Monte Carlo study of impact ionization in n-type InAs induced by intense ultrashort terahertz pulses

Electron impact ionization induced in n-type InAs by single-cycle pulses of picosecond and subpicosecond duration has been investigated by Monte Carlo method. It is established that the rate of generation of electron–hole pairs decreases with the decrease of the pulse duration. The impact ionization threshold field is found to depend linearly on frequency for the fields oscillating at frequencies much higher than reciprocal momentum relaxation time. Good agreement between calculations and available experimental data has been obtained.     

Phenylethenyl‐Substituted Triphenylamines: Efficient,Easily Obtainable,and Inexpensive Hole‐Transporting Materials

Star‐shaped charge‐transporting materials with a triphenylamine (TPA) core and various phenylethenyl side arm(s) were obtained in a one‐step synthetic procedure from commercially available and relatively inexpensive starting materials. Crystallinity, glass‐transition temperature, size of the π‐conjugated system, energy levels, and the way molecules pack in the solid state can be significantly influenced by variation of the structure of these side arm(s). An increase in the number of phenylethenyl side arms was found to hinder intramolecular motions of the TPA core, and thereby provide significant enhancement of the fluorescence quantum yield of the TPA derivatives in solution. On the other hand, a larger number of side arms facilitated exciton migration through the dense side‐arm network formed in the solid state and, thus, considerably reduces fluorescence efficiency by migration‐assisted nonradiative relaxation. This dense network enables charges to move more rapidly through the hole‐transport material layer, which results in very good charge drift mobility (μ up to 0.017 cm² V ^?1 s^?1).     

Diboraanthracene-Doped Polymer Systems for Colour-Tuneable Room-Temperature Organic Afterglow

Organic ultralong room temperature phosphorescence (RTP), or organic afterglow, is a unique phenomenon, gaining widespread attention due to its far-reaching application potential and fundamental interest. Here, two laterally expanded 9,10-dimesityl-dihydro-9,10-diboraanthracene (DBA) derivatives are demonstrated as excellent afterglow materials for red and blue-green light emission, which is traced back to persistent thermally activated delayed fluorescence and RTP. The lateral substitution of polycyclic DBA scaffold, together with weak transversal electron-donating mesityl groups, ensures the optimal molecular properties for (reverse) intersystem crossing and long-lived triplet states in a rigid poly(methyl methacrylate) matrix. The achieved afterglow emission quantum yields of up to 3 % and 15 %, afterglow lifetimes up to 0.8 s and 3.2 s and afterglow durations up to 5 s and 25 s (for red and blue-green emitters, respectively) are attributed to the properties of single molecules.     

Electromagnetic field and dispersion characteristic analysis of absorbing onion-like carbon tube waveguides

Here, we present our calculation results of the electromagnetic field distributions and the dispersion characteristics of open cylindrical tube waveguides. The analyzed waveguides are made of the onion-like carbon (OLC) material. The solution of the boundary problem was fulfilled by the partial area method (Nickelson et al. in electrodynamical analysis of open lossy metamaterial waveguide and scattering structures. InTech, UK, pp 27–58 [1]). We have determined the complex roots of the dispersion equation using of the Muller method. It discovered the very complicated dependencies of the phase and attenuation constants on the waveguide radii. Such dependencies arise because the OLC material is the highly dispersive and absorbing one. We have investigated the high-frequency cutoff frequency of the propagating hybrid modes HE₁₁ and HE₁₂ dependent on the tube waveguide external and internal radii. We found that it is possible to reach the one-mode regime of OLC tube waveguide.     

Numerical Study of Open Slot Lines with Asymmetrically Placed Strip Conductor in 37.5–78 GHz Frequency Band

Slot lines with strip conductors asymmetrically placed on a finite size substrate have been rigorously studied by means of the singular integral equation method (SIE). Dependences of the propagation constant on the frequency for different slot line sizes (substrate width, strip conductor width as well as the distance from the substrate edge, slot width and substrate thickness) were determined in the 37.5 – 78 GHz frequency. What we analized was a) how the substrate thickness and slot width affected the number of modes propagating in the slot line and b) what the size of the line should be to ensure propagating of only one mode (this is important in practice).     

Phase constant peculiarities of cylindrical zero-index anisotropic metamaterial waveguide

Here, we present the phase constant dependencies of propagating eigenmodes of an open cylindrical anisotropic metamaterial waveguide when the metamaterial permittivity and permeability tensor components may accept values close or equal to zero. Dispersion characteristics of rod and hollow-core waveguides with the radii 0.5, 2.5, and 5 mm at the left-handed polarization of the microwave are shown here. There are unusual shapes of eigenmode dispersion characteristics and anomalous sectors of the characteristics at certain frequencies. The first eigenmode of the rod waveguide with the lowest cutoff frequency is a particularly important mode because it is a single one in the frequency range 1.0?C1.9 GHz, and some small variations on the frequency produce large changes in the phase constant. We can observe packages of dispersion characteristic branches when their cutoff frequencies closed to the metamaterial electric and magnetic plasma frequencies between 1.9 and 3.5 GHz. There are only three modes in the hollow core anisotropic metamaterial waveguide at the frequency range 1.4?C2.8 GHz.     

Influence of MSL Substrate Semiconductor Specific Resistivity on Electrodynamical Characteristics

In this paper the electrodynamical characteristics of open microstrip lines (MSLs) with semiconductor n–Si substrates were investigated using the method of singular integral equations (SIE) in the quasi–TEM approximation. Our algorithm allows anyone to consider in their calculations the real geometry and sizes of MSLs. There is an agreement between our numerical and experimental results. The dependences of a complex wave impedance, a wavelength, and an attenuation constant of a MSL were analyzed on the frequency f until f = 70 GHz. Our calculations were fulfilled for MSLs with low ohmic semiconductor substrates and for several specific resistivities of the substrate material. We specified the dependences of electrodynamical characteristics on frequencies when they became constant at the start of a certain frequency and then they remained constant at higher frequencies. These frequency values were dependent on the substrate specific resistivity.