Experimental demonstration of influence of underwater turbulence on ghost imaging
It is difficult to obtain a clear image in underwater turbulence environment with classical imaging methods due to the absorption, scattering, and underwater turbulence on the propagation beam. However, ghost imaging (GI), a non-locally imaging technique, has shown the turbulence-free ability in atmospheric turbulence by exploiting the second-order correlation between the signal beam and the reference beam. In this paper, we experimentally investigate the imaging quality of GI affected by the underwater environment, where the underwater environment is simulated by a 1 m×0.4 m×0.4 m tank with distilled water. The water temperature is controlled by a heater inside the tank, and a temperature gradient is obtained by putting the heater at different positions of the tank. The water vibration is produced by a heavy force, and the turbid medium is obtained by dissolving very small specks of CaCO3 in the water. A set of Hadamard speckle pattern pairs are generated and modulated on the incident beam, and then the beam illuminates on an unknown object after passing through the simulated underwater environment. With the second-order correlations, the image is reconstructed under different temperature gradients, water vibration, and turbid medium ratios. The results show that GI has the turbulence-free ability under lower temperature gradient, water vibration, and turbid media. The structural similarity image measurement (SSIM) values of the reconstructed images only start to decrease when the temperature gradient is greater than 4.0℃. The same temperature gradient produced at the different positions has a little effect on the quality of the underwater GI.
Multiple trapping using a focused hybrid vector beam
We propose a simple and efficient method that uses a single focused hybrid vector beam to confine metallic Rayleigh particles at multiple positions. We study the force mechanisms of multiple trapping by analyzing the gradient and scattering forces. It is observed that the wavelength and topological charges of the hybrid vector beam regulate the trapping positions and number of optical trap sites. The proposed method can be implemented easily in three-dimensional space, and it facilitates both trapping and organization of particles. Thus, it can provide an effective and controllable means for nanoparticle manipulation.
CsPbBr3 nanocrystal for mode-locking Tm-doped fiber laser
CsPbBr3 nanocrystal is used as the saturable absorber (SA) for mode-locking Tm-doped fiber laser in a ring fiber cavity. The modulation depth, saturable intensity, and non-saturable loss of the fabricated SA are 14.1%, 2.5 MW/cm2, and 5.9%, respectively. In the mode-locking operation, the mode-locked pulse train has a repetition rate of 16.6 MHz with pulse width of 24.2 ps. The laser wavelength is centered at 1992.9 nm with 3-dB spectrum width of 2.5 nm. The maximum output power is 110 mW with slope efficiency of 7.1%. Our experiment shows that CsPbBr3 nanocrystal can be used as an efficient SA in the 2-μm wavelength region.
Stable continuous-wave single-frequency intracavity frequency-doubled laser with intensity noise suppressed in audio frequency region
We demonstrated a continuous wave (cw) single-frequency intracavity frequency-doubled Nd:YVO4/LBO laser with 532 nm output of 7.5 W and 1.06 μm output of 3.1 W, and low intensity noise in audio frequency region. To suppress the intensity noise of the high power 532 nm laser, a laser frequency locking system and a feedback loop based on a Mach-Zehnder interferometer were designed and used. The influences of the frequency stabilization and the crucial parameters of the MZI, such as the power splitting ratio of the beam splitters and the locking state of the MZI, on the intensity noise of the 532 nm laser were investigated in detail. After the experimental optimizations, the laser intensity noise in the frequency region from 0.4 kHz to 10 kHz was significantly suppressed.
Passively Q-switched diode-pumped Tm, Ho: LuVO4 laser with a black phosphorus saturable absorber
We presented a passively Q-switched (PQS) diode-pumped c-cut Tm, Ho:LuVO4 laser with a black phosphorus saturable absorber for the first time. Under PQS mode, an average output power of 0.86 W and a peak power of 2.32 W were acquired from the Tm, Ho:LuVO4 laser with the pump power of 14.55 W, corresponding to a pulse width of 2.89 μs, a pulse repetition rate of 71.84 kHz, and a pulse energy of about 6.70 μJ.
