Photoconductivity of Si/Ge/Si structures with 1.5 and 2 ML of Ge layer

A photoconductivity (PC) of Si/Ge/Si structures with narrow Ge layer [thickness's 1.5 and 2 monolayers (ML)] on interband light intensity has been investigated for the different values of lateral voltage U, and temperature T. In contrast to the Si/Ge structure with 2 ML, where only monotonous PC growth was registered, for the 1.5 ML structure a stepped and a fluctuated PC were observed. These PC features are explained by a percolation of photoexcited carriers via the localized states induced by one monolayer scale Si/Ge interface roughnesses.     

Finite barrier width effects on exciton states and optical properties in wurtzite InGaN/GaN quantum well

Exciton states and optical properties in wurtzite (WZ) InGaN/GaN quantum well (QW) are investigated theoretically, considering finite barrier width and built-in electric field effects. Numerical results show that when the barrier width increases, the ground-state exciton binding energy, the interband transition energy and the integrated absorption probability increase first and then they are insensitive to the variation of the barrier width. For any barrier width, the ground-state exciton binding energy and the integrated absorption probability have a maximum when the well width is 1 nm; moreover, the integrated absorption probability goes to zero when the well width is larger than 6 nm. In addition, the competition effects between the built-in electric field and quantum confinement are also investigated in the WZ InGaN/GaN QW.     

Ultrafast relaxation of hot optical phonons in monolayer and multilayer graphene on different substrates

Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.     

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2 MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50 W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70 W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.     

Rotational anisotropy in second harmonic intensity from Cu nanowire arrays on the NaCl (1 1 0) substrates

We have measured the azimuthal angle dependence of the second harmonic (SH) intensity from Cu nanowires on the faceted NaCl (1 1 0) substrates in air at the fundamental photon energy of 1.17 eV. The SH intensity patterns showed two main lobes for p-in/p-out, s-in/p-out, and s-in/s-out polarization configurations. From the results of the experiment and the pattern analysis we have found that the observed SH light is enhanced by the electric field components along the substrate normal.     

Control by osmolarity and electric field strength of electro-induced gene transfer and protein release in fission yeast cells

A high electric pulse was applied to the uptake of DNA into cells, the sterilization of cells, and the release of protein from cells. These applications to fission yeast showed a strong dependence on both the osmolarity of pulsing sorbitol solution and the intensity of the electric pulse. In electroporation, high transformation efficiency was obtained with a wide range of sorbitol (0.6–1.6 M) at 10.0 kV/cm for about 5 ms. Furthermore, the highest efficiency was achieved in 1.5 M sorbitol at a higher strength, 12.5 kV/cm, although the cell survival rate dropped. The release of protein generally increased with increasing electric field strength, due mainly to leakage from dead cells under hypotonic conditions. However, protein was released significantly in 1.5 M sorbitol at a lower strength, 7.5 kV/cm, although a high survival rate was maintained. Thus, the application of the high electric pulse to fission yeast under hypertonic conditions increased the uptake and release of macromolecules controlled by the electric field strength.     

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25 eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5 V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.     

Electronic structures of Si(1 1 1) in the 7×7↔“1×1” phase transition studied by surface second-harmonic generation

Surface second-harmonic generation (SHG) of Si(1 1 1)-7×7 shows an increase in intensity for the surface-state transition (56%) and the strain-induced E₀′ interband transition (32%) in response to the phase transition to “1×1” taking place around 1100 K. The SHG surface-state transition in “1×1” is assigned as the redshifted S₃→U₁ transition in 7×7 from the observation of no discernible changes in the resonant characteristics. From the symmetry and atomic geometry of the electronic states responsible for SHG, the intensity jump is related to the dissolution of the stacking fault in the “1×1” phase.     

Exciton binding energies in GaN/AlxGa1 − xN pseudomorphic quantum wells

Interband transitions of pseudomorphic GaN/Al_xGa_1 − xN quantum wells are analysed theoretically with respect to the piezoelectric field utilizing a 6  ×  6 Rashba–Sheka–Pikus (RSP) Hamiltonian. Band structure modifications due to the built-in Stark effect explain a shift of the emission peak in GaN/Al_0.15Ga_0.85N of up to 400 meV. Quantum well exciton binding energies are calculated by the variational method and are discussed in terms of spatial separation of electrons and holes by the built-in electric field, as well as the interaction between valence subbands.     

