Quantifying bound and leaky states of resonant optical lattices by Rytov-equivalent homogeneous waveguides.

The properties of periodic optical lattices are generally investigated with rigorous numerical methods. For physical insight and understanding of the complex processes underlying observed spectra, simple analytical models are needed. Accordingly, we show that by applying full Rytov effective-medium formalisms, the bound and leaky states of resonant photonic lattices can be quantified with high precision. Thus, all key properties are embodied in Rytov-equivalent homogeneous waveguides. The symmetric effective-medium theory (EMT) model quantifies rigorously computed guided-mode resonance (GMR) reflectance loci defining the leaky states. The asymmetric EMT formula similarly quantifies the bound state in the continuum (BIC) loci. Even with the period and wavelength on similar scales, the analytical EMT refractive index solutions agree exactly with rigorous solutions. We apply the Rytov formulas to explain the resonant leaky band structure including appearance of GMR and BIC states as well as band transitions and band closure points. The wavenumbers of the equivalent waveguides represent the BIC embedded eigenvalues as quantified here.

Evidence that the transition of HIV-1 gp41 into a six-helix bundle, not the bundle configuration, induces membrane fusion.

Many viral fusion proteins exhibit a six-helix bundle as a core structure. HIV Env-induced fusion was studied to resolve whether membrane merger was due to the transition into the bundle configuration or occurred after bundle formation. Suboptimal temperature was used to arrest fusion at an intermediate stage. When bundle formation was prevented by adding inhibitory peptides at this stage, membranes did not merge upon raising temperature. Inversely, when membrane merger was prevented by incorporating lysophosphatidylcholine (LPC) into cell membranes at the intermediate, the bundle did not form upon optimizing temperature. In the absence of LPC, the six-helix bundle did not form when the temperature of the intermediate was raised for times too short to promote fusion. Kinetic measures showed that after the temperature pulse, cells had not advanced further toward fusion. The latter results indicate that bundle formation is the rate-limiting step between the arrested intermediate and fusion. Electrical measures showed that the HIV Env-induced pore is initially large and grows rapidly. It is proposed that bundle formation and fusion are each contingent on the other and that movement of Env during its transition into the six-helix bundle directly induces the lipid rearrangements of membrane fusion. Because peptide inhibition showed that, at the intermediate stage, the heptad repeats of gp41 have become stably exposed, creation of the intermediate could be of importance in drug and/or vaccine development.

Photosensitization of red blood cell hemolysis by lutetium texaphyrin.

Lutetium (III) texaphyrin photosensitizes postirradiation or "delayed" photohemolysis (DPH) of human and bovine red blood cells at 730 nm by a Type-2 pathway mediated by singlet molecular oxygen. The DPH rate increases with increasing incubation temperature and with the second power of the incident fluence. The experimental DPH curves are in good agreement with a multi-hit kinetics model based on target theory.

2.07-microm cw diode-laser-pumped Tm, Ho:YLiF(4) room-temperature laser.

Continuous-wave action is obtained at 2.07 microm from a 2-mm-long Tm-sensitized Ho:YLiF(4) crystal at room temperature when longitudinally pumped by a pair of diode-laser arrays. Laser output power at 300 K is 26 mW, with a 30% slope efficiency and a lasing threshold of 108 mW. A maximum output power of 187 mW is obtained from a 4-mm-long crystal at 77 K, with a 67% slope efficiency. A preliminary demonstration of cavity Q switching produced 165 microJ of pulse energy at a repetition rate of 100 Hz.

3.5-W Q-switched 532-nm Nd:YAG laser pumped with fiber-coupled diode lasers.

A diode-pumped laser with greater than 11 W of cw 1064-nm output power and 3.5 W of frequency-doubled average power at a 50-kHz pulse repetition frequency has been developed. A single Nd:YAG rod was pumped with the combined output of three fiber-coupled diode-laser arrays. Each pump laser was capable of 10 W of cw output power. The fiber output of each pump laser was first collimated and then focused with a single lens onto one end of a Nd:YAG rod. The resonator mirrors for the L-shaped cavity were selected such that thermal lensing in the laser crystal was mostly compensated. The 532-nm output beam quality factor (M(2)) was less than 1.5.

100-megabit-per-second resonant-cavity phase modulator for coherent optical communications.

