Two-dimensional materials have attracted significant interest recently for their unique optical properties compared to their bulk counterparts. Specifically, the family of transition metal dichalcogenides (TMD), such as MoS2 and WS2, have large second order nonlinear susceptibility. Extraordinary second harmonic generation and sum frequency generation have been observed. Here we investigate the second order nonlinearity of 2D materials, including TMD layered materials with dopants and defects. Experimental results and preliminary theoretical analysis will be discussed.
Layered two-dimensional (2D) materials possess unique optical properties. For example, the monolayer family of Transition Metal Dichalcogenide (TMD) materials (such as WS2 and MoS2) is well known for the existence of a direct band gap. In this talk, we discuss the characterization of the second order nonlinear susceptibility in mono- or few-layer TMDs, which have been recently shown to exhibit extraordinary second harmonic generation. We investigate the large nonlinear response due to resonance enhancement. Theoretical analysis will be discussed and compared with experimental results.
We report a quenching phenomenon within the visible region of the electromagnetic spectrum in the photonic response of a passive Fe3O4-silicone elastomer composite film due to magnetically aligned Fe3O4 nanoparticles. We performed systematic studies of the polarization dependence, the effect of particle size, and an in- and out-of-plane particle alignment on the optical response of the Fe3O4-silicone elastomer composites using a UV/vis/NIR spectrometer. We observed systematic redshifts in the response of the out-of-plane composite films with increasing particle alignment and weight that are attributed to dipole-induced effects. There were no observable shifts in the spectra of the in-plane films, suggesting the orientation of the magnetic dipole and the induced electric dipole play a crucial role in the optical response. A dramatic suppression to near quenching of the photonic response occurred in films containing moderate concentrations of the aligned nanoparticles. This is attributed to the interplay between the intra- and the interparticle dipoles. This occurred even when low magnetic fields were used during the curing process, suggesting that particle alignment and particle size limitation are critical in the manipulation of the photonic properties. A dipole approximation model is used to explain the quenching phenomenon. An active system of such a composite has a potential application in magneto-optic switches.
Two-Dimensional (2D) layered materials have garnered interest due to their novel optical and electronic properties. In
this work, we investigate Second Harmonic Generation (SHG) in Tungsten Disulfide (WS2) monolayers grown on
SiO2/Si substrates and suspended on a transmission electron microscopy grid; we find an unusually large second order
susceptibility, which is nearly three orders of magnitude larger than common nonlinear crystals. We have also developed
a Green’s function based formalism to model the harmonic generation from a 2D layer .
Nonlinear effects are consequence of interaction of height intensities of energy with the matter. Self-diffraction is
nonlinear effect and rings are produced. We analyzed the increase of rings due to changes in intensity of CW Ar laser
that modify the nonlinear refractive index. The Carbon Nanotubes (CNTs) were dispersed on different solvents: a) water,
b) ethanol, c) isoprophanol, and d) acetone. The concentrations were 10ml:1mg in all samples. The dependence between
power and concentration of CNTs is shown.
In situ experiments, based on electron irradiation at high temperature in a transmission electron microscope, are used
to investigate isolated, packed and crossing single-wall nanotubes. During continuous, uniform atom removal, surfaces of isolated single-wall nanotubes heavily reconstruct leading to drastic dimensional changes. In bundles, coalescence of single-wall nanotubes is observed and induced by vacancies via a zipper-like mechanism. 'X', 'Y',
and 'T' carbon nano-structures are also fabricated by covalently connecting crossed single-wall nanotubes in order to pave the way towards controlled fabrication of nanotube based molecular junctions and network architectures exhibiting exciting electronic and mechanical behavior. Each experiment is followed by quantum modeling in order to investigate the effect of the irradiation process at the atomic level.
Particular attention will be focused on efficient self-assembly pyrolytic routes to large arrays (<2.5 cm2) of aligned C, CNx and BxCyNz nanotubes (15-80 nm od and < 100 microns length). In general, these 'hollow' fibres do not easily break upon bending and may behave as shock absorbing fillers in the fabrication of robust composites. The electronic and field emission properties, as well as the density of states (DOS) of CNx and BCx nanotubes using scanning tunneling spectroscopy (STS) will be presented. We further demonstrate that the presence of N and B are responsible for introducing donor and acceptor states near the Fermi Level. Novel applications of these doped materials will also be discussed. Finally, it will be shown that high electron irradiation during annealing at 700 - 800 °C, is capable of coalescing and joining single-walled nanotubes (SWNTs). Vacancies induce the merge via a zipper-like mechanism, imposing a continuous reorganization of atoms on individual tube lattices within the adjacent tubes. Other topological defects induce the polymerization of tubes and creation of 'Y', 'T' and 'X' nanotube junctions. The latter results pave the way to the fabrication of nanotube contacts, nanocircuits and strong 3D composites using irradiation doses under annealing conditions.