School of Physics - Research Publications
Now showing items 1-12 of 664
Epitaxial Formation of SiC on (100) Diamond
(American Chemical Society (ACS), 2020-07-28)
We demonstrate locally the coherent formation of silicon carbide (SiC) on diamond, a rare example of heteroepitaxy with a lattice mismatch that exceeds 20%. High-resolution transmission electron microscopy confirms the quality and atomic structure near the interface. Guided by molecular dynamics simulations, a theoretical model is proposed for the interface wherein the large lattice strain is alleviated via point dislocations in a two-dimensional plane without forming extended defects in three dimensions. The possibility of realizing heterojunctions of technologically important materials such as SiC with diamond offers promising pathways for thermal management of high-power electronics. At a fundamental level, the study redefines our understanding of SiC and diamond heteroepitaxy and furthers our understanding of large lattice mismatched interfaces.
Metasurface with metallic nanoantennas and graphene nanoslits for sensing of protein monolayers and sub-monolayers
(OSA - Optical Society of America, 2020-06-08)
Biomolecule sensing plays an important role in both fundamental biological studies and medical diagnostic applications. Infrared (IR) spectroscopy presents opportunities for sensing biomolecules as it allows their fingerprints to be determined by directly measuring their absorption spectra. However, the detection of biomolecules at low concentrations is difficult with conventional IR spectroscopy due to signal-to-noise considerations. This has led to recent interest on the use of nanostructured surfaces to boost the signals from biomolecules in a method termed surface enhanced infrared spectroscopy. So far, efforts have largely involved the use of metallic nanoantennas (which produce large field enhancement) or graphene nanostructures (which produce strong field confinement and provide electrical tunability). Here, we propose a nanostructured surface that combines the large field enhancement of metallic nanoantennas with the strong field confinement and electrical tunability of graphene plasmons. Our device consists of an array of plasmonic nanoantennas and graphene nanoslits on a resonant substrate. We perform systematic electromagnetic simulations to quantify the sensing performance of the proposed device and show that it outperforms designs in which only plasmons from metallic nanoantennas or plasmons from graphene are utilized. These investigations consider the model system of a representative protein-goat anti-mouse immunoglobulin G (IgG) - in monolayer or sub-monolayer form. Our findings provide guidance for future biosensors for the sensitive quantification and identification of biomolecules.
Direct fabrication of 3D graphene on nanoporous anodic alumina by plasma-enhanced chemical vapor deposition
(NATURE PUBLISHING GROUP, 2016-01-25)
High surface area electrode materials are of interest for a wide range of potential applications such as super-capacitors and electrochemical cells. This paper describes a fabrication method of three-dimensional (3D) graphene conformally coated on nanoporous insulating substrate with uniform nanopore size. 3D graphene films were formed by controlled graphitization of diamond-like amorphous carbon precursor films, deposited by plasma-enhanced chemical vapour deposition (PECVD). Plasma-assisted graphitization was found to produce better quality graphene than a simple thermal graphitization process. The resulting 3D graphene/amorphous carbon/alumina structure has a very high surface area, good electrical conductivity and exhibits excellent chemically stability, providing a good material platform for electrochemical applications. Consequently very large electrochemical capacitance values, as high as 2.1 mF for a sample of 10 mm(3), were achieved. The electrochemical capacitance of the material exhibits a dependence on bias voltage, a phenomenon observed by other groups when studying graphene quantum capacitance. The plasma-assisted graphitization, which dominates the graphitization process, is analyzed and discussed in detail.
