At the frequencies from 0.1 to 1 THz, we measured the angular distributions of terahertz (THz) emission from DC-biased femtosecond filament. The external electric field (DC bias) was increased from 0 ...to 3.3 kV/cm and provided continuous transition from forward conical emission, corresponding to the unbiased single-color filament, to on-axis emission, corresponding to the DC-biased one. We decomposed the measured far-field THz distributions into the quadrupole and dipole contributions, the latter being increased with increasing biasing field. The superposition of quadrupole and dipole local sources was integrated numerically over the plasma channel length and fit to the experimentally obtained angular distributions. The transition from the conical to the on-axis emission occured at the external field of (3.2 ± 0.8) kV/cm in the range of frequencies studied.
•Sensitized Mn2+ luminescence is studied in lithium tetraborate.•Existence of complex luminescence centres is proposed.•Slow or ultrafast Mn2+ luminescence decays occur depending on the excitation ...energy.•Mechanisms for excitation transfer and luminescence decay acceleration are proposed.
Sub-nanosecond luminescence of Mn2+ in Mn-, Sn/Mn-, and Cu/Mn-doped Li2B4O7 ceramics is observed under laser excitation. Picosecond pulsed laser radiation (350–370 nm) generates the frequency-doubled radiation directly in a single microcrystal selected in a ceramic sample, with this secondary radiation falling within the impurity-induced VUV excitation bands of doped Li2B4O7. These excitation bands are attributed to the creation of an impurity-bound intra-anionic exciton in MeiO4–BO3–MnLi complex centres (Mei = Mn, Sn, or Cu). The exciton, in turn, interacts with a nearby Mn2+ ion due to mixing of the Mn2+ excited states with the excitonic states. Exciton annihilation involves Mn2+ energy levels as transient states and causes a sub-nanosecond radiative transition which is accelerated, being accompanied by rearrangement of the surrounding atoms. Excitation within the lower-energy bands is related to the other part of the complex luminescence centre, this is metal-to-ligand intra-anionic charge transfer, which makes an electron accessible to MnLi, while the hole is transferred only during relaxation of the excited structure. Mn2+ radiative transitions are not forced in the latter case, and slow Mn2+ luminescence decay is observed.
Two-dimensional distribution patterns of terahertz radiation generated in a laser single-color filament plasma are measured at several frequencies. In the low-frequency region (0.1–0.5 THz), the ...radiation propagates in a cone with a minimum on the axis. At higher frequencies, the terahertz radiation pattern depends significantly on the laser pulse polarization. In the case of linear polarization, the axial symmetry is broken: terahertz radiation propagates into two maxima located along the axis perpendicular to the laser polarization. In the case of circular polarization, the axial symmetry of the terahertz radiation distribution is restored.
Abstract
In this paper, we consider the properties of a flat crystal structure associated with the matrix representation of finite continued fractions generating unimodular morphisms of a flat ...integer lattice. The used matrix representations of the continued fractions and their properties are obtained in 1. The constructed model allows us to explain the existing limitations of the sets of Weiss parameters (the rational ratio of the lengths of the edges of the forming cell) of crystals by the Gauss-Kuzmin distribution of natural numbers in the representation of continued fractions.
At the selected frequencies from 0.3 to 10 THz we measured the two-dimensional (2D) distributions of fluence and polarization of terahertz (THz) emission from a single-color femtosecond filament. At ...the majority of frequencies studied, the THz beam has a donut-like shape with azimuthal modulations and radial polarization. At the maximal modulation, THz beam takes the form of the two lobes and polarization of the THz field degenerates into orthogonal to the laser pulse polarization direction. Violation of the radially polarized donut beam shape is due to destructive interference of THz waves driven by light pressure directed along the laser beam propagation axis and ponderomotive force parallel to the laser polarization.
We have experimentally obtained two-dimensional distributions of terahertz radiation generated by one or four filaments formed by phase optical elements in air. It has been demonstrated that the use ...of the phase mask reduces the propagation angles of terahertz beam by approximately one and a half times, which is due to the interference of terahertz radiation from four sources. The use of the Dammann grating slightly enlarges these angles.
•Multi-filamentation of high power femtosecond laser pulses in water.•Fabrication of sub-diffraction surface ripples on wet aluminum by high optical harmonics.
Relief ripples with sub-diffraction ...periods (≈λlas/3, λlas/4) were produced on a aluminum surface immersed in water and irradiated in a multi-filamentation regime by focused 744nm femtosecond laser pulses with highly supercritical, multi-GW peak powers. For the VUV (8.5eV) surface plasmon resonance on the wet aluminum surface, such small-scale surface nanogratings can be produced by high – second and third – optical harmonics, coming to the surface from the optical filaments in the water layer. Then, the sub-diffraction surface ripples may appear through interference of their transverse electric fields with the longitudinal electric fields of their counterparts, scattered on the surface roughness and appeared as the corresponding high-energy, high-wavenumber surface polaritons.
A technique is presented to create uninterrupted long ultraviolet filaments in air using appropriately structured transmission mesh. The mesh with different cell sizes was inserted into 10-cm ...parallel beam of 0.2-J, 248-nm, and 870-fs pulse propagating along ~100-m corridor. Transverse positions of multiple filaments formed by the optimum size cells were reproducible within at least 15 m along the propagation path. 3D+time simulations confirmed uninterrupted plasma channels with fixed positions in the transverse space similar to the experiment. Unoptimized cell size resulted in filaments shifting towards the cell center and destruction of uninterrupted filaments.
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