New Publication: Diffraction by a truncated planar array of dipoles: A Wiener–Hopf approach

Having recently passed his viva and recently joined University of Pennsylvania in a posdoctoral role, PGR Miguel Camacho Aguilar, who graduates this summer, has just published a paper on Diffraction by a truncated planar array of dipoles: A Wiener–Hopf approach in the Special Issue on Canonical Scattering of the journal Wave Motion.

Abstract

We present a rigorous solution to the problem of scattering of a semi-infinite planar array of dipoles, i.e., infinite in one direction and semi-infinite in the other direction, thus presenting an edge truncation, when illuminated by a plane wave. Such an arrangement represents the canonical problem to investigate the diffraction occurring at the edge-truncation of a planar array. By applying the Wiener–Hopf technique to the Z-transformed system of equations derived from the electric field integral equation, we provide rigorous close form expressions for the dipoles’ currents. We find that such currents are represented as the superposition of the infinite array solution plus a perturbation, which comprises both edge diffraction and bound surface waves excited by the edge truncation. Furthermore, we provide an analytical approximation for the double-infinite sum involved in the calculation which drastically reduces the computational effort of this approach and also provides physically-meaningful asymptotics for the diffracted currents.

Keep up to date with Miguel’s latest research at https://scholar.google.co.uk/citations?user=62eJgVAAAAAJ&hl=en. Miguel’s thesis title was “Microwave response of finite periodic metal structures”.

 

New Publication: Origins of All-Optical Generation of Plasmons in Graphene

Congratulations to fourth year CDT PGR Craig Tollerton who has recently published an article on Origins of All-Optical Generation of Plasmons in Graphene in Scientific Reports journal. Abstract below.

Abstract

Graphene, despite its centrosymmetric structure, is predicted to have a substantial second order nonlinearity, arising from non-local effects. However, there is disagreement between several published theories and experimental data. Here we derive an expression for the second order conductivity of graphene in the non-local regime using perturbation theory, concentrating on the difference frequency mixing process, and compare our results with those already published.

Figure 1- Illustration of electromagnetic fields ( E → E→ ) (applicable to pump, probe, and DFG) propagating in the x-z plane. All the fields are p-polarized and the directions of propagation and polarizations are indicated by the red and black arrows respectively. The angles of incidence and transmission are defined in the figure as θ and ϕ.

We find a second-order conductivity (σ(2) ≈ 10−17 AmV−2) that is approximately three orders of magnitude less than that estimated from recent experimental results. This indicates that nonlinear optical coupling to plasmons in graphene cannot be described perturbatively through the electronic nonlinearity, as previously thought. We also show that this discrepancy cannot be attributed to the bulk optical nonlinearity of the substrate. As a possible alternative, we present a simple theoretical model of how a non-linearity can arise from photothermal effects, which generates a field at least two orders of magnitude larger than that found from perturbation theory.

For information on Craig’s previous publications, please check out his Google Scholar page.

New Publication: Laser-writable high-k dielectric for van der Waals nanoelectronics

Congratulations to second year XM² postgraduate researcher Konstantinos-Andreas Anastasiou, whose article Laser-writable high-k dielectric for van der Waals nanoelectronics has been published in Science Advances.

State of the art van der Waals heterostructures rely on the use of hexagonal boron nitride as a gate dielectric, a tunnel barrier or a high-quality substrate material. The material is transferred mostly by chemical vapour deposition on top of the two-dimensional (2D) crystals, a technique which typically contains impurities that lead to leakage current in transistor devices. Other common deposition techniques used for SiO2 and HfO2 are not directly compatible with 2D materials and they tend to damage or modify the electronic properties of the underlying 2D crystal. In this paper, the authors demonstrate a method to embed and pattern a multifunctional few-nanometer-thick high-k oxide within various van der Waals devices without degrading the properties of the neighboring 2D. Abstract below.

Abstract

Similar to silicon-based semiconductor devices, van der Waals heterostructures require integration with high-k oxides. Here, we demonstrate a method to embed and pattern a multifunctional few-nanometer-thick high-k oxide within various van der Waals devices without degrading the properties of the neighboring two-dimensional materials. This transformation allows for the creation of several fundamental nanoelectronic and optoelectronic devices, including flexible Schottky barrier field-effect transistors, dual-gated graphene transistors, and vertical light-emitting/detecting tunneling transistors. Furthermore, upon dielectric breakdown, electrically conductive filaments are formed. This filamentation process can be used to electrically contact encapsulated conductive materials. Careful control of the filamentation process also allows for reversible switching memories. This nondestructive embedding of a high-k oxide within complex van der Waals heterostructures could play an important role in future flexible multifunctional van der Waals devices.

