Cubic Nonlinearity of Graphene-Oxide Monolayer.

The cubic nonlinearity of a graphene-oxide monolayer was characterized through open and closed z-scan experiments, using a nano-second laser operating at a 10 Hz repetition rate and featuring a Gaussian spatial beam profile. The open z-scan revealed a reverse saturable absorption, indicating a positive nonlinear absorption coefficient, while the closed z-scan displayed valley-peak traces, indicative of positive nonlinear refraction. This observation suggests that, under the given excitation wavelength, a two-photon or two-step excitation process occurs due to the increased absorption in both the lower visible and upper UV wavelength regions.

Exciton Dephasing in Tungsten Diselenide Atomic Layer.

An intrinsic exciton dephasing is the coherence loss of exciton dipole oscillation, while the total exciton dephasing originates from coherence loss due to exciton-exciton interaction and excitonphonon coupling. In this article, the total exciton dephasing time of tungsten diselenide (WSe₂) atomic layers was analyzed as functions of excitation intensity with exciton-exciton coupling strength and temperature with exciton-phonon coupling strength. It was hypothesized that the total exciton dephasing time is shortened as the exciton-exciton interaction and the exciton-phonon coupling are increased.

Cubic Nonlinearity of Molybdenum Disulfide Nanoflakes.

The cubic optical nonlinearity of molybdenum disulfide (MoS₂) nanoflakes was characterized by Z-scan and I-scan with resonant excitation. The excitation source was a ~6 ns laser at 532 nm with a 10 Hz repetition rate. The open and closed Z-scan analyzed the nonlinear absorption and nonlinear refraction properties of MoS₂ nanoflakes.

Piezoelectricity in WSe/MoS heterostructure atomic layers.

A two-dimensional heterostructure of WSe2/MoS2 atomic layers has unique piezoelectric characteristics which depend on the number of atomic layers, stacking type and interlayer interaction size. The van der Waals heterostructure of p- and n-type TMDC atomic layers with different work functions forms a type-II staggered gap alignment. The large band offset of the conduction band minimum and the valence band maximum between p-type WSe2 and n-type MoS2 atomic layers leads to large electric polarization and piezoelectricity.

Bandgap Tunability of Transition Metal Dichalcogenide Atomic Layers.

The temperature-dependent bandgap of transition metal dichalcogenides (TMDCs, MX2; M = Mo or W; X = S, Se, or Te) is analyzed using the O'Donnell and Chen relation with parameters including the average acoustic phonon energy (〈ħω〉) and the electron-phonon coupling strength (s). Wider (narrower) tunability of the bandgap results from the larger (smaller) electron-phonon coupling strength for a constant acoustic phonon energy. A 1.

Exciton Formation Entropy Changes in Transition Metal Dichalcogenide Atomic Layers.

The atomic layers of transition metal dichalcogenides (TMDCs, MX2; M = Mo or W; X = S, Se, or Te) are of great interest in the areas of photonics and optoelectronics due to the correlation between valley orbital, spin, and optical helicity; the compositional tuning of exciton bandgaps in visible and near-infrared spectra; and the bandgap modification from indirect for bilayer or multilayer to direct for monolayer. The derivative of the O'Donnell and Chen relation is analyzed as a function of temperature and gives the relationship between the change in entropy of exciton formation and the bandgap energy. The analysis suggests the change in entropy of exciton formation with higher energy phonons (~100 meV) is constant until ~90 K while lower energy phonons (~10 meV) approaches a constant value of -2skB between ~250 K and ~300 K where s is the strength of electron-phonon interaction and kB is the Boltzmann constant.

Piezoelectricity enhancement and bandstructure modification of atomic defect-mediated MoS monolayer.

Piezoelectricity appears in the inversion asymmetric crystal that converts mechanical deformation to electricity. Two-dimensional transition metal dichalcolgenide (TMDC) monolayers exhibit the piezoelectric effect due to inversion asymmetry. The intrinsic piezoelectric coefficient (e) of MoS is ∼298 pC m.

Plasmon-Coupled CdSe/ZnS and CdTe/CdS/ZnS Coreshells for Hybrid Light Emitting Devices.

Plasmon-coupled CdSe/ZnS and CdTe/CdS/ZnS coreshells are investigated for their optoelectronic applications because of their high color purity, wide optical tunability, large PL enhancement, and compact and easy integration into electronic devices. The quantum confinement of carriers within quantum dots (QDs) with sizes near the exciton Bohr radius (CdSe ~ 5.8 nm, CdTe ~ 7 nm) exhibits the features of discrete energy states and blue-shift from the bulk bandgap (CdSe ~718 nm, CdTe ~ 863 nm) in the optical spectrum.

Hybrid optical materials of plasmon-coupled CdSe/ZnS coreshells for photonic applications.

A hybrid optical nanostructure of plasmon-coupled SQDs was developed for photonic applications. The coupling distances between the mono-layers of Au nanoparticles with a surface concentration of ~9.18 × 10 nm and CdSe/ZnS SQDs with that of ~3.

Optical nonlinearities of Au nanoparticles and Au/Ag coreshells.

Au nanoparticles exhibited both negative and positive nonlinear absorptions with ground-state plasmon bleaching and free-carrier absorption that could be origins of the saturable and reverse-saturable optical properties. Au/Ag coreshells displayed only positive nonlinear absorption and reverse-saturable optical properties as a function of excitation intensity at the edge of surface-plasmon resonance, which implies no ground-state plasmon bleaching and the existence of two-photon absorption.

Slow light and superluminality in Kerr media without a pump.

Subluminal and superluminal propagation of a light pulse in Kerr materials has been investigated. Group velocities as slow as much less than 1 mm per second to as fast as negative several thousands meters per second can easily be obtained in the Kerr medium, which possesses a large nonlinear refractive index and long relaxation time, such as Cr3+-doped alexandrite, ruby, and GdAlO3. The physical mechanism is the strong highly dispersive coupling between different frequency components of the pulse.