Magnetic nanoarrays on flexible substrates.

Abstract: In this work, we used nanosphere lithography to fabricate large area 2-D magnetic nanoparticle (MNP) arrays on a flexible polyimide substrate (Kapton). Samples were fabricated by assembling polystyrene (PS) spheres on thin films of Co capped with Au. Etched PS spheres were used to mask Co-Au particle arrays.

Biomimetic Colorants and Coatings Designed with Cephalopod-Inspired Nanocomposites.

Brilliant and dynamic colors in nature have stimulated the design of dyes and pigments with broad applications ranging from electronic displays to apparel. Inspired by the nanostructured pigment granules present in cephalopod chromatophore organs, we describe the design and fabrication of biohybrid colorants containing the cephalopod-specific pigment, xanthommatin (Xa), encased within silica-based nanostructures. We employed a biomimetic approach to encapsulate Xa with amine-terminated polyamidoamine (PAMAM) dendrimer templates, which helped stabilize the pigment during encapsulation.

Quantum-Well Bound States in Graphene Heterostructure Interfaces.

We present experimental evidence of electronic and optical interlayer resonances in graphene van der Waals heterostructure interfaces. Using the spectroscopic mode of a low-energy electron microscope (LEEM), we characterized these interlayer resonant states up to 10 eV above the vacuum level. Compared with nontwisted, AB-stacked bilayer graphene (AB BLG), an ≈0.

Single-Electron Tunneling PbS/InP Heterostructure Nanoplatelets for Synaptic Operations.

Power consumption, thermal management, and wiring challenge of the binary serial architecture drive the search for alternative paradigms to computing. Of special interest is neuromorphic computing, in which materials and device structures are designed to mimic neuronal functionalities with energy-efficient non-linear responses and both short- and long-term plasticities. In this work, we explore and report on the enabling potential of single-electron tunneling (SET) in PbS nanoplatelets epitaxially grown in the liquid phase on InP, which present these key features.

Nanorectenna spectrally-selective plasmonic hot electron response to visible-light lasers.

Active metasurfaces with novel visible and infrared (vis/IR) functionalities represent an exciting, growing area of research. Rectification of vis/IR frequencies would produce needed direct current (DC) with no inherent frequency limitation (e.g.

Differential phase-shift-keying demodulation by coherent perfect absorption in silicon photonics.

We demonstrate a novel differential phase-shift-keying (DPSK) demodulator based on coherent perfect absorption (CPA). Our DPSK demodulator chip device, which incorporates a silicon ring resonator, two bus waveguide inputs, and monolithically integrated detectors, operates passively at a bit rate of 10 Gbps at telecommunication wavelengths, and fits within a mm-scale footprint. Critical coupling is used to achieve efficient CPA by tuning the gap between the ring and bus waveguides.

Enhanced coupling of broadband light into amorphous silicon via periodic nanoplasmonic arrays.

Achieving enhanced coupling of solar radiation over the full range of the silicon absorption spectrum up to the bandgap is essential for increased efficiency of solar cells, especially thin film versions. While many designs for enhancing trapping of radiation have been explored, detailed measurements of light scattering inside silicon cells is still lacking. Here, we demonstrate experimentally and computationally that plasmonic-assisted localized and traveling modes can efficiently couple red and infrared radiation into ultrathin amorphous silicon (a-Si) layers.

Two-color field enhancement at an STM junction for spatiotemporally resolved photoemission.

We report measurements and numerical simulations of ultrafast laser-excited carrier flow across a scanning tunneling microscope (STM) junction. The current from a nanoscopic tungsten tip across a ∼1  nm vacuum gap to a silver surface is driven by a two-color excitation scheme that uses an optical delay-modulation technique to extract the two-color signal from background contributions. The role of optical field enhancements in driving the current is investigated using density functional theory and full three-dimensional finite-difference time-domain computations.

Engineering the Structural and Electronic Phases of MoTe through W Substitution.