Fig. 1 Heterostructure processing and characterization. (A) The heterostructure is fabricated via dry transfer peeling from poly(dimethylsiloxane) membrane (left), the area containing HfS2 is exposed to laser light (center), and the HfS2 is converted into HfOx (right). (B) BF STEM image showing a cross section of a Gr/HfOx device after laser-assisted oxidation (left) and EDX elemental analysis (right). a.u., arbitrary units. (C) Optical image of a graphene-HfS2/MoS2 heterostructure before (top) and after (bottom) oxidation. Black outlines the region of the graphene back gate, green outlines the HfO2, and red outlines the MoS2. (D) Current (Isd) versus applied voltage (Vsd) for the heterostructure in (C) before (red) and after (green) photo-induced oxidation. Inset shows the stacking sequence. (E) Top: Optical micrograph of a HfS2 flake encapsulated between hBN and graphene (green, HfS2; yellow, hBN; red, graphene). Bottom: Optical micrograph of the same heterostructure imaged within our vacuum chamber showing laser irradiation effects in vacuum (blue hatched area) and in air (red hatched area). Note: No obvious oxidation effects are observed when irradiated in vacuum (P ~ 10−5 mbar). (F) Two-terminal resistance versus gate voltage for a graphene on hBN (d ~ 40 nm)/SiO2 (290 nm) FET measured at T = 266 K in a helium atmosphere (blue curve) and after placing a thin HfS2 flake and subjecting it to laser oxidation (red curve; sweep rate = 10 V/min). Inset shows a Raman spectrum of graphene after oxidation plotted on a logarithmic scale showing the G peak and a negligible D peak.
Fig. 1 Heterostructure processing and characterization. (A) The heterostructure is fabricated via dry transfer peeling from poly(dimethylsiloxane) membrane (left), the area containing HfS2 is exposed to laser light (center), and the HfS2 is converted into HfOx (right). (B) BF STEM image showing a cross section of a Gr/HfOx device after laser-assisted oxidation (left) and EDX elemental analysis (right). a.u., arbitrary units. (C) Optical image of a graphene-HfS2/MoS2 heterostructure before (top) and after (bottom) oxidation. Black outlines the region of the graphene back gate, green outlines the HfO2, and red outlines the MoS2. (D) Current (Isd) versus applied voltage (Vsd) for the heterostructure in (C) before (red) and after (green) photo-induced oxidation. Inset shows the stacking sequence. (E) Top: Optical micrograph of a HfS2 flake encapsulated between hBN and graphene (green, HfS2; yellow, hBN; red, graphene). Bottom: Optical micrograph of the same heterostructure imaged within our vacuum chamber showing laser irradiation effects in vacuum (blue hatched area) and in air (red hatched area). Note: No obvious oxidation effects are observed when irradiated in vacuum (P ~ 10−5 mbar). (F) Two-terminal resistance versus gate voltage for a graphene on hBN (d ~ 40 nm)/SiO2 (290 nm) FET measured at T = 266 K in a helium atmosphere (blue curve) and after placing a thin HfS2 flake and subjecting it to laser oxidation (red curve; sweep rate = 10 V/min). Inset shows a Raman spectrum of graphene after oxidation plotted on a logarithmic scale showing the G peak and a negligible D peak.

 

 

New Publication: Metamaterial-enhanced infrared attenuated total reflection spectroscopy

Congratulations to XM² PGR Cheng Shi (4th year) whose paper Metamaterial-enhanced infrared attenuated total reflection spectroscopy has been accepted by Nanoscale and is due to be published next month. Abstract below.

Abstract

The use of Fourier transform infrared spectroscopy with attenuated total reflection (FTIR-ATR) allows solid or liquid samples to be characterised directly without specific sample preparation. In such a system, the evanescent waves generated through total internal reflection within a crystal interact with the sample under test. In this work we explore the use of a mid-infrared metasurface to enhance the interaction between molecular vibrations and the evanescent waves. A complementary ring-resonator structure was patterned onto both silicon and SiO2/Si substrates, and the spectral properties of both devices were characterised using a FTIR-ATR system. Minima in reflectance were observed corresponding to the resonance of the metasurface on the silicon substrate, and to the hybrid resonance of phonon modes and metasurface resonances on the SiO2/Si substrate, in good agreement with simulations. Preliminary experiments were undertaken using mixtures containing trace amounts of butyl acetate diluted with oleic acid. Without the use of a metasurface, the minimum concentration of butyl acetate that could be clearly detected was 10%, whereas the use of the metasurface on the SiO2/Si substrate allowed the detection of 1% butyl acetate. This demonstrates the potential of using metasurfaces to enhance trace chemical detection in FTIR-ATR systems.

Cheng’s previous publications include Metamaterial-based graphene thermal emitter (co-author) for Nano Research journal.