We propose plasmonic switches based on nanoantennas with fractal features on top of vanadium dioxide (VO2) thin films. These plasmonic switches can be devised by utilizing various kinds of fractals like – the Sierpenski fractal or the Koch fractal. When exposed to heat, voltage, or infrared radiation, the VO2 thin film undergoes a phase transition from its insulator state to its metal state, thus leading to switching in the total optical behavior of the proposed switch. In this paper, the switching performance characteristics of the near-field plasmonic switches (NFPS) are numerically analyzed. As the iterations of the fractal features of the switch are increased, the electric-field intensity is enhanced during ON state of the NFPS and the electric-field intensity is reduced during OFF state of the NFPS. We also employ Finite Difference Time Domain (FDTD) analysis to numerically analyze the VO2 layer thickness effect on the performance of the NFPS. These plasmonic switches possess the potential to be used as elementary switching devices in computing networks and optical communication networks.
We present 2-D arrays of plasmonic nanostructures ─ gold nanopillars on a gold coated substrate ─ coated with a thin film of VO2 (vanadium dioxide), as novel optical switches. The incident optical radiation is coupled into plasmonic modes due to the small gaps between the adjacent nanostructures, which leads to high electromagnetic fields in the gap regions. As VO2 changes phase from the semiconducting to the metallic phase on heating or on application of voltage, there is a change in the overall plasmonic properties of the VO2 coated 2-D plasmonic nanostructures, and a change in the reflectance spectra. We employ Rigorous Coupled Wave Analysis (RCWA) to calculate the switchability, i.e., the differential reflectance between the semiconducting and metallic state of the VO2 coated nanostructures. An exhaustive analysis of the differential reflectance spectra is carried out, by optimizing the different geometrical parameters of the structure. Moreover, we demonstrate that these 2-D arrays of plasmonic nanostructures can be employed for switching with unpolarized light. These nanostructures can also be employed for multi-wavelength switching if a degree of asymmetry is introduced with different gaps or periodicities in the two dimensions. Thus, we propose 2-D VO2 coated nanostructures which can be employed as plasmonic switches with unpolarized light.
The current photovoltaic module market is dominated by silicon solar cells, whose development is limited by high costs of manufacturing processes. The search for easy low temperature fabrication techniques has spurred the development of solar cells based on organic semiconductor polymers. Recent studies have reported polymer based solar cells with comparable power conversion efficiencies to those of commercially available silicon solar cell modules. In the face of these advances, higher efficiencies are still desirable to better utilize the available solar energy for power generation. Organic semiconducting polymers have a high coefficient of absorption, but short carrier path lengths which necessitates the fabrication of thin layers for optimal power generation. The introduction of plasmonic effects in these organic solar cells leads to an increase in the optical path length of the incident light in the active layer, thereby increasing the short circuit current density. In this work, an organic solar cell is presented which contains metal-dielectric core-shell plasmonic nanoparticles. Finite difference time domain (FDTD) modelling has been used to simulate the models of light interaction with the organic solar cells containing different metal@dielectric nanoparticle composites. The different parameters of the nanoparticle composites in the organic solar cells were varied to the study the absorption enhancement in the active layer medium. The results, thus obtained for enhanced performance, were used for the chemical synthesis of the metal@dielectric nanoparticle composites and fabrication of organic solar cells with high power conversion efficiency.
An integrated high-speed novel electro-optic power splitter based on interleaved PN junction in a silicon microring resonator has been proposed, which can be used as an ultra-fast, dynamically tunable optical power splitter to efficiently distribute power between various optical stations in Network-on-chip (NoC). The main working principle behind such a device is that we can change the refractive index of the PN junction waveguide under an application of an external reverse bias voltage, enabling the resonator to work as a power splitter. The interleaved PN junction waveguides utilizing the features of greater overlap of the optical waveguide mode with the depletion layer compared to normal PN junction, provide the opportunity to maximize the change in refractive index with minimum applied reverse bias voltage. The carriers (electrons and holes) are depleted near to the p region and n region interface on applying a reverse bias voltage. The depletion region increases with the applied reverse bias voltage, which enhances more interaction with the propagating light. To evaluate the performance of the splitter, we have calculated the change in the effective refractive index and the optical loss due to the free carrier concentrations. The proposed optical power splitter is capable of tuning the power splitting ratio from 0.15 to 7.82, which is a very wide range of tunability with low power consumption (35 mW) and low external voltage (0-3.5 V). We have proposed the use of a high-speed Digital to Analog converter (DAC) to apply an external reverse bias voltage to the interleaved PN junction waveguide.