MoTe is an exfoliable transition metal dichalcogenide (TMD) that crystallizes in three symmetries: the semiconducting trigonal-prismatic 2H- or α-phase, the semimetallic and monoclinic 1T- or β-phase, and the semimetallic orthorhombic γ-structure. The 2H-phase displays a band gap of ∼1 eV making it appealing for flexible and transparent optoelectronics. The γ-phase is predicted to possess unique topological properties that might lead to topologically protected nondissipative transport channels.

Color Richness in Cephalopod Chromatophores Originating from High Refractive Index Biomolecules.

Cephalopods are arguably one of the most photonically sophisticated marine animals, as they can rapidly adapt their dermal color and texture to their surroundings using both structural and pigmentary coloration. Their chromatophore organs facilitate this process, but the molecular mechanism potentiating color change is not well understood. We hypothesize that the pigments, which are localized within nanostructured granules in the chromatophore, enhance the scattering of light within the dermal tissue.

Opportunities and Limitations for Nanophotonic Structures To Exceed the Shockley-Queisser Limit.

Nanophotonic engineering holds great promise for photovoltaics, with several recently proposed approaches that have enabled efficiencies close to the Shockley-Queisser limit. Here, we theoretically demonstrate that suitably designed nanophotonic structures may be able to surpass the 1 sun Shockley-Queisser limit by utilizing tailored directivity of the scattering response of nanoparticles. We show that large absorption cross sections do not play a significant role in the efficiency enhancement, and on the contrary, directivity enhancement constitutes the nanoscale equivalent to concentration in macroscopic photovoltaic systems.

Rigorous theoretical analysis of a surface-plasmon nanolaser with monolayer MoS gain medium.

Lasers based on monolayer (ML) transition-metal dichalcogenide semiconductor crystals have the potential for low threshold operation and a small device footprint; however, nanophotonic engineering is required to maximize the interaction between the optical fields and the three-atom-thick gain medium. Here, we develop a theoretical model to design a direct bandgap optically pumped nanophotonic integrated laser. Our device utilizes a gap-surface-plasmon optical mode to achieve subwavelength optical confinement and consists of a high-index GaP nanowire atop an ML MoS film on an Ag substrate.

Experimental demonstration of coherent perfect absorption in a silicon photonic racetrack resonator.

We present the first experimental demonstration of coherent perfect absorption (CPA) in an integrated device using a silicon racetrack resonator at telecommunication wavelengths. Absorption in the racetrack is achieved by Si-ion-implantation, allowing for phase controllable amplitude modulation at the resonant wavelength. The device is measured to have an extinction of 24.

Direct Measurement of the Tunable Electronic Structure of Bilayer MoS2 by Interlayer Twist.

Using angle-resolved photoemission on micrometer-scale sample areas, we directly measure the interlayer twist angle-dependent electronic band structure of bilayer molybdenum-disulfide (MoS2). Our measurements, performed on arbitrarily stacked bilayer MoS2 flakes prepared by chemical vapor deposition, provide direct evidence for a downshift of the quasiparticle energy of the valence band at the Brillouin zone center (Γ̅ point) with the interlayer twist angle, up to a maximum of 120 meV at a twist angle of ∼40°. Our direct measurements of the valence band structure enable the extraction of the hole effective mass as a function of the interlayer twist angle.

Optical parametric amplification via non-Hermitian phase matching.

We introduce the notion of dissipative optical parametric amplifiers (DOPA) and demonstrate that, even in the absence of the Hermitian phase-matching condition in these structures, the signal beam can be amplified when the idler mode suffers optical attenuation. We discuss the optical implementation of this concept in waveguide platforms, and we propose different methods to control the optical loss of these configurations only at the wavelength of the idler component. Surprisingly, this spectrally selective dissipation process allows the signal beam to draw more energy from the pump and, as a result, attains net amplification.

Development and application of spherically curved charge-coupled device imagers.

Operation of a CCD imager on a curved focal surface offers advantages to flat focal planes, especially for lightweight, relatively simple optical systems. The first advantage is that the modulation transfer function can approach diffraction-limited performance for a spherical focal surface employed in large field-of-view or large-format imagers. The second advantage is that a curved focal surface maintains more uniform illumination as a function of radius from the field center.

Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2H-NbSe2.

