Recent advances in surface enhanced Raman spectroscopy (SERS): finite difference time domain (FDTD) method for SERS and sensing applications

There have been significant advancements in the field of surface-enhanced Raman spectroscopy (SERS). Despite being an ultra-sensitive analytical technique, challenges, such as how to get a proper match between the SERS substrate and light for better signal enhancement to obtain a stable, sensitive SERS substrate, prevent its widespread applications. Finite-difference time-domain (FDTD) method, a numerical tool for modeling computational electrodynamics, has recently been used to investigate SERS for understanding the underlying physics, and optimally design and fabricate SERS substrates for molecular analysis. In this review, we summarize the trend of using FDTD method in SERS studies by providing an introduction of fundamental principles, the studies of optical responses, electromagnetic (EM) field distribution, enhancement factor (EF) of SERS, the application in design and fabrication of SERS substrates, and SERS for biosensing and environmental analysis. Finally, the critical issues of using inherently approximate FDTD method and future improvement for solving EM problems and SERS applications are discussed

Modifying Single-Molecule Fluorescence with a Plasmonic Optical Antenna: Theory, Methodology, and Measurement

Nanophotonics is the study and technological application of light on the nanometer scale. This dissertation brings together two disparate branches of nanophotonics: plasmonics and single-molecule super-resolution microscopy. Plasmonics studies the collective oscillations of free electrons in a conductor, which enable light to be manipulated on subwavelength length scales. Plasmonics, and in particular plasmonic optical antennas, have generated a huge amount of interest due to their rich new physics and countless applications, ranging from surface-enhanced spectroscopies, to novel cancer therapies, and to quantum information platforms. With single-molecule fluorescence super-resolution microscopy, the optical properties of individual molecules can be studied with nanometer-scale resolution, far better than the micron scale of traditional microscopy. Super-resolution microscopy has revolutionized cellular biomedicine, ushering in a new generation of fundamental discoveries and associated medical therapies. Super-resolution microscopy is also increasingly enabling discoveries and advances across disciplines, allowing direct visualizations of phenomena ranging from chemical imaging of surface reactions to nanoscale fluid dynamics. By bringing together these two fields, this dissertation supports two synergistic directions for applications of this science: enhancing the resolution of single-molecule fluorescence super-resolution imaging and using a novel technique to directly study how a single emitter interacts with an optical antenna. In this dissertation, I present a new theoretical approach to understand the interaction of a single fluorescent molecule with an optical antenna, a broadly applicable new image analysis algorithm, and experimental measurements of antenna-modified fluorescence. The theoretical framework expands an established theory of antenna-modified fluorescence to incorporate the variability of real experiments. This research has uncovered a mislocalization effect: differences between the actual position of an emitter and the apparent, super-resolved position of the emitter image. I therefore present computational methods to predict the emission mislocalization of single fluorescent molecules coupled to an optical antenna and compare these predictions to experiments. I describe the SMALL-LABS algorithm, a general data analysis approach to accurately locating and measuring the intensity of single molecules, regardless of the shape or brightness of an obscuring background. Finally, I present the results of experiments studying the polarization dependence of coupling a single fluorescent molecule to a gold nanorod plasmonic optical antenna, and I compare these measurements with theoretical predictions. This work advances the fundamental science of nanophotonics and will pave the way for next generation super-resolution imaging and optical antenna technologies.PHDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145846/1/isaacoff_1.pd

Time-domain Modeling of Light Matter Interactions in Active Plasmonic Metamaterials

Metamaterials are artificially engineered to obtain unprecedented electromagnetic control leading to new and exciting applications. In order to further the understanding of fundamental optical phenomena and explore the effects of dynamically changing media on light propagation, numerous modeling methods have been developed. Among them, due to the nature of transient, nonlinear, and impulsive behavior, the time domain modeling approach is viewed as the most viable method. In this work, we develop a time-domain model (method of finite-difference time-domain (FDTD)) of light matter interactions in active plasmonic metamaterials. In order to model the dispersion of plasmonic nanostructure in the time-domain, we introduce a generalized dispersive material model built on Padé approximants. The developed 3D FDTD solver is then applied to study several plasmonic nanostructures and metamaterials, such as metal-dielectric composite films, random nano-nets for transparent conducting electrodes, and a graphene photodetector enhanced by a fractal plasmonic metasurface. In addition to this we also developed a multi-physics time-domain model to investigate the properties of a silver nanohole array coated with Rhodamine-101 dye. With accurate modeling of the retrieved kinetic parameters, the simulated emission intensity shows clear lasing, which is in good agreement with our experimental measurements. By tracing the population inversion and polarization dynamics, the amplification and lasing regimes inside the nanohole cavity can be clearly distinguished. With the help of our systematic approach, we further the understanding of time-resolved physics in active plasmonic nanostructures with gain