Forward-headed structure change of acetic acid-water binary system by stimulated Raman scattering
The acetic acid-water binary system is a classical hydroxy-carboxy mixed system, while new and interesting phenomena appear under stimulated Raman scattering (SRS). Compared with the weaker signal of the acetic acid-water binary system obtained in spontaneous Raman scattering, SRS provides a finer band and a relatively distinct structural transition point. The structural transformation points are respectively at 30% and 80% by volume ratio under the condition of spontaneous Raman spectroscopy, while they are respectively at 15% and 25% under the condition of SRS. This phenomenon is attributed to the generation of laser induced plasma and shockwave induced dynamic high pressure environment during SRS.
Refractive index sensor based on high-order surface plasmon resonance in gold nanofilm coated photonic crystal fiber
We propose a novel kind of wide-range refractive index optical sensor based on photonic crystal fiber (PCF) covered with nano-ring gold film. The refractive index sensing performance of the PCF sensor is analyzed and simulated by the finite element method (FEM). The refractive index liquid is infiltrated into the cladding air hole of the PCF. By comparing the sensing performance of two kinds of photonic crystal fiber structures, a wide range and high sensitivity structure is optimized. The surface plasmon resonance (SPR) excitation material is chose as gold, and large gold nanorings are embedded around the first cladding air hole of the PCF. The higher order surface plasmon modes are generated in this designed optical fiber structure. The resonance coupling between the fundamental mode and the 5th order surface plasmon polariton (SPP) modes is excited when the phase matching condition is matched. Therefore, the 3rd loss peaks appear obvious red-shift with the increase of the analyte refractive index, which shows a remarkable polynomial fitting law. The fitnesses of two structures are 0.99 and 0.98, respectively. When the range of refractive indices is from 1.40 to 1.43, the two kinds of sensors have high linear sensitivities of 1604 nm/RIU and 3978 nm/RIU, respectively.
Pancharatnam-Berry metasurface for terahertz wave radar cross section reduction
The digital coding metasurfaces need several kinds of meta-particle structures to obtain corresponding electromagnetic wave responses and require time-consuming optimization. In this paper, we present train-symbol-shaped meta-particles with various orientations utilizing Pancharatnam-Berry (PB) phase to achieve 1-, 2-, and 3-bit digital coding metasurfaces. Terahertz wave scattering patterns of the coding metasurfaces with regular and random sequences are given and discussed. They have strongly suppressed backward scattering with approximately -13.5 dB radar cross section (RCS) reduction in a wide band range from 0.85 THz to 1.6 THz. The proposed digital coding metasurfaces provide a simple way and new opportunities for manipulating terahertz wave scattering with polarization independence.
Resolving multi-orbital effects on high harmonic generation from aligned N2 molecules in linearly and elliptically polarized intense laser fields
We perform an experimental study of the multi-orbital effect on the high-order harmonic generation (HHG) from aligned N2 molecules in both linearly and elliptically polarized intense laser fields. Measured by a home-built extreme ultraviolet (XUV) flat grating spectrometer with the pump-probe method, the angular distributions of different orders of HHG are obtained, which show distinctive behaviors for harmonics in the plateau and the cut-off regions. The ellipticity dependence of HHG is investigated by aligning the molecular axis parallel or perpendicular to the laser polarization. Our results indicate that both the highest occupied molecular orbital (HOMO) as well as the lower one (HOMO-1) contribute to the HHG of N2 molecules, in either linearly or elliptically polarized intense laser field. The study paves the way for understanding the ultrafast electron dynamics of molecules exposed to an intense laser field.
Diode-pumped Kerr-lens mode-locked Ti: sapphire laser with broad wavelength tunability
We report a direct blue-diode-pumped wavelength tunable Kerr-lens mode-locked Ti:sapphire laser. Central wavelength tunability as broad as 89 nm (736-825 nm) is achieved by adjusting the insertion of the prism. Pulses as short as 17 fs are generated at a central wavelength of 736 nm with an average output power of 31 mW. The maximum output power is 46.8 mW at a central wavelength of 797 nm with a pulse duration of 46 fs.
