Session Z30: Optoelectronic Devices and Applications

Near-Field Generation and Detection of Surface Plasmon Polaritons on Silver Nanowires

Chemically-grown silver nanowires are highly crystalline and excellent waveguides for surface plasmon polaritions (SPPs). As their radii approaches the nanowire limit, their SPP modes become highly confined, but it becomes increasingly difficult to scatter light into and out of these modes. We demonstrate nanowire junction-based techniques for generating and detecting SPPs in the near field, thereby circumventing the need for scattering. For near-field SPP generation, we use a silver nanowire as both an SPP waveguide and an electrode for an electroluminescent Schottky junction. For near-field SPP detection, the silver nanowire doubles as a local gate for a photosensitive nanowire. We discuss the mechanism of SPP generation and detection, including a gain mechanism in the detector and a memory effect in the emitter, which is related to filamentary current paths.   

Experimental demonstration of TE-excited surface plasmon polariton wave

Plasmonics, as the most rapidly developing subject in nanophotonics and nano-scale optoelectronics, is finding ample applications ranging from bio-imaging, sensing, solar cell to chip scale optoelectronic integration. However, the inherent polarization feature of surface plasmon polariton (SPP) dictates that it can only be excited by incident light with TM polarization, thus limiting the excitation efficiency to 50{\%} at most if the incident light is unpolarized. Here, we propose a novel plasmonic nanostructure that can overcome this inherent limitation for SPP excitation. The proposed structure supports highly efficient SPP-TE coupling, due to an excited hybrid mode inside the plasmonic structure. This unique TE-excited SPP was successfully verified both in numerical simulation and in experiment using the Kretschmann configuration as a sharp dip was identified in the reflection spectrum, consistent with our theoretical prediction. Furthermore, we show that SPPs could be simultaneously excited with both TE and TM polarization and thus the excitation efficiency could approach 75{\%}.   

Plasmonic Biosensors based on Multi-Layered Metallodielectric Nanostructures

Nanoplasmonics found many applications in diverse topics of optics such as biosensing, solar cells, etc. Studies built up so far is focused on 2D nanostructures, however expanding into the third dimension will provide higher degrees of freedom in the design space. Third dimension is mostly avoided because of fabrication related issues and therefore novel approaches are required. In this work, we have investigated the hybrid multi-layered metallic structures. Transmission spectra provided the conventional extraordinary optical transmission peaks and in addition the newly found modes, which are observed due to the coupling of nanohole and nanoparticle layers. In this talk, we will present the effects of these plasmonic and photonic interactions between nanostructure layers. The newly found mode is explained as the fundamental Fabry-Perot mode of the nanocavity. Numerical analysis shows that the field pattern overlap in the dielectric is superior to any other mode, therefore making this resonance highly sensitive to refractive index changes. Also we will present new results from another coupled 3D structure, multi-layered nanohole arrays.   

Simulation and Testing of Type-II Strained-Layer Superlattices for Low Temperature Thermophotovoltaic Cells.

The focus of this paper is the characterization of a novel, low band gap, long-wavelength, Thermophotovoltaic (TPV) cell design. These cells are based on type-II strained layer superlattice (SLS) structures where the effective bandgap is adjustable and a function of the thickness of the individual layers, creating minibands. Additionally, the type-II nature of the SLS causes the charge carriers to be spatially separated, which minimizes Auger recombination, allowing for the creation and operation of lower temperature TPV cells. The simulation was done using nextnano, while the testing was done on a custom-designed, cryogenic, high vacuum thermal simulator, specifically developed for characterizing low temperature TPV. These cells have the ability to extract energy from long wavelength photons, which will enable devices to harvest energy from more common sources than previously possible. Through this work, energy harvesting could occur at body temperature and below. Here, we characterize several TPV samples with wavelengths up to 10 microns. The capability to extract energy from longer wavelengths opens up new possibilities for TPVs, such as cooling microprocessors and other low temperature applications, or enabling devices like wireless sensors and biological implants to power themselves using heat from their ambient surroundings.   

Enhancing Thermophotovoltaics: 2D photonic crystals and Surface Plasmon Resonance to Increase the efficiency of GaSb

For many years researchers have attempted to efficiently harvest waste heat via thermophotovoltics (TPVs). The low quantum efficiency (QE; i.e. the probability that a photon will be absorbed) in most cells is probably the biggest limiting factor in achieving an economically viable device and directly affects the conversion efficiency (CE; i.e. the probability that a photon will be converted into a carrier that is collected). In many cases, top of the line TPV cells might only have a CE of 20 percent. Recent advances have enabled the creation of novel structures to enhance the absorption and the conversion of the incident thermal photons. In particular, photonic crystals (PhC) and surface plasmon (SP) interface enhancements have been shown to increase the efficiency of photon to current conversions for infrared photodetectors. Here, we report on the enhancement of photon conversion by integration of PhC and SP structures into the TPV cells. Photonic crystals consisting of rods of either air or dielectric surface-passivation material are placed into the base semiconductor TPV cells to increase duration of thermal photon absorption, resulting in significantly enhanced QE and CE. The ability to harvest waste heat for energy will help make many processes more energy efficient, a critical component in ushering the USA into an era of energy independence.   