Organic semiconducting polymers, due to their tunable optical and electronic properties and ease of fabrication processes, are useful in numerous photonic applications. They have found increased interest in the field of photovoltaics due to their comparative low cost as compared to current commercial silicon solar cell modules. The introduction of plasmonic effects in these organic polymer-based solar cells leads to better performance characteristics of these cells. The plasmonic nanoparticles, which can be placed in the different layers of the organic solar cell (OSC), scatter light into the active layer thereby increasing the optical path length of the incident light leading to higher absorption and short circuit current density of the OSC. In this paper, an organic solar cell based on a low bandgap polymer blend and containing complex plasmonic metal nanoparticles has been presented. Finite difference time domain (FDTD) method has been used to simulate models to study the interaction of incident light with the OSCs containing the plasmonic nanoparticles and then compare their performance with that of the OSCs without the nanoparticles. The effect of varying nanoparticle and solar cell parameters on the absorption enhancement of the OSC was studied to determine the best configuration for fabrication. Short circuit current density enhancement of 19.3% was obtained in the OSC containing the nanoparticles. The plasmonic nanoparticles, thus obtained, were synthesized by chemical processing to be introduced in OSCs with different active layer materials for high power conversion efficiency.
We propose plasmonic nanostructures - a 2D array of circular ‘nanopillars inside square nanoholes’ - as polarization independent SERS substrates for portable detection of chemical and biological molecules. These substrates were fabricated in a reproducible and controllable manner on a wafer-scale using a combination of deep-UV lithography, reactive ion etching (RIE) and E-Beam evaporation. The SERS spectra were measured using a portable Raman spectrometer to demonstrate portable SERS based sensing and the limit of detection was found to be ~ 13.14 femtogram for the detection of 2,4-DNT. Furthermore, numerical modeling of the proposed substrates was carried out using Finite Difference Time Domain (FDTD) modeling to study the effect of structural parameters on the electromagnetic enhancement factor and the resonance wavelength. Moreover, based on numerical simulations and experimental results, it was found that the SERS signal from these SERS substrates is only slightly dependent on polarization. Thus, the proposed SERS substrate can be employed for polarization independent SERS-based trace detection of chemical and biological molecules in real-time field settings using a portable Raman spectrometer.
KEYWORDS: Solar cells, Plasmonics, Absorption, Nanostructures, Finite-difference time-domain method, Thin film solar cells, Microcrystalline materials, Silicon solar cells, Surface plasmons
Due to the high cost of conventional crystalline silicon solar cells, researchers have been attracted towards the development of thin-film Si solar cells, where a several hundred nanometers thick amorphous Si (a-Si) or microcrystalline Si (μc-Si) solar cell layer is deposited by plasma-enhanced chemical vapor deposition (PECVD). This paper presents the use of plasmonic nanostructures in μc-Si p-i-n junction thin-film solar cells to increase the absorption in a broad spectral range. Finite-difference time-domain (FDTD) simulation results demonstrate a broadband absorption enhancement in these solar cells due to plasmonic nanostructures. The enhancement in the absorption is attributed to the enhanced electromagnetic fields in the active layer due to the excitation of surface plasmon modes and photonic Bloch modes at multiple wavelengths. Moreover, the plasmonic nanostructures lead to a significant enhancement in the shortcircuit current density of the μc-Si thin-film solar cell.
