Session F12: Nanostructures and Metamaterials 5

Strong coupling between Surface Plasmon Polaritons and Excitons for Silver Nanowires

When dye molecules or quantum dots are placed in photonic or plasmonic cavities, new hybrid states can be formed that are mixtures of the excitonic and the plasmonic/photonic states. These states have unusual properties, and have been the subject of a number of recent studies. A particularly useful way to investigate coupled plasmonic-excitonic systems is to measure a dispersion curve (frequency versus wavevector). These curves show an avoided crossing in the presence of strong coupling. Analysis of the avoided crossing yields the coupling strength or “Rabi Frequency” for the system. In this talk, recent measurements of dispersion curves for the “leaky” surface plasmon polariton (SPP) modes of Ag nanowires will be discussed. In these experiments the SPP modes are launched by focusing a laser at the end of the nanowire with a high numerical aperture objective. Scattered light from the leaky mode is collected by the same objective and sent to a camera. The wavevector for the leaky mode was measured by imaging the back focal plane of the objective onto the camera. Performing these measurements for a series of laser wavelengths generates a dispersion curve. The experiments were conducted on bare nanowires, and on nanowires coated by a thin dye-doped polymer film. The dye-coated nanowires show clear evidence of an avoided crossing. The measured dispersion curves are compared to finite element simulations, which provide a more accurate description of the system compared to the conventional coupled oscillator model for plasmon-exciton coupling.
  

Landau damping of surface plasmons in metal nanostructures

We present a quantum-mechanical model for surface-assisted plasmon decay in metal nanostructures of arbitrary shape. We obtain an explicit expression, in terms of local fields inside the metal structure, for surface absorbed power and surface scattering rate for nanostructures with characteristic size larger than the nonlocality scale, and calculate the Landau damping rate for several common nanoparticle shapes. We show that surface scattering is highly sensitive to the local field polarization and can be incorporated into the metal dielectric function along with phonon and impurity scattering. Our model can be used for calculations of plasmon-assisted hot carrier generation rates in photovoltaics and photochemistry applications.   

Non-Thermal Plasma Synthesis of Plasmonic Zirconium Nitride Nanoparticles and Oxidation Mitigation

Localized surface plasmon resonance (LSPR) has garnered interest in a variety of fields recently, such as photocatalysis, photovoltaics, biophotonics, spectroscopy, sensing, and wave-guiding. Cost and production concerns motivate the search for plasmonic materials that absorb visible light alternative to precious metals, like Group IV transition metal-nitrides. We present a novel technique for the synthesis of plasmonic zirconium nitride (ZrN) nanoparticles using a scalable non-thermal plasma process. The synthesized particles exhibit a plasmonic absorption peak within the visible spectrum, tunable from 530 nm to 700 nm. The crystalline ZrN particles have a cubic rock salt structure and a tunable size distribution below 10 nm. We further developed a modular non-thermal plasma system that coats the particles with amorphous silicon nitride in-flight. This coating acts as an oxygen barrier that prevents oxidation of the ZrN core when the material is exposed to atmosphere and yields blue-shifted and increased-intensity absorption.   

Theory of Weyl-points in Electromagnetic Continua

Recently, Weyl-points have attracted a lot of attention both in electronics and in photonics. In the vicinity of a Weyl point the energy-momentum dispersion is linear and consists of two touching bands. Remarkably, Weyl points are highly robust to any form of perturbation that preserves the Hermitian property of the relevant time evolution operator (the Hamiltonian). Furthermore, a material with Weyl-points supports exotic edge states, known as Fermi arcs, which enable unusual wave phenomena. In this talk, we will present a general and simple criterion to characterize and determine all the Weyl points of a wide class of bianisotropic dispersive electromagnetic continua with no intrinsic periodicity. We apply the developed theory to the case of media invariant under-rotations about some fixed axis, and determine the most general classes of electromagnetic media with Weyl-points. We will present a detailed numerical study that illustrates the robustness of the Weyl points to arbitrary perturbations and illustrates the unusual properties of the considered photonic platforms.   

