Parametric attosecond pulse amplification far from the ionization threshold from high order harmonic generation in He.

Parametric amplification of attosecond coherent pulses around 100 eV at the single-atom level is demonstrated for the first time by using the 3D time-dependent Schrödinger equation in high-harmonic generation processes from excited states of He+. We present the attosecond dynamics of the amplification process far from the ionization threshold and resolve the physics behind it. The amplification of a particular central photon energy requires the seed XUV pulses to be perfectly synchronized in time with the driving laser field for stimulated recombination to the He+ ground state and is only produced in a few specific laser cycles in agreement with the experimental measurements. Our simulations show that the amplified photon energy region can be controlled by varying the peak intensity of the laser field. Our results pave the way to the realization of compact attosecond pulse intense XUV lasers with broad applications.

All-solid-state VUV frequency comb at 160 nm using high-harmonic generation in nonlinear femtosecond enhancement cavity.

We realized a solid-state-based vacuum ultraviolet frequency comb by harmonics generation in an external enhancement cavity. Optical conversions were so far reported by only using gaseous media. We present a theory that allows the most suited solid generation medium to be selected for specific target harmonics by adapting the material's bandgap. We experimentally use a thin AlN film grown on a sapphire substrate to realize a compact frequency comb high-harmonic source in the Deep Ultraviolet (DUV) / Vacuum Ultraviolet (VUV) spectral range. By extending our earlier VUV source [Opt. Express26, 21900 (2018)] with the enhancement cavity, a sub-Watt level Ti:sapphire femtosecond frequency comb is enhanced to 24 W stored average power, its 3rd, 5th, and 7th harmonics are generated, and the targeted 5th harmonic's power at 160 nm increased by two orders of magnitude. The emerging nonlinear effects in the solid medium, together with suitable intra-cavity dispersion management, support optimal enhancement and stable locking. To demonstrate the realized frequency comb's spectroscopic ability, we report on the beat measurement between the 3rd harmonic beam and a 266 nm CW laser reaching about 1 MHz accuracy.

Non-perturbative generation of DUV/VUV harmonics from crystal surfaces at 108 MHz repetition rate.

We demonstrate non-perturbative 3rd (267 nm) and 5th (160 nm) harmonic generation in solids from a Ti:sapphire frequency comb (800 nm) at 108 MHz repetition rate. The experiments show that non-perturbative low harmonics are dominantly generated on the surface and on the interface between solids, and that they are not produced by bulk processes from the near-surface layer of the material. Measurements reveal that due to the lack of phase matching, the generated harmonics in bulk are suppressed by orders of magnitude compared to the signal generated on the surface. Our results pave the way for the development of all-solid-state high repetition rate harmonic sources for vacuum ultraviolet spectroscopy and high precision frequency comb metrology.

Coherent amplification of attosecond light pulses in the water-window spectral region.

We present a theoretical study on coherent extreme ultraviolet (XUV) attosecond pulse amplification mediated by nonlinear parametric enhanced forward scattering occurring in the interaction of a strong femtosecond infrared (IR) laser pulse combined with a weak attosecond XUV pulse train with an atom. We predict large amplification of XUV radiation when the IR strong pulse and the XUV weak pulse are optimally phased. We study high-order harmonic processes (HHG) in He, He(+) and Ne(++), and show how although the HHG yield is largely affected by the particular atom used as target, nonlinear parametric XUV amplification is only weakly affected. We conclude that XUV nonlinear parametric attosecond pulse amplification can be most efficiently observed by using atoms with a high ionization potential and that the nonlinear amplification is robust at high photon energies where HHG is not efficient, such as in the water-window spectral region.

Subdiffractive light pulses in photonic crystals.

We investigate propagation of light pulses in photonic crystals in the vicinity of the zero diffraction point. We show that Gaussian pulses due to nonzero width of their temporal spectrum spread weakly in space and time during the propagation. We also find the family of nonspreading pulses, propagating invariantly in the vicinity of the zero diffraction point of photonic crystals.

CD4+ lymphocytes and tuberculin skin test as survival predictors in pulmonary tuberculosis HIV-infected patients.

BACKGROUND: We analyse whether the tuberculin skin test is a good survival marker in a cohort of pulmonary tuberculosis patients with HIV infection (PTB/HIV). In all, 494 PTB/HIV patients were enrolled in Barcelona (Spain) between January 1992 and December 1994 in the Tuberculosis Program of Barcelona. The main data problem was the large proportion of missing values in the covariates percentage of T CD4+ lymphocytes and the tuberculin test results: only 157 patients (31.8%) had both covariates recorded. METHODS: Patients were dichotomized into two groups according to their level of immunosuppression (< or = 14 and >14% T CD4+ cells). First, we carried out the semiparametric and parametric complete case analysis. After this, we analysed the data assuming a missing at random non-response pattern. We developed a bootstrap approach where missing data in the markers are imputed via a two-way linear model. Using Weibull regression estimation, we used a multiple imputation scheme to estimate the parameters of interest. RESULTS: We found significative differences for the most immunosuppressed group when comparing positive tuberculin patients with those who were tuberculin negative. From a complete case approach and through a multivariate Cox analysis, we obtained a significant relative hazard of 0.3657 (95% CI: 0.13-1.02; P = 0.054). When a Weibull model was fitted, we estimated a constant relative percentile value of pR = 4.1329 (95% CI: 0.97-17.59). From a missing data approach, we obtain a higher constant relative percentile 5.48 (P = 0.079). CONCLUSIONS: The imputation method allows us to assess the protective character of positivity for the tuberculin test for the lowest CD4+ level. These findings strongly suggest the value of the tuberculin skin test as a qualitative measure of the immunological response and its interest for developing countries where specific laboratory tests are not affordable.

Polarization chaos in an optically pumped laser.

We study the steady-state and dynamic behavior of an optically pumped J = 0 ? J = 1 ? J = 0 laser operating with an isotropic ring cavity and an axial magnetic field. The gain anisotropy induced by a linearly polarized pump-laser f ield leads, in the steady state, to locking of the two circularly polarized components of the laser field, which acquires a linear polarization parallel to that of the pump field. In the presence of laser intensity instabilities, however, locking does not occur, and polarization instabilities appear. For the f irst time to our knowledge, polarization chaos has been found in a laser system.

[Use of DNA amplification to diagnose cytomegalovirus in HIV seropositive patients]. / Utilidad de la amplificación de DNA para diagnosticar citomegalovirus en la población seropositiva para el virus de la inmunodeficiencia humana.

BACKGROUND: Cytomegalovirus (CMV) infection is common among HIV seropositive patients, being difficult to diagnose because it requires cell cultures not available in all hospitals. DNA amplification is being applied for diagnosis of infectious diseases with an increase in sensitivity and specificity with respect to previous laboratory methods. METHODS: Polymerase chain reaction (PCR) has been used in comparison with culture isolation, early antigen detection to diagnose CMV infection in 22 HIV infected patients, that suffered from symptoms compatible with CMV infection at the present time, and in other 5 patients suffering from Kaposi sarcoma. PCR was done with primer for CMV IE genomic region. The amplified sequences were detected after hybridization with a gamma-P-32 labelled probe, followed by electrophoresis in a 5% polyacrylamide gel and autoradiography. RESULTS: The PCR allows to detect CMV genome in cases in which other tests are negatives, in blood as well as in urine, included those patients suffering only from febrile symptoms or with other associated pathogen. CONCLUSIONS: PCR is a sensitive method to detect CMV, although it does not establish the responsibility of CMV in HIV infected patients.