Ahmad A. Darweesh

Improved optical enhancement using double-width plasmonic gratings with nanogaps

Plasmonic grating structures have been shown effective at increasing near-field optical enhancement. A double-width plasmonic grating design is introduced, where each period has two alternating metal widths separated by a nanogap. With this new design, analysis has shown that plasmonic resonances couple between each metal section, resulting in even greater optical enhancement compared with single-width gratings. The geometry that gives the greatest optical enhancement has been determined with a computational model. This work demonstrates that the increased enhancement is due to hybridized modes that couple between the two grating segments.

Substrate Oxide Layer Thickness Optimization for a Dual-Width Plasmonic Grating for Surface-Enhanced Raman Spectroscopy (SERS) Biosensor Applications

This work investigates a new design for a plasmonic SERS biosensor via computational electromagnetic models. It utilizes a dual-width plasmonic grating design, which has two different metallic widths per grating period. These types of plasmonic gratings have shown larger optical enhancement than standard single-width gratings. The new structures have additional increased enhancement when the spacing between the metal decreases to sub-10 nm dimensions. This work integrates an oxide layer to improve the enhancement even further by carefully studying the effects of the substrate oxide thickness on the enhancement and reports ideal substrate parameters. The combined effects of varying the substrate and the grating geometry are studied to fully optimize the device’s enhancement for SERS biosensing and other plasmonic applications. The work reports the ideal widths and substrate thickness for both a standard and a dual-width plasmonic grating SERS biosensor. The ideal geometry, comprising a dual-width grating structure atop an optimal SiO2 layer thickness, improves the enhancement by 800%, as compared to non-optimized structures with a single-width grating and a non-optimal oxide thickness.

Modeling and optimization of Au-GaAs plasmonic nanoslit array structures for enhanced near-infrared photodetector applications

Abstract. This theoretical work explores how various geometries of Au plasmonic nanoslit array structures improve the total optical enhancement in GaAs photodetectors. Computational models studied these characteristics. Varying the electrode spacing, width, and thickness drastically affected the enhancement in the GaAs. Peaks in enhancement decayed as Au widths and thicknesses increased. These peaks are resonant with the incident near-infrared wavelength. The enhancement values were found to increase with decreasing electrode spacing. Additionally, a calculation was conducted for a model containing Ti between the Au and the GaAs to simulate the necessary adhesion layer. It was found that optical enhancement in the GaAs decreases for increasing Ti layer thickness. Optimal dimensions for the Au electrode include a width of 240 nm, thickness of 60 nm, electrode spacing of 5 nm, and a minimum Ti thickness. Optimal design has been shown to improve enhancement to values that are up to 25 times larger than for nonoptimized geometries and up to 300 times over structures with large electrode spacing. It was also found that the width of the metal in the array plays a more significant role in affecting the field enhancement than does the period of the array.

Plasmonic Au Array SERS Substrate with Optimized Thin Film Oxide Substrate Layer

This work studies the effect of a plasmonic array structure coupled with thin film oxide substrate layers on optical surface enhancement using a finite element method. Previous results have shown that as the nanowire spacing increases in the sub-100 nm range, enhancement decreases; however, this work improves upon previous results by extending the range above 100 nm. It also averages optical enhancement across the entire device surface rather than localized regions, which gives a more practical estimate of the sensor response. A significant finding is that in higher ranges, optical enhancement does not always decrease but instead has additional plasmonic modes at greater nanowire and spacing dimensions resonant with the period of the structure and the incident light wavelength, making it possible to optimize enhancement in more accessibly fabricated nanowire array structures. This work also studies surface enhancement to optimize the geometries of plasmonic wires and oxide substrate thickness. Periodic oscillations of surface enhancement are observed at specific oxide thicknesses. These results will help improve future research by providing optimized geometries for SERS molecular sensors.

Fabrication and analysis of metallic nanoslit structures: advancements in the nanomasking method

Abstract. This work advances the fabrication capabilities of a two-step lithography technique known as nanomasking for patterning metallic nanoslit (nanogap) structures with sub-10-nm resolution, below the limit of the lithography tools used during the process. Control over structure and slit geometry is a key component of the reported method, exhibiting the control of lithographic methods while adding the potential for mass-production scale patterning speed during the secondary step of the process. The unique process allows for fabrication of interesting geometric combinations such as dual-width gratings that are otherwise difficult to create with the nanoscale resolution required for applications, such as nanoscale optics (plasmonics) and electronics. The method is advanced by introducing a bimetallic fabrication design concept and by demonstrating blanket nanomasking. Here, the need for the secondary lithography step is eliminated improving the mass-production capabilities of the technique. Analysis of the gap width and edge roughness is reported, with the average slit width measured at 7.4±2.2  nm. It was found that while no long-range correlation exists between the roughness of either gap edge, and there are ranges in the order of tens of nanometers over which the slit edge roughness is correlated or anticorrelated across the gap. This work helps quantify the nanomasking process, which aids in future fabrications and leads toward the development of more accurate computational models for the optical and electrical properties of fabricated devices.

