Compact tunable 80 MHz repetition rate vacuum ultraviolet light source up to 10 eV: intracavity high harmonic generation by nonlinear reflection on a AlN nanofilm in a mode locked Ti:sapphire oscillator.

We report the realization of an intra-oscillator high harmonic source based on a Kerr lens mode locked Ti:sapphire laser running at 80 MHz repetition rate. A nonlinear medium consisting of an AlN nanofilm on a thin sapphire substrate is placed inside the oscillator cavity. The harmonics are generated, in reflection geometry, on the AlN nanofilm, directing the harmonic beam out of the cavity. Exploiting the benefits of this approach, a compact size, tunable, high repetition rate and coherent vacuum ultraviolet light source with a spectrum up to the 7th harmonic has been achieved. In particular, the powerful 5th harmonic covering the 145-163 nm range aims to be an attractive tunable light source for spectroscopical applications.

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.

High-harmonic generation and parametric amplification in the soft X-rays from extended electron trajectories.

We report, for the first time, the generation of high-order harmonics in a spectral range between 200 eV and 1 keV with an unusual spectral property: only every 4(th) (4i + 1, i∈ℵ) harmonic line appears, whereas the usual high-harmonic spectra consist of every odd (2i + 1) harmonic. We attribute this unique property to the quantum path interference of two extended electron trajectories that experience multiple re-scattering. In the well-established theory, electrons emitted via tunnel ionisation are accelerated by a laser field, return to the ion and recombine. The acceleration typically lasts for less than one optical cycle, and the electrons radiate in the extreme ultraviolet range at recombination. In contrast, for extended trajectories, electrons are accelerated over two or more optical cycles. Here, we demonstrate that two sets of trajectories dominate and provide substantial contributions to the generated soft X-ray radiation because they fulfil the resonance condition for X-ray parametric amplification.

Parametric amplification of attosecond pulse trains at 11†nm.

We report the first experimental demonstration of the parametric amplification of attosecond pulse trains at around 11 nm. The helium amplifier is driven by intense laser pulses and seeded by high-order harmonics pulses generated in a neon gas jet. Our measurements suggest that amplification takes place only if the seed pulse-trains are perfectly synchronized in time with the driving laser field in the amplifier. Varying the delay, we estimate the durations of the individual extreme ultraviolet pulses within the train to be on the order of 0.2 fs. Our results demonstrate that strong-field parametric amplification can be a suitable tool to amplify weak attosecond pulses from non-destructive pump-probe experiments and it is an important step towards designing amplifiers for realization of energetic XUV pulses with sub-femtosecond duration using compact lasers fitting in university laboratories.

Respiration-related discharge of hyoglossus muscle motor units in the rat.

Although respiratory muscle motor units have been studied during natural breathing, simultaneous measures of muscle force have never been obtained. Tongue retractor muscles, such as the hyoglossus (HG), play an important role in swallowing, licking, chewing, breathing, and, in humans, speech. The HG is phasically recruited during the inspiratory phase of the respiratory cycle. Moreover, in urethane anesthetized rats the drive to the HG waxes and wanes spontaneously, providing a unique opportunity to study motor unit firing patterns as the muscle is driven naturally by the central pattern generator for breathing. We recorded tongue retraction force, the whole HG muscle EMG and the activity of 38 HG motor units in spontaneously breathing anesthetized rats under low-force and high-force conditions. Activity in all cases was confined to the inspiratory phase of the respiratory cycle. Changes in the EMG were correlated significantly with corresponding changes in force, with the change in EMG able to predict 53-68% of the force variation. Mean and peak motor unit firing rates were greater under high-force conditions, although the magnitude of discharge rate modulation varied widely across the population. Changes in mean and peak firing rates were significantly correlated with the corresponding changes in force, but the correlations were weak (r(2) = 0.27 and 0.25, respectively). These data indicate that, during spontaneous breathing, recruitment of HG motor units plays a critical role in the control of muscle force, with firing rate modulation playing an important but lesser role.

Synthesis and Diels-Alder cycloadditions of exo-imidazolidin-2-one dienes.

An efficient and versatile synthesis of novel exo-imidazolidin-2-one dienes is described. This involves the base-assisted condensation/cyclization cascade reaction of the monoimino derivatives of diacetyl with a series of isocyanates. This methodology enables preparation of symmetrical dienes, as long as the substrates have the same N substituent. Moreover, use of different N-substituted starting materials leads to formation of nonsymmetrical dienes. The reactivity of these dienes was evaluated in Diels-Alder reactions, showing a high reactivity.

Influence of prenatal nicotine exposure on development of the ventilatory response to hypoxia and hypercapnia in neonatal rats.

