248 research outputs found
Terahertz radiation from magnetoresistive PrCaMnO thin films
Terahertz (THz) radiation with its spectrum extending up to 1 THz has been
observed by an illumination of femtosecond optical pulses to optical switching
devices fabricated on magnetoresistive manganite thin films;
PrCaMnO. The THz radiation strongly depends on temperature
and its trend reverses sign across charge-orbital and spin ordering
's.Comment: Revtex4, 4 pages including 3 figure
Partial and macroscopic phase coherences in underdoped BiSrCaCuO thin film
A combined study with use of time-domain pump-probe spectroscopy and
time-domain terahertz transmission spectroscopy have been carried out on an
underdoped BiSrCaCuO thin film. It was observed that
the low energy multi-excitation states were decomposed into superconducting gap
and pseudogap. The pseudogap locally opens below K
simultaneously with the appearance of the high-frequency partial pairs around
1.3 THz. With decreasing temperature, the number of the local domains with the
partial phase coherence increased and saturated near 100 K, and the macroscopic
superconductivity appeared below 76 K through the superconductivity fluctuation
state below 100 K. These experimental results indicate that the pseudogap makes
an important role for realization of the superconductivity as a precursor to
switch from the partial to the macroscopic phase coherence.Comment: Revtex4, 4 pages, 4 figure
Laser Terahertz Emission Microscope
Abstract: Laser terahertz (THz) emission microscope (LTEM) is reviewed. Femtosecond lasers can excite the THz waves in various electronic materials due to ultrafast current modulation. The current modulation is realized by acceleration or deceleration of photo-excited carriers, and thus LTEM visualizes dynamic photo-response of substances. We construct free-space type and scanning probe one with transmission or reflection modes. The developed systems have a minimum spatial resolution better than 2 µm, which is defined by the laser beam diameter. We also present some examples of LTEM applications
On the possibility of a terahertz light emitting diode based on a dressed quantum well
We consider theoretically the realization of a tunable terahertz light
emitting diode from a quantum well with dressed electrons placed in a highly
doped p-n junction. In the considered system the strong resonant dressing field
forms dynamic Stark gaps in the valence and conduction bands and the electric
field inside the p-n junction makes the QW asymmetric. It is shown that the
electrons transiting through the light induced Stark gaps in the conduction
band emit photons with energy directly proportional to the dressing field. This
scheme is tunable, compact, and shows a fair efficiency.Comment: 6 pages, 5 figure
Chirality of Matter Shows Up via Spin Excitations
Right- and left-handed circularly polarized light interact differently with
electronic charges in chiral materials. This asymmetry generates the natural
circular dichroism and gyrotropy, also known as the optical activity. Here we
demonstrate that optical activity is not a privilege of the electronic charge
excitations but it can also emerge for the spin excitations in magnetic matter.
The square-lattice antiferromagnet BaCoGeO offers an ideal arena to
test this idea, since it can be transformed to a chiral form by application of
external magnetic fields. As a direct proof of the field-induced chiral state,
we observed large optical activity when the light is in resonance with spin
excitations at sub-terahertz frequencies. In addition, we found that the
magnetochiral effect, the absorption difference for the light beams propagating
parallel and anti-parallel to the applied magnetic field, has an exceptionally
large amplitude close to 100%. All these features are ascribed to the
magnetoelectric nature of spin excitations as they interact both with the
electric and magnetic components of light
Graphene photodetectors for high-speed optical communications
While silicon has dominated solid-state electronics for more than four
decades, a variety of new materials have been introduced into photonics to
expand the accessible wavelength range and to improve the performance of
photonic devices. For example, gallium-nitride based materials enable the light
emission at blue and ultraviolet wavelengths, and high index contrast
silicon-on-insulator facilitates the realization of ultra dense and CMOS
compatible photonic devices. Here, we report the first deployment of graphene,
a two-dimensional carbon material, as the photo-detection element in a 10
Gbits/s optical data link. In this interdigitated metal-graphene-metal
photodetector, an asymmetric metallization scheme is adopted to break the
mirror symmetry of the built-in electric-field profile in conventional graphene
