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Periodic Bifurcation and Soliton Deflexion for Kadomtsev--Petviashvili Equation
DAI Zheng-De, , LI Shao-Lin, LI Dong-Long, ZHU Ai-Jun
Chin. Phys. Lett. 2007, 24 (6):
1429-1432
.
The spatial--temporal bifurcation for Kadomtsev--Petviashvili (KP) equations is considered. Exact two-soliton solution and doubly periodic solution to the KP-I equation, and two classes of periodic soliton solutions in different directions to KP-II are obtained using the bilinear form, homoclinic test technique and temporal and spatial transformation method, respectively. The equilibrium solution u0=-1/6, a unique spatial--temporal bifurcation which is periodic bifurcation for KP-I and deflexion of soliton for KP-II, is investigated.
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Effects of Continuous Size Distributions on Pressures of Granular Gases
LI Rui, ZHANG Duan-Ming, CHEN Zhi-Yuan, SU Xiang-Ying, ZHU Hong-Ying, ZHANG Ling, HUANG Ming-Tao
Chin. Phys. Lett. 2007, 24 (6):
1482-1485
.
Direct Monte Carlo simulations are employed to investigate the granular pressures in granular materials with a power-law particle size distribution. Specifically, smooth circular discs of uniform material density are engaged in a two-dimensional rectangular box, colliding inelastically with each other and driven by a homogeneous heat bath at zero gravity. The resulting pressures are found to decrease as the widths of particle size distribution are increased. Moreover, the granular pressures in power-law systems are found to be unequally distributed among the various sizes of particles, with large particles possessing more pressure than their smaller counterparts. The width-dependent nature of the total pressures is induced by the more dispersion of smaller particles in the system as the particle size distribution is widened.
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Multiscale Entropy under the Inverse Gaussian Distribution: Analytical Results
TANG Ying, PEI Wen-Jiang, XIA Hai-Shan, HE Zhen-Ya
Chin. Phys. Lett. 2007, 24 (6):
1490-1493
.
The multiscale entropy (MSE) reveals the intrinsic multiple scales in the complexity of physical and physiological signals, which are usually featured by heavy-tailed distributions. However, most research results are pure experimental search. Recently, Costa et al. have made the first attempt to present the theoretical basis of MSE, but it only supports the Gaussian distribution [Phys Rev. E 71 (2005) 021906]. We present the theoretical basis of MSE under the inverse Gaussian distribution, a typical model for physiological, physical and financial data sets. The analysis allows for ncorrelated inverse Gaussian process and 1/f noise with the multivariate inverse Gaussian distribution, and then provides a reliable foundation for the potential applications of MSE to explore complex physical and physical time series.
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Collective Band Structures in Neutron-Rich 108Mo Nucleus
DING Huai-Bo, ZHU Sheng-Jiang, J. H. Hamilton, A. V. Ramayya, J. K. Hwang, Y. X. Luo, J. O. Rasmussen, I. Y. Lee, CHE Xing-Lai, WANG Jian-Guo, XU Qiang
Chin. Phys. Lett. 2007, 24 (6):
1517-1520
.
High spin states in the neutron-rich 108Mo nucleus are studied by measuring prompt γ-rays following the spontaneous fission of 252Cf with a Gammasphere detector array. The ground-state band is confirmed, and the one-phonon γ-vibrational band is updated with spin up to 12295;. A new collective band with the band head level at 1422.4keV is suggested as a two-phonon γ-vibrational band. Another new band is proposed as a two-quasi-proton excitation band. Systematic characteristics of the collective bands are discussed.
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Polarization of Radiation Emitted after Electron Impact Excitation
WU Ze-Qing, LI Yue-Ming, DUAN Bin, YAN Jun, ZHANG Hong
Chin. Phys. Lett. 2007, 24 (6):
1560-1562
.
