Optical Absorption in Periodic Graphene Superlattices: Perpendicular Applied Magnetic Field and Temperature Effects

A detailed study of the magneto‐optical absorption is presented for graphene superlattices (SLs) subjected to a perpendicular magnetic field. For a given temperature, this quantity exhibits a resonant peak structure whose characteristics depend on the magnetic field regime, circular polarization of light and SL barrier height. For the intermediate field regime, we demonstrated that the resonant peak structure of is directly correlated to the partial joint density of states. Specifically, the latter exhibits van Hove‐like singularities and peaks at energies where takes its maximum values. We also investigated the magnetoabsorption in the weak field regime for SLs exhibiting one and extra Dirac points in the absence of the field. It was found that for SLs with only one Dirac point, the absorption spectra consist of resonant peaks satisfying the same circular polarization dependent selection rule as that for pristine graphene, except for one of them. For SLs with extra Dirac points, the resonant peaks arise from transitions between singlet subbands or between doublet subbands and satisfy a circular polarization and peak intensity dependent selection rule. It was also found that the resonant structure of can be observed experimentally at room temperature in clean SLs.     

Diamond/graphite content and biocompatibility of DLC films fabricated by PLD

Biocompatibility and physicochemical properties of diamond-like carbon (DLC) thin layers prepared by pulsed laser deposition method were studied. The films of high and low diamond/graphite content were prepared by changing the laser energy density on the graphite target from 4 to 11 J cm⁻². The bonds and surface properties as roughness, atomic force microscopy topology, contact angle parameters, and zeta potential were measured. The cell adhesion/proliferation on DLC layers was tested using normal human fibroblasts and keratinocytes.     

The Cut Metric,Random Graphs,and Branching Processes

In this paper we study the component structure of random graphs with independence between the edges. Under mild assumptions, we determine whether there is a giant component, and find its asymptotic size when it exists. We assume that the sequence of matrices of edge probabilities converges to an appropriate limit object (a kernel), but only in a very weak sense, namely in the cut metric. Our results thus generalize previous results on the phase transition in the already very general inhomogeneous random graph model introduced by the present authors in Random Struct. Algorithms 31:3–122 (2007), as well as related results of Bollobás, Borgs, Chayes and Riordan (Ann. Probab. 38:150–183, 2010), all of which involve considerably stronger assumptions. We also prove corresponding results for random hypergraphs; these generalize our results on the phase transition in inhomogeneous random graphs with clustering (Random Struct. Algorithms, 2010, to appear).     

Room temperature pulsed laser deposition of Si x C thin films in different compositions

Amorphous silicon–carbon alloy films in different compositions were prepared by pulsed laser deposition from two-component targets containing pure silicon and carbon parts. The silicon–carbon ratio in the films was varied by adjusting the number of laser shots on the constituent silicon and carbon targets. The composition, optical properties, thickness, and bonding structure of the films were determined by backscattering spectrometry, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy, respectively. Backscattering spectrometry data were used to determine the deposition rate of silicon and carbon. This enabled the calculation of the number of the shots onto each target to reach a predefined composition. As the film composition changed from carbon to silicon, it was shown that the microscopic and macroscopic properties of the films also changed from a diamond-like carbon phase to an amorphous silicon phase via graphite- and silicon-carbide-like composite.     

1444-nm Q-switched pulse generator based on Nd:YAG/V:YAG microchip laser

Q-switched microchip laser emitting radiation at eye-safe wavelength 1444 nm was designed and realized. This laser was based on composite crystal which consists of 4 mm long Nd:YAG active medium diffusion bonded with 1 mm long V:YAG saturable absorber. The diameter of the composite crystal was 5 mm. The initial transmission of the V:YAG part was T ₀ = 94% @ 1440 nm. The microchip resonator consists of dielectric mirrors, directly deposited onto the composite crystal surfaces. These mirrors were specially designed to ensure desired emission at 1444 nm and to prevent parasitic lasing at other Nd³⁺ transmissions. The output coupler with reflectivity 94% for the generated wavelength 1444 nm was placed on the V³⁺-doped part. The laser was operating under pulsed pumping for the duty-cycle up to 50%. With increasing value of mean pumping power a strong decrease of generated pulse length was observed. The shortest generated pulses were 4.2 ns long (FWHM). Stable pulses with energy 34 μJ were generated with repetition rate up to 1.5 kHz. Corresponding pulse peak power was 8.2 kW. The wavelength of linearly polarized TEM₀₀ laser mode was fixed to 1444 nm.     

Determination of Very Slow Diffusion of Paramagnetic Ions by NMR Spectroscopy

In this paper, we propose a new method of measuring the very slow paramagnetic ion diffusion coefficient using a commercial high-resolution spectrometer. If there are distinct paramagnetic ions influencing the hydrogen nuclear magnetic relaxation time differently, their diffusion coefficients can be measured separately. A cylindrical phantom filled with Fricke xylenol gel solution and irradiated with gamma rays was used to validate the method. The Fricke xylenol gel solution was prepared with 270 Bloom porcine gelatin, the phantom was irradiated with gamma rays originated from a ⁶⁰Co source and a high-resolution 200 MHz nuclear magnetic resonance (NMR) spectrometer was used to obtain the phantom ¹H profile in the presence of a linear magnetic field gradient. By observing the temporal evolution of the phantom NMR profile, an apparent ferric ion diffusion coefficient of 0.50 μm²/ms due to ferric ions diffusion was obtained. In any medical process where the ionizing radiation is used, the dose planning and the dose delivery are the key elements for the patient safety and success of treatment. These points become even more important in modern conformal radio therapy techniques, such as stereotactic radiosurgery, where the delivered dose in a single session of treatment can be an order of magnitude higher than the regular doses of radiotherapy. Several methods have been proposed to obtain the three-dimensional (3-D) dose distribution. Recently, we proposed an alternative method for the 3-D radiation dose mapping, where the ionizing radiation modifies the local relative concentration of Fe²⁺/Fe³⁺ in a phantom containing Fricke gel and this variation is associated to the MR image intensity. The smearing of the intensity gradient is proportional to the diffusion coefficient of the Fe³⁺ and Fe²⁺ in the phantom. There are several methods for measurement of the ionic diffusion using NMR, however, they are applicable when the diffusion is not very slow.     

Propagation of surface plasmon polaritons through gradient index and periodic structures

We study propagation of surface electromagnetic waves along a metallic surface covered by various layered dielectric structures. We show that strong radiative losses, typical for scattering of a surface wave, can be considerably suppressed when a single dielectric step is substituted by gradient index or periodic layered structure.