field-effect-transistor channels, allowing for efficient photo-detection within
the entire area of light illumination. A maximum external photo-responsivity of
6.1 mA/W is achieved at 1.55 {\mu}m wavelength, a very impressive value given
that the material is below one nanometer in thickness. Moreover, owing to the
unique band structure and exceptional electronic properties of graphene, high
speed photodetectors with an ultra-wide operational wavelength range at least
from 300 nm to 6 {\mu}m can be realized using this fascinating material.Comment: 20 pages, 3 figure
Co-production of hydrogen and ethanol from glucose in Escherichia coli by activation of pentose-phosphate pathway through deletion of phosphoglucose isomerase (pgi) and overexpression of glucose-6-phosphate dehydrogenase (zwf) and 6-phosphogluconate dehydrogenase (gnd)
Background: Biologically, hydrogen (H-2) can be produced through dark fermentation and photofermentation. Dark fermentation is fast in rate and simple in reactor design, but H-2 production yield is unsatisfactorily low as < 4 mol H-2/ mol glucose. To address this challenge, simultaneous production of H-2 and ethanol has been suggested. Co-production of ethanol andH(2) requires enhanced formation of NAD(P) H during catabolism of glucose, which can be accomplished by diversion of glycolytic flux from the Embden-Meyerh-of-Parnas (EMP) pathway to the pentose-phosphate (PP) pathway in Escherichia coli. However, the disruption of pgi (phosphoglucose isomerase) for complete diversion of carbon flux to the PP pathway made E. coli unable to grow on glucose under anaerobic condition. Results: Here, we demonstrate that, when glucose-6-phosphate dehydrogenase (Zwf) and 6-phosphogluconate dehydrogenase (Gnd), two major enzymes of the PP pathway, are homologously overexpressed, E. coli.pgi can recover its anaerobic growth capability on glucose. Further, with additional deletions of Delta hycA,Delta hyaAB,Delta hybBC,Delta ldhA, and Delta frdAB, the recombinant.pgi mutant could produce 1.69 mol H-2 and 1.50 mol ethanol from 1 mol glucose. However, acetate was produced at 0.18 mol mol(-1) glucose, indicating that some carbon is metabolized through the Entner-Doudoroff (ED) pathway. To further improve the flux via the PP pathway, heterologous zwf and gnd from Leuconostoc mesenteroides and Gluconobacter oxydans, respectively, which are less inhibited by NADPH, were overexpressed. The new recombinant produced more ethanol at 1.62 mol mol(-1) glucose along with 1.74 mol H-2 mol(-1) glucose, which are close to the theoretically maximal yields, 1.67 mol mol(-1) each for ethanol andH(2). However, the attempt to delete the ED pathway in the.pgi mutant to operate the PP pathway as the sole glycolytic route, was unsuccessful. Conclusions: By deletion of pgi and overexpression of heterologous zwf and gnd in E. coli Delta hycA Delta hyaAB Delta hybBC Delta ldhA Delta frdAB, two important biofuels, ethanol andH(2), could be successfully co-produced at high yields close to their theoretical maximums. The strains developed in this study should be applicable for the production of other biofuels and biochemicals, which requires supply of excessive reducing power under anaerobic conditions
Low-energy charge dynamics in LaCaMnO: THz time-domain spectroscopic studies
Direct experimental estimations of the low-energy ( meV) complex
dielectric constants spectrum and its temperature variation have been
investigated for LaCaMnO thin films using terahertz
time-domain spectroscopy. At low temperatures, a clear Drude-term emerges. With
increasing temperature, the scattering rate increases, while the plasma
frequency decreases, derived both from a simple Drude model. Finally, a
Drude-term submerges well below the insulator-metal transition temperature. On
the basis of the present results, low-energy charge dynamics are discussed.Comment: REVTeX 13 pages, 3 figure
Spectroscopic evidence for a charge-density-wave condensate in a charge-ordered manganite: Observation of collective excitation mode in PrCaMnO by using THz time-domain spectroscopy
THz time-domain spectroscopy was used to directly probe the low-energy
(0.5--5 meV) electrodynamics of the charge-ordered manganite
PrCaMnO. We revealed the existence of a finite peak
structure around 2--3 meV well below the charge gap meV. In analogy
to the low-energy optical properties of the well-studied low-dimensional
materials, we attributed this observed structure to the collective excitation
mode arising from the charge-density-wave condensate. This finding provides the
importance role of the quasi-one dimensional nature of the charge and orbital
ordering in PrCaMnO.Comment: REVTeX4, 8 pages including 7 figures and 2 table
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