Terahertz radiation from magnetoresistive Pr0.7_{\text{0.7}}Ca0.3_{\text{0.3}}MnO3_{\text3} 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; Pr$_{0.7}$Ca$_{0.3}$MnO$_3$. The THz radiation strongly depends on temperature $T$ and its $T$ trend reverses sign across charge-orbital and spin ordering $T$'s.Comment: Revtex4, 4 pages including 3 figure

Partial and macroscopic phase coherences in underdoped Bi2{}_{2}Sr2{}_{2}CaCu2{}_{2}O8+δ{}_{8+{\delta}} 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 Bi$_2$Sr$_2$CaCu$_2$O$_{8+{\delta}}$ thin film. It was observed that the low energy multi-excitation states were decomposed into superconducting gap and pseudogap. The pseudogap locally opens below $T^*{\simeq}210$ 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 Ba$_2$CoGe$_2$O$_7$ 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 La0.7_{\text{0.7}}Ca0.3_{\text{0.3}}MnO3_{\text3}: THz time-domain spectroscopic studies

Direct experimental estimations of the low-energy ($1.5\sim10$ meV) complex dielectric constants spectrum and its temperature variation have been investigated for La$_{0.7}$Ca$_{0.3}$MnO$_3$ 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 Pr0.7_{\text{0.7}}Ca0.3_{\text{0.3}}MnO3_{\text{3}} 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 Pr$_{0.7}$Ca$_{0.3}$MnO$_3$. We revealed the existence of a finite peak structure around 2--3 meV well below the charge gap $\sim300$ 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 Pr$_{0.7}$Ca$_{0.3}$MnO$_3$.Comment: REVTeX4, 8 pages including 7 figures and 2 table