We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe2 that we measure by scanning tunneling spectroscopic imaging. We show, from the momentum and energy dependence of the quasiparticle interference, that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe2. We demonstrate that, by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wave vector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions.

Controlling surface reactions with nanopatterned surface elastic strain.

The application of elastic lattice strain is a promising approach for tuning material properties, but the attainment of a systematic approach for introducing a high level of strain in materials so as to study its effects has been a major challenge. Here we create an array of intense locally varying strain fields on a TiO2 (110) surface by introducing highly pressurized argon nanoclusters at 6-20 monolayers under the surface. By combining scanning tunneling microscopy imaging and the continuum mechanics model, we show that strain causes the surface bridge-bonded oxygen vacancies (BBOv), which are typically present on this surface, to be absent from the strained area and generates defect-free regions.

Si⁺-implanted Si-wire waveguide photodetectors for the mid-infrared.

CMOS-compatible Si⁺-implanted Si-waveguide p-i-n photodetectors operating at room temperature and at mid-infrared wavelengths from 2.2 to 2.3 µm are demonstrated.

Nanoscale strain engineering on the surface of a bulk TiO2 crystal.

Arrays of highly strained 5-25 nm-wide regions have been prepared on rutile TiO2(110) surface through a low energy Ar ion bombardment technique. Using scanning tunneling microscopy (STM) and an innovative STM tip-triggered nanoexplosion approach we show experimentally that the protrusions arise from subsurface Ar-filled pockets. Continuum mechanics modeling gives good estimates of the corresponding elastic deformation.

Engineering metal-nanoantennae/dye complexes for maximum fluorescence enhancement.

We theoretically investigate the fluorescence enhancement of a molecule placed in a variable (4 - 20 nm) gap of a plasmonic dimer, with different dye molecules as well as different nanoparticle geometries, using a fully vectorial three-dimensional finite-difference time-domain (3D FDTD) method. This work extends previous studies on molecular fluorescence in the vicinity of metal interfaces and single nanoparticles and shows how the radiative emission of a molecule can be further enhanced by engineering the geometry of a plasmonic structure. Through the use of rigorous 3D FDTD calculations, in conjunction with analytic guidance based on temporal coupled-mode (TCM) theory, we develop a design procedure for antennae assemblies that is useful both for general understanding of molecule-metal structure interaction and experimental efforts in plasmon-enhanced molecular spectroscopy.

Helium-ion-induced radiation damage in LiNbO₃ thin-film electro-optic modulators.

Helium-ion-induced radiation damage in a LiNbO3-thin-film (10 μm-thick) modulator is experimentally investigated. The results demonstrate a degradation of the device performance in the presence of He(+) irradiation at doses of ≥ 10(16) cm(-2). The experiments also show that the presence of the He(+) stopping region, which determines the degree of overlap between the ion-damaged region and the guided optical mode, plays a major role in determining the degree of degradation in modulation performance.

Direct measurement of the thickness-dependent electronic band structure of MoS2 using angle-resolved photoemission spectroscopy.

We report on the evolution of the thickness-dependent electronic band structure of the two-dimensional layered-dichalcogenide molybdenum disulfide (MoS2). Micrometer-scale angle-resolved photoemission spectroscopy of mechanically exfoliated and chemical-vapor-deposition-grown crystals provides direct evidence for the shifting of the valence band maximum from Γ to K, for the case of MoS2 having more than one layer, to the case of single-layer MoS2, as predicted by density functional theory. This evolution of the electronic structure from bulk to few-layer to monolayer MoS2 had earlier been predicted to arise from quantum confinement.

A 60 Gb/s MDM-WDM Si photonic link with < 0.7 dB power penalty per channel.

Mode-division-multiplexing (MDM) and wavelength-division-multiplexing (WDM) are employed simultaneously in a multimode silicon waveguide to realize on-chip MDM and MDM-WDM transmission. Asymmetric Y-junction MDM multiplexers and demultiplexers are utilized for low coherently suppressed demultiplexed crosstalk at the receiver. We demonstrate aggregate bandwidths of 20 Gb/s and 60 Gb/s for MDM and MDM-WDM on-chip links, respectively, with measured 10(-9) BER power penalties between 0.