In this work, we present surface enhanced Raman scattering (SERS) based sensor chips for applications in nanomedicine. Finite Difference Time Domain (FDTD) simulations in visible, infrared and near-infrared regimes were done to model electric field enhancement in the vicinity of plasmonic nanostructures. Some of the plasmonic nanostructures simulated were present bowtie nanohole arrays and bridged-bowtie nanohole arrays in a gold thin film. Surface enhanced Raman scattering (SERS) substrates based on these nanostructures exhibit large electromagnetic enhancement of SERS. We employ numerical simulations based on the finite difference time domain (FDTD) method to determine the electric field enhancement factors (EFs) and therefore the electromagnetic SERS enhancement factor for these SERS substrates. It was observed that the resonance wavelength of these arrays of nanoholes can be tuned by altering the size of the nanoholes. It was also observed that bridged-bowtie nanohole arrays exhibit very high electric field enhancement factors (EF) for multiple wavelengths. It was observed that bridged-bowtie nanohole arrays exhibit a highest electromagnetic SERS enhancement factor (EF) of ~ 109, which is orders of magnitude higher than what has been previously reported for nanohole arrays as SERS substrates. Hence, these nanostructures can provide SERS enhancement suitable for a few-molecule detection.
We present the design and modeling of novel electro-optic modulators and switches that have large extinction ratios, such that these electro-optic modulators and switches operate at the optical communication wavelength range (around 1550 nm). Firstly, we describe the design of an electro-optic modulator based on a tunable slotted ring resonator, having two pairs of partially overlapping graphene layers above and below of the slotted ring (in some portion of the circumference). We demonstrate that the transmission of light through the through port can be modulated by the application of voltage across the graphene layers. Secondly, we discuss the design of electro-optic switches using phase change materials either in a micro disk resonator or in a photonic crystal slab waveguide. These devices are based on the shift in the resonant frequency of a micro disk resonator and on the shift in the photonic bandgap of the photonic crystal slab waveguide, respectively, when its refractive index changes upon the application of voltage across the phase change material. A three dimensional finite-difference time-domain modeling software (Lumerical FDTD) was used for optical modeling and a commercial device modeling software (Lumerical DEVICE) was for the electrical modeling. The proposed electro-optic modulators and electro-optic switches can be used in optoelectronics, as well in the telecom wavelength range.
In this paper, we describe the design and modeling of novel long-range hybrid plasmonic waveguides that consist of both plasmonic thin films and nano-scale structures of a high refractive index material (such as silicon), with a material of low refractive index (such as silicon di-oxide) lying in the region between the nano-scale structures and the plasmonic thin film. We have employed complex geometry of silicon nanostructures in the vicinity of a plasmonic thin film. The effective refractive index and the corresponding propagation length obtained for these plasmonic waveguides and hybrid plasmonic waveguides were obtained using a full-vector finite difference eigen mode solver. In our simulations, different structural parameters of the the hybrid plasmonic waveguides were varied, and the effect of these parameters ⎯ on the propagation length and effective mode area ⎯ was analyzed. We describe the design of novel hybrid plasmonic waveguides that have a propagation length greater than 1 mm and also have a low effective mode area. The waveguides being proposed by us can be fabricated with relative ease using the standard lithography processes.
A highly sensitive and easy-to-fabricate hydrogen sensor based on a plasmonic ‘gold nanowire array on a palladium layer deposited on a metallic substrate' is proposed. Plasmonic waveguide modes are excited in the gaps between the nanowires in this ‘gold nanowire array on a palladium spacer layer deposited on a metallic substrate' system. As incident light is coupled into the plasmonic modes, a dip in the reflectance spectra is observed at the resonant wavelength, i.e., the wavelength at which the incident light is coupled into plasmonic modes. On exposure to hydrogen, the palladium spacer layer transforms to palladium hydride (PdHx), where x, the atomic ratio of H:Pd, increases as the hydrogen concentration increases. This transformation changes the optical properties of the Pd layer, and hence the position of the resonance wavelengths (λres), i.e., the position of the reflection dips in the reflectance spectra of the Au-Pd-Au system, for various concentrations of hydrogen. The difference between the positions of the resonant wavelengths of PdHx and Pd, (λres(PdHx)−λres(Pd)), is used as a measure of the sensitivity of the proposed hydrogen sensor. Analysis of this shift in the plasmon resonance wavelength is done numerically, using Rigorous Coupled Wave Analysis (RCWA) for various values of d, the side length of the nanowires; t, the thickness of the Pd spacer; g, the gap between the adjacent nanowires and θ, the angle of the incident radiation. It is found that, in the presence of hydrogen, the maximum shift in the resonance wavelength for the proposed sensor is ~41 nm as compared to the case when hydrogen is absent. This shift in the resonance wavelength is higher than many currently employed plasmonic Pd-based hydrogen sensors. Thus, the proposed ‘gold nanowire array on a palladium spacer layer deposited on a metallic substrate' is an easy-to-fabricate, selective and sensitive hydrogen sensor.
