Simarjeet S. Saini

Simultaneous Measurement of Refractive Index, Temperature, and Strain Using Etched-Core Fiber Bragg Grating Sensors

This study demonstrates simultaneous measurement of three parameters viz. refractive index of surrounding medium, temperature, and strain using etched-core fiber Bragg grating sensors. Simultaneous measurement is achieved by exciting higher order modes in the sensor using an asymmetric nonadiabatic taper and measuring difference in their Bragg wavelength shifts due to different parameters. In order to simultaneously measure three parameters, three different order modes were excited in the sensor. The ability to measure multiple parameters is useful in bio-chemical measurements as it allows us to compensate for the change in Bragg wavelength with respect to change in temperature or strain. Thus, the sensors do not need to be stabilized.

Color matrix refractive index sensors using coupled vertical silicon nanowire arrays.

Vivid colors are demonstrated in silicon nanowires with diameters ranging from 105 to 346 nm. The nanowires are vertically arranged in a square lattice with a pitch of 400 nm and are electromagnetically coupled to each other, resulting in frequency-dependent reflection spectra. Since the coupling is dependent on the refractive index of the medium surrounding the nanowires, the arrays can be used for sensing. A simple sensor is demonstrated by observing the change in the reflected color with changing refractive index of the surrounding medium. A refractive index resolution of 5 × 10(-5) is achieved by analyzing bright-field images captured with an optical microscope equipped with a charge coupled device camera.

Enhanced first-order Raman scattering from arrays of vertical silicon nanowires

Vertical ordered silicon nanowire arrays with diameters ranging from 30 to 60 nm are fabricated and display enhanced Raman scattering. The first-order 520 cm(-1) phonon mode shows no significant shift or peak broadening with increasing laser power, suggesting that the excellent defect-free diamond crystalline structure and thermal properties of bulk silicon are maintained. The Raman enhancement per unit volume of the first-order phonon peak increases with increasing nanowire diameter, and has maximum enhancement factors of 7.1 and 70 when compared to the original silicon on insulator (SOI) and bulk silicon wafers, respectively. For the array with 60 nm diameter nanowires, the total Raman intensity is larger than that of the SOI wafer. The results are understood using a model based on the confinement of light and are supported by finite difference time domain (FDTD) simulations.

Color Generation and Refractive Index Sensing Using Diffraction from 2D Silicon Nanowire Arrays

Tunable structural color generation from vertical silicon nanowires arranged in different square lattices is demonstrated. The generated colors are adjustable using well-defined Bragg diffraction theory, and only depend on the lattice spacing and angles of incidence. Vivid colors spanning from bright red to blue are easily achieved. In keeping with this, a single square lattice of silicon nanowires is also able to produce different colors spanning the entire visible range. It is also shown that the 2D gratings also have a third grating direction when rotated 45 degrees. These simple and elegant solutions to color generation from silicon are used to demonstrate a cost-effective refractive index sensor. The sensor works by measuring color changes resulting from changes in the refractive index of the medium surrounding the nanowires using a trichromatic RGB decomposition. Moreover, the sensor produces linear responses in the trichromatic decomposition values versus the surrounding medium index. An index resolution of 10(-4) is achieved by performing basic image processing on the collected images, without the need for a laser or a spectrometer. Spectral analysis enables an increase in the index resolution of the sensor to a value of 10(-6) , with a sensitivity of 400 nm/RIU.

Bunching characteristics of silicon nanowire arrays

Ordered arrays of silicon nanowires were fabricated by etching, and their bunching characteristics were parametrically studied by varying the diameter, the length, and the pitch. The diameter to length ratio was found to be critical for the nanowires to stand vertically without bunching. For a length of 650 nm, 40 nm and larger diameter nanowires were vertical, whereas for a length of 400 nm, 34 nm and larger diameter nanowires were vertical. Further, the phase change between the bunching and vertical nature of nanowires happens abruptly and was verified by finite element modeling of the deflections of the nanowire tips for different diameters. The detailed experimental study provides guidelines for silicon nanowire arrays being considered for different applications including solar cells, optical waveguides, and sensors.