Comparison of three kinds of polarized Bessel vortex beams propagating through uniaxial anisotropic media
A comparison of differently polarized Bessel vortex beams propagating through a uniaxial anisotropic slab is discussed in terms of the vector wave function expansions. The magnitude profiles of electric field components, the transformation of polarization modes, and the distributions of orbital angular momentum (OAM) states of the reflected and transmitted beams for different incident angles are numerically simulated. The results indicate that the magnitude profiles of electric field components for different polarization modes are distinct from each other and have a great dependence on the incident angle, thus the transformation of polarization modes which reflects the change of energy can be affected largely. As compared to the x and circular polarization incidences, the reflected and transmitted beams for the radial polarization incidence suffer the fewest transformation of polarization modes, showing a better energy invariance. The distributions of OAM states of the reflected and transmitted beams for different polarization modes are diverse as well, and the derived OAM states of the transmitted beam for radial polarization present a focusing effect, concentrating on the state between two predominant OAM states.
Properties of metal-insulator-metal waveguide loop reflector
A new type and easy-to-fabricate metal-insulator-metal (MIM) waveguide reflector based on Sagnac loop is designed and investigated. The transfer matrix theoretical model for the transmission of electric fields in the reflector is established, and the properties of the reflector are studied and analyzed. The simulation results indicate that the reflectivity strongly depends on the coupling splitting ratio determined by the coupling length. Accordingly, different reflectivities can be realized by varying the coupling length. For an optimum coupling length of 750 nm, the 3-dB reflection bandwidth of the MIM waveguide reflector is as wide as 1.5 μm at a wavelength of 1550 nm, and the peak reflectivity and isolation are 78% and 23 dB, respectively.
Polarization dependence of gain and amplified spontaneous Brillouin scattering noise analysis for fiber Brillouin amplifier
The polarization dependences of gain and amplified spontaneous Brillouin scattering (ABS) noise for fiber Brillouin amplifier (FBA) are analyzed through theories, simulations, and experiments. Modified vector propagation equations for calculating the gain of the probe signal and the ABS noise are derived and analyzed in the Stokes spaces. In simulations and experiments, we prove that the gain of the probe signal and the ABS noise are strongly dependent on the relative state of polarization (SOP) of the pump and probe signals. The closer the relative SOP of the pump and probe signals is, the more obvious ABS noise suppression effect will be brought by increasing the power of the input probe signal.
Quantum optical interferometry via general photon-subtracted two-mode squeezed states
We investigate the sensitivity of phase estimation in a Mach-Zehnder interferometer with photon-subtracted two-mode squeezed vacuum states. Our results show that, for given initial squeezing parameter, both symmetric and asymmetric photon subtractions can further improve the quantum Cramér-Rao bound (i.e., the ultimate phase sensitivity), especially for single-mode photon subtraction. On the other hand, the quantum Cramér-Rao bound can be reached by parity detection for symmetric photon-subtracted two-mode squeezed vacuum states at particular values of the phase shift, but it is not valid for asymmetric photon-subtracted two-mode squeezed vacuum states. In addition, compared with the two-mode squeezed vacuum state, the phase sensitivity via parity detection with asymmetric photon-subtracted two-mode squeezed vacuum states will be getting worse. Thus, parity detection may not always be the optimal detection scheme for nonclassical states of light when they are considered as the interferometer states.
Using Helmholtz resonator arrays to improve dipole transmission efficiency in waveguide
It is well known that the radiation efficiency of an acoustic dipole is very low, increasing the radiation efficiency of an acoustic dipole is a difficult task, especially in an ordinary waveguide. In addition, current acoustic superlenses all utilize in-phase sources to do the super-resolution imaging, it is almost impossible to realize super-resolution imaging of an acoustic dipole. In this paper, after using the Helmholtz resonator arrays (HRAs) which are placed at the upper and lower surfaces of the waveguide, we observe a large dipole radiation efficiency at the certain frequency, which gives a method to observe an acoustic dipole in the far field and offers a novel model which is promising to realize the superlens with a source of an acoustic dipole. We discuss how the arrangement of HRAs affects the transmission of the acoustic dipole.