This paper presents hybrid plasmonic substrates fabricated by a combination of bottom-up and top-down process of fabrication which can be employed as efficient Surface enhanced Raman scattering (SERS) substrates for chemical sensing. The hybrid approach leads to a cost-efficient fabrication with smaller fabrication times than the pure top-down approach and higher degree of control than the pure bottom-up approach. We demonstrate the achievement of sub-20 nm gaps on a large area with this hybrid methodology. These small gaps lead to the formation of electromagnetic hotspots, i.e., regions of high electromagnetic enhancement. The electromagnetic behavior of these substrates is analyzed theoretically using Finite Difference Time Domain modeling. The sub-20 nm gaps lead to the electromagnetic SERS enhancements of the order of ∼108, and a change in the gap size can tune the plasmon resonance wavelength from the visible to the near-IR region of the spectrum. It is thus shown that these SERS substrates offer high SERS enhancement along with a capability of passive tunability of the plasmon resonance wavelength by changing the geometrical parameters in these substrates.
We present nanostructured gold films ⎯ with complex plasmonic nanostructures present on the surface of the films ⎯ for enhanced SPR-based sensing and imaging of biomolecules attached to the surface of these films. We employed rigorous coupled wave analysis (RCWA) for simulating the nanostructured plasmonic gold films. In our simulations, surface plasmon polaritons were excited on the surface of the nanostructured gold films using the Kretschmann configuration. We observe that these nanostructured gold films show a significant enhancement in the sensitivity of SPR sensing and imaging of biomolecules as compared to planar gold films when optimal geometries and sizes of the plasmonic nanostructures (present on the surface of the gold film) are employed.
Bridged-bowtie nanohole arrays and cross bridged-bowtie nanohole arrays in a gold film are presented as surfaceenhanced Raman scattering (SERS) substrates. We employed the numerical FDTD method to calculate the maximum electromagnetic SERS enhancement factor (EF) as a function of wavelength. It is found that the proposed nanohole arrays do not only display an extremely large enhancement factor but also have the hotspot spread over a larger area compared to the various other nanopillar structures. The calculation of electromagnetic SERS enhancement factor reveals that the cross bridged-bowtie nanohole arrays exhibit the maximum electromagnetic SERS EF of ~ 109 spreading over an area of 100 nm2. In addition, the electromagnetic SERS EF of ~ 108 is spread over 500 nm2 area which is higher than hotspot area in case of nanopillar structures. The resonance wavelength of the nanohole array can be tuned by varying the size of the nanoholes. These nanohole arrays can be employed both in transmission as well as in reflection mode as effective SERS substrates. In addition, bridged-bowtie and cross bridged-bowtie nanohole arrays show the significantly high electromagnetic SERS EF at more than one wavelength and therefore are useful for application involving multiple wavelength SERS response. Furthermore, the cross bridged-bowtie nanohole array exhibit the spatial tunability of hotspot by rotating the direction of polarization of incident field.
We propose plasmonic nanogratings such that they have tunable plasmon resonances in far- and deep-UV wavelength ranges. These plasmonic nanostructures and nanogratings of different plasmonic materials have been simulated using Finite Difference Time Domain modeling and the Rigorous Coupled Wave Analysis. Application of these plasmonic nanostructures for surface plasmon resonance and localized surface plasmon resonance based sensing in the UV wavelength regime has been demonstrated by calculating the shifts in the plasmon resonance wavelength for changes in either the bulk refractive index or the localized refractive index of the medium next to the plasmonic structures.
