Dark modes in symmetric bulk Dirac semimetal dimers excited by cylindrical vector beams

Dark plasmonic modes with sharp spectral resonance peak and exceptionally high quality factor makes it essential for applications in bio-chemical sensing and fluorescence enhancement. Here, we investigate the mid-infrared spectral responses of bulk Dirac semimetal (BDS) dimers under focused azimuthally polarized and radially polarized cylindrical vector beams (CVB). Through numerical simulations, we obtained direct excitation of dark modes and revealed how the beams manipulate the dipole hybridization to produce these modes. By tuning Fermi energy of BDS, the resonant wavelength of the dark modes can be further adjusted. Our results may find the application of CVB in plasmonic sensing.

Multigram Scale Total Synthesis of Piperarborenines C-E

We report the development of a multigram scale total synthesis of heterodimeric β-truxinic imides piperarborenines C-E using a catechol-mediated diastereoselective intramolecular [2+2] photocycloaddition. Key innovations lie in the use of (1) catechol as a highly selective auxiliary for the robust and scalable synthesis of homo- and heterodimeric β-truxinates, (2) UV LEDs for direct excitation in the [2+2] cycloaddition step, and (3) a bis pentafluorophenyl ester and LDA for the challenging installation of the syn dihydropyridinone imides. This approach is exceptionally scalable – requiring minimal chromatography, no photocatalysts, and no cryogenic conditions - and will enable thorough evaluation of the biological properties and anticancer profiles of piperaborenines C-E and derivatives thereof.

Effects of secondary γ-electrons from accelerator grid under ion impingement in gridded ion sources

Thus far, effects of secondary γ-electrons emitted from accelerator grids of gridded ion sources on ionization in discharge chambers have not been studied. The presence and induced processes of such secondary electrons in a microwave electron cyclotron resonance gridded ion source are confirmed by the consistent explanations of: (1) the observed jump of ion beam current (Ib) in case of a low-density plasma appearing at the chamber’s radial center due to the microwave skin effect; (2) the evolution of glow images recorded from the end-view of the ion source during the jump of Ib; (3) the over-large jump step of Ib with the increasing microwave power; (4) the pattern appearing on the temperature sticker exposed to the discharge operated in the regime where the arrayed energetic-electron beamlets are injected into the discharge chamber; (5) the measured step-increment in the voltage drop across the screen grid sheath. A positive feedback loop composed of involved processes are established to elucidate the underlying mechanism. Energetic γ-electrons from the accelerator grid and warm δ-electrons from the opposite antenna do not produce direct excitation and ionization, but they enhance the electrical confinement of cold electrons by elevating the voltage drop across the sheaths at the antenna and screen grid, thus leading to the jump of Ib. The energetic γ-electrons-based model can be also modified to explain abnormal results observed in the other gridded ion sources. Energetic γ-electrons from accelerator grids should be taken into account in understanding gridded ion sources.

Optical Anisotropy in van der Waals materials: Impact on Direct Excitation of Plasmons and Photons by Quantum Tunneling

Inelastic quantum mechanical tunneling of electrons across plasmonic tunnel junctions can lead to surface plasmon polariton (SPP) and photon emission. So far, the optical properties of such junctions have been controlled by changing the shape, or the type of the material, of the electrodes, primarily with the aim to improve SPP or photon emission efficiencies. Here we show that by tuning the tunneling barrier itself, the efficiency of the inelastic tunneling rates can be improved by a factor of 3. We exploit the anisotropic nature of hexagonal boron nitride (hBN) as the tunneling barrier material in Au//hBN//graphene tunnel junctions where the Au electrode also serves as a plasmonic strip waveguide. As this junction constitutes an optically transparent hBN–graphene heterostructure on a glass substrate, it forms an open plasmonic system where the SPPs are directly coupled to the dedicated strip waveguide and photons outcouple to the far field. We experimentally and analytically show that the photon emission rate per tunneling electron is significantly improved (~ ×3) in Au//hBN//graphene tunnel junction due to the enhancement in the local density of optical states (LDOS) arising from the hBN anisotropy. With the dedicated strip waveguide, SPP outcoupling efficiency is quantified and is found to be ∼ 80% stronger than the radiative outcoupling in Au//hBN//graphene due to the high LDOS of the SPP decay channel associated with the inelastic tunneling. The new insights elucidated here deepen our understanding of plasmonic tunnel junctions beyond the isotropic models with enhanced LDOS.

Spectral study of argon-methane mixture plasma jet generated by a DC plasmatron

We present the results of studying optical emission spectra of Ar:CH4 plasma produced on a DC plasmatron for graphene synthesis. We have identified the basic set of spectral lines and bands in the obtained spectra and shown that H lines and C2 bands appear due to direct excitation by an electron strike of corresponding neutral particles. C2 molecular bands were also identified in the spectra with intensity considerably lower compared to previous studies where He: C2H2 mixture was used as plasma-forming gas.

Electrodynamics of dual-band end-fed wire dipole radiators

This paper briefly describes an analysis of main electrodynamic characteristics required in design of dual-band Yagi antenna exciters consisting of two electrically thick conductors. The study of end-fed excitation of the said system of conductors allows to predetermine the design of conceptually new circuits of classic dipole radiators, avoiding closely-spaced arrangement of feeding strips that ensure direct excitation of dipole arms and, therefore, reducing their electromagnetic spurious coupling. The latter prevents asymmetry in surface current amplitude distribution on dipole arms and allows to improve its directivity and matching with a power feeder.

Up-Conversion Luminescence Processes in NaLaF4 Doped with Tm3+ and Yb3+ and Dependence on Tm3+ Concentration and Temperature

In this work, luminescence processes in polycrystalline NaLaF4:Tm3+ and NaLaF4:Tm3+,Yb3+ materials were studied. Luminescence spectra and decay kinetics measurements were performed for NaLaF4 doped with various Tm3+ concentrations (0.01, 0.1, 0.5, 1, and 2 mol%) under direct excitation to 3P0, 1D2, 1G4, and 3H4 states. It was found that some of the Tm3+ excited states are more affected by Tm3+ concentration than other states. Under infrared excitation of Yb3+, energy transfer to Tm3+ occurred and intensive ultraviolet and blue up-conversion luminescence was observed. Possible up-conversion mechanisms are discussed. Spectroscopic measurements show that long-duration excitation radiation reduces ultraviolet up-conversion luminescence intensity, and this intensity reduction is related to sample heating due to high excitation radiation density and a poor heat sink from samples. It was found that sample configuration for spectroscopic measurements is crucial to correctly describe measured up-conversion luminescence spectra.