Strong coupling between height of gaps and thickness of thermal boundary layer in partitioned convection system
A direct numerical simulation (DNS) method is used to calculate the partitioned convection system with Ra number ranging from 107 to 2×109. Using the boundary layer thickness to normalize the height of gaps d, we find a strong consistency between the variation of the TD number (the average value of the temperature in each heat transfer channel is averaged after taking the absolute values) with the change of the height of gaps and the variation of the TD number with the change of Ra number in partitioned convection. For a given thickness of partition, heights of gaps are approximately equal to 0.5 or 1 time of the thermal boundary layer thickness λθ at different Ra numbers. TD number representing temperature characteristics is almost the constant value, which means that TD number is a function of d/λθ only. Analysis of local temperature field of area in gaps shows that the temperature distribution in the gaps are basically the same when d/λθ is certain. The heat transfer Nu number of the system at d/λθ≈ 0.5 is larger than that of d/λθ≈ 1, both of them have the same scaling law with Ra number and Nu~Ra0.25.
Fluctuation of arc plasma in arc plasma torch with multiple cathodes
Fluctuation phenomena commonly exist in arc plasmas, limiting the application of this technology. In this paper, we report an investigation of fluctuations of arc plasmas in an arc plasma torch with multiple cathodes. Time-resolved images of the plasma column and anode arc roots are captured. Variations of the arc voltage, plasma column diameter, and pressure are also revealed. The results indicate that two well-separated fluctuations exist in the arc plasma torch. One is the high-frequency fluctuation (of several thousand Hz), which arises from transferring of the anode arc root. The other is the low-frequency fluctuation (of several hundred Hz), which may come from the pressure variation in the arc plasma torch. Initial analysis reveals that as the gas flow rate changes, the low-frequency fluctuation shows a similar variation trend with the Helmholtz oscillation. This oscillation leads to the shrinking and expanding of the plasma column. As a result, the arc voltage shows a sinusoidal fluctuation.
Enhancement of corona discharge induced wind generation with carbon nanotube and titanium dioxide decoration
Dip-coated double-wall carbon nanotubes (DWCNTs) and titanium dioxide (TiO2) sol have been prepared and smeared onto the tip of a conductive iron needle which serves as the corona discharge anode in a needle-cylinder corona system. Compared with the discharge electrode of a CNT-coated needle tip, great advancements have been achieved with the TiO2/CNT-coated electrode, including higher discharge current, ionic wind velocity, and energy conversion efficiency, together with lower corona onset voltage and power consumption. Several parameters related to the discharge have been phenomenologically and mathematically studied for comparison. Thanks to the morphology reorientation of the CNT layer and the anti-oxidation of TiO2, better performance of corona discharge induced wind generation of the TiO2/CNT-coated electrode system has been achieved. This novel decoration may provide better thoughts about the corona discharge application and wind generation.
First polar direct-drive exploding-pusher target experiments on the ShenGuang laser facility
Low density and low convergence implosion occurs in the exploding-pusher target experiment, and generates neutrons isotropically to develop a high yield platform. In order to validate the performance of ShenGuang (SG) laser facility and test nuclear diagnostics, all 48-beam lasers with an on-target energy of 48 kJ were firstly used to drive room-temperature, DT gas-filled glass targets. The optimization has been carried out and optimal drive uniformity was obtained by the combination of beam repointing and target. The final irradiation uniformity of less than 5% on polar direct-drive capsules of 540 μ in diameter was achieved, and the highest thermonuclear yield of the polar direct-drive DT fuel implosion at the SG was 1.04×1013. The experiment results show neutron yields severely depend on the irradiation uniformity and laser timing, and decrease with the increase of the diameter and fuel pressure of the target. The thin CH ablator does not impact the implosion performance, but the laser drive uniformity is important. The simulated results validate that the γ distribution laser design is reasonable and can achieve a symmetric pressure distribution. Further optimization will focus on measuring the symmetry of the hot spot by self-emission imaging, increasing the diameter, and decreasing the fuel pressure.
Van der Waals interlayer potential of graphitic structures: From Lennard-Jones to Kolmogorov-Crespy and Lebedeva models
The experimental knowledge on interlayer potential of graphitic materials is summarized and compared with the computational results based on phenomenological models. Besides Lennard-Jones approximation, the Mie potential is discussed, as well as the Kolmogorov-Crespy model and equation of Lebedeva et al. An agreement is found between a set of reported physical properties of graphite (layer binding energies, compressibility along c-axis in a broad pressure range, Raman frequencies for bulk shear and breathing modes under pressure), when a proper choice of model parameters is taken. It is argued that anisotropic potentials, Kolmogorov-Crespy and Lebedeva, are preferable for modeling, as they provide a better, self-consistent description. A method of fast numerical modeling, convenient for the accurate estimation of the discussed physical properties, is proposed. It may be useful in studies of other van der Waals homo/heterostructures as well.