Realization of complex conjugate medium by using a ring of exceptional points in non-PT-symmetric photonic crystals

We propose a method to locate exceptional points in general non-Hermitian 2-dimensional (2D) photonic crystals (PCs). We focus on systems carrying a Dirac-like cone at k=0, where a ring of exceptional points can emerge when PT-symmetric complex potentials are introduced. We then build a Hamiltonian to find the ring of exceptional points in non-PT-symmetric PCs analytically. By adjusting the loss/gain ratio of our PC, the ring of exceptional points could be made to sit on the real axis even though the PC is not PT-symmetric. We also explored the properties of the PC from an effective medium theory point of view. For frequencies near the ring of exceptional points, the PC behaves like a homogenized complex conjugate medium, whose refractive index is real while the permittivity and permeability are complex individually. The optical properties of such medium will be discussed.
  

Optoelectronic Properties of Borophene, a Two-dimensional Transparent Metal, from Many-body Perturbation Theory

Borophene is a recently synthesized metallic sheet that displays many similarities to graphene and has been predicted to be complimentary to graphene as a high density of states, optically transparent 2D conductor. We present a first-principles density functional theory and many-body perturbation theory study aimed at understanding the strain-dependent optoelectronic properties of two likely allotropes of borophene that are consistent with experimental scanning tunneling microscopy images. We predict that both structures behave as transparent conductors, with conductivity that is smaller than graphene and limited by electron-phonon scattering. Additionally, we demonstrate that strains consistent with change in substrate can be utilized to modify the optoelectronic properties of borophene, suggesting that modification of growth conditions can be used to tune these properties.   

Nondispersive Raman D Peak Observed in Chirally- Enriched, Single-Wall Carbon Nanotube Solutions

Aqueous two-phase extraction provides enriched single-wall carbon nanotubes (SWCNTs) with unprecedented chiral enrichment¹. These SWCNTs possess distinctive electronic transitions that result in fingerprint optical modes. Resonant Raman spectroscopy (RRS) probes SWCNT properties such as metallicity, chirality, degree of bundling, dielectric environment inside or outside of the tube, defect density, impurity level, and the level of electron-phonon coupling. Via tunable laser sources spanning the entire visible range, we resonantly probe aqueous dispersions of single and few chirality SWCNTs. Recent findings enabled by the quality of the separated SWCNT dispersions will be highlighted. First, an under-recognized complexity in the evaluation of Raman spectra for the assignment of (n,m) population distributions². Strong structural dependencies affect the intensity ratio of the RBM to G modes and can mislead interpretations. A second example leverages the sensitivity of Raman spectral features to symmetry breaking in the sp2 carbons, a measure of defects. A test of the depressive nature of the D or defect modes, a long-held assumption, was performed. High quality, well-separated SWCNTs enable new physics to be observed via RRS. ¹Adv Mater 2014, 18, 2800 ²ACS Nano 2016, 10, 525
  

Impact of Structure and Surface Chemistry on Optical Properties and Trap States in Semiconductor Quantum Dot Systems

Doped and un-doped semiconductor quantum dots (QD) are an emerging material of interest with applications for photovoltaics and photodetectors. They are solution-processed and their electro-optical properties are tunable by size, structure, doping character and, due to their high surface-to-volume, chemical environment as controlled by capping ligands. This work examines the interplay of these factors to understand the impact on absorption and trap states of QD thin films. Crystalline germanium QD were synthesized using long-chain oleylamine ligands. QD uncapped and capped with oleylamine and dodecanethiol ligands were studied with extended x-ray absorption fine structure (EXAFS) which yielded detailed structural information about small changes in bond length across ligand systems, doping concentrations and QD size. These findings were correlated with absorption and trap state measurements taken with photothermal deflection spectroscopy (PDS), a system that eliminates scattering and transmission effects by measuring absorption through heat effects.
  

Tunable Infrared Metasurfaces from Soft Polymer Scaffolds

We have developed a microelectromechanical systems (MEMS) atomic calligraphy based method for the fabrication of optical metasurfaces on soft polymer scaffolds. The scaffolds are engineered to allow for tuning of the metasurface optical response when strain is applied. Our technique has demonstrated metasurfaces with frequency tunable mid-infrared resonances. Here, we explore how our fabrication method and mechanical tuning can be used to control other optical parameters, such as the polarization of transmitted or reflected light, to produce strain tunable polarizers and chiral metasurfaces. We also highlight efforts to integrate these metasurfaces as part of functional devices, like MEMS actuators, for dynamic control of optical parameters.   

