School of Physics - Research Publications
Now showing items 1-12 of 661
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.
Experimental demonstration of infrared spectral reconstruction using plasmonic metasurfaces
(OPTICAL SOC AMER, 2018-09-15)
We computationally reconstruct short- to long-wave infrared spectra using an array of plasmonic metasurface filters. We illuminate the filter array with an unknown spectrum and measure the optical power transmitted through each filter with an infrared microscope to emulate a filter-detector array system. We then use the recursive least squares method to determine the unknown spectrum. We demonstrate our method with light from a blackbody. We also demonstrate it with spectra generated by passing the light from the blackbody through various materials. Our approach is a step towards miniaturized spectrometers spanning the short- to long-wave infrared based on filter-detector arrays.
All-dielectric nanotweezers for trapping and observation of a single quantum dot
(OPTICAL SOC AMER, 2019-02-18)
We report the optical trapping of a single streptavidin-coated CdSe/ZnS quantum dot whose overall diameter is around 15-20 nm, in a microfluidic chamber by an all-dielectric (silicon) nanotweezer with negligible local heating. The use of fluorescence microscopy allows us to readily observe trapping events, tracking the fluorescence emission from, and the position of, each individual trapped quantum dot as a function of time. The blinking behavior of the quantum dots is observed during the trapping process, that is, in the near field region of the silicon nanoantenna. We furthermore show that the continuous wave infrared laser employed to trap the quantum dots can also excite photoluminescence from them via two-photon absorption. We present Maxwell stress tensor simulations of optical forces applied to a single quantum dot in the nanoantenna's vicinity. This work demonstrates that all-dielectric nanotweezers are a promising means to handle quantum dots in solution, enabling them to be localized for observations over extended periods of time.
Bull's eye grating integrated with optical nanoantennas for plasmonic enhancement of graphene long-wave infrared photodetectors
(AIP Publishing, 2019-03-04)
Two-dimensional (2D) materials have exhibited potential for infrared detection at room temperature, yet their low light absorption impedes their widespread application. In addition, micromechanical cleavage, which is the main method by which high-quality 2D layers are achieved, typically leads to small-area flakes, hampering their application as photodetectors. In this work, we designed a hybrid plasmonic structure, comprising a metallic bull's eye grating and optical nanoantennas, to collect and concentrate light into a piece of single-layer graphene with sub-wavelength lateral extent. This boosts the interaction between the graphene and light, thereby improving its photodetection performance in the technologically important long-wave infrared (LWIR) region. Finite-difference time-domain electromagnetic simulations were performed to this end. The plasmonic structure we present is predicted to enhance the absorption of light by the graphene by ∼558 times, which in turn is predicted to enhance the detectivity of the LWIR photodetector by ∼32 times.
Vertical Ge-Si Nanowires with Suspended Graphene Top Contacts as Dynamically Tunable Multispectral Photodetectors
(AMER CHEMICAL SOC, 2019-03-20)
Numerous applications would be enabled by pixels for multispectral imaging whose spectral responses can be dynamically tuned and that can be potentially manufactured at low cost. Here, we show such a capability, by experimentally demonstrating arrays of vertically oriented germanium–silicon heterojunction nanowires with graphene top contacts. Our devices present opportunities for multispectral imaging because their responsivity spectra can be tailored by choice of nanowire radius for enhanced absorption at certain wavelengths across the visible to short-wave infrared. Importantly, these responsivity spectra can also be dynamically tuned by bias voltage. We demonstrate this experimentally by tuning the responsivity peak of a single pixel across the visible region by varying the bias voltage and by showing that this would allow red/green/blue channels to be reconstructed. This opens the exciting prospect of a single pixel that can resolve color (i.e., replacing the three red/green/blue pixels of traditional approaches) or even resolve several bands for multispectral imaging.