A programme is developed to calculate the polarizations of the radiation emitted after electron impact excitation. The fully relativistic distorted-wave method is used in cross-section calculations. The programme is applied to He- and Li-like ions. The calculated values of line polarization are compared with other theoretical results and experimental values. For He-like U, at lower incident energy, the present polarization agrees with the other theoretical ones within 1%, while at higher energy, the differences increase up to about 10%. For He-like Fe and Ti, the present results of polarization degree for most of the lines agree with the experimental data within the experimental error bars. For the Li-like Ti line q (1s2s2p2P 3/2 to 1s22s), the present value of the polarization agree excellently with another theoretical one, and both the values are consistent with the measured data within the experimental error bar.
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Low-Threshold High-Temperature Operation of ~7.4μm Quantum Cascade Lasers
LI Lu, LIU Feng-Qi, SHAO Ye, LIU Jun-Qi, WANG Zhan-Guo
Chin. Phys. Lett. 2007, 24 (6):
1577-1579
.
We report low-threshold high-temperature operation of 7.4μm strain-compensated InGaAs/InAlAs quantum cascade lasers (QCLs). For an uncoated 22-μm-wide and 2-mm-long laser, the low-threshold current densities, i.e. 0.33kA/cm2 at 81K in pulsed mode and 0.64kA/cm2 at 84K in cw mode, are realized. High-temperature operation of uncoated devices, with a high value of 223K, is achieved in cw mode.
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Passively Q-Switched Nd:KLuW Laser with Semiconductor Saturable Absorber
GUO Lin, WANG Gui-Ling, ZHANG Hong-Bo, GENG Ai-Cong, CHEN Ya-Hui, LU Yuan-Fu, CUI Qian-Jin, ZHOU Yong, CUI Da-Fu, ZHANG Jian-Xiu, ZHANG Huai-Jin, WANG Ji-Yang, XU Zu-Yan
Chin. Phys. Lett. 2007, 24 (6):
1583-1585
.
We demonstrate a passively Q-switched Nd:KLuW laser with a semiconductor saturable absorber mirror (SESAM) at wavelength 1070nm. At a pump power of 1.3W, the pulse width is measured to be about 17ns with repetition rate of 10kHz and with the average output power of 260mW. To our knowledge, this is the first demonstration of Nd:KLuW used for passively Q-switched laser with an SESAM.
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Long Optical Delay Lines Enhanced by Ring Configuration in Optical Fibres
DONG Yong-Kang, LU Zhi-Wei, LI Qiang, GAO Wei
Chin. Phys. Lett. 2007, 24 (6):
1586-1588
.
A long optically controlled delay line enhanced by ring configuration is demonstrated by using the group-velocity control of signal pulses based on stimulated Brillouin scattering. In experiment, two optical fibre ring cavities are used: one is used as the Brillouin laser, providing single-mode Stokes wave as probe wave; the other is used as the Brillouin amplifier, working as slow light medium. We achieve a maximum time delay of 215ns using the ring Brillouin amplifier, five times larger than the input probe pulse width of 40ns. In the meantime, a considerable pulse broadening is observed, which agrees well with the theoretical prediction based on linear theory.
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Fibre Bragg Gratings Inscribed in Homemade Microstructured Fibres
JIN Long, KAI Gui-Yun, LI Jin-Yan, CHEN Wei, LIU Jian-Guo, LIU Yan-Ge, WANG Zhi, ZHANG Jian, LIU Bo, YUAN Shu-Zhong, DONG Xiao-Yi
Chin. Phys. Lett. 2007, 24 (6):
1603-1606
.
Fibre Bragg gratings (FBGs) are inscribed in homemade microstructured fibres by the standard phase mask method. Enhanced couplings between the forward fundamental mode and backward cladding modes are obtained. The mode coupling and spectral characteristics are investigated experimentally. The cladding mode resonances can be affected by filling active materials into the air holes, which will be useful for the implementation of tunable photonic devices in optical fibre communication and sensing systems.
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A New Doppler Shift Spectroscopy for Measurement of Neutral Beam Profile
SHI Yue-Jiang, WANG Guang-Qi, FU Jia, WU Zhen-Wei, CHANG Jia-Feng, SUN Dan-Peng, GAO Wei, HUANG Juan, ZHOU Qian, GAO Xiang, WAN Bao-Nian
Chin. Phys. Lett. 2007, 24 (6):
1629-1632
.
