Silicon Nanowires for Integrated Photonics: Bridging Nano and Micro Photonics

Silicon Nanowires (SiNWs) with ability to confine carriers and photons in two directions while allowing propagation in third dimension offer interesting modified optical properties such as increased material absorption and optical non-linearities with regard to that of bulk silicon. Enhanced optical properties in SiNWs open a window not only to improve the performance of existing devices but also to realize novel structures. As such, I chose to investigate SiNWs for their applications in photonics, especially for sensing and non-linear devices. My goal was to conduct fundamental research on the optical properties of these SiNWs, and then develop an integration platform to realize practical devices. The platform should be compatible with IC manufacturing. Electron Beam Lithography (EBL) using a Poly Methyl Methacrylate (PMMA) resist followed by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) is used for SiNWs fabrication. Now we are able to fabricate nanowires as small as 15 nm in diameter with the smallest separation of 50 nm. In addition, the interface between SiNWs and Si substrate is optically smooth enabling us to fundamentally understand optical properties of these structures. During the course of this project, I have contributed new fundamental knowledge about SiNWs. For example, Second Harmonic Generation (SHG) is demonstrated in SiNWs, which is absent in bulk silicon. This is achieved by self-straining the nanowires and is the first demonstration of this kind. Second-order non-linearities are more efficient for optical signal processing than third-order ones (which have been used for silicon photonics devices so far). Therefore, these results open a new area of research in silicon. In addition to second order nonlinearity, high enhancement of Raman scattering is achieved in SiNWs fabricated on Silicon on Insulator (SOI) substrate. This can find promising applications in sensing and nonlinear based devices such as optical switches and logic gates. Further, polarization resolved reflections from these nanowire arrays were measured and significant differences were observed for the reflection characteristics for the sand p-polarized beams. In order to understand these reflections, an effective index model is proposed based on calculations using Finite Difference Time Domain (FDTD) method. Results of this analysis provide useful information for designing of many optical devices using SiNWs such as solar cells and photodetectors. As another part of this thesis, vivid colors in mutually coupled SiNWs is demonstrated where nanowire diameters range from 105 nm to 345 nm. 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 using a simple charge coupled device (CCD) camera. Although, there were some paradigm shifting results during my fundamental studies, it became very apparent that SiNWs suffer from a major issue inhibiting their use in photonics devices. Below the diameter of 100 nm where these enhanced material properties were observed, SiNW is a poor optical waveguide with less than 1 % of light confined. The low confinement factor means that though the intrinsic properties of SiNWs increase, the overall device performance is not significantly enhanced. To overcome this issue, a new platform technology is invented, called Silicon Nanowire Optical Waveguide (SNOW). It combines the material advantages of nanostructures with the optical properties of conventional waveguides, and consists of arrays of nanowires in close proximity. It is shown that such a structure can guide an optical mode using the FDTD method. This waveguide structure can be used as a versatile platform to manufacture various devices such as sensors, switches, modulators, grating, and delay lines. For instance, a novel bio-sensor is proposed and designed whose sensitivity is enhanced by a factor of 20, compared to conventional silicon-wire waveguides

High efficiency near diffraction-limited mid-infrared flat lenses based on metasurface reflectarrays

A limiting factor in the development of mid-infrared optics is the lack of abundant materials that are transparent, low cost, lightweight, and easy to machine. In this paper, we demonstrate a metasurface device that circumvents these limitations. A flat lens based on antenna reflectarrays was designed to achieve near diffraction-limited focusing with a high efficiency (experiment: 80%, simulation: 83%) at 45(o) incidence angle at {\lambda} = 4.6 {\mu}m. This geometry considerably simplifies the experimental arrangement compared to the common geometry of normal incidence which requires beam splitters. Simulations show that the effect of comatic aberrations is small compared to parabolic mirrors. The use of single-step photolithography allows large scale fabrication.Comment: 9 page

The effect of silicon on some morpho-physiological and phytochemical traits of Purple Coneflower (Echinacea purpurea L.) under salinity stress

The most important abiotic stresses is salinity and more importantly affect plant growth and limit the plants yield capacity. Silicon is the second most abundant element in the soil and alleviates the biotic and abiotic stresses in plants. The aim of this study is to assess the effects of salt stress and silicon on the some morphological and phytochemical characteristics of purple coneflower (Echinacea purpurea L.). This study was conducted in completely randomized based factorial design with 4 levels of salinty and 4 levels of silicon and 4 replications. Application of stress in hydroponic culture medium (perlite and cocopeat) with Hoagland nutrition solution. The result showed, salinity significantly affected morphophysiological and phytochemical characteristics and silicon affected all characteristics except leaf burn percent. Also, the interactions of these two factors on each trait except shoot wet and dry weight and leaf burn percentage were significant So that the negative effects of salinity stress on growth, morpho-physiological and phytochemical traits of the plant were greatly reduced by silicon and improved the growth and increase of yield. Also, the interaction of these two factors on all traits except the wet and dry weight and leaf burn were affected and the negative effect of salinity stress, especially in the least to medium levels, on growth and morphology, and phytochemical characteristics of the plant were decreased and improved growth. So that the best salinity level in increasing biochemical factors, 25 mM NaCl and the best level for silicon, was 2.25 mM in reducing the negative effects of salinity stress

Broadband and chiral binary dielectric meta-holograms

Subwavelength structured surfaces, known as meta-surfaces, hold promise for future compact and optically thin devices with versatile functionalities. By revisiting the concept of detour phase, we demonstrate high-efficiency holograms with broadband and chiral imaging functionalities. In our devices, the apertures of binary holograms are replaced by subwavelength structured microgratings. We achieve broadband operation from the visible to the near infrared and efficiency as high as 75% in the 1.0 to 1.4 mm range by compensating for the inherent dispersion of the detour phase with that of the subwavelength structure. In addition, we demonstrate chiral holograms that project different images depending on the handedness of the reference beam by incorporating a geometric phase. Our de- vices’ compactness, lightness, and ability to produce images even at large angles have significant potential for impor- tant emerging applications such as wearable optics.Engineering and Applied Science

Metasurface enabled wide-angle fourier lens

Adv.Mater, 30, 1706368 (2018

Ray-optical transformation optics with ideal thin lenses makes omnidirectional lenses

We present the theory of ray-optical transformation optics (RTO) with ideal thin lenses, and show that ideal-thin-lens RTO devices are omnidirectional lenses. Key to designing such devices are two theorems, the loop-imaging theorem and the edge-imaging theorem, which ensure that the interior physical space is distorted in the same way for all viewing directions. We discuss the possibility of realising such devices using lens holograms or Fresnel lenses, as both are in principle capable of changing the directions of rays incident from a specific point precisely like an ideal thin lens, thereby enabling macroscopic and broad-band RTO devices that work for at least one viewing position. Even when restricted in this way, our work opens up new possibilities in ray optics. Our devices have the potential to form the basis of new microscope objectives, virtual-reality headsets, and medical spectacles