Numerical simulations of single and multiple scattering by fractal ice clusters

We consider the scattering model in the form of a vertically and horizontally homogeneous particulate slab of an arbitrary optical thickness composed of widely separated fractal aggregates built of small spherical ice monomers. The aggregates are generated by applying three different approaches, including simulated cluster-cluster aggregation (CCA) and diffusion-limited aggregation (DLA) procedures. Having in mind radar remote-sensing applications, we report and analyze the results of computations of the backscattering circular polarization ratio obtained using efficient superposition T-matrix and vector radiative-transfer codes. The computations have been performed at a wavelength of 12.6 cm for fractal aggregates with the following characteristics: monomer refractive index m=1.78+i0.003, monomer radius r=1 cm, monomer packing density p=0.2, overall aggregate radii R in the range 4≤R≤10 cm and fractal dimensions D_f=2.5 and 3.We show that for aggregates generated with simulated CCA and DLA procedures, the respective values of the backscattering circular polarization ratio differ weakly for D_f=2.5, but the differences can increase somewhat for D_f=3, especially in case of an optically semi-infinite medium. For aggregates with a spheroidal overall shape, the dependence of the circular polarization ratio on the cluster morphology can be quite significant and increases with increasing the aspect ratio of the circumscribing spheroid.     

The influence of a demagnetizing field on hysteresis losses in a dense assembly of superparamagnetic nanoparticles

The electrodynamic method is used to measure the hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D=25 nm in the frequency range f=10–150 kHz and for magnetic field amplitudes H₀=100–300 Oe. It is found that the specific loss power is determined by a demagnetizing factor of a whole sample. It diminishes approximately 4.5 times when the sample aspect ratio decreases from L/d=11.4 to L/d≈1, where L and d are the sample length and diameter, respectively. For H₀≤300 Oe the maximal specific loss power 120 W/g is obtained for the sample with L/d=11.4 at f=120 kHz. For comparison, the assembly specific absorption rate has been determined also by means of direct measurement of the temperature difference between the inner and outer surfaces of a flat cuvette containing magnetic nanoparticles. For both methods of measurement close values for the specific absorption rate are obtained for samples with similar demagnetizing factors.     

Novel device to measure critical point "Onset" of capillary wave and interpretation of Faraday instability wave by numerical analysis

A capillary wave was created on a surface by vibrating from the bottom of a container. When the amplitude of the container vibration approached the critical point, called the onset state, the surface broke up and bursted into very small drops on the air. The numerical analysis was used to determine the amplitude of the onset. The onset point was found to be 0.349 μm at f = 500 kHz. The critical amplitude h_cr was determined by using a multi-Fourier horn nozzle (MFHN) device. The onset point was measured to be 0.37 μm using a laser Doppler vibrometer (LDV) with the MFHN at f = 486 kHz. These drops indicate that particle size distributions of 10.8 μm and 7.0 μm were produced by the MFHN at f = 289 kHz and f = 486 kHz, respectively. These results agreed with those obtained using Kelvin’s equation, which predicted D = 0.34λ.     

Operation of thin-film gated SOI lateral PIN photodetectors with gate voltage applied and intrinsic length variation

This paper describes the operation principle of thin-film gated SOI lateral PIN photodetectors, and an analytical model of depletion voltage is presented and validated by two-dimensional Atlas simulations. With gate voltage applied to achieve fully depleted (FD) condition in intrinsic region, the variation of intrinsic length (L_i) on photocurrent and dark current characteristics, sensitivity, and speed is addressed. With L_i between 1 and 10 μm, the simulated results predict internal quantum efficiency (QI) in excess of 95% even near 100% at a 400 nm wavelength. Also, QI can yield over 87% for the long channels. Under FD condition, the total −3 dB frequency value can achieve 16 GHz (19 GHz) for L_i = 1  and 4.1 GHz (6.2 GHz) for L_i = 2 μm with V_K = 1.0 V (2.0 V). And a high ratio of more than 10⁷ between illuminated and dark currents can be yielded for all detectors realized in 0.18 μm SOI CMOS technology.     

Chemical etching to dissolve dislocation cores in multicrystalline silicon

Multicrystalline silicon wafers are used for approximately half of all solar cells produced at present. These wafers typically have dislocation densities of up to ∼10⁶ cm⁻². Dislocations and associated impurities act as strong recombination centres for electron–hole pairs and are one of the major limiting factors in multicrystalline silicon substrate performance. In this work we have explored the possibility of using chemical methods to etch out the cores of dislocations from mc-Si wafers. We aim to maximise the aspect ratio of the depth of the etched structure to its diameter. We first investigate the Secco etch (1K₂Cr₂O₇ (0.15 M): 2HF (49%)) as a function of time and temperature. This etch removes material from dislocation cores much faster than grain boundaries or the bulk, and produces tubular holes at dislocations. Aspect ratios of up to ∼7:1 are achieved for ∼15 μm deep tubes. The aspect ratio decreases with tube depth and for ∼40 μm deep tubes is just ∼2:1, which is not suitable for use in bulk multicrystalline silicon photovoltaics. We have also investigated a range of etches based on weaker oxidising agents. An etch comprising 1I₂ (0.01 M): 2HF (49%) attacked dislocation cores, but its etching behaviour was extremely slow (<0.1 μm/h) and the pits produced had a low aspect ratio (<2:1).     

Synthesis of crystalline SrMoO4 nanowires from polyoxometalates

Crystalline SrMoO₄ nanowires were synthesized via a facile hydrothermal process at 180 °C for 10 h. α-(NH₄)₆-P₂Mo₁₈O₆₂·nH₂O, one of polyoxometalates with Dawson structure, was employed as the source of molybdates. The diameter and length of the obtained SrMoO₄ nanowires are about 20 nm and 5-10 μm, respectively. HRTEM results show that the SrMoO₄ nanowires are of high crystallinity with rough surface. However, when Na₂MoO₄·2H₂O was used, there are only SrMoO₄ nanorods with smaller aspect ratio (200/70 nm) in the similar hydrothermal process. The probable growth mechanism was discussed.     

Crosstalk in double-pumped fiber optic parametric amplifiers for wavelength division multiplexing systems

We study experimentally inter-channel crosstalk in double-pumped fiber optic parametric amplifiers constructed with conventional dispersion shifted fibers (DSFs) having different lengths (L_A = 13.8, L_B = 6.8, L_C = 4.3, and L_D = 0.8 km). For long fibers (L_A and L_B), eye diagram measurements in a 5-channel (100 GHz spacing) system show that in order to have negligible crosstalk, the output signal power per channel, P_s, should be limited to P_s < 0 dBm. By decreasing the fiber length (to L_C) it is possible to increase the output signal power and/or the number of signals while keeping the crosstalk on negligible levels. This trend was further confirmed by using a very short DSF (L_D = 0.8 km).Finally, we experimentally demonstrate that a general trend in 2P-FOPAs is that spurious FWM increases with the number of signal channels up to a given number of channels when a saturation regime is reached. This saturation of the generation of spurious tones occurs when the bandwidth occupied by the signals exceeds ∼4-5 nm.     

Influence of Nb substitution for Fe on the magnetic and magneto-impedance properties of amorphous and annealed Fe76.5−xSi13.5B9Cu1Nbx (x=0–7) ribbons

We report a systematic study of the influence of Nb substitution for Fe on the magnetic properties and magneto-impedance (MI) effect in amorphous and annealed Fe_76.5−xSi_13.5B₉Cu₁Nb_x (x=0, 1, 2, 3, 4, 5, 6, and 7) ribbons. The amorphous ribbons were annealed at different temperatures ranging from 530 to 560 °C in vacuum for different annealing times between 5 and 20 min. We have found that for the as-quenched amorphous ribbons, the substitution of Nb for Fe first increases the saturation magnetization (M_s) and decreases the coercivity (H_c) until x=3, for which the largest M_s∼152 emu/g and the smallest H_c∼1.3 Oe are obtained, then an opposite trend is found for x>3. The largest MI ratio (ΔZ/Z∼38% at f=6 MHz) is achieved in the amorphous ribbon with x=3. A similar trend has been observed for the annealed ribbons. The most desirable magnetic properties (M_s∼156 emu/g and H_c∼1.8 Oe) and the largest MI ratio (ΔZ/Z∼221% at f=6 MHz) are achieved for the x=3 sample annealed at 540 °C for 15 min. A correlation between the microstructure, magnetic properties, and MI effect in the annealed ribbons has been established.     

Magnetic spectra of Fe-based nanocrystalline wires with axial or transverse domains

The circular permeability μ′=μ′−jμ″ of two Fe-based soft magnetic wires with axial and transverse domains, has been determined from the measurements of impedance Z=R+jX as functions of frequency (f=10-10⁵ Hz) and AC current amplitude (I=0.1-100 mA). From the magnetic spectra of μ′−f and μ″−f for a few circular fields (H_φ=0.4, 1.2, 4, 12, 40 A/m), we found that the sample with axial domain structure exhibits a relaxational feature, while for the one with transverse domain resonance-like spectra were observed when the circular field H_φ≥4 A/m. These results have been discussed in terms of domain structure and circular magnetization processes.     

Effect of surface roughness on nucleation and growth of vanadium pentoxide nanowires

The effect of surface roughness on subsequent growth of vanadium pentoxide (V₂O₅) nanowires is examined. With increasing surface roughness, both the number density and aspect ratio of V₂O₅ nanowires increase. Structures and morphology of obtained nanowires were characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The nanowires are approximately 40-90 nm in diameter and 2 μm in length. X-ray diffraction (XRD) analysis indicates that the obtained nanowires are orthorhombic structure with (0 0 1) out-of-plane orientation. The luminescence property of V₂O₅ nanowires has been investigated by photoluminescence (PL) at 150 K and 300 K. PL results show intense visible emission, which is attributed to different inter-band transitions between the V 3d and O 2p band. This simple fabrication approach might be useful for fabrication of large area V₂O₅ nanowires arrays with high density.     

Magnetic properties of Nd–Fe–B film magnets prepared by RF sputtering

Highly oriented films of ∼6 μm in thickness consisting of the Nd₂Fe₁₄B compound phase were obtained by a three-dimensional sputtering method at room temperature and the subsequent crystallization by annealing. The c-axis orientation and coercivity of film samples were sensitive to the sputtering parameters and annealing conditions. The optimum temperature and time for annealing were 650 °C and 30 min to show the highest coercivity without any deterioration for the orientation of Nd₂Fe₁₄B grains, and furthermore the degree of c-axis orientation was increased by decreasing the Ar gas pressure or input power for sputtering. The resultant film magnets with good magnetic properties of B_r=∼1.06 T, H_C=∼371 kA/m, and (BH)_max=∼160 kJ/m³ were obtained under the optimized parameters for sputtering.     

Application of 2D graded eye-shape scatterers for slow light effect in photonic crystal line defect waveguide

With operating frequency f = 1thz, two-dimensional (2D) graded eye-shape scatterers were firstly applied into photonic crystal waveguide (PCW) for slow light effect in two ways: (1) selecting group index n_g from 31.4 to 95.0, low-dispersion bandwidth (n_g varies within a 10% range) was got from 0.736 μm to 2.334 μm, and ultralow-dispersion bandwidth (n_g varies within a 1% range) was got from 0.438 μm to 0.945 μm by grading the eye-shape scatterers along the longitudinal direction; (2) selecting group index n_g from 32.1 to 98.3, low-dispersion bandwidth was got from 0.559 μm to 1.765 μm, and ultralow-dispersion bandwidth was got from 0.296 μm to 0.661 μm by grading the eye-shape scatterers along the transverse direction. The 2D graded structures can also used in asymmetrical structures and heterostructures for slow light effect.     

Numerical analysis of long wavelength infrared HgCdTe photodiodes

We present a detailed investigation of the performance limiting factors of long and very long wavelength infrared (LWIR and VLWIR) p on n Hg_1−xCd_xTe detectors through numerical simulations at 77 K incorporating all considerable generation-recombination (G-R) mechanisms including trap assisted tunneling (TAT), Shockley-Read-Hall (SRH), Auger and radiative processes. The results identify the relative strengths of the dark current generation mechanisms by numerically extracting the contribution of each G-R mechanism to the detector characteristics with various cut-off wavelengths (λ_c) and practically achievable material parameters.The results show that the dominant sensitivity degrading trap level depends on the detector cut-off wavelength being ∼0.7E_g for LWIR HgCdTe sensors (λ_c = ∼10 μm) instead of 0.5E_g which is generally believed to be the most efficient R-G level. TAT related 1/f noise dominates the sensor noise even under small reverse bias voltages at a trap density as low as 1 × 10¹⁴ cm⁻³ for sensors with λ_c > 11 μm. Considering the fact that trap densities below this level are rarely reported for HgCdTe material, exceptionally trap-free material is required to achieve desirable imaging performance with these sensors. Simulation results show that Auger mechanism has twofold effect on the sensitivity of the sensor by increasing the dark current and decreasing the photo current of the detector.     