A new diagnostic based on Doppler shift is designed to measure the power profile of a hydrogen or deuterium neutral beam on the magnetic confined fusion machines. The interference filters and multi-channel photon detector array (PDA) are the main components of this diagnosis. The multi-channel PDA measures the line integrated Doppler Hα signal emitted by the neutral beam at one section in two directions. The local intensity of neutral beam can be obtained with the tomography technique. Compared to the conventional calorimeter diagnoses, this diagnosis can provide the beam profile without blocking the injection of neutral beam.
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Effect of Substrate Bias on Microstructures of Zirconia Thin Films Deposited by Cathodic Vacuum Arc
LI Xiang-Zhou, ZHANG Xian-Hui, HE Ping, NIU Er-Wu, XIA Yuan-Yu, HUANG Jun, FENG Ke-Cheng, YANG Si-Ze
Chin. Phys. Lett. 2007, 24 (6):
1633-1636
.
Zirconium oxide (ZrO2) thin films are deposited at room temperature by cathodic arc at substrate biases of 0V, -60V and -120V, respectively. The crystal structure, composition, morphology, and deposition rate of the as-deposited thin films are systematically investigated by x-ray diffraction, x-ray photoelectron spectroscopy (XPS) as well as scanning electron microscopy. The results show that the crystal structure, morphology and deposition rate of the films all are dependant on substrate bias. With the increase of bias voltage from 0V to -120V, the zirconium oxide thin film grown on silicon wafer first exhibits monoclinic lattice and tetragonal lattice, further evolves monoclinic phase with the preferred orientation along the (-111) and (-222) directions at -60V and finally along nearly one observed preferred (002) direction under -120V. In addition, the variations of morphology with bias voltage are correlated to changes of the film structure. The results of XPS demonstrate that Zr elements are almost oxidized completely in the films achieved under -120V bias.
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Power Consideration for Pulsed Discharges in Potassium Seeded Argon
XIA Sheng-Guo, HE Jun-Jia, LIU Ke-Fu
Chin. Phys. Lett. 2007, 24 (6):
1637-1640
.
Minimization of energy consumed in plasma generation is critical for applications, in which a large volume of plasmas is needed. We suggest that a high electron density atmospheric pressure plasmas can be generated by pulsed discharges in potassium seeded argon at an elevated temperature with a very small power input. The ionization efficiency and power budget of pulsed discharges in such plasmas are analytically studied. The results show that ionization efficiency of argon, especially at small reduced electric field E/N (the ratio of the electric field to the gas number density), is improved ffectively in the presence of small amount of potassium additives. Power input of pulsed discharge to sustain a prescribed average level of ionization in potassium seeded argon is three orders of magnitude lower than that in pure argon. Further, unlike in pure argon, it is found that very short high-voltage pulses with very high repetition rates are unnecessary in potassium seeded argon. A pulse with 100ns of pulse duration, 5kHz of repetition rate, and 2Td (1Td=1×10-21Vm2) of E/N is enough to sustain an electron density of 1019m-3 in 1 atm 1500K Ar+0.1% K mixture, with a very small power input of about 0.08×104W/m3.
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Growth of Semi-Insulating GaN by Using Two-Step AlN Buffer Layer
ZHOU Zhong-Tang, GUO Li-Wei, XING Zhi-Gang, DING Guo-Jian, ZHANG Jie, PENG Ming-Zeng, JIA Hai-Qiang, CHEN Hong, ZHOU Jun-Ming
Chin. Phys. Lett. 2007, 24 (6):
1641-1644
.
