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Biomedical subjects

G Korn

Publications and source records attributed to G Korn.

At least 19 recordsLinked to original sources
PubMed2006

[Femtosecond laser for refractive corneal surgery: foundations, mode of action and clinical applications].

Modern corneal laser surgery for the correction of optical errors of the eye requires a precise, reliable and reproducible creation of corneal lenticels (flaps). The use of ultra-short laser pulses with pulse durations of a few 100 femtoseconds (10(-13) s) allows for non-thermal cuts of ocular tissue. Mean flap thicknesses as small as 100 microm with a reproducibility of 10 microm (standard deviation) can be created by using mechano-optical adaptations through the eye. Thus, the femtosecond laser can be considered a good alternative approach with a safety in clinical use that is comparable with that of mechanical microkeratomes.

Epithelium, Corneal↗
PubMed2003

Rescattering of ultralow-energy electrons for single ionization of Ne in the tunneling regime.

Electron emission for single ionization of Ne by 25 fs, 1.0 PW/cm(2) laser pulses at 800 nm has been investigated in a kinematically complete experiment using a "reaction microscope." Mapping the complete final state momentum space with high resolution, a distinct local minimum is observed at P(e parallel )=0, where P(e parallel ) is the electron momentum parallel to the laser polarization. Whereas tunneling theory predicts a maximum at zero momentum, our findings are in good agreement with recent semiclassical predictions which were interpreted to be due to "recollision."

Journal Article↗
PubMed2002

Coincident fragment detection in strong field photoionization and dissociation of H2.

Electron-ion momentum spectroscopy is used to investigate the correlated electronic and nuclear motion in fragmentation of H2 in 4 x 10(14) W/cm(2), 25 fs laser pulses at 795 nm. Reaction channel dependent photoelectron spectra indicate that besides the main, stepwise H2 ionization H2(+) dissociation mechanism resulting in the products H(1s) + H(+) + e(-) a second new mechanism has to be assumed. The momentum distribution of H(+) ions in the dissociation channels H(1s) + H(+) + e(-) and 2H(+) + 2e(-) is found to be independent of the kinetic energy of the photoelectrons.

Journal Article↗
PubMed2002

Generation of single intense short optical pulses by ultrafast molecular phase modulation.

Pulses with durations below 4 fs have been generated using the method of ultrafast molecular phase modulation. A laser pulse shorter than the molecular vibrational or rotational period obtains spectral broadening during propagation along a hollow waveguide filled with previously impulsively excited Raman active gases. The induced time dependent phase, frequency, and frequency chirp are controllable by changing the delay between excitation and probe pulse within the molecular vibrational period.

Journal Article↗
PubMed2002

Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers.

We report on an experimental study of supercontinuum generation in photonic crystal fibers with low-intensity femtosecond pulses, which provides evidence for a novel spectral broadening mechanism. The observed results agree with our theoretical calculations carried out without making the slowly varying envelope approximation. Peculiarities of the measured spectra and their theoretical explanation demonstrate that the reason for the white-light generation in photonic crystal fibers is fission of higher-order solitons into redshifted fundamental solitons and blueshifted nonsolitonic radiation.

Journal Article↗
PubMed2001

Separation of recollision mechanisms in nonsequential strong field double ionization of Ar: the role of excitation tunneling.

Vector momentum distributions of two electrons created in double ionization of Ar by 25 fs, 0.25 PW/cm(2) laser pulses at 795 nm have been measured using a "reaction microscope." At this intensity, where nonsequential ionization dominates, distinct correlation patterns are observed in the two-electron momentum distributions. A kinematical analysis of these spectra within the classical "recollision model" revealed an (e,2e)-like process and excitation with subsequent tunneling of the second electron as two different ionization mechanisms. This allows a qualitative separation of the two mechanisms demonstrating that excitation-tunneling is the dominant contribution to the total double ionization yield.

Journal Article↗
PubMed2000

fs-pulse synthesis using phase modulation by impulsively excited molecular vibrations

We report the temporal characteristics of laser radiation transmitted through impulsively excited SF6 and exhibiting sideband Raman lines. Even without special dispersion control we observed a sequence of compressed fs pulses following with the period of the excited A(1g) vibrational mode of SF6. The use of both negative and positive group velocity dispersion compensation for the temporal compression was found to be as appropriately efficient. The results prove our new concept of the ultrafast molecular phase modulator.

