Plasmonic Properties of Al2O3 Nanoshell with a Metallic Core

Background: Al is the promising candidate for the deep UV and longer wavelength range plasmonic applications. But it is difficult to have the pure Aluminium nanostructure as it is easily oxidized forming a thin layer of Al2O3. In this paper we have evaluated the field enhancement of oxide layer on metallic shell (Al-Al2O3 and Au- Al2O3) for single and dimer core-shell configuration and shown potential of oxide layer in SERS. Methods: The Finite Difference Time Domain (FDTD) has been used to evaluated the LSPR and field enhancement of single and dimer Al-Al2O3 and Au- Al2O3 nanostructure. Results: The results exhibit the tunable plasmon resonance on varying the inner and outer radii of the Al2O3 shell. A redshift and decrease in enhancement were observed as shell thickness increases whereas on increasing the core size the enhancement gets increased in the case of Au-Al2O3 and gets a decrease in Al-Al2O3 due to quadrupole contribution. But on comparing the Au-Al2O3 with Al-Al2O3 for the same particle size, Al-Al2O3 shows larger enhancement because Au has to compete with its inter band transition. Conclusion: By optimizing the thickness of the shell and core size, it can be concluded that an ultrathin shell of Al2O3 can give higher enhancement. With Al as a core metal the enhancement increases as compared to Au-Al2O3. Since a single Al-Al2O3 nanoshell has shown a huge enhancement we have considered the multimer configuration of two identical nanoshell. Due to coupling between two nanoshell a huge increase in enhancement factor ~1012 was observed for Al-Al2O3 dimer nanoshell in the UV region.

Strain and Electric Field Modulated Indirect to Direct Band Transition of Monolayer GaInS2

Strain and electric field dependent electronic and optical properties have been calculated using density functional theory (DFT) and time-dependent DFT (TD-DFT) for GaInS2 monolayer. GaInS2 monolayer shows an indirect band gap of 1.79 eV where valance band maxima (VBM) and conduction band maxima (CBM) rest between K and Γ point and at Γ point, respectively. Under a particular tensile strain (8%), a phase change from semiconductor to semimetal has been noticed. While at 4% compressive strain, the material changes from indirect to direct band gap of 2.22 eV having VBM and CBM at Γ point. With further increase in compressive strain, CBM shifted from Γ to M point, which leads to an indirect band gap again. The electric field also affects the band structure of monolayer GaInS2 and shows the transition between indirect to direct band gap at positive electric field of 4 V/nm, which acts normal to the surface. The strain-dependent optical properties are also calculated, which suggests that the absorption coefficient increases with compressive strain.

Front-induced transitions control THz waves

Relativistically moving dielectric perturbations can be used to manipulate light in new and exciting ways beyond the capabilities of traditional nonlinear optics. Adiabatic interaction with the moving front modulates the wave simultaneously in both space and time, and manifests a front-induced transition in both wave vector and frequency yielding exotic effects including non-reciprocity and time-reversal. Here we introduce a technique called SLIPSTREAM, Spacetime Light-Induced Photonic STRucturEs for Advanced Manipulation, based on the creation of relativistic fronts in a semiconductor-filled planar waveguide by photoexcitation of mobile charge carriers. In this work, we demonstrate the capabilities of SLIPSTREAM for the manipulation of terahertz (THz) light pulses through relativistic front-induced transitions. In the sub-luminal front velocity regime, we generate temporally stretched THz waveforms, with a quasi-static field lasting for several picoseconds tunable with the front interaction distance. In the super-luminal regime, the carrier front outpaces the THz pulse and a time-reversal operation is performed via a front-induced intra-band transition. We anticipate our platform will be a versatile tool for future applications in the THz spectral band requiring direct and advanced control of light at the sub-cycle level.

Layer-engineered interlayer excitons

Photoluminescence (PL) from excitons serves as a powerful tool to characterize the optoelectronic property and band structure of semiconductors, especially for atomically thin two-dimensional transition metal dichalcogenide (TMD) materials. However, PL quenches quickly when the thickness of TMD materials increases from monolayer to a few layers, due to the change from direct to indirect band transition. Here, we show that PL can be recovered by engineering multilayer heterostructures, with the band transition reserved to be a direct type. We report emission from layer-engineered interlayer excitons from these multilayer heterostructures. Moreover, as desired for valleytronics devices, the lifetime, valley polarization, and valley lifetime of the generated interlayer excitons can all be substantially improved as compared with that in the monolayer-monolayer heterostructure. Our results pave the way for controlling the properties of interlayer excitons by layer engineering.

Magneto-optical absorption coefficients in monolayer MoSe2

We study the linear, third-order nonlinear, and total absorption coefficients (OACs) caused by intra- and inter-band transitions in monolayer MoSe2 in the presence of a magnetic field by using the compact density matrix approach. The results show that the OACs display the blue-shift behaviour with an increase in the magnetic field. The Zeeman fields do not affect the peak positions but reduce peak intensities slightly. Besides, the strong spin-orbit coupling in monolayer MoSe2 causes the peaks to differ significantly due to spin-up and spin-down. The OACs due to intra-band transition display only one peak in the THz range, while the inter-band spectra show a series of peaks in the near-infrared optical range, making monolayer MoSe2 a promising candidate for novel optoelectronic applications.

First Principles Study on Electronic and Optical Properties of Quinary Copper-based Sulfides and Selenides Cu2HgGe(S1-xSex)4

Electronic and optical properties of Cu2HgGe(S1-xSex)4 compounds (x = 0, 0.25, 0.5, 0.75, and 1) were revealed by density functional theory (DFT), in which the Heyd-Scuseria-Ernzerhof hybrid functional was used. Dependence of band gap on the Se constituent in Cu2HgGe(S1-xSex)4 was reported. The substitution of Se element basically cause a slightly lattice expansion and minor change of the band gap. Meanwhile, the overlap of Cu and S/Se states becomes more dense leading to better electron/hole pair separation and inter-band transition of photo-excited electrons. The Cu2HgGe(S0.75Se0.25)4 compound was predicted to be very promising absorber due to the low band gap, high absorption rate, and low reflectivity in the incoming light energy range from 0 eV to 2 eV.