Semi-insulating GaN is grown by using a two-step AlN buffer layer by metalorganic chemical vapour deposition. The sheet resistance of as-grown semi-insulating GaN is dramatically increased to 1013Ω/sq by using two-step AlN buffer instead of the traditional low-temperature GaN buffer. The high sheet resistance of as-grown GaN over 1013Ω/sq is due to inserting an insulating buffer layer (two-step AlN buffer) between the high-temperature GaN layer and a sapphire substrate which blocks diffusion of oxygen and overcomes the weakness of generating high density carrier near interface of GaN and sapphire when a low-temperature GaN buffer is used. The result suggests that the high conductive feature of unintentionally doped GaN is mainly contributed from the highly conductive channel near interface between GaN and the sapphire substrate, which is indirectly manifested by room-temperature photoluminescence excited by an incident laser beam radiating on growth surface and on the substrate. The functions of the two-step AlN buffer layer in reducing screw dislocation and improving crystal quality of GaN are also discussed.
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Microstructure of Epitaxial Er2O3 Thin Film on Oxidized Si (111) Substrate
XUE Xian-Ying, WANG Yu-Zhu, JIA Quan-Jie, WANG Yong, CHEN Yu, JIANG Xiao-Ming, ZHU Yan-Yan, JIANG Zui-Min
Chin. Phys. Lett. 2007, 24 (6):
1649-2652
.
Er2O3 thin films are grown on oxidized Si (111) substrates by molecular beam epitaxy. The sample grown under optimized condition is characterized in its microstructure, surface morphology and thickness using grazing incidence x-ray diffraction (GIXRD), atomic force morphology and x-ray reflectivity. GIXRD measurements reveal that the Er2O3 thin film is a mosaic of single-crystal domains. The interplanar spacing d in-plane residual strain tensor ε| and the strain relaxation degree ξ are calculated. The Poisson ratio μ obtained by conventional x-ray diffraction is in good agreement with that of the bulk Er2O3. In-plane strains in three sets of planes, i.e. (440), (404), and (044), are isotropic.
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Effects of Dislocation on High Temperature Transport Characteristics of Unintentionally Doped GaN
WANG Mao-Jun, SHEN Bo, XU Fu-Jun, WANG Yan, XU Jian, HUANG Sen, YANG Zhi-Jian, QIN Zhi-Xin, ZHANG Guo-Yi
Chin. Phys. Lett. 2007, 24 (6):
1682-1685
.
High temperature transport characteristics of unintentionally doped GaN have been investigated by means of high temperature Hall measurements from room temperature to 500°C. The increment of electron concentration from room temperature to 500°C is found to vary largely for different samples. The dispersion of temperature dependence of electron concentration is found to be directly proportional to the density of dislocations in GaN layers calculated by fitting the FWHM of the rocking curves in x-ray diffraction measurements (XRD). The buildup levels in persistent photoconductivity (PPC) are also shown to be directly proportional to the density of dislocations. The correlation of XRD, Hall and PPC results indicate that the high temperature dependence of electron density in unintentional doped GaN is directly dislocation related.
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Growth and Characterization of AlGaN/AlN/GaN HEMT Structures with a Compositionally Step-Graded AlGaN Barrier Layer
MA Zhi-Yong, WANG Xiao-Liang, HU Guo-Xin, RAN Jun-Xue, XIAO Hong-Ling, LUO Wei-Jun, TANG Jian, LI Jian-Ping, LI Jin-Min
Chin. Phys. Lett. 2007, 24 (6):
1705-1708
.
A new AlGaN/AlN/GaN high electron mobility transistor (HEMT) structure using a compositionally step-graded AlGaN barrier layer is grown on sapphire by metalorganic chemical vapour deposition (MOCVD). The structure demonstrates significant enhancement of two-dimensional electron gas (2DEG) mobility and smooth surface morphology compared with the conventional HEMT structure with high Al composition AlGaN barrier. The high 2DEG mobility of 1806cm2/Vs at room temperature and low rms surface roughness of 0.220nm for a scan area of 5μm×5μm are attributed to the improvement of interfacial and crystal quality by employing the step-graded barrier to accommodate the large lattice mismatch stress. The 2DEG sheet density is independent of the measurement temperature, showing the excellent 2DEG confinement of the step-graded structure. A low average sheet resistance of 314.5Ω/square, with a good resistance uniformity of 0.68%, is also obtained across the 50mm epilayer wafer. HEMT devices are successfully fabricated using this material structure, which exhibits a maximum extrinsic transconductance of 218mS/mm and a maximum drain current density of 800mA/mm.
