to generate new hybridized resonances. In particular, pairing nanoparticles with different optical, chemical, electrical, or mechanical properties as a heterodimer allows one to customize nanophotonic systems that utilize the desired features of individual components simultaneously. In this thesis, I present two examples to show the flexibility and modularity of such plasmonic heterodimers. In the first example, I demonstrate the use of Al-Pd nanodisk heterodimers as an antenna-reactor photocatalyst. A photocatalyst harvests energy from light to drive chemical reactions. Conventional catalysts are made of transition metal nanoparticles, which only interact weakly with light. On the contrary, plasmonic metals such as Al, Au, and Ag interact strongly with light, but are far poorer catalysts. By combining plasmonic and catalytic metal nanoparticles in one entity, the plasmonic antenna can drive a polarization in the catalytic reactor, creating a forced plasmon. This process dramatically enhances the optical response of the reactor, making it an efficient photocatalyst. Precisely defined, self-aligned, and strongly coupled Al-Pd nanodisk heterodimers can be prepared at the wafer scale using hole-mask colloidal lithography. Light-induced hydrogen dissociation reaction was performed as a model reaction to evaluate the performance of this photocatalyst. The wavelength- and polarization-dependent reaction rate closely follows the Al-mediated optical absorption of the Pd nanodisk. The high structural uniformity of the heterodimers also enables microscopic quantification of reaction rates and quantum efficiencies at single nanostructure level. In the second example, I investigate the optical properties of Al-Au nanodisk heterodimers. Both components of this heterodimer support surface plasmon resonances, but in different wavelength ranges. Forced plasmon can be created when the on-resonance nanodisk drives the off-resonance nanodisk through near field coupling. The hybridized resonances are not only observed with far field extinction spectroscopy, but also probed in the near field by photo-induced force microscopy. Moreover, when the interdisk spacing is very small and the near field interaction extremely strong, the Au nanodisk of the heterodimer can be repositioned and reshaped under laser illumination. This is attributed to a joint effect of photothermal softening of the Au lattice and the optical forces applied to the Au nanodisk. This thesis paves the way of designing and utilizing plasmonic heterodimers for a rich abundance of applications including photocatalysis, solar energy harvesting, sensing, and optically-induced nanomanufacturing.

Browsing by Author "Halas, Naomi J"

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