Highly ordered vertical GaAs nanowire arrays with dry etching and their optical properties

We report fabrication methods, including metal masks and dry etching, and demonstrate highly ordered vertical gallium arsenide nanowire arrays. The etching process created high aspect ratio, vertical nanowires with insignificant undercutting from the mask, allowing us to vary the diameter from 30 nm to 400 nm with a pitch from 250 nm to 1100 nm and length up to 2.2 μm. A diameter to pitch ratio of ∼68% was achieved. We also measured the reflectance from the nanowire arrays and show experimentally diameter-dependent strong absorption peaks resulting from resonant optical mode excitations within these nanowires. The reflectance curves match very well with simulations. The work done here paves the way towards achieving high efficiency solar cells and tunable photodetectors using III-V nanowires.

Highly Enhanced Raman Scattering of Graphene using Plasmonic Nano-Structure

Highly enhanced Raman scattering of graphene on a plasmonic nano-structure platform is demonstrated. The plasmonic platform consists of silver nano-structures in a periodic array on top of a gold mirror. The gold mirror is used to move the hot spot to the top surface of the silver nano-structures, where the graphene is located. Two different nano-structures, ring and crescent, are studied. The actual Raman intensity is enhanced by a factor of 890 for the G-peak of graphene on crescents as compared to graphene on a silicon dioxide surface. The highest enhancement is observed for the G-peak as compared to the 2D-peak. The results are quantitatively well-matched with a theoretical model using an overlap integral of incident electric field intensities with the corresponding intensities of Raman signals at the G- and 2D-peaks. The interaction of light with nano-structures is simulated using finite element method (FEM).

Colorimetric sensors using nano-patch surface plasmon resonators

A two-dimensional array of gold nano-patches on a highly reflective mirror is proposed for refractive index sensing based on changes in the reflected colors. The grating on the mirror creates localized surface plasmon resonances resulting in a minimum in the visible reflectance spectra. The wavelength of the resonance can be tuned by changing the width of the nano-patches and is also dependent on the refractive index of the surrounding medium. The color variation due to change in the refractive index is measured and used to realize a simple low-cost sensor with a refractive index resolution better than 10⁻⁵ just using image processing. The efficacy of the proposed sensor is also demonstrated for surface sensing by depositing thin layers of silicon dioxide. The color difference due to the addition of a 3 nm thick layer of silicon dioxide is detectable by the naked eye and deposition thickness of 2 Å can be resolved using image processing.

Compact mode expanders using resonant coupling between a tapered active region and an underlying coupling waveguide

A novel technique for enhanced laser-fiber coupling, based on resonant power coupling between a tapered active waveguide and an underlying coupling waveguide, is presented. Spot-sizes are transformed from 2.0/spl times/1.1 /spl mu/m in the active region to 6.0/spl times/3.1 /spl mu/m in the coupling waveguide, over a length of 200 /spl mu/m, with a mode transformation loss of only 0.36 dB. Butt-coupling efficiencies of 55% (2.6 dB loss) are estimated to standard cleaved single-mode fibers at 1.55 /spl mu/m. The proposed device requires a single epitaxial growth and conventional processing techniques, making it amenable for low-cost manufacturing.

Silicon nanowire arrays with enhanced optical properties.

Vertical silicon nanowire arrays were fabricated leading to an enhanced photoluminescence (PL) emission and second-order nonlinear optical response. PL from the nanowires was increased by a factor of 50 as compared to bulk silicon. The second order nonlinearity was demonstrated in second-harmonic generation and rotational anisotropic measurements. Enhancement by at least a factor of 80 was achieved as compared to bulk silicon for the p-polarized input and s-polarized output. These enhancements in the silicon characteristics should enable highly desired applications using a silicon platform, such as nonlinear and active devices.