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Microwave Response of Bi2Sr2CaCu2O8+δ Surface Intrinsic Josephson Junctions with Bending Pancake Vortices
WEI Yan-Feng, ZHAO Shi-Ping, ZHU Xiao-Bo, CHEN Geng-Hua, REN Yu-Feng, YU Hong-Wei, YANG Qian-Sheng
Chin. Phys. Lett. 2007, 24 (6):
1709-1712
.
We study the microwave response of surface intrinsic Josephson junctions on Bi2Sr2CaCu2O8+δ, in which bending pancake vortex lines are introduced in a controllable way. It is found that the bending vortices can greatly influence the response. In some cases, typical Shapiro steps that lie far above the quasiparticle branch are observed, with the step interval satisfying the Josephson relation and their amplitude versus the square root of microwave power following the Bessel function behaviour. In the other cases, current steps that lie on the quasiparticle branch are observed, but only one or two steps appear at the same time under the variation of the microwave power.
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Light-Induced Domain Inversion in Mg-Doped near Stoichiometric Lithium Niobate Crystals
LIU Hong-De, KONG Yong-Fa, HU Qian, WU Ri-Wen, WANG Wen-Jie, LI Xiao-Chun, CHEN Shao-Lin, LIU Shi-Guo, XU Jing-Jun
Chin. Phys. Lett. 2007, 24 (6):
1720-1723
.
We investigate the influence of visible light on domain inversion in Mg-doped near stoichiometric lithium niobate crystals and find that the switching electric field decreases about 70% above a threshold light intensity. This effect helps us optically control domain switching and produce bulk domain structures on the micrometre scale. Finally, we introduce a model of photo-induced carriers to explain the origin of the reduction of switching electric field.
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Evolution of Photocurrent during Coadsorption of Cs and O on GaAs (100)
ZOU Ji-Jun, CHANG Ben-Kang, YANG Zhi, DU Xiao-Qing, GAO Pin, QIAO Jian-Liang
Chin. Phys. Lett. 2007, 24 (6):
1731-1734
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The photocurrent and spectral response characteristics of gallium arsenide (GaAs) are obtained by a multi-information measurement system, and the evolution of the photocurrent versus the Cs:O flux ratio is investigated. The experimental results show that the photocurrent increases approximately exponentially after the first exposure to Cs until a maximum sensitivity is reached, the detailed evolution process and the ultimate photocurrent are different for different samples. These differences are analysed, and according to the process of coadsorption of Cs and oxygen on GaAs, an equation is presented to explain the increase of photocurrent.
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High-Reflectivity AlGaN/AlN Distributed Bragg Reflector in Ultraviolet Region
JI Xiao-Li, JIANG Ruo-Lian, XIE Zi-Li, LIU Bin, ZHOU Jian-Jun, LI Liang, HAN Ping, ZHANG Rong, ZHENG You-Dou, GONG Hai-Mei
Chin. Phys. Lett. 2007, 24 (6):
1735-1737
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Thirty-pair Al0.3 Ga 0.7 N/AlN distributed Bragg reflectors centred at 320nm are designed and grown on sapphire substrates by metalorganic chemical vapour deposition. No cracks are observed in the main area of the 2-inch wafer except for about 4mm margin under an optical microscope. Regular stack of alternating layers is shown by scanning electron microscopy. Clear two-dimensional growth steps and very low surface roughness are shown by atomic force microscopy (AFM). Well-defined periodicity is shown by high resolution x-ray diffraction. High reflectivity of 93% at 313nm with a bandwidth of 13nm is obtained.
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Effect of Carbon Source with Different Graphitization Degrees on the Synthesis of Diamond
LIU Wan-Qiang, MA Hong-An, LI Xiao-Lei, LIANG Zhong-Zhu, LIUMi-Lan, LI Rui, JIA Xiao-Peng,
Chin. Phys. Lett. 2007, 24 (6):
1749-1752
.
