Highly efficient THz four-wave mixing in doped silicon.

Third-order non-linearities are important because they allow control over light pulses in ubiquitous high-quality centro-symmetric materials like silicon and silica. Degenerate four-wave mixing provides a direct measure of the third-order non-linear sheet susceptibility χL (where L represents the material thickness) as well as technological possibilities such as optically gated detection and emission of photons. Using picosecond pulses from a free electron laser, we show that silicon doped with P or Bi has a value of χL in the THz domain that is higher than that reported for any other material in any wavelength band.

Properties of Conjugated Materials from Quantum Chemistry Coupled to Molecular Dynamics Generated Ensembles.

We provide a set of molecular dynamics simulations employing a force field specifically parameterized for organic π-conjugated materials. The resulting conformation ensemble was coupled to quantum chemistry calculations, and quantities of interest for optoelectronic applications, namely, ground- and excited-state energies, oscillator strengths, and dipole moments were extracted. This combined approach allowed not only exploration of the configurational landscape but also of the resulting electronic properties of each frame within the simulation and thus probe the link between conformation and property.

Self-trapping and excited state absorption in fluorene homo-polymer and copolymers with benzothiadiazole and tri-phenylamine.

Excited state absorption (ESA) is studied using time-dependent density functional theory and compared with experiments performed in dilute solutions. The molecules investigated are a fluorene pentamer, polyfluorene F8, the alternating F8 copolymer with benzothiadiazole F8BT, and two blue-emitting random copolymers F8PFB and F8TFB. Calculated and measured spectra show qualitatively comparable results.

General Force-Field Parametrization Scheme for Molecular Dynamics Simulations of Conjugated Materials in Solution.

We describe a general scheme to obtain force-field parameters for classical molecular dynamics simulations of conjugated polymers. We identify a computationally inexpensive methodology for calculation of accurate intermonomer dihedral potentials and partial charges. Our findings indicate that the use of a two-step methodology of geometry optimization and single-point energy calculations using DFT methods produces potentials which compare favorably to high level theory calculation.

Si:P as a laboratory analogue for hydrogen on high magnetic field white dwarf stars.

Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10(5) T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.

Quantitative description of interactions between linear organic chromophores.

To model intermolecular excitation transfer between organic chromophores in the framework of Förster theory, the interaction matrix element is needed for all relative orientations and separations of chromophores. Simulations of extended multi-chromophoric systems thus require a fast but reliable approximation scheme to calculate these dipole interactions. By means of a comparative study of the dipole approximation with quantum chemistry, we demonstrate that the usual line-dipole theory, while suitable for short molecules, breaks down for longer molecules with inter-molecular separations similar to or smaller than the length of the interacting chromophores; a limit that is typically found in conjugated polymer thin films.

Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene molecules.

Star-shaped molecules are of growing interest as organic optoelectronic materials. Here a detailed study of their photophysics using fluorescence depolarisation is reported. Fluorescence depolarisation dynamics are studied in branched oligofluorene-truxene molecules with a truxene core and well-defined three-fold symmetry, and are compared with linear fluorene oligomers.

Optical excitations in star-shaped fluorene molecules.

A detailed study of the low-energy optical transitions in two families of star-shaped molecules is presented. Both families have 3-fold rotational symmetry with oligofluorene arms attached to a central core. In one family, the core of the molecule is a rigid meta-linked truxene, while the other is a meta-linked benzene moiety.

Effect of exciton self-trapping and molecular conformation on photophysical properties of oligofluorenes.

Electronic absorption and fluorescence transitions in fluorene oligomers of differing lengths are studied experimentally and using density functional theory (DFT) and time-dependent DFT. Experimental values are determined in two ways: from the measured molar absorption coefficient and from the radiative rate deduced from a combination of fluorescence quantum yield and lifetime measurements. Good agreement between the calculated and measured transition dipoles is achieved.

Wavelet transforms for optical pulse analysis.

An exploration of wavelet transforms for ultrashort optical pulse characterization is given. Some of the most common wavelets are examined to determine the advantages of using the causal quasi-wavelet suggested in Proceedings of the LEOS 15th Annual Meeting (IEEE, 2002), Vol. 2, p.

Coherent dynamics of the localized vibrational modes of hydrogen in CaF2.

We report the observation of giant quantum coherence effects in the localized modes of ionized hydrogen in synthetic fluorite. Infrared free induction decay experiments on the substitutional H- center show dramatic modulations at negative delay times due to interference between multiple vibrational levels. Spectrally resolving the degenerate four wave mixing signal allows unambiguous assignments of the participating vibrational states.

Nature of HLA-associated predisposition to childhood acute lymphoblastic leukemia.

A molecular analysis was carried out in 63 sequentially diagnosed childhood acute lymphoblastic leukemia (ALL) patients and 1011 controls to investigate the homozygosity rate for HLA-DR53. HLA-DR53 is associated with acute myeloblastic leukemia at the protein level, and our previous study has shown its association with early-onset chronic myeloid leukemia only in homozygous form at the DNA level. In the present study, the homozygosity rates for DR53 were 17.

A molecular analysis of the telomeric end of the major histocompatibility complex. DNA typing of HLA-A3 subtypes and -B7.

A molecular investigation of the HLA-A, -B, -C, I82, D6S128, and D6S265 loci was carried out using RFLP and PCR analyses in 331 healthy individuals, 21 HLA-A3B7 homozygous subjects, and 17 HLA homozygous cell lines. New polymorphic RFLP patterns were noted at the HLA-A and -B loci which were useful for distinguishing subtypes of HLA-A3 and -B44. Strictly specific bands were observed for HLA-A3, -A11, -B7, -B57.

Homozygous MHC genotypes and longevity.

To investigate the relevance of the major histocompatibility complex (MHC) in longevity, we carried out a molecular analysis of the MHC in 432 unrelated healthy individuals. The comparison of individuals < or = 25 years and > 25 years showed that the 5.8-kb DQA1 allele, which corresponds to HLA-DR53, was negatively associated with longevity (p = 0.

Human major histocompatibility complex contains several leukemia susceptibility genes.

In mice, homozygosity for the Mhc haplotype H-2k is associated with increased susceptibility to spontaneous and virus-induced leukemia, lymphoma and other neoplasms in the predisposed host. The influence of the Mhc on malignant development in these models is to shorten the latency after virus inoculation. Here, we present evidence that a similar phenomenon results in early-onset of human leukemia.

Autoantibody associations with MHC class II antigens in scleroderma and autoimmune vasculitis.

HLA-DR haplotypes in patients with scleroderma and vasculitis were compared with those in healthy controls from the Scottish population to investigate whether any associations exist between MHC antigens and development of specific autoantibodies. In patients with systemic vasculitis the presence of any antibodies against neutrophil cytoplasmic antigens (ANCA) was associated with an increased frequency of DR8 [p < 0.004], and no patients expressed the DR5 antigen.

The spectrum of disease associated with a positive ANCA.

This is a retrospective analysis of all patients with a raised titre of ANCA in a single centre over a two-year period. Sixty-five patients were identified and clinical data is presented for 58 of these-34 male and 24 female. The median age is 56 years (13-83).

MHC class III polymorphisms in selection of donors for BMT.

Current practice for the selection of unrelated donors involves serological typing of HLA-A, -B and -DR antigens, DNA analysis of the class II region and the MLR. However, even after matching for the class II loci at the DNA level, a significant proportion of matched unrelated pairs remain MLR reactive. Ideal matching for BMT would be a match for the whole MHC haplotype rather than individual HLA loci.