In this paper, we present a new design for an electro-optic modulator ⎯ operating at the telecomm wavelength of 1550 nm and having a very high extinction ratio ⎯ based on photonic crystal (PhC) slab waveguide and phase change material Germanium Selenide (GeSe) embedded in core silicon layer. The device is based on the shifting of the photonic bandgap of the PhC slab waveguide when the refractive index of the GeSe layer changes on application of electric field. Since GeSe changes from its phase crystalline to amorphous on application of an electric field, its refractive index also changes when this phase transition occurs. As a result of a large refractive index contrast between the two phases, the change in the effective refractive index in the PhC slab waveguide is also very high. With two self-sustainable states, the hybrid modulator shows broadband switching capability and an On/Off extinction ratio > 37 dB around a wavelength of 1550 nm.
We present a plasmonic switch based on a combination of plasmonic nanoantennas and a phase-change material such as vanadium dioxide (VO2) that exhibits great potential for switching the near-field around the nanoantenna at ultrafast time-scales. In order to characterize the switch, we employed the FDTD method to calculate the intensity switching ratio in the vicinity of the nanoantennas, i.e. the ratio of the electric-field intensity between the metallic state (On-state) and the semiconductor state (Off-state) of the VO2 material. The proposed switch exhibits an intensity switching ratio which is much higher as compared to those reported previously.
In this letter, we present a novel optical power splitter having an arbitrary split-ratio that can be tuned over a wide range by employing relatively low voltage levels. It is based on a slotted ring resonator. A 120 nm electro-optic polymer-filled slot is created throughout the circumference of the ring. The hybrid ring resonator is made to work between the full and off resonance states, allowing it to work as a power splitter. This is done by changing the refractive index of the electrooptic polymer inside the slot by the application of an external electric field. The splitter combines the electro-optic functionality of the polymer with the high index contrast of the silicon, resulting in a low tuning voltage power splitter. Over a small voltage range of 0-1 V, it is possible to change the split-ratio of this splitter from 0.031-16.738, making it 10 times better than other competing designs. In addition, it takes less than 500 ps to reconfigure the splitter.
We propose and design long-range hybrid plasmonic waveguides (HPW) consisting of a combination of plasmonic thin film and nano-scale structures of a high refractive index material (such as silicon), with a low refractive index material (such as silica) surrounding the nano-scale structures and the plasmonic thin film. The effective refractive index and the corresponding propagation length obtained for these plasmonic waveguides, obtained using a full-vector finite difference eigen mode (FDE) solver, demonstrates the viability of these hybrid plasmonic waveguides in applications that demands long propagation range with reasonable field confinement. These waveguides not only have high propagation lengths ⎯ even greater than 1 mm for certain geometrical parameters of the plasmonic waveguides ⎯ but can also have tight mode confinement (low effective mode area). Moreover, the proposed hybrid plasmonic waveguides can also be easily fabricated using the conventional nanolithography processes. Moreover, we study the effect of the variation of different waveguide parameters on the propagation length and effective mode area.
We present plasmonics-enhanced organic solar cells (OSCs) containing nanostructures of plasmonic metals in the hole transport layer extending to the active layer of the solar cell. Finite-difference time-domain (FDTD) modeling was employed to simulate the interaction of incident light with the plasmonic nanostructures, leading to a broadband absorption enhancement in the OSCs. We studied the effect of employing nanostructures of different sizes and materials on the absorption enhancement in the OSCs. In some OSCs, we demonstrate 32% increase in the short circuit current density due to the presence of plasmonic nanostructures.
We present Indium-rich InGaN thin-film solar cells containing plasmonic and dielectric nanostructures such as Ag and ITO nanopillars. Finite-difference time-domain (FDTD) simulations were carried out for solar cells containing these nanostructures on the back side and on the front side of the solar cells, and an improvement in the performance of the solar cells was compared for the different geometries and sizes of these nanostructures. In order to develop highefficiency InGaN solar cells, the indium content in the InGaN active layer needs to be increased in order to cover the large solar spectral range. Recently, several reports have demonstrated the growth of single-crystalline Indium-rich InGaN alloys without phase separation by controlling the growth temperature and the pressure. Our FDTD simulation results demonstrate that the Ag nanostructures on the back side of the solar cell lead to an enhanced surface plasmonbased scattering mostly for longer wavelengths of light including band edge of active material, while the ITO nanostructures on the front side lead to enhanced scattering of a middle wavelength range from 450 nm to 700 nm. Hence, a combination of Ag and ITO nanostructures leads to a significant broadband absorption enhancement in the active-medium of the solar cells which in turn leads to a significant enhancement (~ 25 %) in the short circuit current density (Jsc) of these solar cells.
