Stephen J. Bauman

Fabrication of Sub-Lithography-Limited Structures via Nanomasking Technique for Plasmonic Enhancement Applications

Fabrication of sub-10 nm features is advantageous for continued improvement of plasmonic enhancement applications. This letter demonstrates a technique, called nanomasking, which can fabricate nanostructures and gaps below the resolution limit of the lithography technique used in the process: nanogaps that are less than 10 nm in width, and nanostructures with 15 nm widths. This method improves upon existing techniques for creating metallic features at this scale, and it is also scalable for mass production. Unique structures that can be fabricated with this technique have been optically investigated to provide evidence as to their potential plasmonic applications.

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 structures fabricated via nanomasking sub-10 nm lithography technique

Making use of a newly established nanomasking technique, nanoscale features (sub-10 nm) have been fabricated with the potential to act as plasmonic enhancement structures. The technique makes use of a two-step lithography process to simultaneously produce many plasmonic hotspots with two-dimensional features over a large area, showing promise for mass production scalability. This technique is highly reproducible, reliably patterning multiple nanostructures and nanogaps over a potentially wafer-scale area without significantly increasing the number of steps required. Fabrication results show promise for scalability towards applications such as biosensing, photovoltaics, and enhanced spectroscopies.

Optical nanogap matrices for plasmonic enhancement applications

Plasmonic structures can be used to enhance electromagnetic radiation, and nanoscale (<5 nm) gaps can increase this enhancement even further. Fabrication of these desired structures involves using a relatively new, previously developed self-aligned process to overcome typical electron beam lithography resolution limits. The resulting nanogap structures have been shown to exhibit enhanced optical emission. This technique enables the fabrication of a large-area two-dimensional matrix of such nanostructures which could prove useful for photovoltaics, plasmonically enhanced Raman spectroscopy, biosensing, and other optoelectronic applications. Computational electromagnetic simulations of the structures will prove useful for predicting behavior upon interaction with light and for experimental comparison.

Tuning Infrared Plasmon Resonance of Black Phosphorene Nanoribbon with a Dielectric Interface

We report on the tunable edge-plasmon-enhanced absorption of phosphorene nanoribbons supported on a dielectric substrate. Monolayer anisotropic black phosphorous (phosphorene) nanoribbons are explored for light trapping and absorption enhancement on different dielectric substrates. We show that these phosphorene ribbons support infrared surface plasmons with high spatial confinement. The peak position and bandwidth of the calculated phosphorene absorption spectra are tunable with low loss over a wide wavelength range via the surrounding dielectric environment of the periodic nanoribbons. Simulation results show strong edge plasmon modes and enhanced absorption as well as a red-shift of the peak resonance wavelength. The periodic Fabry-Perot grating model was used to analytically evaluate the absorption resonance arising from the edge of the ribbons for comparison with the simulation. The results show promise for the promotion of phosphorene plasmons for both fundamental studies and potential applications in the infrared spectral range.

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.