Surperhard monoclinic BC6N allotropes: First-principles investigations
Via structural searching methodology and first-principles calculations, we predicted two new BC6N allotropes, a C-centered monoclinic BC6N (Cm-BC6N) and a primitive-centered monoclinic BC6N (Pm-BC6N). The lattice vibrations, elastic properties, ideal strength, theoretical hardness, and electronic structure of the predicted BC6N were investigated systematically. Our results reveal that Cm-BC6N is more favorable energetically than graphite-like g-BC6N above 20.6 GPa, which is lower than the transition pressures of r-BC6N, t-BC6N, and Pm-BC6N. Both Cm-BC6N and Pm-BC6N are indirect semiconductors with band gaps of 2.66 eV and 0.36 eV, respectively. Cm-BC6N exhibits the excellent ideal shear strength of 53.9 GPa in (011), much greater than that of Pm-BC6N (25.0 GPa in (010) shear direction), and Cm-BC6N shows a much lower anisotropy in shear strength than Pm-BC6N. The Vickers hardness of Cm-BC6N is estimated to be above 80 GPa, which is more outstanding than those of t-BC6N and r-BC6N.
Crystal melting processes of propylene carbonate and 1,3-propanediol investigated by the reed-vibration mechanical spectroscopy for liquids
The melting of crystals is one of the most common and general phase transition phenomena. However, the mechanism of crystal melting is not well understood, and more experimental measurements and explorations are still needed. The mechanical spectra of propylene carbonate and 1,3-propanediol during the crystal melting processes are measured by the reed vibration mechanical spectroscopy for liquids (RMS-L) for the first time. The experimental results show that as the temperature increases, the real part of the complex Young modulus first decreases slowly, and then quickly drops to zero; meanwhile, its imaginary part increases slowly at first, then goes up and drops quickly to zero, showing a peak of internal friction. Preliminary analyses indicate that both the real and imaginary parts can present some characteristics of the melting process, such as the transition from the disconnected liquid regions to the connected liquid regions, that from the connected crystal regions to the disconnected crystal regions, and so on. In addition, the results show that the melting rate per unit volume of crystalline phase versus temperature satisfies the Arrhenius relation at the initial stage of melting, and deviates from this relation as the temperature increases to a certain value. Therefore, the RMS-L will provide an effective supplement for the further study of melting.
Structural transitions in NaNH2 via recrystallization under high pressure
Multiple phase transitions are detected in sodium amide (NaNH2), an important hydrogen storage material, upon compression in diamond anvil cells (DAC) by using Raman spectroscopy and x-ray diffraction (XRD) measurements. Additional Bragg reflections appear on lower and higher angle sides of the original ones at~1.07 GPa and 1.84 GPa, accompanied by obvious changes in Raman spectroscopy, respectively. It reveals that NaNH2 undergoes the high-pressure phase sequence (α-β-γ) up to 20 GPa at room temperature. Spectral analysis indicates an orthorhombic structure with PBAN space group for the γ phase. We also experimentally observe high pressure induced recrystallization in alkaline amide compounds for the first time.
Laser scattering, transmittance and low thermal expansion behaviors in Y2-x(ZnLi)xMo3O12 by forming regular grains
Ceramics usually have irregular grains, cracking, or porosity, which result in their lightproof. Y2Mo3O12 ceramics have more porosity due to the heavy hygroscopicity. Introducing ZnLi to Y2Mo3O12 could form regular grains, reduce cracking and porosity. With increasing the content of ZnLi, the grain shapes self-assembly gradually and then the laser scattering and transmittance improve. The laser scattering property and transmittance of diverging rays become the best in ceramics Y2-x(ZnLi)xMo3O12 (x=1.0 and 1.2) with regular grains and low thermal expansion. The formation mechanism of regular grains is ascribed to the substitutions of Zn2+ and Li+ for Y3+ in Y2Mo3O12 resulting in the preferential growth. The investigation in laser scattering, transmittance and low thermal expansion behaviors of Y2-x(ZnLi)xMo3O12 could pave a way to weaken the strong-laser attack from the high-power laser weapon and the other.