Hybrid diamond/ carbon fiber microelectrodes enable multimodal electrical/chemical neural interfacing
Implantable medical devices are now in regular use to treat or ameliorate medical conditions, including movement disorders, chronic pain, cardiac arrhythmias, and hearing or vision loss. Aside from offering alternatives to pharmaceuticals, one major advantage of device therapy is the potential to monitor treatment efficacy, disease progression, and perhaps begin to uncover elusive mechanisms of diseases pathology. In an ideal system, neural stimulation, neural recording, and electrochemical sensing would be conducted by the same electrode in the same anatomical region. Carbon fiber (CF) microelectrodes are the appropriate size to achieve this goal and have shown excellent performance, in vivo. Their electrochemical properties, however, are not suitable for neural stimulation and electrochemical sensing. Here, we present a method to deposit high surface area conducting diamond on CF microelectrodes. This unique hybrid microelectrode is capable of recording single-neuron action potentials, delivering effective electrical stimulation pulses, and exhibits excellent electrochemical dopamine detection. Such electrodes are needed for the next generation of miniaturized, closed-loop implants that can self-tune therapies by monitoring both electrophysiological and biochemical biomarkers.
Efficient polarization beam splitter pixels based on a dielectric metasurface
(OPTICAL SOC AMER, 2015-04-20)
The polarization dependence of the reflection, refraction, and diffraction of electromagnetic waves from materials is measured in applications that extend from small (e.g., ellipsometry of semiconductor chips) to large scales (e.g., remote sensing for planetary science and weather radar). Such applications employ polarimeters that are in turn based on devices with polarization-selective absorption or reflection/refraction properties (e.g., prisms). The latter devices are generally bulky, thereby limiting their integration into compact systems. The former devices are inherently lossy, as they function by absorbing the unwanted polarization. Here, we experimentally demonstrate a conceptually novel method for pixellevel polarimetry. Each pixel contains amorphous-silicon nanoridges and deflects incident light in a polarizationdependent manner. As photons are sorted by polarization rather than filtered, the approach permits high efficiency. A high transmission efficiency of 90% and a high extinction ratio of 15 times are demonstrated.
Optical Trapping of Nanoparticles Using All-Silicon Nanoantennas
(AMER CHEMICAL SOC, 2018-12-01)
The ability to optically trap nanoscale particles in a reliable and noninvasive manner is emerging as an important capability for nanoscience. Different techniques have been introduced, including plasmonic nanostructures. Nano-optical tweezers based on plasmonics face the problem of Joule heating, however, due to high losses in metals. Here we experimentally demonstrate the optical trapping and transport of nanoparticles using a nonplasmonic approach, namely, a silicon nanoantenna. We trap polystyrene nanoparticles with diameters of 20 and 100 nm and use fluorescence microscopy to track their positions as a function of time. We show that multiple nanoparticles can be trapped simultaneously with a single nanoantenna. We show that the infrared trapping laser beam also produces fluorescent emission from trapped nanoparticles via two-photon excitation. We present simulations of the nanoantenna that predict enhanced optical forces with insignificant heat generation. Our work demonstrates that silicon nanoantennas enable nanoparticles to be optically trapped without deleterious thermal heating effects.
Generalized Method of Images and Reflective Color Generation from Ultrathin Multipole Resonators
(AMER CHEMICAL SOC, 2018-06-01)
The multipole expansion has found limited applicability for optical dielectric resonators in inhomogeneous environment, such as on the surface of substrates. Here, we generalize the method of images to multipole analysis for light scattering by dielectric nanoparticles on conductive substrates. We present examples illustrating the physical insight provided by our method, including selection rules governing the excitation of the multipoles. We propose and experimentally demonstrate a new mechanism to generate high resolution surface color. The dielectric resonators employed are very thin (less than 50 nm), i.e., similar in thickness to the plasmonic resonators that are currently being investigated for structural color. The generalized method of images opens up new prospects for design and analysis of metasurfaces and optical dielectric resonators.
