Top-down fabricated tapered GaAs nanowires with sacrificial etching of the mask.

A novel fabrication method using controlled sacrificial etching of the mask is utilized to fabricate tapered vertical GaAs nanowire arrays. Experimental measurements of the absorption characteristics show that the tapered nanowires absorb over a broadband range as compared to cylindrical ones. The broadband characterization is verified by using optical modeling and results from improved coupling of the nanowires due to distinct radial HE modes being excited separately in the taper and the cylindrical part. The absorption is found to be more broadband as compared to conical nanowires studied so far.

A platform for colorful solar cells with enhanced absorption.

We demonstrate submicron thick platform integrating amorphous silicon nanowires and thin-films achieving vivid colors in transmission and reflection. The platform nearly doubles the absorption efficiency compared to the starting thin-film without much compromising with color diverseness. The structural colors can be changed over a wide range by changing the diameters of the nanowires while still keeping the absorption efficiency higher than starting thin-film. The optical response of the platform is conceptually understood for different diameters combined with different thin-film thicknesses indicating the presence of leaky waveguide modes and coupled cavity modes. Our proposed platform can enable architectural low price colorful solar cells on transparent substrates.

Adjustable optical response of amorphous silicon nanowires integrated with thin films.

We experimentally demonstrate a new optical platform by integrating hydrogenated amorphous silicon nanowire arrays with thin films deposited on transparent substrates like glass. A 535 nm thick thin film is anisotropically etched to fabricate vertical nanowire arrays of 100 nm diameter arranged in a square lattice. Adjusting the nanowire length, and consequently the thin film thickness permits the optical properties of this configuration to be tuned for either transmission filter response or enhanced broadband absorption. Vivid structural colors are also achieved in reflection and transmission. The optical properties of the platform are investigated for three different etch depths. Transmission filter response is achieved for a configuration with nanowires on glass without any thin film. Alternatively, integrating thin film with nanowires increases the absorption efficiency by ∼97% compared to the thin film starting layer and by ∼78% over nanowires on glass. The ability to tune the optical response of this material in this fashion makes it a promising platform for high performance photovoltaics, photodetectors and sensors.

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.

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 Šcan be resolved using image processing.

Optical bio-chemical sensors on SNOW ring resonators.

In this paper, we propose and analyze novel ring resonator based bio-chemical sensors on silicon nanowire optical waveguide (SNOW) and show that the sensitivity of the sensors can be increased by an order of magnitude as compared to silicon-on-insulator based ring resonators while maintaining high index contrast and compact devices. The core of the waveguide is hollow and allows for introduction of biomaterial in the center of the mode, thereby increasing the sensitivity of detection. A sensitivity of 243 nm/refractive index unit (RIU) is achieved for a change in bulk refractive index. For surface attachment, the sensor is able to detect monolayer attachments as small as 1 Å on the surface of the silicon nanowires.

Silicon nanowire optical waveguide (SNOW).

In this paper, we propose a novel optical waveguide consisting of arrays of silicon nanowires in close proximity. We show that such a structure can guide an optical mode provided the electric field is polarized along the length of the nanowires. Furthermore, such guidance can happen even if the nanowires are arranged randomly albeit at a higher scattering loss. On the other hand, high radiation losses are observed if the electric field is polarized in the transverse direction to the nanowires. We calculate the optical radiation loss for different structures using Finite Difference Time Domain (FDTD) method. We also show that the arrayed nanowire region can be approximated using an effective index bulk waveguide. The approximation allows for design and optimization of optical structures using integrated optics methodology resulting in significant savings in time and resources. The advantage of the proposed waveguide structure is that it allows for increased optical confinement while using the enhanced optical interactions of nanowire structures compared to single nanowire photonic waveguide for diameters smaller than 100 nm. For a diameter of 50 nm for the silicon nanowire, an optical confinement factor of 33 % was achieved in the proposed waveguide as opposed to 0.1 % that is achieved for a single nanowire photonic waveguide. A radiation loss of 0.12 cm(-1) is achieved for nanowires of the same diameter spaced 75 nm apart. While our analysis is done on silicon nanowires at 1550 nm, the proposed structures can be extended to other materials and wavelength regimes also.

All-optical logic gates using nonlinear effects in silicon-on-insulator waveguides.

We propose multiple all-optical logic operations in complementary metal oxide semiconductor compatible silicon-on-insulator waveguides based on three nonlinear phenomena, stimulated Raman scattering, free carrier absorption, and cross phase modulation. The performance of three optical logic operations is simulated by use of the finite-difference time-domain method. We achieved an extinction ratio of approximately 13 dB between two logic levels.

Modified Fabry-Perot interferometric method for waveguide loss measurement.

An extension to the Fabry-Perot interferometric method is demonstrated to calculate the optical loss and the reflectivity for optical waveguides simultaneously. The method uses an excitation of the waveguide with a broadband amplified spontaneous emission source (a superluminescent diode in our case) and curve fitting to account for the change of input power, thereby simplifying the measurement procedure. The use of a broadband source as opposed to tunable lasers allows for simultaneous measurements over multiple wavelengths and decreased sensitivity to reflections in the cavity. Further, waveguides of different lengths are measured to calculate the optical loss and the reflectivity simultaneously. It is shown that, if the value for reflectivity is assumed, there could be a large error in the measurement of loss especially for short waveguides. Optical loss for ridge waveguides is measured and compared by using a tunable laser as the input source. The method can be used for a generic case where it is suspected that the input power changes during the measurement.