Anuj Dhawan

Room temperature pulsed laser deposited indium gallium zinc oxide channel based transparent thin film transistors

Indium gallium zinc oxide deposited by pulsed laser deposition at room temperature was used as a channel layer to fabricate transparent thin film transistors with good electrical characteristics: field effect mobility of 11cm2V−1s−1 and subthreshold voltage swing of 0.20V∕decade. By varying the oxygen partial pressure during deposition the conductivity of the channel was controlled to give a low off-current of ∼10pA and a drain current on/off ratio of ∼5×107. Changing the channel layer thickness was a viable way to vary the threshold voltage. The effect of the gate dielectric on the electrical behavior was also explored.

Narrow groove plasmonic nano-gratings for surface plasmon resonance sensing

We present a novel surface plasmon resonance (SPR) configuration based on narrow groove (sub-15 nm) plasmonic nano-gratings such that normally incident radiation can be coupled into surface plasmons without the use of prism-coupling based total internal reflection, as in the classical Kretschmann configuration. This eliminates the angular dependence requirements of SPR-based sensing and allows development of robust miniaturized SPR sensors. Simulations based on Rigorous Coupled Wave Analysis (RCWA) were carried out to numerically calculate the reflectance - from different gold and silver nano-grating structures - as a function of the localized refractive index of the media around the SPR nano-gratings as well as the incident radiation wavelength and angle of incidence. Our calculations indicate substantially higher differential reflectance signals, on localized change of refractive index in the narrow groove plasmonic gratings, as compared to those obtained from conventional SPR-based sensing systems. Furthermore, these calculations allow determination of the optimal nano-grating geometric parameters - i. e. nanoline periodicity, spacing between the nanolines, as well as the height of the nanolines in the nano-grating - for highest sensitivity to localized change of refractive index, as would occur due to binding of a biomolecule target to a functionalized nano-grating surface.

Plasmonic Structures Based on Subwavelength Apertures for Chemical and Biological Sensing Applications

Periodic arrays of apertures with subwavelength dimensions and submicron periodicity were fabricated on gold-coated tips of silica optical fibers using focused ion beam (FIB) milling. Interaction of light with subwavelength structures such as an array of nanoapertures in an optically thick metallic film leads to the excitation of surface plasmon waves at the interfaces of the metallic film and the surrounding media, thereby leading to a significant enhancement of light at certain wavelengths. The spectral position and magnitude of the peaks in the transmission spectra depend on the refractive index of the media surrounding metallic film containing the nanohole array. This lays the foundation for the development of fiber-optic chemical and biological sensors that sense the change in refractive index of the medium around the metallic film. This is demonstrated by testing the sensors with solutions of alcohols with different refractive indices and by the attachment of biomolecules to the sensor surface. The bulk refractive index sensitivity of these nanoaperture array-based sensors is shown to be higher than what has been typically reported for metallic nanoparticle-based plasmonic sensors.

Focused ion beam fabrication of metallic nanostructures on end faces of optical fibers for chemical sensing applications

Focused ion beam (FIB) fabrication of fiber optic sensors, mainly chemical sensors, which are based on plasmonics-active nanostructures formed on the cleaved tips of optical fibers, is reported. The nanostructures fabricated included nanoholes in optically thick metallic films as well as metallic nanopillars and nanorods. The sensing mechanism is based on detecting shifts in surface plasmon resonances (SPRs) associated with nanoholes in metallic films and localized SPRs of metallic nanopillars and nanorods, when the refractive index of the medium surrounding the nanostructures is changed. These sensors can be employed for the detection of chemical agents in air as well as liquid media surrounding the sensors. FIB milling was employed to fabricate ordered arrays of nanoholes in optically thick (100–240nm) metallic films deposited on cleaved end faces of multimode, four-mode, and single-mode optical fibers. Separately, metallic nanorods and nanopillars were formed by first depositing a metallic (gold or sil...

Plasmon resonances of gold nanoparticles incorporated inside an optical fibre matrix

Metallic nanoparticles were incorporated into the core of standard telecommunication grade optical fibres. This creates a simple, yet robust, platform which can be used to investigate the properties of nanoparticles, for sensing, spectroscopy, and optical switching applications. The optical response of gold nanoparticles embedded in the optical fibre matrix was evaluated as a function of temperature and the use of the structure as an inline fibre-optic temperature sensor is described. A redshift in the localized surface plasmon (LSP) resonance related peak, as well as broadening of the plasmon resonance, was observed upon increasing the temperature of the nanoparticle containing fibre. The shift and broadening of the plasmon resonance were attributed to the temperature dependence of dielectric constants of metallic nanoparticles and the silica matrix and to plasmon-phonon interactions.

