David A. Nielsen

Estrogen regulation of gene transcription and mRNA stability

This chapter discusses that to study the molecular basis for steroid hormone regulation of gene expression, model systems are employed based on the estrogen control of liver cell differentiation and the induction of the liver messenger RNAs (mRNAs) coding for the Xenopus laevis egg yolk precursor protein, vitellogenin, for the serum retinol binding protein and for the Xenopus estrogen receptor. The experimental system initially employed in studies is the estrogen induction of the vitellogenin mRNAs in liver cells of the amphibian, X. laevis. The chapter reviews that rationale for a detailed study of the regulatory mechanisms used to achieve the induction of vitellogenin mRNA is derived from both in vivo studies and experiments using primary liver explant cultures. In the experiments explained in the chapter, quantitative RNA dot hybridization is used to measure the kinetics of vitellogenin mRNA induction. The data demonstrates that estrogen induces vitellogenin mRNA from undetectable levels to approximately 50,000 molecules per cell, at which time it comprises approximately half of the cells mRNA.

High-speed wavelength conversion in quantum dot and quantum well semiconductor optical amplifiers

We experimentally investigate wavelength conversion in quantum dot and quantum well optical amplifiers via four-wave mixing. Our results show superior conversion efficiency in a quantum dot device compared to a quantum well device with identical gain. Furthermore, a small-signal modulation bandwidth >25GHz was measured with greater than 100% efficiency. Cross talk between two simultaneously input beams was found to be 20dB below the signal power demonstrating the possibility for high-speed, multichannel performance. Cross-gain modulation measurements were performed as well and show a much smaller bandwidth of 1GHz indicating that four-wave mixing is superior for high-speed signals.

Electrically tunable slow and fast lights in a quantum-dot semiconductor optical amplifier near 1.55 μm

We have demonstrated both slow light in the absorption regime and fast light in the gain regime of a 1.55 microm quantum-dot semiconductor optical amplifier at room temperature. The theory with coherent population oscillations and four-wave mixing effects agrees well with the experimental results. We have observed a larger phase delay at the excited state than that at the ground state transition, likely due to the higher gain and smaller saturation power of the excited state.

Strong tunable slow and fast lights using a gain-clamped semiconductor optical amplifier

Previously demonstrated slow light is still far from applications, particularly due to the limited bandwidth and control speed. Although semiconductor-based slow light has the high bandwidth and sub-nanosecond control speed, slow light was observed only in the absorption regime with attenuation, while fast light observed in the gain regime with amplification. The large power difference in two regimes makes the use of the optical delay impractical. We report novel slow light in the gain regime, with a high power comparable to that of fast light, utilizing the anomalous gain characteristic in a gain-clamped semiconductor optical amplifier. The slow light is tunable to fast light with the current as the only variable. Additional high speed operation, fast delay control, and wide range of operation wavelength make the present approach practical.

Fast-Light to Slow-Light Switching in a Laser Cavity

We report on experimental work investigating slow- and fast-light in a ring-laser cavity that utilizes a semiconductor optical amplifier (SOA) as its primary gain medium. By allowing a pulse to propagate around the loop multiple times, we are able to achieve a tunable delay-bandwidth product near 10 for a 10-ps pulse. Our results show a shift between fast-light, as expected in a typical SOA, to slow-light as the pulse propagates around the loop. This slow-light phenomenon in a gain medium is not expected from an SOA, but arises due to the interaction between the lasing mode and the signal mode.

Comparison of four-wave mixing in quantum dots and quantum wells for wavelength conversion

We experimentally investigate wavelength conversion in quantum-dot and quantum-well optical amplifiers via four-wave mixing. Our results show superior conversion efficiency in a quantum dot device compared to a quantum well device with identical gain.

Four-wave mixing in quantum dots for wavelength conversion

By utilizing the four-wave mixing effect in a quantum dot semiconductor optical amplifier, we have demonstrated multi-channel wavelength conversion with greater than 100% efficiency and small signal modulation bandwidths in excess of 25 GHz.

Multiple-bits slow light by cascading semiconductor optical amplifiers in a loop configuration

We achieved slow light of 8.3 pulses of a 12-ps pulse in a series cascade of QW-SOAs in a loop configuration. The linear relationship between the number of optical passes through the SOA and time delay is experimentally demonstrated.

Estrogen Control of Vitellogenin Gene Transcription and mRNA Stability

We have analyzed the regulatory strategies used to achieve the massive, estrogen-mediated, induction of the mRNA coding for the egg yolk precursor protein, vitellogenin, in primary liver cultures of the amphibian Xenopus laevis. The induction of vitellogenin mRNA is achieved by three basic mechanisms: (a) an increase of at least several thousand fold in the absolute rate of vitellogenin gene transcription, which rises from undetectable levels to approximately 5 transcripts per gene per min., (b) an increase of approximately 20 fold in the absolute rate of total nuclear RNA synthesis, and (c) a selective stabilization of vitellogenin mRNA against cytoplasmic degradation. Vitellogenin mRNA is degraded with a half life of approximately 3 weeks in the presence of estrogen and exhibits a half life of 16 hours after estrogen is removed from the culture medium. The 30 fold increase in the stability of vitellogenin mRNA in the presence of estrogen is a specific, reversible cytoplasmic effect of the steroid hormone. Our measurements of the absolute rate of nuclear vitellogenin gene transcription and of the rate of appearance of the mRNA in the cytoplasm demonstrate that the average efficiency with which individual intervening sequences are excised from vitellogenin transcripts in vivo is at least 99%.