In a recent study (Huang YH et al. Respir Physiol Neurobiol 143: 1-8, 2004), we showed that prenatal nicotine exposure (PNE) increased the frequency of spontaneous apneic events on the first 2 days of life in unanesthetized neonatal rats. Here we test the hypothesis that PNE blunts chemoreceptor reflexes. Ventilatory responses to three levels each of hypoxia (inspired O(2) fraction: 16, 12, and 10%) and hypercapnia (3, 6, and 9% inspired CO(2) fraction, all in 50% O(2), balance N(2)), and one level each of combined hypoxia-hypercapnia (H/H; 12% inspired O(2) fraction/5% inspired CO(2) fraction) and hyperoxia (50% O(2), 50% N(2)) were recorded with head-out plethysmography in neonatal rats exposed to either nicotine (N = 12) or physiological saline (N = 12) in the prenatal period. Recordings were made on postnatal day 1 (P1), P3, P6, P9, P12, and P18, in each animal. The change in ventilation in response to hypoxia was blunted in PNE animals on P1 and P3, but there were no other treatment effects. Hyperoxia significantly depressed ventilation in both groups from P3-P18, but there were no significant treatment effects. The ventilatory response to 3, 6, and 9% inspired CO(2) was significantly blunted in PNE animals at all ages studied, due exclusively to a blunted tidal volume response. PNE also blunted the ventilatory response to H/H at all ages, due primarily to blunting of the tidal volume response. PNE had no significant effect on body mass or metabolic rate, except that PNE animals had a slightly higher mass on P18 and a lower metabolic rate on P1. As shown by others, PNE has small and inconsistent effects on hypoxic ventilatory responses, but here we show that responses to hypercapnia and H/H are consistently blunted by PNE due to a diminished tidal volume response. The combination of reduced hypoxic and hypercapnic sensitivity over the first 3 days of life may define an especially vulnerable developmental period.

The effects of a supervised toothbrushing programme on the caries increment of primary school children, initially aged 5-6 years.

Children in the London Boroughs of Kensington, Chelsea and Westminster have one of the highest levels of caries in England and Wales. In 1997/98, the mean dmft for 5-year-old children was 2.83 with only 45.9% of the children being caries free. The aim of this study was to determine whether teacher-supervised toothbrushing, once a day, at school, during term time, with commercial toothpaste containing 1,450 ppm fluoride, could reduce dental caries in primary school children when compared with children from the same community who did not receive this intervention. A total of 517 children (mean age 5.63 years) were recruited for the study. Class teachers were trained individually by the same dental hygienist in an appropriate toothbrushing technique for young children. Children in the intervention group brushed once a day at school. All examinations were by visual assessment only. All teeth present were assessed using the BASCD criteria. For children in the intervention group, the overall caries increment (2.60) was significantly less (10.9%; p < 0.001) than for children in the non-intervention group (2.92). Among different tooth surfaces, the difference in caries increment between the intervention group (0.78) and the non-intervention group (1.03) was greatest for the proximal surfaces (21.4%; p < 0.01). In conclusion, this study suggests that a programme of daily teacher-supervised toothbrushing with fluoride toothpaste can be effectively targeted into socially deprived communities and a significant reduction in dental caries can thereby be achieved especially among caries-susceptible children.

Laser technology: source of coherent kiloelectronvolt X-rays.

Generating X-rays that have the properties of laser light has been a long-standing goal for experimental science. Here we describe the emission of highly collimated, spatially coherent X-rays, at a wavelength of about 1 nanometre and at photon energies extending to 1.3 kiloelectronvolts, from atoms that have been ionized by a 5-femtosecond laser pulse. This means that a laboratory source of laser-like, kiloelectronvolt X-rays, which will operate on timescales relevant to many chemical, biological and materials problems, is now within reach.

Differences in hormonal and inflammatory parameters in male Lewis and Long Evans rats with adjuvant arthritis.

The purpose of the present study was (i) to compare secretory responses of prolactin and corticosterone to the acute stress of immobilization in male rats of the Lewis and Long Evans strains and (ii) to compare secretion of the two hormones in rats with fully developed adjuvant arthritis (AA) and their relationship with the intensity of the inflammatory reaction. A short immobilization of 5 min induced equal elevations of both hormones in both strains, but 20-min immobilization produced significantly stronger responses in Long Evans rats than in Lewis rats. AA inhibited prolactin secretion equally in both strains (from 11.6 +/- 1.3 ng/ml to 4.2 +/- 0.6 ng/ml in Lewis rats, p < 0.01, and from 3.7 +/- 0.6 to 2.12 +/- 0.1 ng/ml in Long Evans rats, p < 0.05), but caused a conspiciously larger elevation of corticosterone in the Long Evans than in the Lewis animals (11.5 +/- 1.2 microg/dl in Long Evans rats versus 5.1 +/- 0.5 microg/dl in Lewis rats, p < 0.01) while basal levels were similar. The larger corticosterone response in the Long Evans rats was associated with a stronger inflammatory reaction assessed by hind paw swelling (2.3 +/- 0.1 ml for Long Evans rats versus 1.8 +/- 0.08 ml for Lewis rats, p < 0.01) and plasma levels of nitric oxide (47.5 +/- 5.7 microM for Long Evans rats versus 28.7 +/- 2.5 microM for Lewis rats, p < 0.01) than in the Lewis males with lower corticosterone levels. In conclusion, there are significant, obviously genetically based, differences in the corticosterone responses to both immobilization and AA between the two strains, with the Long Evans rats reacting more strongly than the Lewis rats. The lack of the expected inverse relationship between corticosterone levels and the intensity of the inflammation indicates that the activity of corticosterone is not its primary determinant and that other important factors are involved.