Plasmonic Nanoplatforms for Biochemical Sensing and Medical Applications

Plasmonics, the science of the excitation of surface plasmon polaritons (SPP) at the metal-dielectric interface under intense beam radiation, has been studied for its immense potential for developing numerous nanophotonic devices, optical circuits and lab-on-a-chip devices. The key feature, which makes the plasmonic structures promising is the ability to support strong resonances with different behaviors and tunable localized hotspots, excitable in a wide spectral range. Therefore, the fundamental understanding of light-matter interactions at subwavelength nanostructures and use of this understanding to tailor plasmonic nanostructures with the ability to sustain high-quality tunable resonant modes are essential toward the realization of highly functional devices with a wide range of applications from sensing to switching. We investigated the excitation of various plasmonic resonance modes (i.e. Fano resonances, and toroidal moments) using both optical and terahertz (THz) plasmonic metamolecules. By designing and fabricating various nanostructures, we successfully predicted, demonstrated and analyzed the excitation of plasmonic resonances, numerically and experimentally. A simple comparison between the sensitivity and lineshape quality of various optically driven resonances reveals that nonradiative toroidal moments are exotic plasmonic modes with strong sensitivity to environmental perturbations. Employing toroidal plasmonic metasurfaces, we demonstrated ultrafast plasmonic switches and highly sensitive sensors. Focusing on the biomedical applications of toroidal moments, we developed plasmonic metamaterials for fast and cost-effective infection diagnosis using the THz range of the spectrum. We used the exotic behavior of toroidal moments for the identification of Zika-virus (ZIKV) envelope proteins as the infectious nano-agents through two protocols: 1) direct biding of targeted biomarkers to the plasmonic metasurfaces, and 2) attaching gold nanoparticles to the plasmonic metasurfaces and binding the proteins to the particles to enhance the sensitivity. This led to developing ultrasensitive THz plasmonic metasensors for detection of nanoscale and low-molecular-weight biomarkers at the picomolar range of concentration. In summary, by using high-quality and pronounced toroidal moments as sensitive resonances, we have successfully designed, fabricated and characterized novel plasmonic toroidal metamaterials for the detection of infectious biomarkers using different methods. The proposed approach allowed us to compare and analyze the binding properties, sensitivity, repeatability, and limit of detection of the metasensing device

Bead Mediated Microscopy: from high resolution microscopy to nano-Raman

Solid-state physics, material science, as well as biology, need continuously more and more information from their samples. High spatial resolution information such as optical or electrical properties, chemical species identification as well as topography are important information that optical microscopy or Scanning Probe Microscopy (SPM) can provide. Although electron microscopy (SEM and TEM) certainly assumes a position of absolute importance in the field, its cost and its need to be used by highly specialised personnel still make it an instrument of limited everyday use. On the contrary, probe microscopy has now become of very high diffusion in research labs. To develop my thesis I focused myself on three main and somehow related microscopy techniques: high resolution Raman microscopy, Scanning Near-field Optical Microscopy (SNOM), and Tip Enhanced Raman Spectroscopy (TERS). All of them are state-of-the-art on surface optical analysis techniques but still present relevant limits; among others, respectively: spatial resolution, local power density, complexity and field of applicability. My approach wants to combine some aspects of these techniques to go beyond their limits. Raman spectroscopy is a powerful optical technique, which measures the inelastic scattering of an incoming EM radiation due to the vibrational modes of the molecules present on the surface of a sample. Thanks to its high specificity, it is very powerful in identifying the chemical components of a sample. Several organic and inorganic molecules have their typical Raman spectral peaks, hence, by the Raman spectra, it\u2019s possible to provide a qualitative and quantitative analysis of the elements of a sample. High spatial resolution Raman setups uses the combination of a confocal microscope with a spectrometer assisted by a series of long pass and band pass filters. Despite its extreme versatility, basing Raman spectroscopy on a confocal system also constrains it to acquire its limit in spatial resolution determined by the limit of diffraction. To overcome this limit the most used techniques in SPM are Scanning Near-field Optical Microscopy (SNOM) and Tip Enhanced Raman Spectroscopy (TERS). Both of them exploits evanescent field, which is an electric field that is created by oscillating charges and/or currents and does not propagate in the far field as a classical electromagnetic wave, but is spatially concentrated very near to its source. This confinement allows to obtain field sources definitely smaller than in confocal systems. In SNOM technique, the excitation light is focused through an aperture smaller than the wavelength, creating an evanescent field strongly localized near the aperture itself. Scanning the sample in this near range brings the spatial resolution down to the aperture dimension. The main disadvantage of aperture SNOM is that the overall optical efficiency of probes is very low. The excitation power cannot be too high in order to prevent any damage of the probe, hence the energy that reaches the sample is usually not enough for Raman analysis. TERS instead is more suitable for this purpose. It basically exploits Surface Enhanced Raman Spectroscopy (SERS) principles, using a laser irradiated gold sharp tip to obtain a local enhancement at its apex. Its good efficiency permits to analyze Raman effects with a spatial super-resolution, but, on the other hand, TERS probes usually lack of reprodubility and require very skilled and specialised users. My PhD project has been focused to investigate and optimize an original approach to perform high resolution optical microscopy and Raman spectroscopy, well below the diffraction limit. The concept is to exploit the optical proprieties of a dielectric micro bead lens to achieve a powerful nanoscale near field confinement of light and the Scanning Probe Microscopy (SPM) technique to scan a sample to acquire optical maps. When a dielectric micro bead is hit by an Electromagnetic (EM) wave its effect is to transmit and concentrate the incident EM radiation in a specific area called nanojet, at first glance similar to that created with a standard lens. Some optical proprieties of the nanojets have been already introduced in the literature, but their application in the world of SPM, their employment in Raman microscopy and their combination with nanostructures to improve the spatial resolution are novel features whose investigation is promising. I gave to this technique the name of Beam Mediated Microscopy (BeMM). The combination of super resolution bead mediated SPM with Raman spectroscopy opens interesting perspectives about powerful surface analysis for samples that need a versatile optical probe with a high spatial resolution and soft interaction with the sample, like soft matter substrates or biosamples

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis