Biquadratic exchange coupling in an unequal Fe/Cr/Fe(1 0 0) trilayer

We have investigated the magnetic properties of a (1 0 0)-oriented unequal trilayer, Fe(45 Å)/Cr(30 Å)/Fe(15 Å), by means of Brillouin light scattering and magnetization measurements. The experimental results show that this sample highlights the effect of biquadratic coupling which aligns the magnetization of the Fe layers at 90° to each other. We extracted the bilinear and biquadratic coupling strengths by fitting the experimental results with a theory that treats the static and dynamic responses on an equal footing. Our results confirm that the model describes both the static and dynamic properties even when the magnetization of the layers is aligned at 90°. The coupling strengths, and their temperature dependence, are discussed and compared with other results reported in the literature.     

Analysis of multimode BDK doped POF gratings for temperature sensing

We report a temperature sensor based on a Bragg grating written in a benzil dimethyl ketal (BDK) doped multimode (MM) polymer optical fiber (POF) for the first time to our knowledge. The thermal response was further analyzed in view of theory and experiment. In theory, with the order of the reflected mode increasing from 1st to 60th order, for MM silica fiber Bragg grating (FBG) the temperature sensitivity will increase linearly from 16.2 pm/°C to 17.5 pm/°C, while for MM polymer FBG the temperature sensitivity (absolute value) will increase linearly from ?79.5 pm/°C to ?104.4 pm/°C. In addition, temperature sensitivity of MM polymer FBG exhibits almost 1 order larger mode order dependence than that of MM silica FBG. In experiment, the Bragg wavelength shift will decline linearly as the temperature rises, contrary to that of MM silica FBG. The temperature sensitivity of MM polymer FBG is ranged from ?0.097 nm/°C to ?0.111 nm/°C, more than 8 times that of MM silica FBG, showing great potential used as a temperature sensor.     

Physical,electrochemical and supercapacitive properties of activated carbon pellets from pre-carbonized rubber wood sawdust by CO2 activation

The physical and electrochemical properties of the activated carbon pellet electrodes have been investigated. Activated carbon pellets were prepared from single step carbonization process of pre-carbonized rubber wood sawdust at a temperature of 800 °C that followed with a CO₂ activation process at temperature in the range of 700–1000 °C. The BET characterization on the sample found that the surface area of the carbon pellet increased with the increasing of the activation temperature. The optimum value was as high as 683.63 m² g⁻¹. The electrical conductivity was also found to increase linearly with the increasing of the activation temperature, namely from 0.0075 S cm⁻¹ to 0.0687 S cm⁻¹ for the activation temperature in the range of 700–1000 °C. The cyclic voltammetry characterization of the samples in aqueous solution of 1 M H₂SO₄ also found that the specific capacitance increased with the increasing of the activation temperature. Typical optimum value was shown by the sample activated at 900 °C with the specific capacitance was as high as 33.74 F g⁻¹ (scan rate 1 mV s⁻¹). The retained ratio was as high as 32.72%. The activated carbon pellet prepared from the rubber wood sawdust may found used in supercapacitor applications.     

Effect of temperature and injection current on characteristics of TO-CAN packaged Fabry–Perot laser diode

We fabricated and investigated the thermal characteristics of TO-CAN (transistor-outline-can) packaged long wavelength GaInAsP/InP Fabry–Perot laser diode. Characteristic temperature of 55 K and peak wavelength shift of 0.4 nm/°C depending on the temperatures were obtained. Furthermore, overheating of semiconductor laser originated from thermal resistance is analyzed and evaluated by measuring junction temperature. Results from experiment and estimation are compared at extreme operating conditions in which show agreement within 1 °C.     

A phase stable short pulse generator using a DPMZM and phase modulators for application in 160 GBaud DQPSK systems

A method of 40 GHz phase stable short pulses generation is experimentally demonstrated. It is based on a dual parallel Mach–Zehnder modulator (DPMZM) driven by only one electrical sinusoidal clock and two cascaded phase modulators. The generated pulses are characterized with full-width-at-half-maximum pulse width of 1.9 ps, extinction ratio of 27 dB, timing jitter of 36 fs and signal to noise ratio over 30 dB. The high quality and phase stability of the pulses are further experimentally verified in a 4 × 40 GBaud differential quadrature phase shift keying (DQPSK) optical-time-division-multiplexing (OTDM) system.     

Effects of thickness of β-barium borate and angle of non-collinearity on the fs pulse generation by optical parametric amplification

Non-collinear optical parametric amplifiers (NOPAs) are used for the generation of tunable femtosecond pulses. The spectra of the uncompressed output from a lab-built NOPA in the 470–650 nm range have been recorded. Theoretical simulations for the effect of the length of the β-barium borate (BBO) crystal as well as the non-collinear angles between the pump and seed wavelengths have been carried out. For these we have obtained the initial experimental data from a 2 mm-thick BBO crystal when pumped with the second harmonic of the Ti:sapphire laser pulses of 100 fs duration. The pulse splitting length (PSL) and the group velocity mismatch (GVM) have been considered in simulations of the output. It was found that the crystal length of 1.3 mm and the crystal tilt of approximately 3.7° are optimal for the generation of pulses of ～11 fs at 600 nm.     

Influence of magnetic anisotropy constant and particle domain magnetization on magneto-dielectric response of substituted manganese ferrite particles dispersed in kerosene

This paper presents a dielectric and magneto-dielectric study of magnetic fluids made of three different, partially substituted, manganese ferrite particles dispersed in kerosene. Measurements were performed using a wholly automated spectrometer in the frequency range 1 mHz–10 MHz and a temperature range from −200°C up to 100°C. We can distinguish the two phases (solid and liquid) in pure kerosene and three phases for all the magnetic fluid samples. We observed the effect of anisotropy constant and domain magnetization of the particles on magneto-dielectric measurements. We also observed the temperature sensitivity of these fluids.     

Numerical analysis of a broadband spectrum generated in a standard fiber by noise-like pulses from a passively mode-locked fiber laser

This paper covers a numerical analysis of supercontinuum spectrum generation in a piece of standard fiber by using as the pump noise-like pulses produced by a passively mode-locked fiber laser. An experimental study was also carried out, yielding results that support the numerical results. In the numerical study we estimated that the spectral extension of the generated supercontinuum reaches ~ 1000 nm, and that it presents a high flatness over a region of ~ 220 nm (1630 nm-1850 nm) when we use as the pump noise-like pulses with a wide optical bandwidth (~ 50 nm) and a peak power of ~ 2 kW. Experimentally, the output signal spectrum extends from ~ 1530 nm to at least 1750 nm and presents a high flatness over a region of 1640 nm to 1750 nm for the same value of numerical input power, 1750 nm being the upper limit of the optical spectrum analyzer. The numerical analysis presented here is thus an essential part to overcome the severe limitation in measuring capabilities and to understand the phenomena of supercontinuum generation, which is mainly related to Raman self-frequency shift. Finally, this work demonstrates the potential of noise-like pulses from a passively mode-locked fiber laser for broadband spectrum generation.     

Effect of annealing on the magnetic properties of Cu-capped ultrathin Co films

We report the effect of intermixing of Cu on the magnetic properties of ultrathin Co films deposited on Si(1 0 0). Rutherford backscattering was employed to determine the extent of intermixing, which increased from 7% in an as-grown sample to nearly 23% when annealed at 400 °C. The as-grown sample showed a higher value of magnetization of 530 emu/cm³, which dropped to 20 emu/cm³ on annealing at 400 °C. The low temperature magnetization behavior of the as-grown Co films showed the presence of both positive and negative exchange bias due to the formation of antiferromagentic domains in parallel with ferromagnetic domains. This behavior is explained using the Malozemoff Random Field Model, which predicted values of exchange bias closely matching the present experimental findings.     

Sensor application-related defect chemistry and electromechanical properties of langasite

The operation of langasite (La₃Ga₅SiO₁₄) resonators as sensors at elevated temperature and controlled atmospheres is examined. This paper focuses on mapping the regimes of gas-insensitive operation of uncoated langasite resonators and the correlation to langasite's defect chemistry for temperatures up to 1000 °C. As a measure of sensitivity, the fundamental resonant mode at 5 MHz is estimated to be determined to within ± 4 Hz by network analysis for resonators operated in air at temperatures below 1000 °C. The calculated frequency shift induced by redox-related reactions in langasite only exceeds the limit of ± 4 Hz below p_O2 ≈ 10^− 17 bar at 1000 °C, below 10^− 24 bar at 800 °C and below 10^− 36 bar at 600 °C. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen-containing atmospheres applied here, langasite can be regarded as a stable resonator material above oxygen partial pressures of about 10^− 13 and 10^− 20 bar at 800 and 600 °C, respectively.     

Fabry–Pérot cavities based on chemical etching for high temperature and strain measurement

In this paper, two hybrid multimode/single mode fiber Fabry–Pérot (FP) cavities were compared. The cavities fabricated by chemical etching are presented as high temperature and strain sensors. In order to produce this FP cavity a single mode fiber was spliced to a graded index multimode fiber with 62.5 μm core diameter. The Fabry–Pérot cavities were tested as a high temperature sensor in the range between room temperature and 700 °C and as strain sensors. A reversible shift of the interferometric peaks with temperature allowed to estimate a sensitivity of 0.75 ± 0.03 pm/°C and 0.98 ± 0.04 pm/°C for the sensor A and B respectively. For strain measurement sensor A demonstrated a sensitivity of 1.85 ± 0.07 pm/μ? and sensor B showed a sensitivity of 3.14 ± 0.05 pm/μ?. The sensors demonstrated the feasibility of low cost fiber optic sensors for high temperature and strain.     

XPS investigation of vacuum annealed vertically aligned ultralong ZnO nanowires

The surface properties of vertically aligned ZnO nanowires grown by chemical vapour deposition on GaN using a gold layer as a catalyst are investigated by X-ray Photoelectron Spectroscopy as a function of annealing temperature in ultra high vacuum (UHV). The nanowires are 8.5 μm long and 60 nm wide. 87% of the surface carbon content was removed after annealing at 500 °C in UHV. Analysis of the gold intensity suggests diffusion into the nanowires after annealing at 600 °C. Annealing at 300 °C removes surface water contamination and induces a 0.2 eV upward band bending, indicating that adsorbed water molecules act as surface electron donors. The contaminants re-adsorbed after 10 days in UHV and the surface band bending caused by the water removal was reversed. The UHV experiment also affected the nanowires arrangement causing them to bunch together. These results have clear implications for gas sensing applications with ZnO NWs.     

Desorption of O2 from a stepped platinum(113) surface during 308 nm laser irradiation

Angular and velocity distributions of desorbing O₂ during irradiation of 308 nm laser pulses were studied on a stepped Pt(1 1 3) surface. With increases in the coverage, three desorption components collimated at around 12°, 30° and 50° successively appeared when the desorption angle was changed in a plane along the step edge. The translational temperature also showed maxima at these collimation angles, and the values were slightly lower than previous results for 193 nm irradiation. Some possible desorption mechanisms are discussed.     

Effect of annealing on natural calcitic crystals—A thermostimulated luminescence (TSL) study

The quality crystals (Calcitic limestone) were selected using the UV–visible methylene blue adsorption method. The thermostimulated luminescence (TSL) glow curve characteristics of six well crystallized limestone samples were analyzed. The glow curves of unannealed sample show only one peak in the range 320–330 °C. The sample irradiated with a gamma dose of 100 Gy shows two additional peaks in the range of 113–125 °C and 242–260 °C when recorded with linear heating rate of 10 °C/s. The annealed sample also shows the same trend as that of irradiated sample. Annealing treatment above 250 °C increases the sensitivity of all TSL peaks except 320 °C. On the other hand, annealing at 750 °C caused a collapse in the TSL sensitivity. The enhancement in TSL sensitivity was found to depend on the annealing temperature and time. Annealing treatment at 650 °C for 4 h followed by quenching in air is the optimum condition for TSL sensitization. The response to gamma irradiation is linear in the range from 0.5 Gy to 10⁴ Gy. The emission spectra of all the samples show an emission at around 610 nm but with different intensities for each TSL peak. With reference to earlier work, it may be assumed that the recombination site always involves Mn²⁺ ions. The observation made through infra-red (IR) and X-ray diffraction (XRD) studies with thermal treatment shows the structural changes of calcite from D_3h to C_s symmetry at 750 °C. The Thermogravimetric-Differential Thermal Analysis (TG-DTA) analysis shows the calcite gets disordered at 760 °C. Hence, the collapse in the TSL sensitivity at 750 °C is due to structural change or structural disorderedness.     

Automatic coarse-alignment for TEM tilt series of rod-shaped specimens collected with a full angular range

An automatic coarse-alignment method for a tilt series of rod-shaped specimen collected with a full angular range (from α = ?90° to +90°, α is the tilt angle of the specimen) is presented; this method is based on a cross-correlation method and uses the outline of the specimen shape. Both the rotational angle of the tilt axis and translational value of each image can be detected in the images without the use of markers. This method is performed on the basis of the assumption that the images taken at α = ?90° and α =  + 90° are symmetric about the tilt axis. In this study, a carbon rod on which gold particles have been deposited is used as a test specimen for the demonstration. This method can be used as an automatic coarse-alignment method prior to the application of a highly accurate alignment method because the alignment procedure can be performed automatically except for the initial setup of some parameters.     

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Resonator micro optic gyro with double phase modulation technique using an FPGA-based digital processor

Experiments on resonator micro-optic gyro (RMOG) with a digital proportional integral (PI) feedback scheme are performed. In this experimental setup, the key rotation sensing element is a polarization maintaining silica waveguide ring resonator (WRR) with a ring length of 7.9 cm and a diameter of 2.5 cm. A good linearity of 0.0015% over a wide range of ± 2 × 10⁴ °/s can be achieved for the RMOG theoretically. The optimal digital PI feedback scheme is adopted in the frequency servo loop to reduce the reciprocal frequency fluctuations due to the WRR resonance frequency and laser frequency drifts. Residual equivalent input fluctuation can be reduced as low as 0.03 °/s/√Hz based on the optimal digital PI feedback scheme, which is close to the shot noise limited spectral density 0.02 °/s/√Hz of the RMOG with the input optical power of 0.2 mW. Relationship between RMOG output signal and angular rate is obtained from ± 0.1 °/s to ± 5 °/s. The standard deviation of the residuals between RMOG output results and linear fit curve is 0.066 °/s. For an integration of the processing circuit, all the processing circuit is implemented by a field programmable gate array (FPGA) instead of instruments. The output of this digitalized RMOG is obtained over a range of ± 550 °/s. The linearity of this digitalized RMOG is 0.0169%.     

Comprehensive visualization of detonation-diffraction structures and sizes in unstable and stable mixtures

The geometry and characteristic length of diffraction and re-initiation during a two-dimensional detonation propagation were revealed by visualization. C₂H₄ + 3O₂ (unstable), 2C₂H₂ + 5O₂ + 7Ar (stable) and 2C₂H₂ + 5O₂ + 21Ar (stable) were used as the test mixtures. Experiments were performed over the deviation angle range from 30° to 150° and the initial pressure range from 15.8 to 102.3 kPa. By self-emitting photography, we confirmed that the geometry and the characteristic length of diffraction are not different among test gases, with the exception of the fan-like structure of re-initiation that occurred regardless of whether the mixture was unstable or stable. We conducted a compensative experiment by changing the deviation angle and initial pressure, and summarized the detonation diffraction by shadowgraph. At deviation angles larger than 60°, we measured the distances from the vertex of the channel corner to the point where the transverse detonation wave reflected on the under wall (= wall reflection distance) and confirmed that wall reflection distances are approximately in the range of 10–15 times the cell width, whether the mixture is unstable or stable.