A nanoscale Inverse-Extraordianry Optoconductance (I-EOC) efficient room temperature photodetector

We present here a new nanoscale efficient photon sensor based on a new form of extraordinary optoconductance phenomenon, (EOC), in nanoscopic metal-semiconductor hybrid structures (MSH) at room temperature. Our macroscopic devices (dimension $>$ 500 nm) exhibit a normal EOC in which the effective resistance decreases with increased illumination intensity, whereas nanoscopic structures (dimension $<$ 500 nm) of the same geometric design exhibit an inverse and much larger response in which the effective resistance increases with illumination intensity. This inverse EOC (I-EOC) effect is driven by the cross-over from ballistic to diffusive transport of the photo-induced carriers. We observe at room temperature a maximum I-EOC of $9460\%$ for a 250 nm device under 633 nm illumination corresponding to a specific detectivity of $D^* = 3.2\times 10^{11}$ cmHz$^{1/2}$/W with a dynamic response of 40 dB making this sensor technologically competitive for a wide range of nanophotonic applications.   

Non-Symmorphic and Quasi-periodic PhoXonic Crystals

PhoXonic(X=n,t) Crystals allow for the manipulation of elastic and electromagnetic wave propagation. This has led to an abundance of novel effects such as negative refraction, artificial birefringence and complete band-gaps. The key to these effects lies in the design of the artificial structure of the medium. A rational approach towards this task may be adopted by choosing the correct symmetry; hence both the dispersion relations and the normal modes can be controlled and interpreted in the symmetry framework. By continuous deforming a structure from i) The periodic approximant of a quasi-periodic structure to its maximal subgroup and ii) a starting symmorphic plane group into a related non-symmorphic plane group, we demonstrate control over where to open up complete in-plane (TM) and out-of-plane gaps (TE) for 2D phononic(photonic) systems and enforce artificial degeneracy of certain bands through ``sticking'' along certain directions. We can also selectively enhance curvatures for certain bands, providing a handle for ``mode-engineering.'' We also identify features of the dispersion relations that are i)invariant to deformations preserving discrete space group symmetry and ii)invariant to deformations preserving topology. All these features of the band structure become transparent within the symmetrical framework, pointing a rational approach towards designing phoXonic devices.   

Seed Layer Dependence of ZnO Nanorod Growth

ZnO is a wide band gap semiconductor for optoelectronic applications such as solar cells, transparent conducting electrodes, and chemicals sensors. In past decades, significant progress has been achieved in controlled growth of ZnO nanorods and nanotubes. In this study we investigate the optimization of the growth properties such as orientation, diameter and shape of ZnO nanorods grown by a low temperature, chemical bath deposition technique. Our group fabricated nanorods on a glass substrate with a seed layer of ZnO deposited by RF and DC sputtering in a formamide solution bath (5{\%}v/v) with zinc metal foil at 65\r{ }C for 24 hours. Scanning electron microscopy (SEM) images of ZnO nanorods reveal that the orientation and size of nanorods grown on various seed layers depend greatly on the initial seed layer of (doped) ZnO. Our research investigates the substrate dependence by experimenting with multiple seed layer deposition methods such as DC and RF coating, yielding both doped and undoped ZnO seed layers. The dependence on growth parameters, such as the concentration of formamide solution and heating methods, will be also characterized.   

Directed Growth of ZnO Nanobridge Sensors using Carbonized Photoresist

Metal oxide nanowires (NWs) are a natural candidate for high sensitivity sensor applications due to their inherently high surface-to-volume ratio. However, developmental challenges still remain for wafer-scale methods to align and integrate NWs to lithographically-defined contacts.~ Recently, selective growth of ZnO NWs was achieved without a metal catalyst using lithographically-patterned carbonized photoresist (C-PR), but electrical measurements were not reported. We have used C-PR to construct directly-integrated ZnO nanobridge devices. PR was spun onto SiO2 coated Si samples, then patterned and carbonized in a reducing atmosphere. Vapor-solid transport was used to grow nanowires between C-PR pads to form nanobridge devices. Current-voltage measurements revealed a Schottky contact between the C-PR and NWs.~ Operation of these nanobridge devices as bottom gate (Si substrate) modulated transistors, UV sensors (up to two orders of magnitude current increase), and gas / humidity sensors is demonstrated.   

Improved Open Circuit Voltage in Hybrid Photovoltaics by Surface Modification of ZnO with Mercurochrome

Inorganic--organic PV cells such as ZnO/P3HT provide an attractive alternative to conventional silicon solar cells due to their solution based, low-temperature fabrication, and scalability in manufacturing. ZnO/P3HT heterojunctions however suffer poor PV performance compared to all-organic cells which use PCBM as the electron acceptor. One pathway for improving performance in these hybrid devices replies on surface modification of ZnO with electron acceptors such as C60 to aid in charge transfer from the electron donating polymer to ZnO. In this study ZnO sol-gel films are modified with mercurochrome resulting in a decrease in ZnO work function as measured by Kelvin probe and concurrently an increase in open circuit voltage (Voc). Additionally, EQE measurements show that part of the current in the modified cells results from absorption by mercurochrome. Potential mechanisms for the increased Voc in these modified hybrid cells will be discussed.   

Light-induced binding of metal nanoparticles via surface plasmons

Recently, nanomachines based on the interaction of nanosize objects with nanostructrued surfaces have attracted much attention. In this work, we study theoretically the light-induced binding forces between a metallic nanosphere and a planar structure, and also between nanoparticles in a diatomic plamonic chain of shelled and unshelled metallic nanoparticles placed alternatively. These forces are calculated by Bergman-Milton spectral representation and multiple image methods within the long wavelength limit. When we tune the incident frequency to the surface plasmon resonant frequency, a stable local minimum in the potential energy is found. It signifies a binding between nanoparticles (nanostructures), which indicates a possible stable structure of the metallic clusters. Such binding is caused by the excitation of collective plasmon modes, which depends on the interparticle distances. This study has potential applications in plasmonic waveguides and colloidal metallic clusters on the nanoscales.