We describe plasmonic switches consisting of 1-D arrays of plasmonic nanostructures such that they have thin films of vanadium-dioxide (VO2) in the vicinity of the plasmonic nanostructures. A multi-wavelength plasmonic switch is presented based on one dimensional plasmonic, asymmetric narrow-groove nanogratings (ANGN), coated with a thin layer of VO2. Incident optical radiation is coupled into plasmonic waveguide modes in metallic narrow-groove nanogratings leading to a localization of electromagnetic fields inside the narrow grooves. The switching is exhibited due to coating of a thin layer of VO2 ⎯ a material whose phase changes from semiconductor to metal on exposure to heat, IR radiation or voltage. As the phase of VO2 changes, it undergoes a change in its dielectric and optical properties. This phase transition in the thin layer of VO2 coated on the nanograting changes the overall optical response from the nanograting, thus exhibiting a switching in the reflectance spectra. The switchability is analyzed through the differential reflectance spectrum which is obtained by subtracting the reflectance spectra of VO2 (M) coated ANGNs from the reflectance spectra of VO2 (S) coated ANGNs. Asymmetry is created in these narrow-groove nanogratings by choosing different values for the narrow-groove gaps. Rigorous coupled wave analysis (RCWA) and finite difference time domain (FDTD) modeling demonstrates that ⎯ due to the presence of asymmetric groove widths ⎯ the incident light is coupled into plasmonic modes in all the grooves at different resonant wavelengths. The presence of several resonant wavelengths in reflectance spectra of ANGNs gives rise to multiple dips and peaks in the differential reflectance spectra, thus exhibiting multiple switching wavelengths. Thus, these asymmetric plasmonic narrow-groove nanogratings can be employed for switching at multiple wavelengths.
This paper describes methodologies for fabricating of highly efficient plasmonics-active SERS substrates - having
metallic nanowire structures with pointed geometries and sub-5 nm gap between the metallic nanowires enabling
concentration of high EM fields in these regions - on a wafer-scale by a reproducible process that is compatible with
large-scale development of these substrates. Excitation of surface plasmons in these nanowire structures leads to
substantial enhancement in the Raman scattering signal obtained from molecules lying in the vicinity of the
nanostructure surface. The methodologies employed included metallic coating of silicon nanowires fabricated by
employing deep UV lithography as well as controlled growth of silicon germanium on silicon nanostructures to form
diamond-shaped nanowire structures followed by metallic coating. These SERS substrates were employed for detecting
chemical and biological molecules of interest. In order to characterize the SERS substrates developed in this work, we
obtained SERS signals from molecules such as p-mercaptobenzoic acid (pMBA) and cresyl fast violet (CFV) attached
to or adsorbed on the metal-coated SERS substrates. It was observed that both gold-coated triangular shaped nanowire
substrates as well as gold-coated diamond shaped nanowire substrates provided very high SERS signals for the
nanowires having sub-15 nm gaps and that the SERS signal depends on the closest spacing between the metal-coated
silicon and silicon germanium nanowires. SERS substrates developed by the different processes were also employed for
detection of biological molecules such as DPA (Dipicolinic Acid), an excellent marker for spores of bacteria such as
Anthrax.