Josephson effect in the strontium titanate/lanthanum aluminate junction
We report theoretical studies on the newly discovered novel Josephson effect and scanning tunneling spectroscopy (STS) at the interface of strontium titanate/lanthanum aluminate (STO/LAO). With a phenomenological boson-fermion model, the density of states is calculated and the results are consistent with the STS experiments. A typical calculation of Josephson effect is performed, and it is in qualitative agreement with the experiments. The calculations indicate that the gap states come from the pairing of quasi-particles with a finite total momentum and the Josephson current comes from the tunneling of quasi-particle pairs with zero momentum. The quasi-particles are Bogoliubov quasi-particles. Moreover, the fits using Kulik's formula imply that the Josephson junction at the STO/LAO interface has a point contact with the clean superconductor limit.
The unique magnetic damping enhancement in epitaxial Co2Fe1-xMnxAl films
Uniform precession dynamics and its magnetic damping are investigated in epitaxial Co2Fe1-xMnxAl films by using the time-resolved magneto-optical Kerr effect under out-of-plane configuration. The decay time of uniform precession mode decreases, and thus the magnetic damping increases with the increase of external field. Moreover, the decay time decreases as x decreases, so that the enhancement of magnetic damping occurs in Fe-rich sample. Furthermore, the decay time decreases as the excitation fluence increases, which drops rapidly at low magnetic field comparing with the slow reduction at high magnetic field. This unique magnetic damping enhancement is attributed to the enhancement of homogeneous magnetization.
Magnetic properties of the double perovskite compound Sr2YRuO6
We study the magnetic properties of the double perovskite ruthenate compound Sr2YRuO6 using Monte Carlo simulations (MCS). We elaborate the ground state phase diagrams for all possible and stable configurations. The magnetizations and the susceptibilities as a function of temperature for the studied system are also reported. The effects of the exchange coupling interactions and the crystal field are examined and discussed. On the other hand, since the compound Sr2YRuO6 exhibits an antiferromagnetic behavior, we find its Néel temperature, TN ≈ 31 K, which is in good agreement with the experimental results in the literature. To complete this study, the hysteresis loops and the coercive field as a function of the external magnetic field are also obtained for fixed values of the physical parameters.
Improvement of TE-polarized emission in type-Ⅱ InAlN-AlGaN/AlGaN quantum well
The optical properties of the type-Ⅱ lineup InxAl1-xN-Al0.59Ga0.41N/Al0.74Ga0.26N quantum well (QW) structures with different In contents are investigated by using the six-by-six K-P method. The type-Ⅱ lineup structures exhibit the larger product of Fermi-Dirac distribution functions of electron fcn and hole (1-fvUm) and the approximately equal transverse electric (TE) polarization optical matrix elements (|Mx|2) for the c1-v1 transition. As a result, the peak intensity in the TE polarization spontaneous emission spectrum is improved by 47.45%-53.84% as compared to that of the conventional AlGaN QW structure. In addition, the type-Ⅱ QW structure with x~0.17 has the largest TE mode peak intensity in the investigated In-content range of 0.13-0.23. It can be attributed to the combined effect of|Mx|2 and fcn (1-fvUm) for the c1-v1, c1-v2, and c1-v3 transitions.
Characteristics of urea under high pressure and high temperature
The properties of urea under high pressure and high temperature (HPHT) are studied using a China-type large volume cubic high-presentation apparatus (CHPA) (SPD-6×600). The samples are characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and Raman spectroscopy. By directly observing the macroscopic morphology of urea with SEM, it is confirmed that the melting point of urea rises with the increase of pressure. The XRD patterns of urea residues derived under different pressures show that the thermal stability of urea also increases with the increase of pressure. The XRD pattern of the urea residue confirms the presence of C3H5N5O (ammeline) in the residue. A new peak emerges at 21.80°, which is different from any peak of all urea pyrolysis products under normal pressure. A more pronounced peak appears at 708 cm-1 in the Raman spectrum, which is produced by C-H off-plane bending. It is determined that the urea will produce a new substance with a C-H bond under HPHT, and the assessment of this substance requires further experiments.
