1,024 research outputs found
Activation of σ28-dependent transcription inEscherichia coliby the cyclic AMP receptor protein requires an unusual promoter organization
The Escherichia coli aer regulatory region contains a single promoter that is recognized by RNA polymerase containing the flagellar sigma factor, σ28. Expression from this promoter is dependent on direct activation by the cyclic AMP receptor protein, which binds to a target centred 49.5 base pairs upstream from the transcript start. Activator-dependent transcription from the aer promoter was reconstituted in vitro, and a tethered inorganic nuclease was used to find the position of the C-terminal domains of the RNA polymerase α subunits in transcriptionally competent open complexes. We report that the ternary activator-RNA polymerase-aer promoter open complex is organized differently from complexes at previously characterized promoters. Among other E. coli promoters recognized by RNA polymerase containing σ28, only the trg promoter is activated directly by the cyclic AMP receptor protein. The organization of the different promoter elements and the activator binding site at the trg promoter is the same as at the aer promoter, suggesting a common activation mechanism
Direct determination of the ambipolar diffusion length in strained InxGa1−xAs/InP quantum wells by cathodoluminescence
The ambipolar diffusion length is measured in strained InxGa1−xAs/InP quantum wells for several mole fractions in the interval 0.3<x<0.8 by cathodoluminescence. The ambipolar diffusion length is found to have a significantly higher value in the lower indium mole fraction samples corresponding to tensile-strained wells. This longer diffusion length for the tensile samples is consistent with results of carrier lifetime experiments by M. C. Wang, K. Kash, C. E. Zah, R. Bhat, and S. L. Chuang [Appl. Phys. Lett. 62, 166 (1993)]
Cascaded wavelength conversions using four-wave mixing in semiconductor optical amplifiers
Wavelength conversion in wavelength-division multiplexed (WDM) communication systems would provide significant network performance improvement. Optoelectronic, cross-gain saturation, and cross phase saturation wavelength converters are candidate technologies that have been well characterized, however, they are not “transparent” to either bit-rate or modulation format. Complete transparency is offered only by ultrafast wave mixing techniques-in the present case four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs)
Design and characterization of whispering-gallery spiral waveguides
Whispering gallery delay lines have demonstrated record propagation length on a silicon chip and can provide a way to transfer certain applications of optical fiber to wafer-based systems. Their design and fabrication requires careful control of waveguide curvature and etching conditions to minimize connection losses between elements of the delay line. Moreover, loss characterization based on optical backscatter requires normalization to account for the impact of curvature on backscatter rate. In this paper we provide details on design of Archimedean whispering-gallery spiral waveguides, their coupling into cascaded structures, as well as optical loss characterization by optical backscatter reflectometry
Microwave synthesizer using an on-chip Brillouin oscillator
Low-phase-noise microwave oscillators are important to a wide range of subjects, including
communications, radar and metrology. Photonic-based microwave-wave sources now provide
record, close-to-carrier phase-noise performance, and compact sources using microcavities
are available commercially. Photonics-based solutions address a challenging scaling problem
in electronics, increasing attenuation with frequency. A second scaling challenge, however, is
to maintain low phase noise in reduced form factor and even integrated systems. On this
second front, there has been remarkable progress in the area of microcavity devices with
large storage time (high optical quality factor). Here we report generation of highly coherent
microwaves using a chip-based device that derives stability from high optical quality factor.
The device has a record low electronic white-phase-noise floor for a microcavity-based
oscillator and is used as the optical, voltage-controlled oscillator in the first demonstration of
a photonic-based, microwave frequency synthesizer. The synthesizer performance is comparable
to mid-range commercial devices
Low-noise Brillouin laser on a chip at 1064 nm
We demonstrate narrow-linewidth-stimulated Brillouin lasers at 1064 nm from ultra-high-Q silica wedge disk resonators on silicon. Fundamental Schawlow–Townes frequency noise of the laser is on the order of 0.1 Hz^2/Hz. The technical noise spectrum of the on-chip Brillouin laser is close to the thermodynamic noise limit of the resonator (thermorefractive noise) and is comparable to that of ultra-narrow-linewidth Nd:YAG lasers. The relative intensity noise of the Brillouin laser also is reduced by using an intensity-stabilized pump laser. Finally, low-noise microwave synthesis up to 32 GHz is demonstrated by heterodyne of first and third Brillouin Stokes lines from a single resonator
Cross talk penalty in two-channel wavelength conversion by four-wave mixing in a strained semiconductor optical amplifier
A crucial function in wavelength-division multiplexed (WDM) all-optical networks is a wavelength converter. This function enhances wavelength routing options and improves network reconfigurability. Here we present a systematic study of the cross talk penalty as a function of the pump-to-signal power ratio for two 2.5-Gbit/s ASK channels separated by 1.5 nm
Re-evaluation of the surface ruptures of the November 1951 earthquake series in eastern Taiwan, and its neotectonic implications
The earthquakes of November 1951 constitute the most destructive seismic episode in the recorded history of the Longitudinal Valley, eastern Taiwan. However, information about their source parameters is sparse. To understand the relationship between the 1951 ruptures and new interpretations of the regional neotectonic architecture of the Longitudinal Valley, we re-evaluated the November 1951 ruptures by analyzing old documents, reports and photographs, and by interviewing local residents who experienced the earthquake. As a result, we have revised significantly the rupture map previously published. We divide the surface ruptures from south to north into the Chihshang, Yuli, and Rueisuei sections. The first shock of the 1951 series probably resulted from the Chihshang rupture, and the second shock probably resulted from the Yuli and Rueisuei ruptures. The lengths of these ruptures indicate that the two shocks had similar magnitudes. The Chihshang and Rueisuei ruptures are along segments of the Longitudinal Valley fault, a left-lateral oblique fault along which the Coastal Range thrusts westward over the Longitudinal Valley. The Yuli rupture, on the other hand, appears to be part of a separate, left-lateral strike-slip Yuli fault, which traverses the middle of the Longitudinal Valley. The complex behavior of these structures and interaction between them are important in understanding the future seismic hazard of the area
Phonon laser action in a tunable, two-level photonic molecule
The phonon analog of an optical laser has long been a subject of interest. We
demonstrate a compound microcavity system, coupled to a radio-frequency
mechanical mode, that operates in close analogy to a two-level laser system. An
inversion produces gain, causing phonon laser action above a pump power
threshold of around 50 W. The device features a continuously tunable, gain
spectrum to selectively amplify mechanical modes from radio frequency to
microwave rates. Viewed as a Brillouin process, the system accesses a regime in
which the phonon plays what has traditionally been the role of the Stokes wave.
For this reason, it should also be possible to controllably switch between
phonon and photon laser regimes. Cooling of the mechanical mode is also
possible.Comment: 4 pages, 4 figure
Cascaded wavelength conversion by four-wave mixing in a strained semiconductor optical amplifier at 10 Gb/s
We demonstrate for the first time cascaded wavelength conversion by four-wave mixing in a semiconductor optical amplifier. Bit-error-rate performance of <10^-9 at 10 Gb/s is achieved for two conversions of up to 9 nm down and up in wavelength. For two wavelength conversions of 5 nm down and up, a power penalty of 1.3 dB is measured. A system of two wavelength converters spanning 40 km of single-mode fiber is also demonstrated
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