Fabrication of a linear array of metallic nanodots and nanopillars for plasmonic waveguides is reported in this
paper by two different processes - FIB milling of deposited thin films and electron beam-induced deposition of
metallic nanostructures from a organometallic precursor gas introduced into the chamber. In the first FIB fabrication
method, metallic nanorods and nanopillars were fabricated by depositing a 30-150 nm layer of a metallic (gold or
silver) film on a planar substrate and subsequently employing FIB milling to pattern out the metallic nanopillars from
the film. Employing FIB allowed formation of nanostructures such that the plasmon resonances associated with the
nanostructures could be engineered and precisely controlled by controlling the nanostructure size and shape. Multistep
FIB fabrication procedures were developed to form the nanostructures of complex geometries on planar
substrates. The second fabrication processed used to create nanodots and nanopillars for plasmonic waveguides
discussed in this paper is direct deposition of metal nanostructures, created when an electron beam (e-beam) is used to
dissociate metal from an organometallic precursor gas in a predefined reaction region. Ionization energy required for
decomposition of the Au precursor, i.e. Dimethyl Au (III) Fluoro Actylacetonate, is matched with that of the
secondary electrons (between 5-50 eV) that are generated by exposing the substrate to a focused electron beam.
Plasmon resonances are computed for prolate spheroidal nanoshells. Both longitudinal and transverse resonances are
investigated as a function of aspect ratio. Formulas for the surface charge density on the outside and inside shell surfaces
are derived.
This paper describes the development of fiber optic sensor probes and planar substrates containing patterned
nanostructures such as nanoholes in gold films, as well as gold nanoparticles, nano-pillars, nanorods, and nano-islands.
Several methods of producing gold nanofeatures on fiber tips and planar substrates were investigated such as annealing
of thin gold films and focused ion beam (FIB) milling. A Hitachi FB-2100 FIB milling machine with a gallium ion
source was employed to form the nanoparticles from 20-100 nm gold films deposited on the fiber tip. Nano-engineered
gold features were also formed by coating planar substrates and fiber tips with thin gold films (4-10 nm) and annealing
these thin films. Excitation of surface plasmons in gold nanostructures leads to substantial enhancement in the Raman
scattering signal obtained from molecules attached to the nanostructure surface. In this work, a comparison was made
between the SERS signals obtained from the gold substrates developed by employing the different procedures mentioned
above. Fiber samples and planar substrates with these nanostructures were coated with SERS active dyes such as pmercaptobenzoic
acid (pMBA) and cresyl fast violet (CFV). It was observed that the SERS signal obtained from these
gold nanofeatures was much higher than that obtained from a continuous gold film and that the SERS enhancement was
shape and size dependent.
This paper describes theoretical and experimental evaluations of electromagnetic fields around metallic
nanostructures, such as nanorods, nano-pillars, and a collection of nanorods separated by nano-scale distances.
Nanostructures having different sizes and shapes were evaluated. The spacing between nanorods and elliptical nanopillars
was varied such that the effect of nanoparticle spacing on the electromagnetic fields in the regions between the
nanostructures could be studied. Gold was the metal employed in our work as it demonstrates substantial plasmon
excitation and is chemically stable. Calculations of the electromagnetic fields in the vicinity of the different metallic
nanostructures were made by employing Finite Difference Time Domain (FDTD). Refractive index of the media
surrounding the nanostructures was varied for these calculations. These calculations were carried out at different
wavelengths in the visible and near-infrared spectral regimes. In order to fabricate these nanostructures on silica
substrates, focused ion beam (FIB) milling was employed. These structures were fabricated on gold-coated planar silica
and mica substrates and tips of four mode and multimode optical fibers. In our experimental evaluations of the different
metallic nanostructures, surface enhanced Raman scattering (SERS) signals from the different metallic nanostructures
were obtained and were correlated to the spacing distance between the different metallic nanostructures.
In optical fiber evanescent wave sensors, the interaction with the surrounding environment is usually obtained by tapering an optical fiber, which significantly weakens the structure. This paper describes different processes for developing optical fiber probes with gold nanoparticles on the fiber tip including focused ion beam lithography and annealing of continuous gold films by employing plasma arcs, high temperature, or a focused ion beam. Along with the tip based sensors, robust in-line optical fiber sensors were developed by fusing multimode fibers to coreless fibers and forming nanoparticles on the surface of the coreless fibers. The fiber-optic sensors were placed in mediums of different refractive indices to evaluate their chemical sensing capability.
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