Electric field enhancement with plasmonic colloidal nanoantennas excited by a silicon nitride waveguide

We investigate the feasibility of CMOS-compatible optical structures to develop novel integrated spectroscopy systems. We show that local field enhancement is achievable utilizing dimers of plasmonic nanospheres that can be assembled from colloidal solutions on top of a CMOS-compatible optical waveguide. The resonant dimer nanoantennas are excited by modes guided in the integrated silicon nitride waveguide. Simulations show that 100 fold electric field enhancement builds up in the dimer gap as compared to the waveguide evanescent field amplitude at the same location. We investigate how the field enhancement depends on dimer location, orientation, distance and excited waveguide modes

Two-scale structure for giant field enhancement: combination of Rayleigh anomaly and colloidal plasmonic resonance

We demonstrate theoretically and experimentally a two-scale architecture able to achieve giant field enhancement by simultaneously exploiting both the Rayleigh anomaly and localized surface plasmon resonance. Metallic oligomers composed of colloidal nanospheres are well-known for the ability to strongly enhance the near-field at their plasmonic resonance. However, due to intrinsic nonlocality of the dielectric response of the metals along with their inherent loss, the achievable field enhancement has an ultimate constraint. In this paper we demonstrate that combining plasmonic resonance enhancements from oligomers, with feature size of tens of nanometers, with a Rayleigh anomaly caused by a 1-D set of periodic nanorods, having a period on the order of the excitation wavelength, provides a mean to produce enhancement beyond that constrained by losses in near field resonances. Metallic oligomers are chemically assembled in between the periodic set of nanorods that are fabricated using lithographic methods. The nanorod periodicity is investigated to induce the Rayleigh anomaly at the oligomers plasmonic resonance wavelength to further enhance the field in the oligomers hot spots. A thorough study of this structure is carried out by using an effective analytical-numerical model which is also compared to full-wave simulation results. Experimental results comparing enhancements in surface enhanced Raman scattering measurements with and without nanorods demonstrate the effectiveness of a Rayleigh anomaly in boosting the field enhancement. The proposed structure is expected to be beneficial for many applications ranging from medical diagnostics to sensors and solar cells

Optical Nanoantennas for Sensors, Microscopy and Spectroscopy

Metallic nanosctructures possess electromagnetic properties useful for various applications, such as surface enhanced Raman spectroscopy (SERS), harmonic generation and solar cells. Specifically, because of their ability in confining electromagnetic fields to nanoscopic dimensions, they provide an exquisite platform for conducting studies on molecules placed in their vicinity. In this dissertation, I introduce and study various plasmonic nanostructures to achieve electric and magnetic field enhancement for spectroscopy and microscopy applications. First, I investigate feasibility of CMOS-compatible optical structures to develop novel integrated spectroscopy systems. I show that local field enhancement is achievable utilizing dimers of plasmonic nanospheres that can be assembled from colloidal solutions on top of a CMOS-compatible optical waveguide. The resonant dimer nanoantennas are excited by modes guided in the integrated silicon nitride waveguide. Specifically, I investigate how the field enhancement depends on dimer location, orientation, distance and excited waveguide mode. However, the field enhancement achievable with using oligomers is limited due to inherent losses of plasmonic particles. Thus, I study a novel structure called two-scale, in which Rayleigh anomaly caused by a 1D set of periodic nanorods is utilized. A thorough study of this structure is carried out by using an effective analytical-numerical model which is also compared to full-wave simulation results. Experimental results comparing enhancements in surface enhanced Raman scattering measurements with and without nanorods demonstrate the effectiveness of a Rayleigh anomaly in boosting the field enhancement.The other side of the dissertation is dedicated to magnetic field enhancement. I propose various metallic and dielectric nanostructures for local magnetic field enhancement at optical frequencies. I show that dielectric structures can be a good alternative for their plasmonic counterpart due to their low loss. The idea behind each structure is supported by results from full-wave simulations. More importantly, I utilize azimuthally polarized beam as a way of boosting local magnetic field and isolating it from electric field. The magnetic field enhancement of these structures can be utilized in studying magnetic dipole transitions, magnetic imaging and enhanced spectroscopy applications

Electric Field Enhancement by Two-scale Structure

We propose a novel multi-length-scale architecture for giant electric field enhancement. We investigate the capability of our structure to boost the electric field analytically and using fullwave simulations and verify our results with surface-enhanced Raman spectroscopy experiment

Bridging the Gap between Crosslinking Chemistry and Directed Assembly of Metasurfaces Using Electrohydrodynamic Flow

Advances in understanding chemical and physical driving forces in self-assembly allow the fabrication of unique nanoarchitectures with subwavelength building blocks as the basis for plasmonic and metamaterial devices. Chemical crosslinking of colloidal nanospheres has produced among the smallest gap spacings, necessary to obtain regions of greatly enhanced electric field, hotspots, which are critical to tailor light-matter interactions. However, obtaining uniform electromagnetic response of dense nanoantennas over large area for use in devices remains challenging. In this work, electrohydrodynamic (EHD) flow and chemical crosslinking is combined to form dense, yet discrete, Au nanosphere clusters (oligomers) on a working electrode. EHD provides a long range driving force to bring nanospheres together and anhydride crosslinking yields 0.9 nm gap spacings. Using selective chemistry, nanospheres are simultaneously crosslinked onto a block copolymer template, producing oligomers with a narrower wavelength band width and higher hotspot intensity than monolayer structures produced without a template. We investigate nanoantenna response via full wave simulations, ultraviolet-visible spectroscopy, and surface enhanced Raman scattering (SERS). Nanoantennas exhibit uniform hotspot intensity and gap spacing. Simulations show field enhancements of 600, correlating well with measured average SERS enhancement of 1.4x10^9. Nanoantenna substrates produce a SERS signal with a relative standard deviation of 10% measured over a 1 mm2 area, crucial for nano-optical devices such as optical sensors, among other applications. Understanding long range (EHD flow) and short range (chemical crosslinking) driving forces provides the control for assembling colloidal nanoparticles in architectures for large area plasmonic and metasurface device fabrication