The design of a body RF coil for low-field open MRI using pseudo electric dipole radiation and simulated annealing

The paper presents a new body RF coil design scheme for a low-field open MRI system. The RF coil is composed of four rectangular loops which are made of wide copper strips located near the surfaces of the bottom and top pole faces of the permanent magnet. The body RF coil has been designed by using the pseudo electric dipole radiation (PEDPR) method with the Metropolis algorithm. In the calculation of the RF fields via the finite difference time domain (FDTD) method, the computational time increases as the RF frequency becomes lower. Moreover, the computational process using the FDTD method takes a very long time when the RF coil is optimized. The optimization requires varying the configuration of the RF coil system and performing successive calculations of field strength and field homogeneity. When we perform these successive calculations, the computational time can be reduced by using the PEDPR method, where the segmented current elements of the RF coil are treated as pseudo electric dipole radiation sources. Because the RF coil is made of wide strips, the variation of the current density on the strip has been considered in the B₁-field calculation. For each configuration of the RF coil system, the current distribution is calculated via circuit analysis, where each copper strip is considered as a parallel combination of current element lines. The preliminary field calculation study by the FDTD method verifies both the circuit analysis method for the current distribution and the PEDPR method for the radiation field strength. The optimization of the RF coil configuration is performed by the Simulated Annealing (SA) process using the Metropolis algorithm. Simulations have been performed for a 10 MHz RF frequency. The optimized RF coil has four rectangular loops of 37 cm × 100 cm with 6.5 cm wide strips which are separated vertically 49 cm and horizontally center-to-center 63 cm. In the 25 cm diameter of spherical volume (DSV), the design results show a good field inhomogeneity of the B₁-field below 0.49 dB (5.8%).     

Superhydrophobic silica nanoparticles as ultrasound contrast agents

Microbubbles have been widely studied as ultrasound contrast agents for diagnosis and as drug/gene carriers for therapy. However, their size and stability (lifetime of 5–12 min) limited their applications. The development of stable nanoscale ultrasound contrast agents would therefore benefit both. Generating bubbles persistently in situ would be one of the promising solutions to the problem of short lifetime. We hypothesized that bubbles could be generated in situ by providing stable air nuclei since it has been found that the interfacial nanobubbles on a hydrophobic surface have a much longer lifetime (orders of days). Mesoporous silica nanoparticles (MSNs) with large surface areas and different levels of hydrophobicity were prepared to test our hypothesis. It is clear that the superhydrophobic and porous nanoparticles exhibited a significant and strong contrast intensity compared with other nanoparticles. The bubbles generated from superhydrophobic nanoparticles sustained for at least 30 min at a MI of 1.0, while lipid microbubble lasted for about 5 min at the same settings. In summary MSNs have been transformed into reliable bubble precursors by making simple superhydrophobic modification, and made into a promising contrast agent with the potentials to serve as theranostic agents that are sensitive to ultrasound stimulation.     

A first principle calculation of electronic and dielectric properties of electrically gated low-buckled mono and bilayer silicene

The structural, electronic and dielectric properties of mono and bilayer buckled silicene sheets are investigated using density functional theory. A comparison of stabilities, electronic structure and effect of external electric field are investigated for AA and AB-stacked bilayer silicene. It has been found that there are no excitations of electrons i.e. plasmons at low energies for out-of-plane polarization. While for AB-stacked bilayer silicene 1.48 eV plasmons for in-plane polarization is found, a lower value compared to 2.16 eV plasmons for monolayer silicene. Inter-band transitions and plasmons in both bilayer and monolayer silicene are found relatively at lower energies than graphene. The calculations suggest that the band gap can be opened up and varied over a wide range by applying external electric field for bilayer silicene. In infra-red region imaginary part of dielectric function for AB-stacked buckled bilayer silicene shows a broad structure peak in the range of 75–270 meV compared to a short structure peak at 70 meV for monolayer silicene and no structure peaks for AA-stacked bilayer silicene. On application of external electric field the peaks are found to be blue-shifted in infra-red region. With the help of imaginary part of dielectric function and electron energy loss function effort has been made to understand possible interband transitions in both buckled bilayer silicene and monolayer silicene.     

Broad-band microwave detection with a novel 2D hot-electron device

We present the operation of a new AlGaAs–GaAs multiquantum well hot-electron microwave detector. The working principle of this device is based on the interaction of two-dimensional free carriers inside the wells with the in-plane electric field.We shall report room-temperature responsivity of several 10³V W^− 1, comparable to that of conventional solid-state devices. The different physical principle of operation, however, yields for the present detector a broader frequency range, extending up to the submillimetre band, and short response times which can be estimated around 10 ps. Finally, we report a characterization of the device from the point of view of noise.     

Fermi surfaces of single layer dielectrics on transition metals

Single sheets of hexagonal boron nitride on transition metals provide a model system for single layer dielectrics. The progress in the understanding of h-BN layers on transition metals of the last 10 years is shortly reviewed. Particular emphasis lies on the boron nitride nanomesh on Rh(1 1 1), which is a corrugated single sheet of h-BN, where the corrugation imposes strong lateral electric fields. Fermi surface maps of h-BN/Rh(1 1 1) and Rh(1 1 1) are compared. A h-BN layer on Rh(1 1 1) introduces no new bands at the Fermi energy, which is expected for an insulator. The lateral electric fields of h-BN nanomesh violate the conservation law for parallel momentum in photoemission and smear out the momentum distribution curves on the Fermi surface.     