Effects of chronic food restriction stress and chronic psychological stress on the development of adjuvant arthritis in male long evans rats.

The aim of the present experiments was to study the effect of stress of chronic food restriction (FR) and of repeated psychological challenge (PS) on the development of adjuvant arthritis in the Long Evans male rats. In the FR series, four groups of animals were compared: non-treated control (C) and arthritic (AA) rats, both with free access to food and water, and two analogous groups with a 40% food restriction-FR and AA-FR. All animals were killed 22 days after injection of cFA. In the PS series, stress was induced by random daily exposures of the rats to isolation, over-crowding, food/water deprivation, foot shock, tilting, fear for 14 days before cFA injection and 12 days thereafter (groups: C, AA, PS, and AA-PS). Arthritis causes swelling of the hindpaw, which was prevented in the AA-FR group. PS causes more severe disease symptoms: AA-PS rats had more severe hindpaw swelling than AA rats. Forty percent food restriction associated with elevated CORT levels mitigated inflammatory parameters activated during AA. PS worsened the disease. These results suggested that activated CORT is not the only cause of disease suppression, but some metabolic changes during FR play a role.

Stress of chronic food restriction attenuates the development of adjuvant arthritis in male Long Evans rats.

OBJECTIVE: To study the effect of the stress of chronic food restriction on the development of adjuvant arthritis in Long Evans male rats. METHODS: Four groups of animals were compared: non-treated control (C) and arthritic (AA) rats, both with free access to food and water and two analogous groups with a 40% food restriction, i.e. non-treated (FR) and arthritic (AA-FR) animals. All rats were killed 22 days following the injection of complete Freund's adjuvant. The parameters measured were: serum levels of albumin (ALB), nitrate, glucose, insulin, corticosterone (CORT), prolactin (PRL) and PRL mRNA in the adenopituitaries. In addition the activity of gamma-glutamyl transpeptidase (GGTP) was measured in the spleen. The pain threshold was determined by the tailflick method. The body weight of the animals was recorded on day 0, 3, 7, 11, 15 and 18 of the disease. RESULTS: Arthritis caused swelling of the hind paw (2.37 +/- 0.15 ml vs 1.1 +/- 0.05ml in controls, p < 0.01) which was prevented in the AA-FR group (1.44 +/- 0.13 ml, not significant against controls). Arthritis increased serum NO and reduced ALB levels; both changes were significantly restored in the FR-AA group. Food restriction did not alter the activation of GGTP, or the decrease of PRL mRNA observed in the AA group. Serum CORT was elevated in rats with food restriction (15.49 +/- 2.1 vs. 3.7 +/- 0.87 microg/dL) and remained enhanced to the same extent in AA and FR-AA groups. The tailflick latency prolonged in the AA group was reduced by food restriction. CONCLUSION: These results show that 40% food restriction associated with elevated CORT levels mitigated the inflammatory parameters activated during AA.

Pulse characteristics of an optical parametric oscillator pumped by sub-30-fs light pulses.

We obtained nearly transform-limited light pulses of 34 fs near 1.2 microm by pumping an optical parametric oscillator with a 2-mm-long KTP crystal by 26-fs pulses from a Ti:sapphire laser. The average power of the pulses that were obtained was greater than 50 mW, at an 80-MHz repetition rate. Attempts to downscale the pulse duration by decreasing the pump-pulse duration revealed remarkable limitations of the attainable pulse length for sub-30-fs pump pulses, in accordance with a recent theoretical study [J. Opt. Soc. Am. B 17, 741 (2000)].

The neurosurgical management of pain: a critical review.

The historical background of neurological surgery for pain demonstrates a remarkable honesty in outcome assessment. Because of apparent concentration on technological break-throughs, the contemporary focus is less directed to socially important issues. Because of the importance of outcome management in the current political environment, it is important to return to basic principles. A more socially and economically responsible approach is desperately needed in the neurosurgical community at this time. Standardization of outcome reporting and issues of societal impact must be included to justify the real value of new technology.

Perspectives on surgical indications. Implications for controls.

Surgical indications do not forecast outcomes as well as they should. A review of the literature demonstrates a significant rate of overutilization of coronary angiography, coronary artery surgery, cardiac pacemaker insertion, upper gastrointestinal endoscopies, carotid endarterectomies, back surgery, and pain-relieving procedures. The tendency appears to be a reduction in developing specific indications, and the evolution of a "last resort" concept. Arguments against this approach and a plea for the development of tighter focused indications are presented. Implications for cost saving and reduction in suffering are discussed.

Pericardial effusion as a clinical sign of Legionnaires' disease.

Legionnella often causes systemic manifestations. The clinical spectrum now includes cardiac legionnellosis. The first case of pericardial effusion was reported in September, 1981. To date, few additional cases have been reported. We hereby report a case of asymptomatic pericardial effusion with simultaneous pulmonary involvement. Pericardial involvement, if sought, would probably be more frequent than supposed in Legionnaires' disease.