Bridging the Gap between Crosslinking Chemistry and Directed Assembly of Metasurfaces Using Electrohydrodynamic Flow

Advances in understanding chemical and physical driving forces in self-assembly allow the fabrication of unique nanoarchitectures with subwavelength building blocks as the basis for plasmonic and metamaterial devices. Chemical crosslinking of colloidal nanospheres has produced among the smallest gap spacings, necessary to obtain regions of greatly enhanced electric field, hotspots, which are critical to tailor light-matter interactions. However, obtaining uniform electromagnetic response of dense nanoantennas over large area for use in devices remains challenging. In this work, electrohydrodynamic (EHD) flow and chemical crosslinking is combined to form dense, yet discrete, Au nanosphere clusters (oligomers) on a working electrode. EHD provides a long range driving force to bring nanospheres together and anhydride crosslinking yields 0.9 nm gap spacings. Using selective chemistry, nanospheres are simultaneously crosslinked onto a block copolymer template, producing oligomers with a narrower wavelength band width and higher hotspot intensity than monolayer structures produced without a template. We investigate nanoantenna response via full wave simulations, ultraviolet-visible spectroscopy, and surface enhanced Raman scattering (SERS). Nanoantennas exhibit uniform hotspot intensity and gap spacing. Simulations show field enhancements of 600, correlating well with measured average SERS enhancement of 1.4x10^9. Nanoantenna substrates produce a SERS signal with a relative standard deviation of 10% measured over a 1 mm2 area, crucial for nano-optical devices such as optical sensors, among other applications. Understanding long range (EHD flow) and short range (chemical crosslinking) driving forces provides the control for assembling colloidal nanoparticles in architectures for large area plasmonic and metasurface device fabrication

Self-Assembled Plasmonic Nanogaps

Pseudomonas aeruginosa is an opportunistic, biofilm-forming pathogen. P. aeruginosa produces pyocyanin, a secondary metabolite as part of its quorum sensing signaling system activated during biofilm formation. Self-organized plasmonic nanogaps provide enhancements to Raman scattering signals to achieve 10 ng/mL limit of detection of pyocyanin that enables early detection of biofilm formation. We report, for the first time, in-line biofilm detection in microfluidic channels as early as three hours from the onset of bacterial culturing and quantification spanning five-orders of magnitude of pyocyanin concentration during biofilm formation of PA14

Surface enhanced Raman scattering for detection of Pseudomonas aeruginosa quorum sensing compounds

Pseudomonas aeruginosa (PA), a biofilm forming bacterium, commonly affects cystic fibrosis, burn victims, and immunocompromised patients. PA produces pyocyanin, an aromatic, redox active, secondary metabolite as part of its quorum sensing signaling system activated during biofilm formation. Surface enhanced Raman scattering (SERS) sensors composed of Au nanospheres chemically assembled into clusters on diblock copolymer templates were fabricated and the ability to detect pyocyanin to monitor biofilm formation was investigated. Electromagnetic full wave simulations of clusters observed in scanning electron microcopy images show that the localized surface plasmon resonance wavelength is 696 nm for a dimer with a gap spacing of 1 nm in an average dielectric environment of the polymer and analyte; the local electric field enhancement is on the order of 400 at resonance, relative to free space. SERS data acquired at 785 nm excitation from a monolayer of benzenethiol on fabricated samples was compared with Raman data of pure benzenethiol and enhancement factors as large as 8×109 were calculated that are consistent with simulated field enhancements. Using this system, the limit of detection of pyocyanin in pure gradients was determined to be 10 parts per billion. In SERS data of the supernatant from the time dependent growth of PA shaking cultures, pyocyanin vibrational modes were clearly observable during the logarithmic growth phase corresponding to activation of genes related to biofilm formation. These results pave the way for the use of SERS sensors for the early detection of biofilm formation, leading to reduced healthcare costs and better patient outcomes

In pursuit of photo-induced magnetic and chiral microscopy★

Light-matter interactions enable the perception of specimen properties such as its shape and dimensions by measuring the subtle differences carried by an illuminating beam after interacting with the sample. However, major obstacles arise when the relevant properties of the specimen are weakly coupled to the incident beam, for example when measuring optical magnetism and chirality. To address this challenge we propose the idea of detecting such weakly-coupled properties of matter through the photo-induced force, aiming at developing photo-induced magnetic or chiral force microscopy. Here we review our pursuit consisting of the following steps: (1) Development of a theoretical blueprint of a magnetic nanoprobe to detect a magnetic dipole oscillating at an optical frequency when illuminated by an azimuthally polarized beam via the photo-induced magnetic force; (2) Conducting an experimental study using an azimuthally polarized beam to probe the near fields and axial magnetism of a Si disk magnetic nanoprobe, based on photo-induced force microscopy; (3) Extending the concept of force microscopy to probe chirality at the nanoscale, enabling enantiomeric detection of chiral molecules. Finally, we discuss difficulties and how they could be overcome, as well as our plans for future work