Direct Particle Tracking Observation and Brownian Dynamics Simulations of a Single Nanoparticle Optically Trapped by a Plasmonic Nanoaperture
(AMER CHEMICAL SOC, 2018-07-01)
Optical trapping using plasmonic nanoapertures has proven to be an effective means for the contactless manipulation of nanometer-sized particles under low optical intensities. These particles have included polystyrene and silica nanospheres, proteins, coated quantum dots and magnetic nanoparticles. Here we employ fluorescence microscopy to directly observe the optical trapping process, tracking the position of a polystyrene nanosphere (20 nm diameter) trapped in water by a double nanohole (DNH) aperture in a gold film. We show that position distribution in the plane of the film has an elliptical shape. Comprehensive simulations are performed to gain insight into the trapping process, including of the distributions of the electric field, temperature, fluid velocity, optical force, and potential energy. These simulations are combined with stochastic Brownian diffusion to directly model the dynamics of the trapping process, that is, particle trajectories. We anticipate that the combination of direct particle tracking experiments with Brownian motion simulations will be valuable tool for the better understanding of fundamental mechanisms underlying nanostructure-based trapping. It could thus be helpful in the development of the future novel optical trapping devices.
Polarization-resolved black phosphorus/molybdenum disulfide mid-wave infrared photodiodes with high detectivity at room temperature
(NATURE PUBLISHING GROUP, 2018-10-01)
Infrared photodetectors are currently subject to a rapidly expanding application space, with an increasing demand for compact, sensitive and inexpensive detectors. Despite continued advancement, technological factors limit the widespread usage of such detectors, specifically, the need for cooling and the high costs associated with processing of iii–v/ii–vi semiconductors. Here, black phosphorous (bP)/MoS2 heterojunction photodiodes are explored as mid-wave infrared (MWIR) detectors. Although previous studies have demonstrated photodiodes using bP, here we significantly improve the performance, showing that such devices can be competitive with conventional MWIR photodetectors. By optimizing the device structure and light management, we demonstrate a two-terminal device that achieves room-temperature external quantum efficiencies (ηe) of 35% and specific detectivities (D*) as high as 1.1 × 1010 cm Hz1/2 W−1 in the MWIR region. Furthermore, by leveraging the anisotropic optical properties of bP we demonstrate the first bias-selectable polarization-resolved photodetector that operates without the need for external optics.
Multifunctional Dielectric Metasurfaces Consisting of Color Holograms Encoded into Color Printed Images
A hologram records the wavefront of light from an object, but it is usually not an image itself, and looks unintelligible under diffuse ambient light. Here a new paradigm to encode a color hologram onto a color printed image is experimentally demonstrated. The printed image can be directly viewed under white light illumination, while a low‐crosstalk color holographic image can be seen when the device is illuminated with red (R), green (G), and blue (B) laser beams. The device is a dielectric metasurface that consists of titanium dioxide (TiO2) cones on a glass substrate. The dimensions of the TiO2 cones are chosen to allow them to support visible‐wavelength resonances, thereby producing the desired reflection spectra and thus the color printed image. The detour phase method is furthermore used to encode the hologram into the metasurface. The approach is conceptually different from previously demonstrated color printed images or holograms and presents opportunities for optical document security and data storage applications.
Controlling the Light Absorption in a Photodetector Via Nanowire Waveguide Resonances for Multispectral and Color Imaging
(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018-11-01)
The responsivity spectrum of a photodetector is one of its key specifications. It ultimately originates from the combination of the absorption spectrum of the photosensitive region and the internal quantum efficiency. Many applications of photodetectors would benefit from an improved ability to tailor the responsivity spectrum. This is particularly true for color and multispectral imaging. The absorption spectrum of a bulk (unstructured) semiconductor is fixed however, being determined by its complex refractive index. Here, we review recent work that demonstrates that the absorption spectrum of a photodetector can be controlled via waveguide resonances in semiconductor nanowires. We discuss the physical interpretation for this phenomenon. We review work in which p-i-n photodiodes were incorporated into vertically oriented silicon nanowires, and then used for color imaging. We review work in which tandem-style photodetectors were demonstrated, with a p-i-n silicon nanowire photodiode formed above an n-i-p planar silicon photodiode. We review work in which narrowband photodetection across the visible-to-infrared was demonstrated using germanium nanowires. Finally, we describe related work in which silicon nanowires have been explored for other applications, namely solar cells.