Determining the work function of a carbon-cone cold-field emitter by in situ electron holography

Cold-field emission properties of carbon cone nanotips (CCnTs) have been studied in situ in the transmission electron microscope (TEM). The current as a function of voltage, i(V), was measured and analyzed using the Fowler–Nordheim (F–N) equation. Off-axis electron holography was employed to map the electric field around the tip at the nanometer scale, and combined with finite element modeling, a quantitative value of the electric field has been obtained. For a tip-anode separation distance of 680 nm (measured with TEM) and a field emission onset voltage of 80 V, the local electric field was 2.55 V/nm. With this knowledge together with recorded i(V) curves, a work function of 4.8 ± 0.3 eV for the CCnT was extracted using the F–N equation.     

Supersymmetric quantum-well shape optimization for intersubband bound–continuum second harmonic generation

A method is described for the optimized design of quantum-well structures, with respect to maximizing the second-order susceptibilities relevant for second harmonic generation. The possibility is explored of obtaining resonantly enhanced nonlinear optical susceptibilities in quantum wells with two bound states and a continuum resonance state as the dominant third state. The method relies on applying the isospectral (energy structure preserving) transformations to an initial Hamiltonian in order to generate a parameter-controlled family of Hamiltonians. By changing the values of control parameters one changes the potential shape and thus the values of matrix elements relevant to susceptibility to be maximized. The method was used for the design of Al_xGa_1 − xAs -based QWs. The results indicate the possibility of employing continuum states in resonant second harmonic generation at higher photon energies,ℏω =  200–300 meV.     

Optical nutation in a magnetized semiconductor quantum well structure

Using the semiclassical coherent radiation—semiconductor interaction model, optical nutation has been analysed in aGaAs / Al_xGa_1 − xAs quantum well structure (QWS) assumed to be immersed in a moderately strong magnetic field and irradiated by a not-too-strong near band gap resonant femtosecond pulsed Ti–sapphire laser. The finite potential well depth of the QWS and the Wannier–Mott excitonic structure of the crystal absorption edge is taken into account. The excitation intensity is assumed to be below the Mott transition where the various many-body effects have been neglected with adequate reasoning. Numerical analysis made for a GaAs quantum well of thickness  ∼  100 Åand the confining layers ofAl_xGa_1 − xAs withx =  0.3 at intensity I ≤  5  ×  10⁶Wcm ^− 2reveals that the real and imaginary parts of the transient complex-induced polarization are enhanced with an increase in the magnetic field and their ringing behaviour confirms the occurrence of optical nutation in the QWS.     

Simulation of the acceleration mechanism by light-propulsion for the powder particles at laser direct material deposition

The results of the numerical analysis of heat- and mass-transfer processes at powder particles' motion in a gas flow and laser beam by light-propulsion force during the laser cladding and direct material deposition are presented. Under consideration were the stainless steel particles, the radiation power range of the CO₂ laser were 1000, 3000 and 5000 W. Finally, the particles of 45 μm in diameter reach the maximum velocity of about 80, 220, 280 m/s. It is shown that as particles are heated by the laser up to the temperature approaching the boiling point, the particles' velocity in the light field by the vapor recoil pressure may increase significantly. The radius of the particles slightly varies due to the evaporation; the losses in the clad material mass are negligibly small. Comparisons of numerical results with known experimental data on light-propulsion acceleration of single particles (aluminum, aluminum oxide and graphite) under the influence of pulse laser radiation are also presented. Particle acceleration resulting from the laser evaporation depends on the particle diameter, powder material properties, focusing degree and attenuation laser beam intensity by the direction of its propagation.     

Excitonic optical absorption in wurtzite InGaN/GaN quantum wells

Within the framework of the effective-mass and envelope function theory, exciton states and optical properties in wurtzite (WZ) InGaN/GaN quantum wells (QWs) are investigated theoretically considering the built-in electric field effects. Numerical results show that the built-in electric field, well width and in composition have obvious influences on exciton states and optical properties in WZ InGaN/GaN QWs. The built-in electric field caused by polarizations leads to a remarkable reduction of the ground-state exciton binding energy, the interband transition energy and the integrated absorption probability in WZ InGaN/GaN QWs with any well width and In composition. In particular, the integrated absorption probability is zero in WZ InGaN/GaN QWs with any In composition and well width L > 4 nm. In addition, the competition effects between quantum confinement and the built-in electric field (between quantum size and the built-in electric field) on exciton states and optical properties have also been investigated.     

Fault detection of antenna arrays using infrared thermography

A fast and easy method for fault detection in antenna arrays using infrared thermography is presented. A thin, minimally perturbing, microwave absorption screen made of carbon loaded polymer is kept close in front of the faulty array. Electromagnetic waves falling on the screen increase its temperature. This temperature profile on the screen is identical to electric field intensity profile at the screen location. There is no temperature rise observed on the screen corresponding to non-radiating (faulty) elements and hence can be easily detected by IR thermography. The array input power is modulated at a low frequency which permits thermography to detect even weak fields. It also improves the resolution of thermal images. The power fed to the array is only 30 dBm. In order to show the utility of this technique, an example of 14 GHz 4 × 4 patch antenna array is given. The simulations are carried in CST Microwave Studio 2013. A good agreement between simulation and experimental results is observed.