Journal Article↗
PubMed2000

From above threshold ionization to statistical electron emission: the laser pulse-duration dependence of C60 photoelectron spectra

The photoelectron spectra of C60 ionized using a 790 nm laser with pulse durations varying from 25 fs to 5 ps have been determined. For 25 fs pulses, in the absence of fragmentation, the ionization mechanism is direct multiphoton ionization with clear observation of above threshold ionization. As the pulse duration is increased, this becomes dominated by a statistical ionization due to equilibration among the electronic degrees of freedom. For pulse durations on the order of a ps coupling to the vibrational degrees of freedom occurs and the well-known phenomenon of delayed (&mgr;s) ionization is observed.

Journal Article↗
PubMed2000

Momentum distributions of ne(n+) ions created by an intense ultrashort laser pulse

Vector momentum distributions of Ne(n+) (n = 1,2,3) ions created by 30 fs, approximately 1 PW/cm(2) laser pulses at 795 nm have been measured using recoil-ion momentum spectroscopy. Distinct maxima along the light polarization axis are observed at 4.0 and 7.5 a.u. for Ne2+ and Ne3+ production, respectively. Hence, mechanisms based on an instantaneous release of two (or more) electrons can be ruled out as a dominant contribution to nonsequential strong-field multiple ionization. The positions of the maxima are in accord with kinematical constraints set by the classical "rescattering model."

Journal Article↗
PubMed1994

The influence of protease inhibitors of the organism, especially bovine aprotinin, on the production of virulent hog cholera virus in tissue cultures.

The influence of biological protease inhibitors, especially aprotinin, on the production of virulent hog cholera virus in cell cultures. Production of number and size of fluorescent plaques after infection PK 15 cells with HC virus depended on properties of fetal calf sera added to the medium. By affinity chromatography on bovine alpha-chymotrypsin bound to CM-cellulose inhibitory proteins against chymotrypsin-like proteases could be eliminated from inhibiting sera. The fraction free from inhibitors did not inhibit plaque formation of HC virus in contrast to the fraction containing the eluted inhibitor(s). The inhibitory properties of fetal calf sera could be measured by a plaque inhibition test. By this test no inhibitory component could be demonstrated in 30 individual serum samples from pigs which on subsequent infection with HC virus exhibited virus growth to high titres. The serum from one pig, however, was found to inhibit HC virus plaque formation in PK 15 cells. When infected, this animal reacted with an unexpected low virus replication and mild disease symptoms. The production of virulent HC virus could likewise be inhibited by addition to the medium of the bovine protease inhibitor aprotinin; the degree of inhibition being dependent on the concentration. Virulent virus was produced again after elimination of this inhibitor from the medium. A weak inhibitory effect on plaque formation was also achieved by addition of the protease inhibitor chymostatin (3 mg/5 mg) and human alpha 1x plasma inhibitor to the medium of infected cells. The inhibitory effect appears to be very specific.

Animals↗
PubMed1986

Influence of sleep state and age on body motility in normal premature and full-term neonates.

To assess the distribution of body motility according to sleep states and age, we performed polygraphic tracings in 39 normal premature and full-term infants during the neonatal period, grouped according to conceptional age in weeks (wCA): 31-34, 35-36, 37-38, and 39-41 wCA. Movements of the upper and lower limbs (UL, LL) were recorded using piezo-electric crystals. Time passed with movements (TWM) was automatically measured at an accuracy of 0.01 seconds. Sleep states were defined according to EEG and REMs criteria. We did not find any significant difference when TWM was compared between age-groups before the 38 wCA, on both UL and LL. TWM decreases significantly after 39 wCA (p less than 0.01). In all infants studied TWM was significantly higher in active sleep (AS) than in quiet sleep (QS). TWM in indeterminate sleep (IS) was usually similar to that in AS. The prevalence of body motility during AS compared to QS further confirms the observation of differences in the control of physiological variables according to sleep states as early as 31 wCA (the lower limit of our study).

Gestational Age↗
PubMed1985

[Sleep organization in normal premature newborn infants. Polygraphic study].