Surface-enhanced Raman spectroscopy substrate fabricated via nanomasking technique for biological sensor applications

The nanomasking fabrication technique has been shown to be capable of producing many sub-10 nm gaps between metallic structures over a wafer-scale area. This provides the opportunity to utilize the technique in spectroscopy signal enhancement applications. Here we describe a device designed via nanomasking that holds potential as a surface enhanced Raman spectroscopy (SERS) substrate for biosensing or other applications. The high density of plasmonic hotspot nanogaps improves the feasibility of these types of patterns for signal enhancement, as it provides ease of use and increased speed of sample deposition for taking spectrum. The ability to fabricate these patterns with high repeatability at mass production scale is another benefit of nanomasking-fabricated spectroscopy substrates. This work demonstrates tests of fabricated devices for use in a custom Raman spectroscopy system as a potential source of signal enhancement. Also, theoretical enhancement results are calculated for comparison via computational electromagnetic studies.

Improved optical enhancement in binary plasmonic gratings with nanogap spacing

This work thoroughly investigates binary nanowire gratings with nanogap spacing. A binary plasmonic grating is a periodic nanostructure for which each period has two different widths. The study has determined that plasmonic gratings with two different widths in each period give rise to optical enhancement that is 2.1 times stronger than that of standard plasmonic grating structures. A map of varying width ratios has been created to illustrate the key geometric characteristic for enhancement optimization. The structure under investigation was a gold structure with a constant height of 15 nm and a nanogap of 5 nm. The period size of the structure depends on the two nanowire widths in each grating period. The optical enhancement (E/E0)2 of the geometry was investigated using a finite element method (FEM) simulation for various wavelengths. The results show a strong correlation between the plasmon wavelength and the periodicity of the gratings. Additionally, the plasmonic charge distributions have been calculated for various periods and geometries. Various resonant modes exist for the charge distribution, significantly affecting the enhancement depending on the nanowire widths.

The Role of Rayleigh-Wood Anomalies and Surface Plasmons in Optical Enhancement for Nano-Gratings

We propose and report on the design of a 1-D metallo-dielectric nano-grating on a GaAs substrate. We numerically study the impact of grating period, slit and wire widths, and irradiating angle of incidence on the optical response. The optimal wire width, w = 160 nm, was chosen based on previous results from investigations into the influence of wire width and nano-slit dimensions on optical and electrical enhancements in metal-semiconductor-metal photodetectors. In this present project, resonant absorption and reflection modes were observed while varying the wire and nano-slit widths to study the unique optical modes generated by Rayleigh-Wood anomalies and surface plasmon polaritons. We observed sharp and diffuse changes in optical response to these anomalies, which may potentially be useful in applications such as photo-sensing and photodetectors. Additionally, we found that varying the slit width produced sharper, more intense anomalies in the optical spectrum than varying the wire width.

Plasmonic resonance shift for various nanodevice geometries

Plasmonic nanodevices are metallic structures that exhibit plasmonic effects when exposed to light, causing scattering and enhancement of that light. These plasmons makes it possible for light to be focused below the diffraction limit. Dark-field spectroscopy has been used to capture the scattering spectra of these structures in order to examine the scattering and resonant frequencies of the plasmons provided by the devices. The geometries of the devices change which wavelengths of light are most readily able to couple to the device, resulting in a change in the wavelength of the scattered light. A variety of device geometries and configurations will be studied, including nanodiscs, nanowires, and plasmonic gratings, along with double-width nanogap plasmonic gratings. These new structures will have features below the fabrication limit of electron-beam lithography, i.e. sub-10 nanometer features. The polarization dependencies of these resonance modes are investigated as well. A relation between device geometry and wavelength will be drawn; in effect, this will allow the selection of geometry of the fabricated device based on the desired wavelength of light to be scattered. Preliminary Raman spectroscopy will also be performed in order to study the device response and usefulness for surface-enhanced Raman spectroscopy.

Dual-width plasmonic gratings with sub-10 nm gaps for biosensor applications

Fabrication of dual-width plasmonic gratings with sub-10 nm gaps has been made possible by a recently developed technique. Studying the effects of various material and geometrical parameters on the optical response of these gratings will prove useful to future fabrication of devices. The ability to tune the widths of both wires in the periodic array allows for optimization of the response based not only on one nanowire geometry, but the hybridization of the two. The structures hold potential to be used as a substrate for surface-enhanced Raman spectroscopy (SERS) in the detection of different chemical analytes, with biosensing as a major area of interest. The ability to tune the structures to different wavelengths makes this a potentially attractive method of fabricating sensor substrates capable of enhancing otherwise weak analyte signals. Here, preliminary computational results are shown for a study of the effects of a SiO2 layer on the substrate containing a plasmonic grating