A resonant-cavity electro-optic phase modulator is designed, implemented, and experimentally verified to operate at a data rate of 100 Mbits/s. The cavity is made up of a highly reflective backmirror and the partially reflective end of an electro-optic crystal. A voltage signal applied to the electro-optic crystal erturbs the effective optical path length inside the cavity and hence its resonance frequency. Because the phase of the reflected optical signal from the cavity is highly dispersive when the cavity is tuned near resonance, a cw incident signal will experience a large phase shift as the cavity is electro-optically tuned on and off resonance. This phase-dispersion effect can be used in the construction of an optical phase modulator capable of modulating the signal at data rate in excess of 100 Mbits/s. The performance of the modulator was measured by first heterodyne detecting the signal to an intermediate frequency andthen measuring the spectral characteristics with a radio frequency spectrum analyzer. The measured phase shift is shown to be in good agreement with theoretical predictions.

Generation of continuous-wave 194-nm radiation by sum-frequency mixing in an external ring cavity.

Several microwatts of tunable cw radiation near 194 nm in a linewidth of less than 2 MHz have been generated by sum-frequency mixing the radiation from a frequency-doubled argon-ion laser with the radiation from a ring dye laser in a crystal of potassium pentaborate. An external ring cavity resonant with the dye laser gives an enhancement factor of about 14 in the sum-frequency-generated radiation power. The Doppler-limited absorption spectrum of the 6s(2)S((1/2))-6p(2)P((1/2)) first resonance line of natural Hg II has been resolved, and the vacuum wave number for the mass-202 isotope has been measured to be 51485.904(20) cm(-1).

Diode-pumped optical parametric oscillator.

Diode-pumped optical parametric oscillation has been demonstrated for the first time to our knowledge in a single Nd:MgO:LiNbO(3) nonlinear crystal. The crystal is pumped by a semiconductor diode laser array at 812 nm. The Nd(3+) ions absorb the 812-nm radiation to generate 1084-nm laser oscillation. On internal Q switching the 1084-nm radiation pumps the LiNbO(3) host crystal that is angle cut at 46.5 degrees and generates optical parametric oscillation. The oscillation threshold that is due to the 1084-nm laser pump with a pulse length of 80 ns in a 1-mm-diameter beam was measured to be approximately 1 mJ and produced 0.5-mJ output at 3400-nm signal wavelength.

The purification and characterization of fetal liver hematopoietic stem cells.

Thy-1loSca-1+Lin-Mac-1+CD4- cells have been isolated from the livers of C57BL-Thy-1.1 fetuses. This population appears to be an essentially pure population of hematopoietic stem cells (HSC), in that injection of only six cells into lethally irradiated adult recipients yields a limit dilution frequency of donor cell-reconstituted mice. Sixty-seven to 77% of clones in this population exhibit long-term multilineage progenitor activity. This population appears to include all long-term multilineage reconstituting progenitors in the fetal liver. A high proportion of cells are in cycle, and the absolute number of cells in this population doubles daily in the fetal liver until 14.5 days postcoitum. At 15.5 days postcoitum, the frequency of this population falls dramatically. Long-term reconstituting HSC clones from the fetal liver give rise to higher levels of reconstitution in lethally irradiated mice than long-term reconstituting HSC from the bone marrow. The precise phenotypic and functional characteristics of HSC vary according to tissue and time during ontogeny.

Identification of a lineage of multipotent hematopoietic progenitors.

All multipotent hematopoietic progenitors in C57BL-Thy-1.1 bone marrow are divided among three subpopulations of Thy-1.1(lo) Sca-1+ Lin(-/lo) c-kit+ cells: long-term reconstituting Mac-1- CD4- c-kit+ cells and transiently reconstituting Mac-1(lo) CD4- or Mac-1(lo) CD4(lo) cells. This study shows that the same populations, with similar functional activities, exist in mice whose hematopoietic systems were reconstituted by hematopoietic stem cells after lethal irradiation. We demonstrate that these populations form a lineage of multipotent progenitors from long-term self-renewing stem cells to the most mature multipotent progenitor population. In reconstituted mice, Mac-1- CD4- c-kit+ cells gave rise to Mac-1(lo) CD4- cells, which gave rise to Mac-1(lo) CD4(lo) cells. Mac-1- CD4- c-kit+ cells had long-term self-renewal potential, with each cell being capable of giving rise to more than 10(4) functionally similar Mac-1- CD4- c-kit+ cells. At least half of Mac-1(lo) CD4- cells had transient self-renewal potential, detected in the spleen 7 days after reconstitution. Mac-1(lo) CD4(lo) cells did not have detectable self-renewal potential. The identification of a lineage of multipotent progenitors provides an important tool for identifying genes that regulate self-renewal and lineage commitment.