Line patterning with large refractive index changes in the deep inside of glass by nanosecond pulsed YAG laser irradiation

Nanosecond (∼100 ns) pulsed (10 Hz) Nd:YAG laser operating at the wavelength (λ) of 1064 nm with pulse energies of 0.16-1.24 mJ/cm² has irradiated 10Sm₂O₃·40BaO·50B₂O₃ glass. It is demonstrated for the first time that the structural modification resulting the large decease (∼3.5%) in the refractive index is induced by the irradiation of YAG laser with λ=1064 nm. The lines with refractive index changes are written in the deep inside of 100-1000 μm depths by scanning laser. The line width is 1-13 μm, depending on laser pulse energy and focused beam position. It is proposed that the samarium atom heat processing is a novel technique for inducing structural modification (refractive index change) in the deep interior of glass.     

Radar polarimetry of Saturn's rings: Modeling ring particles as fractal aggregates built of small ice monomers

We analyze ground-based radar polarimetric observations of Saturn's rings at a wavelength of 12.6 cm by employing the model of a vertically and horizontally plane-parallel homogeneous slab composed of clumpy particles in the form of fractal aggregates of small ice monomers. Our model takes full account of the effects of polarization, multiple scattering, and coherent backscattering. Using efficient superposition T-matrix and vector radiative transfer codes, we perform computations of the backscattering circular polarization ratio for fractal aggregates generated with a cluster–cluster aggregation model and having the following characteristics: monomer refractive index m=1.78+i0.003; monomer packing density p=0.2; fractal dimensions D_f=2.5 and 3; and overall fractal radii R in the range 4?R?10 cm. In order to obtain physically realistic values of single-scattering properties of the aggregates we perform averaging over an ensemble of clusters generated for the same values of fractal parameters but having different geometrical configurations of the monomers. We conclude that in the framework of the above morphological model of Saturn's rings and the specific cluster–cluster aggregation procedure, it may be problematic to obtain a satisfactory and realistic agreement between theoretical computations and the observed values of the radar circular polarization ratio.     

New high negative dispersion photonic crystal fiber

A new high negative dispersion photonic crystal fiber is proposed. It has double-core structure. The inner core has a circle germanium-doped region. The outer core is formed by removing the 3rd ring air-holes around the core. There are two ring air-holes between the two cores, Diameter of the 1st ring air holes is bigger than that of the 2nd ring air-holes, this can make mode coupling between inner mode and outer mode and showed that the high negative PCF is the result of this structure characteristics. There are honeycomb photonic lattice in the PCF's cladding. The influence of the structure parameters deviated from the design those on the chromatic dispersion are evaluated. When the structure parameters Λ=1.50 μm, d_core=2.10 μm, d₁=0.90 μm, d₂=0.44 μm and d₃=1.04 μm, the dispersion coefficient D is −1320 ps/(nm·km) at 1550 nm. This is a new kind of chromatic dispersion compensation PCF.     

Heat release of carbon particle formation from hydrogen-free precursors behind shock waves

The process of heat release during carbon particle formation and growth after pyrolysis of carbon suboxide C₃O₂ behind shock waves was investigated. For this goal, temperature and optical density of gas-particle mixtures initially consisting of 3% C₃O₂ + 5% CO₂ in Ar were measured as a function of time. The temperature was determined by two-channel emission-absorption spectroscopy at λ = 2.7 ± 0.4 μm, corresponding to the CO₂ (1,0,1) vibrational band. In the range of initial temperatures behind the shock waves from 1600 up to 2200 K a significant heating of the mixture during particle formation and growth was observed that increased towards higher temperatures. The analysis of the obtained data in combination with previous results about the temperature dependence of the particle size shows a decrease of the heat release of condensation from ∼200 kJ/mol per atom for particles containing ∼1000 atoms to ∼50 kJ/mol per atom for particle containing ∼10⁶ atoms.     

Circular and rectangular via holes formed in SiC via using ArF based UV excimer laser

Circular via holes with diameters of 10, 25, 50 and 70 μm and rectangular via holes with dimensions of 10 μm × 100 μm, 20 μm × 100 μm and 30 μm × 100 μm and drilled depths between 105 and 110 μm were formed in 300 μm thick bulk 4H-SiC substrates by Ar/F₂ based UV laser drilling (λ = 193 nm) with a pulse width of ∼30 ns and a pulse frequency of 100 Hz. The drilling rate was linearly proportional to the fluence of the laser, however, the rate decreased for the larger via holes. The laser drilling produces much higher etch rates (229-870 μm/min) than conventional dry etching (0.2-1.3 μm/min) and the via entry can be tapered to facilitate subsequent metallization.