Laser Thinning and Patterning of MoS2 with Layer-by-Layer Precision

The recently discovered novel properties of two dimensional materials largely rely on the layercritical variation in their electronic structure and lattice symmetry. Achieving layer-by-layer precision patterning is thus crucial for junction fabrications and device engineering, which hitherto poses an unprecedented challenge. Here we demonstrate laser thinning and patterning with layer-by-layer precision in a two dimensional (2D) quantum material MoS2. Monolayer, bilayer and trilayer of MoS₂ films are produced with precise vertical and lateral control, which removes the extruding barrier for fabricating novel three dimensional (3D) devices composed of diverse layers and patterns. By tuning the laser fluence and exposure time we demonstrate producing MoS₂ patterns with designed layer
numbers. The underlying physics mechanism is identified to be temperature-dependent evaporation of the MoS₂ lattice, verified by our measurements and calculations. Our investigation paves way for 3D device fabrication based on 2D layered quantum materials.   

Crystallographic Orientation Dependent Dry Etch in Single Crystal Diamond

Sculpturing desired shapes in single crystal material is ever more crucial in the realization of complex devices for nanophotonics, quantum computing, and quantum optics. The crystallographic orientation dependent wet etch of single crystalline silicon in potassium hydroxide (KOH) allows a range of shapes formed and has significant impacts on MEMS (microelectromechanical systems), AFM (atomic force microscopy), and microfluidics. Here, a crystal direction dependent dry etching principle is presented, which allows to selectively reveal desired crystal planes in monocrystalline diamond. The etch process does not involve mechanical tilting of sample stages or metallic cages. Using the principle, monolithic diamond nanopillars for magnetometry using nitrogen vacancy centers are fabricated. In these nanopillars, a record half-tapering angle is achieved for a high photon efficiency and high mechanical strength of the nanopillar. These results represent the first demonstration of crystallographic orientation dry etch principle, which opens a new window for shaping specific nanostructures which is at the heart of nanotechnology. It is believed that this principle will prove to be valuable for structuring and patterning of other single crystal materials as well.   

Plasmon Resonance Spectroscopy and Mode Coupling in Metallic Nanostructures Consisting of Nano Arcs or Nano Crescents

Using infrared spectroscopy the localized surface plasmon resonances of nanorods, nanoarcs and nanocrescents were investigated. The reflection/transmission spectra are rich with information regarding the fundamental and higher order modes, their dipole component polarization, and the impact that shape design and material design have on the frequency response. Using electron beam lithography and focused ion beam milling to control the relative position of the nanoscale objects in homodimers, heterodimers and tetramers, we have investigated the coupling of plasmons by the response of the system to near-infrared radiation. This information is utilized in the design of thin-film plasmonic metamaterials with engineered chiral and non-linear optic responses (chiroptic and NLO effects).   

On-chip Scalable Multifunctional Optical Networks Integrated with Quantum Dot Single Photon Source: Simulated Response of Dielectric Nanoantana-Waveguide-Beamsplitter Unit

Recently we demonstrated arrays of Mesa Top Single Quantum Dots (MTSQDs) as spectrally uniform on-chip single photon sources [1] that can be readily overgrown and planarized, enabling monolithic integration in scalable optical networks for quantum information processing (QIP). For realization of such networks, we have proposed a new approach [2] that exploits collective Mie resonances of arrays of subwavelength size dielectric building blocks (DBBs) to provide, simultaneously, the needed source light manipulating functions of enhanced and directional emission (nanoantenna) and waveguiding [1]. In this talk, we extend our design to the nanoantenna-waveguide-beamsplitting-recombining unit that exploits a single collective magnetic dipole mode to provide not only enhancement of the emission rate of the MTSQD to improve photon indistinguishability, and directing and propagating the emitted photons on-chip, but also splitting and recombining single photons enabling photon interference, a key for on-chip path entanglement. Such MTSQD-DBB units will serve as building blocks to realize scalable quantum optical networks for QIP.
[1] J. Zhang et.al, J. Appl. Phys. 120, 243103(2016)
[2] S. Chattaraj et.al, J. Opt. Soc. Am. B 33, 2414(2016)