Using three kinds of graphites with different graphitization degrees as carbon source and Fe--Ni alloy powder as catalyst, the synthesis of diamond crystals is performed in a cubic anvil high-pressure and high-temperature apparatus (SPD-6×1200). Diamond crystals with perfect hexoctahedron shape are successfully synthesized at pressure from 5.0 to 5.5GPa and at temperature from 1570 to 1770K. The synthetic conditions, nucleation, morphology, inclusion and granularity of diamond crystals are studied. The temperature and pressure increase with the increase of the graphitization degree of graphite. The quantity of nucleation and granularity of diamonds decreases with the increase of graphitization degree of graphite under the same synthesis conditions. Moreover, according to the results of the Mossbauer spectrum, the composition of inclusions is mainly Fe3C and Fe-Ni alloy phases in diamond crystals synthesized with three kinds of graphites.
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Growth of a Novel Periodic Structure of SiC/AlN Multilayers by Low Pressure Chemical Vapour Deposition
ZHAO Yong-Mei, SUN Guo-Sheng, LI Jia-Ye, LIU Xing-Fang, WANG Lei ZHAOWan-Shun, LI Jin-Min
Chin. Phys. Lett. 2007, 24 (6):
1753-1755
.
A novel 10-period SiC/AlN multilayered structure with a SiC cap layer is prepared by low pressure chemical vapour deposition (LPCVD). The structure with total film thickness of about 1.45μm is deposited on a Si (111) substrate and shows good surface morphology with a smaller rms surface roughness of 5.3nm. According to the secondary ion mass spectroscopy results, good interface of the 10 period SiC/AlN structure and periodic changes of depth profiles of C, Si, Al, N components are obtained by controlling the growth procedure. The structure exhibits the peak reflectivity close to 30% near the wavelength of 322nm. To the best of our knowledge, this is the first report of growth of the SiC/AlN periodic structure using the home-made LPCVD system.
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Ionizing Dose Effect of Thermal Oxides Implanted with Si+ Ions
CHEN Ming, LUO Hong-Wei, ZHANG Zheng-Xuan, ZHANG En-Xia, YANG Hui, TIAN Hao, WANG Ru, YU Wen-Jie,
Chin. Phys. Lett. 2007, 24 (6):
1775-1777
.
Total ionizing dose effects of Si+ ion implanted thermal oxides are studied by 10keV x-ray irradiation. Photoluminescence (PL) method is engaged to investigate nanostructures of samples. Ar+ implanted samples are also studied by the same way to provide a comparison. The results show that Si+ implantation following with high temperature annealing can significantly reduce the radiation induced flatband shift, which is caused by net positive charge accumulation in oxides. This reduction is attributed to the formation of Si nanoscale structures. Ar+ implantation is also found to reduce the radiation induced flatband shift, while it is different that the reduction with Si+ implantation shows little dependence on implant dose of Ar+ ions. This is explained by possible increase of recombination centres.
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Noise Behaviour of a THz Superconducting Hot-Electron Bolometer Mixer
ZHANG Wen, LI Ning, JIANG Ling, MIAO Wei, LIN Zhen-Hui, YAOQi-Jun, SHI Sheng-Cai, CHEN Jian, WU Pei-Heng, S. I. Svechnikov, Yu. B. Vachtomin, S. V. Antipov, B. M. Voronov, G. N. Gol'tsman
Chin. Phys. Lett. 2007, 24 (6):
1778-1781
.
A quasi-optical superconducting NbN hot-electron bolometer (HEB) mixer is measured in the frequency range of 0.5--2.5THz for understanding of the frequency dependence of noise temperature of THz coherent detectors. It has been found that noise temperature increasing with frequency is mainly due to the coupling loss between the quasi-optical planar antenna and the superconducting HEB bridge when taking account of non-uniform distribution of high-frequency current. With the coupling loss corrected, the superconducting HEB mixer demonstrates a noise temperature nearly independent of frequency.
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98 articles
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