Polygraphic recordings including EEG, eye movements (REMs) and limb movements (recorded by piezo-electric crystals), chin EMG and respiratory rate, were performed in 23 normal neonates of 31 to 38 weeks conceptional age (CA). The concordance between various parameters characteristic for sleep states was analysed. For all infants, it was possible to define periods (greater than 7 min) of active (AS) and quiet sleep (QS) according to EEG and REM criteria. Mean duration of the 1st sleep cycle (including both AS and QS) lasted 57 to 65 min; no significant difference regarding CA was observed. According to EEG and REM criteria, AS accounted for 47 to 52% of each sleep cycle; QS rose from 27% at 31-34 wCA to 33% at 35-36 wCA (P less than 0.05); indeterminate sleep (IS) decreased from 32% at 31-34 wCA to 10% at 35-36 wCA (P less than 0.01). At all ages studied, the observed duration of AS was not modified when other criteria were considered in addition to EEG and REMs. On the contrary, the observed duration of QS was frequently shortened and scattered with interruptions (indeterminate sleep) when respiration and body motility were considered; but the tonic EMG remained for 80% of QS. The concordance between different criteria characteristic for QS was not better at 37-38 wCA compared to 31-34 wCA.

Electroencephalography↗
PubMed1983

Respiratory frequency during sleep in siblings of sudden infant death syndrome victims. A comparison with control, normal infants.

The present study was carried out on 76 polygraphic recordings performed on 38 siblings of sudden infant death syndrome victims and on 38 control (2 days to 18 weeks old) infants. Each sibling corresponded to a control infant according to gender, gestational age at birth and postnatal age criteria. We found that in siblings as in controls, respiratory frequency (RF) was higher in active sleep (AS) compared to quiet sleep (QS) state (P less than 0.05 for 11-18 week siblings, p less than 0.01 for the other groups). During the transitional sleep (TS), RF was on an intermediate level (AS greater than TS greater than QS). There were no significant differences between RF of siblings compared to controls, except that RF during QS in 6-10 week control infants was higher than in siblings of the same age (P less than 0.05). We found a wide variability between RF of different individuals within all the age groups of siblings and of controls (P less than 0.001). However, a high correlation was usually noted between RF found in different sleep states: some infants (siblings or controls) breathed more rapidly and others more slowly in all states studied. In siblings, as previously described in other groups of normal infants, RF seems to be an individual characteristic. In addition, the present work shows that according to RF criterion, healthy siblings are similar to normal infants.

Circadian Rhythm↗
PubMed1981

[Waking-sleeping transition in the newborn baby and in infants before the age of 3 months (author's transl)].

The mode of transition from waking to sleeping was studied using two methods: (1) by the simple observation of behavioural criteria (opening and closing of eyes, crying, motility), and in particular, rapid eye movements (REM); (2) from combined behavioural and polygraphic criteria (recorded REM, chin EMG, respiratory rhythm, EEG). This study shows that sleep may begin without complete eye closure, with half closed eyes or with brief, alternating opening and closing of the eyes. The modifications of the various polygraphic and behavioural parameters depend on the stage of sleep which is the outcome. The changes which most often condition the passage to a given stage of sleep are the following: (a) inhibition of the tonic chin EMG in the transition towards active sleep (AS); (b) modifications of the EEG in the transition towards quiet sleep (QS); (c) stopping of crying and of agitation in the transition towards indeterminate sleep (IS). The comparison of results obtained using the two methods of analysis shows that the simple observation of behavioural parameters overestimates the number of sleep inductions in AS. This method does not take into account discordances between polygraphic parameters. This study confirms the reduction with age of sleep onsets in AS, with a concomitant increase in sleep onsets in QS. It stresses the variability in the mode of onset of sleep in normal children of the same age group, as well as in successive sleep inductions in the same child.

Age Factors↗
PubMed1979

Respiratory variability according to sleep states during mechanical ventilation: a polygraphic study in a baby with bilateral diaphragmatic paralysis.

Polygraphic recordings were performed at 1 and 2 months of age in mechanically ventilated baby (25-27 insufflations per min) with bilateral diaphragmatic paralysis. The EEG and the sleep organisation were normal for the given age. The breathing patterns were state dependent. Autonomous breathing movements and phasic inspiratory intercostal EMG were present during both wakefulness and active sleep (AS). In quiet sleep (QS) the respiration was usually passive, completly dependent on the respirator. Small autonomous breathings can occur in some periods of QS, simultaneously with the appearance of numerous skin potential responses. These results are probably related to the differences between the respiratory control in QS and in AS.

Diaphragm↗