Jaspreet Walia

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.

Highly enhanced Raman scattering from coupled vertical silicon nanowire arrays

Vertical silicon nanowire (SiNW) arrays were fabricated in square lattices with varying diameters, pitches, and lengths, in order to investigate the effects on Raman scattering enhancement. An increase in absolute intensity of the 520 cm−1 vibrational mode by a factor of 15 was achieved for 1.1 μm long SiNWs with diameter of 115 nm arranged 400 nm apart. An oscillatory behaviour in the Raman intensity was also observed with increasing diameter, which is a result of constructive and destructive interferences within the array. A maximum Raman enhancement per unit volume (REV) of 838 was achieved for 115 nm diameter SiNWs with a length of 1.1 μm. The experimental REV results were supported and modelled quantitatively using finite difference time domain simulations.

Temperature and hydration dependence of proton MAS NMR spectra in MCM-41: Model based on motion induced chemical shift averaging

The proton MAS NMR spectra in MCM-41 at low hydration levels (less than hydration amounting to one water molecule per surface hydroxyl group) show complex proton resonance peak structures, with hydroxyl proton resonances seen in dry MCM-41 disappearing as water is introduced into the pores and new peaks appearing, representing water and hydrated silanol groups. Surface hydroxyl group-water molecule chemical exchange and chemical shift averaging brought about by a water molecule visiting different surface hydrogen bonding sites have been proposed as possible causes for the observed spectral changes. In this report a simple model based on chemical shift averaging, due to the making and breaking of hydrogen bonds as water molecules move on the MCM-41 surface, is shown to fully reproduce the NMR spectra, both as a function of hydration and temperature. Surface proton-water proton chemical exchange is not required in this model at low hydration levels.

Enhanced photothermal conversion in vertically oriented gallium arsenide nanowire arrays.

The photothermal properties of vertically etched gallium arsenide nanowire arrays are examined using Raman spectroscopy. The nanowires are arranged in square lattices with a constant pitch of 400 nm and diameters ranging from 50 to 155 nm. The arrays were illuminated using a 532 nm laser with an incident energy density of 10 W/mm(2). Nanowire temperatures were highly dependent on the nanowire diameter and were determined by measuring the spectral red-shift for both TO and LO phonons. The highest temperatures were observed for 95 nm diameter nanowires, whose top facets and sidewalls heated up to 600 and 440 K, respectively, and decreased significantly for the smaller or larger diameters studied. The diameter-dependent heating is explained by resonant coupling of the incident laser light into optical modes of the nanowires, resulting in increased absorption. Photothermal activity in a given nanowire diameter can be optimized by proper wavelength selection, as confirmed using computer simulations. This demonstrates that the photothermal properties of GaAs nanowires can be enhanced and tuned by using a photonic lattice structure and that smaller nanowire diameters are not necessarily better to achieve efficient photothermal conversion. The diameter and wavelength dependence of the optical coupling could allow for localized temperature gradients by creating arrays which consist of different diameters.

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.

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.

Resonant photo-thermal modification of vertical gallium arsenide nanowires studied using Raman spectroscopy

Gallium arsenide nanowires have shown considerable promise for use in applications in which the absorption of light is required. When the nanowires are oriented vertically, a considerable amount of light can be absorbed, leading to significant heating effects. Thus, it is important to understand the threshold power densities that vertical GaAs nanowires can support, and how the nanowire morphology is altered under these conditions. Here, resonant photo-thermal modification of vertical GaAs nanowires was studied using both Raman spectroscopy and electron microscopy techniques. Resonant waveguiding, and subsequent absorption of the excited optical mode reduces the irradiance vertical GaAs nanowires can support relative to horizontal ones, by three orders of magnitude before the onset of structural changes occur. A power density of only 20 W mm(-2) was sufficient to induce local heating in the nanowires, resulting in the formation of arsenic species. Upon further increasing the power, a hollow nanowire morphology was realized. These findings are pertinent to all optical applications and spectroscopic measurements involving vertically oriented GaAs nanowires. Understanding the optical absorption limitations, and the effects of exceeding these limitations will help improve the development of all III-V nanowire devices.

Enhanced Raman scattering from sub-wavelength silicon gratings

A one-dimensional sub-wavelength silicon grating with enhanced Raman response is demonstrated. Furthermore, the polarization response of the Raman is investigated. This study shows that, contrary to intuitive expectation, the Raman intensity can be similar for both input polarizations: parallel and perpendicular to the ruling direction. This similarity is achieved due to inter-ridge coupling and polarization dependent characteristics of the grating. Through optimization of the ridge width and spacing, enhanced Raman intensity is realized in both polarizations, simultaneously. The results are further understood using a finite difference time domain model based on the light interaction with the grating for each polarization.