The n-type Si-based materials applied on the front surface of IBC-SHJ solar cells
Interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cells exhibit excellent performance owing to the IBC and SHJ structures. The front surface field (FSF) layer composed of electric field passivation and chemical passivation has been proved to play an important role in IBC-SHJ solar cells. The electric field passivated layer n+-a-Si:H, an n-type Si alloy with carbon or oxygen in amorphous phase, is simulated in this study to investigate its effect on IBC-SHJ. It is indicated that the n+-a-Si:H layer with wider band gap can reduce the light absorption on the front side efficaciously, which hinders the surface recombination of photo-generated carriers and thus contributes to the improvement of the short circuit current density Jsc. The highly doped n+-a-Si:H can result in the remakable energy band bending, which makes it outstanding in the field passivation, while it makes little contribution to the chemical passivation. It is noteworthy that when the electric field intensity exceeds 1.3×105 V/cm, the efficiency decrease caused by the inferior chemical passivation is only 0.16%. In this study, the IBC-SHJ solar cell with a front n+-a-Si:H field passivation layer is simulated, which shows the high efficiency of 26% in spite of the inferior chemical passivation on the front surface.
Structural and dielectric properties of giant dielectric Na1/2Sm1/2Cu3Ti4O12 ceramics prepared by reactive sintering methods
Na0.5Sm0.5Cu3Ti4O12 (NSCTO) ceramics have been prepared by reactive sintering of amorphous powder. Spark plasma sintering (SPS) for 10 min at 1025℃ and conventional sintering (CS) for 10 h at 1090℃ have been employed. X-ray diffraction measurements confirmed the pure CCTO-like phase for SPS and CS NSCTO ceramics. The SPS ceramic showed an average grain size of 500 nm, which is much smaller than that of the CS (~5 μm) sample. The impedance spectroscopy measurements revealed an electrically inhomogeneous structure in the prepared ceramics. While the resistivities of grains of both ceramic samples were in the same order of magnitude, the resistivity of grain-boundaries of the CS ceramic was three orders of magnitude greater than that of the SPS ceramic. Both of the samples showed giant dielectric constant (>103) over wide ranges of temperatures and frequencies. Nevertheless, the room-temperature dielectric loss of the SPS NSCTO (3.2 at 1.1 kHz) ceramic sample was higher than that of the CS NSCTO (0.08 at 1.1 kHz) ceramic sample due to the reduced grain-boundary resistivity of the former. Two dielectric relaxations were detected for each sample and attributed to the relaxations in grains and grain-boundaries. The dielectric behavior of the SPS and CS NSCTO ceramics could be interpreted in terms of the internal barrier layer capacitor (IBLC) model.
Efficient molecular model for squeeze-film damping in rarefied air
Based on the energy transfer model (ETM) proposed by Bao et al. and the Monte Carlo (MC) model proposed by Hutcherson and Ye, this paper proposes an efficient molecular model (MC-S) for squeeze-film damping (SQFD) in rarefied air by releasing the assumption of constant molecular velocity in the gap. Compared with the experiment data, the MC-S model is more efficient than the MC model and more accurate than ETM. Besides, by using the MC-S model, the feasibility of the empirical model proposed by Sumali for SQFD of different plate sizes is discussed. It is proved that, for various plate sizes, the accuracy of the empirical model is relatively high. At last, the SQFD of various vibration frequencies is discussed, and it shows that, for low vibration frequency, the MC-S model is reduced to ETM.
