2,975 research outputs found
Self-assembled ErAs islands in GaAs for optical-heterodyne THz generation
We report photomixer devices fabricated on a material consisting of self-assembled ErAs islands in GaAs, which is grown by molecular beam epitaxy. The devices perform comparably and provide an alternative to those made from low-temperature-grown GaAs. The photomixer's frequency response demonstrates that the material is a photoconductor with subpicosecond response time, in agreement with time-resolved differential reflectance measurements. The material also provides the other needed properties such as high photocarrier mobility and high breakdown field, which exceeds 2×10^5 V/cm. The maximum output power before device failure at frequencies of 1 THz was of order 0.1 µW. This material has the potential to allow engineering of key photomixer properties such as the response time and dark resistance
Design and characterization of optical-THz phase-matched traveling-wave photomixers
Design and characterization of optical-THz phase-matched traveling-wave photomixers for difference-frequency generation of THz waves are presented. A de-biased coplanar stripline fabricated on low-temperature-grown GaAs is illuminated by two non-collinear laser beams which generate moving interference fringes that are accompanied by THz waves. By tuning the offset angle between the two laser beams, the velocity of the interference fringe can be matched to the phase velocity of the THz wave in the coplanar stripline. The generated THz waves are radiated into free space by the antenna at the termination of the stripline. Enhancement of the output power was clearly observed when the phase-matching condition was satisfied. The output power spectrum has a 3-dB bandwidth of 2 THz and rolls off as ~9 dB/Oct which is determined by the frequency dependent attenuation in the stripline, while the bandwidth of conventional photomixer design has the limitation by the RC time constant due to the electrode capacitance. The device can handle the laser power of over 380 mW, which is 5 times higher than the maximum power handring capability of conventional small area devices. The results show that the traveling-wave photomixers have the potential to surpass small area designs, especially at higher frequencies over I THz, owing to their great thermal dissipation capability and capacitance-free wide bandwidth
Traveling-Wave Photomixers Based On Noncollinear Optical/Terahertz Phase-Matching
Traveling-wave THz photomixers based on angle-tuned optical/THz phase-matching are experimentally demonstrated. A dc-biased coplanar stripline terminated by a planar antenna is fabricated on low-temperature-grown GaAs. A distributed area between the striplines is illuminated by two noncollinear laser beams which generate interference fringes accompanied by THz waves. The velocity of the optical fringe is matched to the THz-wave velocity in the stripline by tuning the incident angle of the laser beams. The device can handle the laser power over 300 mW and provides the THz output of ~0.1 µW with the 3-dB bandwidth of 2 THz. The experimental results show that traveling-wave photomixers have the potential to surpass conventional small area designs
A traveling-wave THz photomixer based on angle-tuned phase matching
A traveling-wave THz photomixer based on a free-space optical-THz phase-matching scheme is proposed. A dc-biased coplanar strip line fabricated on low-temperature-grown GaAs serves as the active area of the device, and is illuminated by two noncollinear laser beams which generate interference fringes that are accompanied by THz waves. The device with the laser-power-handling capability over 300 mW and a 3-dB bandwidth of 1.8 THz was experimentally demonstrated. The results show that traveling-wave photomixers have the potential to surpass small-area designs
Dust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies
To study the effect of metallicity on the mass-loss rate of asymptotic giant
branch (AGB) stars, we have conducted mid-infrared photometric measurements of
such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies
with the 10-m camera VISIR at the VLT. We derive mass-loss rates for 29
AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated
from the and colours. Radiative transfer models are used to
check the consistency of the method. Published IRAS and Spitzer data confirm
that the same tight correlation between colour and dust mass-loss
rates is observed for AGB stars from galaxies with different metallicities,
i.e. the Galaxy, the LMC and the SMC.
The derived dust mass-loss rates are in the range 5 to
3 Myr for the observed AGB stars in Sgr dSph
and around 5 Myr for those in Fornax; while
values obtained with the two different methods are of the same order of
magnitude. The mass-loss rates for these stars are higher than the nuclear
burning rates, so they will terminate their AGB phase by the depletion of their
stellar mantles before their core can grow significantly. Some observed stars
have lower mass-loss rates than the minimum value predicted by theoretical
models.Comment: 12 pages, 9 figures, accepted for publication in MNRA
Localization on the D-brane, two-dimensional gauge theory and matrix models
We consider the effective topological field theory on Euclidean D-strings
wrapping on a 2-cycle in the internal space. We evaluate the vev of a suitable
operator corresponding to the chemical potential of vortices bounded to the
D-strings, and find that it reduces to the partition function of generalized
two-dimensional Yang-Mills theory as a result of localization. We argue that
the partition function gives a grand canonical ensemble of multi-instanton
corrections for four-dimensional N=2 gauge theory in a suitable large N limit.
We find two-dimensional gauge theories that provide the instanton partition
function for four-dimensional N=2 theories with the hypermultiplets in the
adjoint and fundamental representations. We also propose a partition function
that gives the instanton contributions to four-dimensional N=2 quiver gauge
theory. We discuss the relation between Nekrasov's instanton partition function
and the Dijkgraaf-Vafa theory in terms of large N phase transitions of the
generalized two-dimensional Yang-Mills theory.Comment: 40 pages, 5 figures, LaTeX2e, typos corrected, references added,
Final version to appear in Physical Review
Electronic inhomogeneity and competing phases in electron-doped superconducting Pr0.88LaCe0.12CuO4
We use neutron scattering to demonstrate that electron-doped superconducting
Pr0.88LaCe0.12CuO4 in the underdoped regime is electronically phase separated
in the ground state, showing the coexistence of a superconducting phase with a
three-dimensional antiferromagnetically ordered phase and a
quasi-two-dimensional spin density wave modulation. The Neel temperature of
both antiferromagnetic phases decreases linearly with increasing
superconducting transition temperature (Tc) and vanishes when optimal
superconductivity is achieved. These results indicate that the electron-doped
copper oxides are close to a quantum critical point, where the delicate
energetic balance between different competing states leads to microscopic
heterogeneity.Comment: 14 pages, 4 figures, accepted to Phys. Rev. B as a rapid
communicatio
The primary structure of three hemoglobin chains from the indigo snake (Drymarchon corais erebennus, Serpentes): First evidence for αD chains and two β chain types in snakes
The hemoglobin of the indigo snake (Drymarchon corais erebennus, Colubrinae) consists of two components, HbA and HbD, in the ratio of 1:1. They differ in both their alpha and beta chains. The amino acid sequences of both alpha chains (alpha(A) and alpha(D)) and one beta chain (betaI) were determined. The presence of an alpha(D)chain in a snake hemoglobin is described for the first time. A comparison of all snake beta chain sequences revealed the existence of two paralogous beta chain types in snakes as well, which are designated as betaI and betaII type. For the discussion of the physiological properties of Drymarchon hemoglobin, the sequences were compared with those of the human alpha and beta chains and those of the closely related water snake Liophis miliaris where functional data are available. Among the heme contacts, the substitution alpha(D)58(E7)His-->Gln is unusual but most likely without any effect. The residues responsible for the main part of the Bohr effect are the same as in mammalian hemoglobins. In each of the three globin chains only two residues at positions involved in the alpha1/beta2 interface contacts, most important for the stability and the properties of the hemoglobin molecule, are substituted with regard to human hemoglobin. On the contrary, nine, eleven, and six alpha1/beta1 contact residues are replaced in the alpha(A), alpha(D), betaI chains, respectively
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