Optical characterization of Eu-doped β-Ga2O3 thin films

Europium-doped β-Ga2O3 thin films were grown on double-side polished c-axis (0001) sapphire substrates by pulsed laser deposition at 850°C. Transmission measurements of the films revealed a sharp band edge with a band gap at 5.0eV. The films exhibited intense red emission at 611nm (2.03eV) due to the transitions from D05 to F27 levels in europium, with intensities that increased with the concentration of europium. Time-resolved photoluminescence measurements revealed a temperature-insensitive lifetime of 1.4ms, which is much longer than the lifetimes of europium luminescence observed in GaN hosts.

Woven Fabric-Based Electrical Circuits Part II: Yarn and Fabric Structures to Reduce Crosstalk Noise in Woven Fabric-Based Circuits

One important problem in electronic textiles is crosstalk and lack of signal integrity between conductive lines. Two significant advantages of electronic textiles over tradi tional circuit boards are flexibility and the ability to scale to large areas. Capacitive and inductive crosstalk is aggravated by long parallel conductors, and varies as the electronic textile is flexed into different configurations. This paper evaluates crosstalk between woven parallel conductors. Two new thread structures—coaxial and twisted pair copper threads—to minimize cross talk are developed and evaluated. Significant reductions in crosstalk are obtained with the coaxial and twisted pair thread structures when compared with bare copper threads or insulated conductive threads.

Fabrication of nanodot plasmonic waveguide structures using FIB milling and electron beam-induced deposition

Fabrication of metallic Au nanopillars and linear arrays of Au-containing nanodots for plasmonic waveguides is reported in this article by two different processes-focused ion beam (FIB) milling of deposited thin films and electron beam-induced deposition (EBID) of metallic nanostructures from an organometallic precursor gas. Finite difference time domain (FDTD) modeling of electromagnetic fields around metallic nanostructures was used to predict the optimal size and spacing between nanostructures useful for plasmonic waveguides. Subsequently, a multi-step FIB fabrication method was developed for production of metallic nanorods and nanopillars of the size and geometry suggested by the results of the FDTD simulations. Nanostructure fabrication was carried out on planar substrates including Au-coated glass, quartz, and mica slides as well as cleaved 4-mode optical fibers. In the second fabrication process, EBID was utilized for the development of similar nanostructures on planar Indium Tin Oxide and Titanium-coated glass substrates. Each method allows 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. Linear arrays of low aspect ratio nanodot structures ranging in diameter between 50-70 nm were fabricated using EBID. Preliminary dark field optical microscopy demonstrates differences in the plasmonic response of the fabricated structures.

Angle-dependent resonance of localized and propagating surface plasmons in microhole arrays for enhanced biosensing

AbstractThe presence of microhole arrays in thin Au films is suited for the excitation of localized and propagating surface plasmon (SP) modes. Conditions can be established to excite a resonance between the localized and propagating SP modes, which further enhanced the local electromagnetic (EM) field. The co-excitation of localized and propagating SP modes depends on the angle of incidence (θexc) and refractive index of the solution interrogated. As a consequence of the enhanced EM field, enhanced sensitivity and an improved response for binding events by about a factor of 3 to 5 was observed with SPR sensors in the Kretschmann configuration for a set of experimental conditions (λSPR, θexc, and η). Thus, microhole arrays can improve sensing applications of SPR based on classical prism-based instrumentation and are suited for SP-coupled spectroscopic techniques. FigCo-excitation of localized and propagating SP enhances sensitivity of SPR

Methodologies for Developing Surface-Enhanced Raman Scattering (SERS) Substrates for Detection of Chemical and Biological Molecules

This paper describes methodologies for developing efficient surface-enhanced Raman scattering (SERS) substrates such as annealing thin gold films for developing gold nanoislands, fabrication of nanopillars arrays and roughened films by employing focused ion beam (FIB) milling of gold films, as well as overcoating deep-UV-fabricated silicon nanowires with a layer of gold film. Excitation of surface plasmons in these gold nanostructures leads to substantial enhancement in the Raman scattering signal obtained from molecules lying in the vicinity of the nanostructure surface. In this paper, we perform comparative studies of SERS signals from molecules such as p-mercaptobenzoic acid and cresyl fast violet attached to or adsorbed on various gold SERS substrates. It was observed that gold-coated silicon nanowire substrates and annealed gold island substrates provided considerably higher SERS signals as compared to those from the FIB patterned substrates and planar gold films. The SERS substrates developed by the different processes were employed for detection of biological molecules such as dipicolinic acid, an excellent marker for spores of bacteria such as Anthrax.