Thermal resistance matrix representation of thermal effects and thermal design of microwave power HBTs with two-dimensional array layout
Based on the thermal network of the two-dimensional heterojunction bipolar transistors (HBTs) array, the thermal resistance matrix is presented, including the self-heating thermal resistance and thermal coupling resistance to describe the self-heating and thermal coupling effects, respectively. For HBT cells along the emitter length direction, the thermal coupling resistance is far smaller than the self-heating thermal resistance, and the peak junction temperature is mainly determined by the self-heating thermal resistance. However, the thermal coupling resistance is in the same order with the self-heating thermal resistance for HBT cells along the emitter width direction. Furthermore, the dependence of the thermal resistance matrix on cell spacing along the emitter length direction and cell spacing along the emitter width direction is also investigated, respectively. It is shown that the moderate increase of cell spacings along the emitter length direction and the emitter width direction could effectively lower the self-heating thermal resistance and thermal coupling resistance, and hence the peak junction temperature is decreased, which sheds light on adopting a two-dimensional non-uniform cell spacing layout to improve the uneven temperature distribution. By taking a 2×6 HBTs array for example, a two-dimensional non-uniform cell spacing layout is designed, which can effectively lower the peak junction temperature and reduce the non-uniformity of the dissipated power. For the HBTs array with optimized layout, the high power-handling capability and thermal dissipation capability are kept when the bias voltage increases.
Performance improvement of 4H-SiC PIN ultraviolet avalanche photodiodes with different intrinsic layer thicknesses
Four 4H-SiC p-i-n ultraviolet (UV) avalanche photodiode (APD) samples PIN-0.1, PIN-0.35, PIN-0.5, and PIN-1.0 with different intrinsic layer thicknesses (0.1 μm, 0.35 μm, 0.5 μm, and 1.0 μm, respectively) are designed and fabricated. Single photon detection efficiency (SPDE) performance becomes better as the intrinsic layer thickness increases, which is attributed to the inhibitation of tunneling. Dark count origin is also investigated, an activation energy as small as 0.22 eV of the dark count rate (DCR) confirms that the trap-assisted tunneling (TAT) process is the main source of DCR. The temperature coefficient ranges from -2.6 mV/℃ to 18.3 mV/℃, demonstrating that the TAT process is dominant in APDs with thinner intrinsic layers. Additionally, the room temperature maximum quantum efficiency at 280 nm differs from 48% to 65% for PIN-0.35, PIN-0.5, and PIN-1.0 under 0 V bias, and UV/visible rejection ratios higher than 104 are obtained.
Intrinsic transverse relaxation mechanisms of polarized alkali atoms enclosed in radio-frequency magnetometer cell
The intrinsic transverse relaxation mechanisms of polarized alkali atoms enclosed in the radio-frequency magnetometer cell are investigated. The intrinsic transverse relaxation rate of cesium atoms as a function of cell temperature is obtained. The absorption of alkali atoms by the glass wall and the reservoir effect are the main error factors which contribute to the disagreements between theory and experiments. A modified relaxation model is presented, in which both the absorption of alkali atoms by the glass wall and the reservoir effect are included. This study provides a more accurate description of the intrinsic transverse relaxation mechanisms of polarized alkali atoms, and enlightens the optimization of the cell design.
Terahertz coherent detection via two-color laser pulses of various frequency ratios
The mechanism of terahertz (THz) pulse coherent detection via two-color laser pulses of various frequency ratios in gas plasma is theoretically investigated. Our investigations demonstrate that except for the commonly used frequency ratio of 2, other uncommon frequency ratios can also be utilized to detect THz pulse, such as 2n, n+1/2 (n ≤ 3, n is a positive integer). The well-developed transient photocurrent model is extended to our terahertz detection process. Based on this model, our simulation results can be explained by analyzing the process of asymmetric electron ionization and electron acceleration.
A new cellular automaton model accounting for stochasticity in traffic flow induced by heterogeneity in driving behavior
A new reliable cellular automaon (CA) model designed to account for stochasticity in traffic flow induced by heterogeneity in driving behavior is presented. The proposed model differs from most existing CA models in that this new model focuses on describing traffic phenomena by coding into its rules the key idea that a vehicle's moving state is directly determined by a driver stepping on the accelerator or on the brake (the vehicle's acceleration). Acceleration obeys a deformed continuous distribution function when considering the heterogeneity in driving behavior and the safe distance, rather than equaling a fixed acceleration value with a probability, as is the rule in many existing CA models. Simulation results show that the new proposed model is capable of reproducing empirical findings in real traffic system. Moreover, this new model makes it possible to implement in-depth analysis of correlations between a vehicle's state parameters.